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Analysis of geographic variation in the antipredator adaptations of the guppy : Poecilia reticulata Seghers, Benoni Hendrik 1973

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AN ANALYSIS OF GEOGRAPHIC VARIATION IN THE ANTIPREDATOR ADAPTATIONS OF THE GUPPY, POECILIA RETICULATA by BENONI HENDRIK SEGHERS B . S c , U n i v e r s i t y of B r i t i s h Columbia, 1967 A THESIS. SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of ZOOLOGY We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA January, 197 3 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced d e g r e e at the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y , t f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Depar tment o f The U n i v e r s i t y o f B r i t i s h C o l u m b i a V a n c o u v e r 8, Canada ABSTRACT The main o b j e c t i v e of t h i s study was to d e s c r i b e and e x p l a i n s e v e r a l f e a t u r e s of geographic v a r i a t i o n among i s o l a t e d and s e m i - i s o l a t e d p o p u l a t i o n s ( T r i n i d a d , West Indies) of the guppy, P o e c i l i a r e t i c u l a t a . Three main aspects of geographic v a r i a t i o n were c o n s i d e r e d : (i) sex r a t i o , ( i i ) body s i z e , and ( i i i ) a n t i p r e d a t o r b e h a v i o r . (i) . Extreme d e v i a t i o n s ( f a v o u r i n g females) from a t h e o r e t i c a l Mendelian sex r a t i o were c o r r e l a t e d with the presence of dense p o p u l a t i o n s of a s m a l l c y p r i n o d o n t i d p r e d a t o r , R i v u l u s h a r t i i . L a b o r a t o r y experiments r e v e a l e d t h a t t h i s v a r i a t i o n was not caused by g e n e t i c d i f f e r e n c e s i n the sex d e t e r m i n a t i o n system. In a d d i t i o n , sex r a t i o s were not c o r r e l a t e d w i t h sexual dimorphism i n c o l o u r . P r e d a t i o n experiments wi t h R i v u l u s demonstrated t h a t male guppies were not s e l e c t i v e l y a t t a c k e d but were l e s s adept a t a v o i d i n g c a p t u r e . S i z e - s e l e c t i v e p r e d a t i o n by R i v u l u s a l s o p l a c e d males a t a s e l e c t i v e disadvantage. Whether conspicuous c o l o r a t i o n i n c r e a s e s the l i a b i l i t y o f males to p r e d a t i o n has y e t to be demonstrated u n e q u i v o c a l l y . ( i i ) P o p u l a t i o n s of guppies sampled i n 1967 and 1969 showed a s t a b l e p a t t e r n of v a r i a t i o n i n body s i z e ; d i f f e r e n c e s of over 41% i n body l e n g t h and 200% i n weight i i i i i were d i s c o v e r e d . In compliance w i t h Bergmann's Rule there was a s i g n i f i c a n t n e g a t i v e c o r r e l a t i o n between body s i z e and stream temperature. Though a s u b s t a n t i a l p o r t i o n of the s i z e v a r i a t i o n can be e x p l a i n e d as a d i r e c t phenotypic response to environmental d i f f e r e n c e s , there i s a l s o good evidence f o r m i c r o e v o l u t i o n a r y d i f f e r e n c e s . Of a m u l t i t u d e of p o t e n t i a l s e l e c t i v e f a c t o r s t h a t might be r e s p o n s i b l e f o r t h i s g e n e t i c d i v e r s i t y , o n l y one, s i z e - s e l e c t i v e p r e d a t i o n , was i n v e s t i g a t e d . F i e l d and l a b o r a t o r y evidence supported the hy p o t h e s i s t h a t l a r g e guppies enjoy an advantage wi t h r e s p e c t t o R i v u l u s p r e d a t i o n but are more v u l n e r a b l e to l a r g e c h a r a c i d and c i c h l i d p r e d a t o r s such as H o p l i a s malabaricus and C r e n i c i c h l a a l t a . In the l a b o r a t o r y , s i z e - s e l e c t i v e p r e d a t i o n appeared t o be caused by d i f f e r e n c e s i n the h a n d l i n g e f f i c i e n c y of the p r e d a t o r s , however, i n nature the i n t e r a c t i o n of s e v e r a l other f a c t o r s must be c o n s i d e r e d . ( i i i ) F i e l d o b s e r v a t i o n s r e v e a l e d t h a t where c h a r a c i d and c i c h l i d p r e d a t o r s were p r e s e n t (and R i v u l u s absent) guppies, (a) were more r e s t r i c t e d t o the stream shore, (b) showed a g r e a t e r tendency t o s c h o o l , (c) avoided a p o t e n t i a l p r e d a t o r a t a g r e a t e r d i s t a n c e , and (d) had a lower alarm t h r e s h o l d . To assess the f u n c t i o n a l and e v o l u t i o n a r y s i g n i f i c a n c e of t h i s b e h a v i o r a l v a r i a t i o n , p r e d a t i o n experiments were conducted w i t h samples of wild-r caught and p r e d a t o r - n a i v e (laboratory-bred) guppies i v o r i g i n a t i n g from 5 n a t u r a l p o p u l a t i o n s . These t e s t s demon^-s t r a t e d t h a t f i s h e i t h e r taken, or descended, from populat-i o n s exposed to c h a r a c i d s and c i c h l i d s were r e l a t i v e l y l e s s v u l n e r a b l e than those exposed t o R i v u l u s . To determine why the p r e d a t o r - n a i v e guppies were s e l e c t e d non-randomly, a comparison wa,s made of t h e i r h a b i t a t p r e f e r e n c e s , s c h o o l i n g b e h a v i o r , r e a c t i o n d i s t a n c e , and escape motor p a t t e r n s . S i g n i f i c a n t i n t e r p o p u l a t i o n d i f f e r e n c e s were found f o r s e v e r a l of the measures; gener-a l l y , these were c o n s i s t e n t w i t h the f i e l d o b s e r v a t i o n s . In a d d i t i o n , i t was hypothesized t h a t c e r t a i n p o p u l a t i o n s may l e a r n to a v o i d p r e d a t o r s more r a p i d l y . I t was concluded t h a t much of the geographic v a r i a ^ t i o n i n a n t i p r e d a t o r behavior i s caused by g e n e t i c d i f f e r e n c e s a t t r i b u t a b l e t o d i f f e r e n t i a l p r e d a t i o n p r e s s u r e s . In some cases, these m i c r o e v o l u t i o n a r y d i f f e r e n c e s are a p p a r e n t l y maintained without a major b a r r i e r to gene flow. ACKNOWLEDGEMENTS Th i s t h e s i s was s u p e r v i s e d by Dr. N. R. L i l e y . I extend s i n c e r e thanks to him f o r i n t r o d u c i n g me t o the problem c o n s i d e r e d i n t h i s study and f o r c r i t i c a l l y r e a d i n g the manuscript. I am a l s o g r a t e f u l to my r e s e a r c h committee, Drs. A. B. Acton, C. S. H o l l i n g , and J . D. McPhail f o r guidance and comments on the t h e s i s . For h o s p i t a l i t y i n the Zoology S e c t i o n , Department of B i o l o g i c a l S c i e n c e s , U n i v e r s i t y of the West I n d i e s , St. Augustine, T r i n i d a d , I am indebted t o Dr. B. D. Ainscough, P r o f . F. G. Cope, Dr. J . S. Kenny, and Mr. R. L. Loregnard. Senator J e f f r e y S t o l l m e y e r k i n d l y p e r m i t t e d me t o c o l l e c t f i s h on h i s e s t a t e and Mr. Hugh Wood, C h i e f F i s h e r i e s O f f i c e r , M i n i s t r y o f A g r i c u l t u r e , allowed me t o main t a i n f i s h a t the Freshwater F i s h e r i e s Research S t a t i o n , Bamboo Grove. Miss Shireen Imam and Mr. Robert W. DeForest gave f r e e l y of t h e i r time t o help with the f i e l d work. I p r o f i t t e d g r e a t l y from the advice and good humour of my f e l l o w students, e s p e c i a l l y Mr, Peter J . B a l l i n , Mr. Kim D. Hyatt, and Dr. Donald L. Kramer. F i n a l l y , I thank my parents f o r t h e i r p a t i e n c e , v and c h e e r f u l encouragement throughout t h i s study. T h i s i n v e s t i g a t i o n was f i n a n c e d by the N a t i o n a l Research C o u n c i l of Canada through o p e r a t i n g grants to Dr. N. R. L i l e y and postgraduate s c h o l a r s h i p s to myself. TABLE OF CONTENTS Page A b s t r a c t . , . . . . . . . . . i i Acknowledgements , v L i s t o f Tables , x i i L i s t of F i g u r e s x i v Chapter 1. I n t r o d u c t i o n . . 1 Main o b j e c t i v e s of the study. . . . . . . . 3 The b i o l o g y of P o e c i l i a r e t i c u l a t a : r e l e v a n t l i t e r a t u r e . . . . . . . . . . . 7 2. M a t e r i a l s and Methods . , , . , 10 I n t r o d u c t i o n . . . . . . . . . . . . . . . . 10 C o l l e c t i o n techniques . . . . . . . . . . . 10 Labor a t o r y p o p u l a t i o n s a t Vancouver . . . . 12 Maintenance of f i s h 13 Obs e r v a t i o n and r e c o r d i n g methods . . . . . 14 E c o l o g i c a l measurements . . . . . . . . . . 15 Measurements on guppies 17 3. The Environment 19 I n t r o d u c t i o n 19 C l a s s i f i c a t i o n o f streams 20 Geographic d i s t r i b u t i o n of f i s h s p e c i e s . . 20 v i i v i i i Chapter Page Predators of the guppy. . . . . . . . . . . 2 3 D i s t r i b u t i o n and n a t u r a l h i s t o r y of the main p r e d a t o r s . . . . . . . . . . 3 1 M i g r a t i o n and d i s p e r s a l o f guppies. . . . . 3 5 Summary of Chapter 3 . . . . . . . . . . . . 3 9 4 . Geographic V a r i a t i o n i n the Sex Ratio, . . . . 4 1 I n t r o d u c t i o n . • 4 1 Sex r a t i o s of n a t u r a l p o p u l a t i o n s . . . . . 4 2 Sex r a t i o s of l a b o r a t o r y stocks . . . . . . 4 6 R e l a t i o n s h i p o f p r e d a t i o n to v a r i a t i o n i n the sex r a t i o . . . . . . . . . . . . . 4 9 S i z e h y pothesis . . . . . . . . . . . . . . 57 Behavior hypothesis . . . . . . 5 8 R e l a t i v e s u r v i v a l of the sexes i n p r e d a t i o n experiments . . . . . . . . . . 5 9 A. Experiments w i t h wild-caught f i s h , . . 6 0 B. Experiments w i t h l a b o r a t o r y stocks , . 6 4 Procedure f o r standard s u r v i v a l t e s t . . . 6 4 D i s c u s s i o n of s u r v i v a l experiments, . . . 7 1 T e s t s of the behavior h y p o t h e s i s . . . . . . 7 5 D i s c u s s i o n of sex d i f f e r e n c e s i n behavior. 82 C o n c l u s i o n and g e n e r a l d i s c u s s i o n of geographic v a r i a t i o n i n the sex r a t i o , . 8 3 Summary of Chapter 4 . . . . . . . . . . . , 8 9 i x Chapter Page 5. Geographic V a r i a t i o n i n Body S i z e . . . . . . 92 I n t r o d u c t i o n . . . . . . . . . . . . . . . . 92 Body s i z e v a r i a t i o n i n n a t u r a l p o p u l a t i o n s . . . . . . . . . . 95 R e l a t i o n s h i p o f body s i z e and temperature . 97 R e l a t i o n s h i p of body s i z e and p r e d a t i o n . . 103 Evidence from the f i e l d f o r s i z e s e l e c t i o n . . . . . . 104 Experimental a n a l y s i s of s i z e s e l e c t i o n . . 107 Mechanisms of pr e d a t o r s e l e c t i o n f o r s i z e d i f f e r e n c e s i n prey. . . . . . . . . 112 C o n c l u s i o n and g e n e r a l d i s c u s s i o n o f geographic v a r i a t i o n i n body s i z e . . . . 123 Summary of Chapter 5 . . . . . . . . . . . . 129 6. Geographic V a r i a t i o n i n Behavior. 131 I n t r o d u c t i o n . , . . 131 F i e l d o b s e r v a t i o n s of behavior f . 132 D i s t r i b u t i o n of guppies i n the stream environment. . . . . . . . . . . 133 Sch o o l i n g behavior 137 Reaction d i s t a n c e t o p o t e n t i a l p r e d a t o r s . . . . . . . . . . . . . . . 141 Motor p a t t e r n s used i n a n t i p r e d a t o r behavior 143 X Chapter Page P o p u l a t i o n d i f f e r e n c e s i n escape motor p a t t e r n s 148 Summary of the f i e l d o b s e r v a t i o n s of b ehavior . . . . . . . . . . . . . . 150 S u r v i v a l value of b e h a v i o r a l d i f f e r e n c e s i n wild-caught f i s h 151 Co n c l u s i o n f o r s u r v i v a l experiments wi t h wild-caught guppies , . » 159 R e l a t i v e s u r v i v a l of l a b o r a t o r y s t o c k s . . , 161 D i s c u s s i o n of r e s u l t s of s u r v i v a l experiments . . . . . . . . . . . . . . . 166 Geographic v a r i a t i o n i n the a n t i p r e d a t o r behavior of l a b o r a t o r y stocks . . . . . . 169 (a) M i c r o h a b i t a t s e l e c t i o n . 170 (b) Reaction d i s t a n c e t o p r e d a t o r s . . . . 182 (c) Escape motor p a t t e r n s of i n d i v i d u a l f i s h 188 (d) S c h o o l i n g b e h a v i o r . 208 Co n c l u s i o n and ge n e r a l d i s c u s s i o n of geographic v a r i a t i o n i n beh a v i o r . . . . . 217 Summary of Chapter 6 . . . . . . . . . . . . 226 7. General D i s c u s s i o n and C o n c l u s i o n s . . . . . . 231 The environment 231 C o l o r a t i o n and the sex r a t i o , . . . . . . . 232 Body s i z e 234 x i Chapter Page A n t i p r e d a t o r behavior , 235 Suggestions f o r f u t u r e r e s e a r c h 239 L i t e r a t u r e c i t e d , 244 Appendix 261 LIST OF TABLES Table Page 1. C l a s s i f i c a t i o n and p h y s i c a l f e a t u r e s of streams i n the Northern Range (March-June, 1 9 6 9 ) . , . . . , . , . , . , . 21 2. N a t u r a l h i s t o r y of the main f i s h p r e d a t o r s 32 3. The p r i n c i p a l f i s h p r e d a t o r s o c c u r r i n g a t the main study areas. . . . . . 36 4. S i t e attachment of guppies i n the P e t i t e Curucaye R. . . . . . . . . . . . . . 38 5A. The sex r a t i o s of n a t u r a l p o p u l a t i o n s (1967) , . 43 5B. The sex r a t i o s of n a t u r a l p o p u l a t i o n s (1969) 44 6. The sex r a t i o s of l a b o r a t o r y p o p u l a t i o n s . , . 48 7. The r e l a t i o n s h i p of the sex r a t i o t o the presence of f i s h p r e d a t o r s . . . . . . . 50 8. R i v u l u s abundance i n two streams i n 1967 and 1969 53 9. Sex r a t i o of guppies a t Blue B a s i n i n 1967 and 1969 , 54 10. P r e d a t i o n by R i v u l u s on male and female guppies i n a 40 l i t e r aquarium . . . . . . . 61 11. The o r i g i n , sex, and s i z e of i n d i v i d u a l C r e n i c i c h l a used i n experiments a t Vancouver 67 12. Reaction d i s t a n c e of naive guppies to a dead C r e n i c i c h l a 81 13. Predators and prey used i n Experiment 5.1. . . 109 x i i x i i i Table Page 14. D i r e c t i o n of change of mean body s i z e i n 18 s i z e s e l e c t i o n t e s t s , . 112 15. The r e l a t i o n s h i p of body s i z e and r e a c t i o n d i s t a n c e to a p r e d a t o r . . . . . . . 115 16. P r e d a t i o n e f f i c i e n c y of C r e n i c i c h l a on l a r g e and s m a l l guppies . . . . . . . . . 117 17. P r e d a t i o n . e f f i c i e n c y of Astyanax on l a r g e and s m a l l guppies. . . . . . . . . . . 119 18. P r e d a t i o n e f f i c i e n c y of R i v u l u s on l a r g e and s m a l l guppies 122 19. The d i s t r i b u t i o n of 5 p o p u l a t i o n s of guppies i n r e l a t i o n to water v e l o c i t y , depth, and d i s t a n c e from the shore . . . . . 138 20. The development of s c h o o l i n g behavior i n 5 p o p u l a t i o n s of guppies . . . . . . „ . , . 141 21. R e l a t i v e m o r t a l i t y of wild-caught guppies exposed to p r e d a t o r s 156 22. R e l a t i v e m o r t a l i t y of naive l a b o r a t o r y stocks of guppies exposed to C r e n i c i c h l a . . 165 23. A comparison of the frequency of occurrence of 5 stocks of guppies i n r e l a t i o n t o water depth. , 174 24. Escape motor p a t t e r n s of 5 l a b o r a t o r y stocks of guppies exposed to a dead C r e n i c i c h l a 190 25. Comparison of p r e d a t o r y success and prey escape behavior f o r 5 l a b o r a t o r y stocks of guppies. . . . . . . . . . . . . . 196 26. Responses of naive Guayamare and P a r i a guppies to a simulated a e r i a l p r e d a t o r , . , 206 27. Concordance of b e h a v i o r a l measures taken on 5 stocks of guppies 220 LIST OF FIGURES F i g u r e Page 1. Map of the n o r t h e r n h a l f of the i s l a n d of T r i n i d a d , West I n d i e s , showing the major r i v e r systems, . . . . . . . . . . 11 2. The d i s t r i b u t i o n o f f i s h s p e c i e s i n the Northern Range r e g i o n 22 3. A schematic r e p r e s e n t a t i o n of the d i s t r i b u t i o n of the major f i s h s p e c i e s i n the northwest corner of T r i n i d a d 24 4. Photograph of the major f i s h p r e d a t o r s of the guppy . . . . . . . . . . . . . . . . 27 5. A comparison of the sex r a t i o of 13 p o p u l a t i o n s of guppies sampled i n 1967 and resampled i n 1969 45 6. The r e l a t i o n s h i p of the sex r a t i o t o the r e l a t i v e d e n s i t y of R i v u l u s . 52 7. R e l a t i v e s u r v i v a l of male and female guppies i n the experimental s e c t i o n of the P e t i t e Curucaye R i v e r , , , 63 8. R e l a t i v e s u r v i v a l of male and female Caparo stock guppies exposed to C r e n i c i c h l a . 69 9. R e l a t i v e s u r v i v a l of male and female guppies of two sto c k s exposed to R i v u l u s 71 10. Predatory behavior of R i v u l u s exposed to male and female guppies of the Paramaribo stock . . . . . . . . . . . . . . 78 11. The r a t i o o f p r e d a t i o n attempts to s u c c e s s f u l captures f o r R i v u l u s f e e d i n g on male and female guppies . . . . . 79 x i v xy F i g u r e Page 12. The body s i z e of a d u l t male guppies c o l l e c t e d a t 20 s i t e s i n 1969, . . . . . . . 96 13. A comparison of the body s i z e of a d u l t male guppies from 13 p o p u l a t i o n s sampled i n 1967 and resampled i n 1969 98 14. The r e l a t i o n s h i p o f mean body s i z e of a d u l t males to stream temperature . . . . 101 15. The r e l a t i o n s h i p of R i v u l u s body s i z e t o the s i z e o f guppies taken as prey 105 16. S i z e s e l e c t i o n by p r e d a t o r s on 18 experimental p o p u l a t i o n s of guppies I l l 17. The d i s t r i b u t i o n of a d u l t guppies ac r o s s a s e c t i o n of the P a r i a R i v e r , . . . . 136 18. R e l a t i v e s u r v i v a l o f wild-caught female guppies of two p o p u l a t i o n s exposed t o e i t h e r a s i n g l e C r e n i c i c h l a or two Hop 11 as , .• , 158 19. Depth p r o f i l e o f spontaneous swimming behavior o f 5 l a b o r a t o r y stocks of guppies, 173 20. H a b i t a t s e l e c t i o n o f 5 l a b o r a t o r y stocks of guppies p l a c e d i n a depth g r a d i e n t . . . . . . . . . . . . . . . 179 21. Reaction d i s t a n c e o f 5 l a b o r a t o r y stocks of guppies t o a dead C r e n i c i c h l a , . , . 186 22. R e l a t i o n s h i p of escape motor p a t t e r n s to s u r v i v a l time i n two l a b o r a t o r y stocks o f guppies exposed to R i v u l u s . . . . 199 23. T e s t apparatus used f o r measuring the responsiveness of l a b o r a t o r y stocks of Guayamare and P a r i a guppies to a simulated a e r i a l p r e d a t o r . 202 24. Mean index of cohesion f o r f i v e l a b o r a t o r y stocks o f guppies 211 x v i F i g u r e Page 25. Predatory behavior of R i v u l u s exposed t o equal numbers o f P a r i a and Lower A r i p o guppies ( l a b o r a t o r y stock) 216 CHAPTER 1 INTRODUCTION An animal f a c e s many c h a l l e n g e s to s u r v i v a l i n i t s n a t u r a l environment. Numerous b i o t i c and a b i o t i c f a c t o r s i n f l u e n c e the l i k e l i h o o d t h a t an animal w i l l produce v i a b l e o f f s p r i n g . Most animals share a common c h a l l e n g e t o s u r v i v a l — p r e d a t i o n . In t h i s t h e s i s I examine how n a t u r a l p o p u l a t i o n s of a sm a l l t r o p i c a l f i s h a,re adapted to the r i s k of p r e d a t i o n . < B i o l o g i s t s u s u a l l y study p r e d a t o r - p r e y systems a t e i t h e r the nume r i c a l , f u n c t i o n a l , or e v o l u t i o n a r y l e v e l . P o p u l a t i o n e c o l o g i s t s work p r i m a r i l y a t the numerical l e v e l — t h e y attempt t o measure the impact o f pr e d a t o r s on the numbers of prey and how i n t u r n prey d e n s i t y governs the number of p r e d a t o r s t h a t a giv e n area can support (e.g. R i c k e r , 1954; Huffaker e t a l , 1963). At the f u n c t i o n a l l e v e l , f a c t o r s c o n t r o l l i n g the consumption of prey are emphasized, p a r t i c u l a r l y the p h y s i o l o g y and behavior of the pre d a t o r (e.g. I v l e v , 1961; H o l l i n g , 1966; Beukema, 196 8). The t h i r d l e v e l a t which we can view a pred a t o r - p r e y system i s the more g e n e r a l e v o l u t i o n a r y l e v e l ; an attempt i s made to measure the c o n t r i b u t i o n o f p r e d a t i o n to n a t u r a l 1 2 s e l e c t i o n i n the prey (e.g. C a i n and Sheppard, 1954; K e t t l e w e l l , 1961; C u r i o , 1965, 1969, 1970a; Mc P h a i l , 1969). T h i s i s the approach adopted here. The e v o l u t i o n a r y impact of p r e d a t o r s on prey popu-l a t i o n s i s normally d i f f i c u l t t o assess d i r e c t l y . Often i t i s not known what pre d a t o r s are important or how e f f e c t i v e l y the prey p o p u l a t i o n s are i s o l a t e d i n terms of gene exchange. O c c a s i o n a l l y a n a t u r a l s i t u a t i o n i s d i s c o v e r e d where s e v e r a l p o p u l a t i o n s of the same s p e c i e s are i s o l a t e d or semi-i s o l a t e d by r e l a t i v e l y s h o r t d i s t a n c e s and are exposed to d i f f e r e n t p r e d a t i o n p r e s s u r e s . T h i s p r e s e n t s an e x c e l l e n t o p p o r t u n i t y to assess the impact of p r e d a t i o n because i t i s one of the few e c o l o g i c a l f a c t o r s t h a t v a r i e s among the p o p u l a t i o n s . A s i t u a t i o n of t h i s type was r e p o r t e d by C. P. Haskins and co-workers (1961) f o r p o p u l a t i o n s of the guppy, P o e c i l i a r e t i c u l a t a P e t e r s , a t r o p i c a l freshwater f i s h n a t i v e to the r i v e r s of n o r t h - e a s t South America and some Caribbean i s l a n d s . On the i s l a n d of T r i n i d a d , West I n d i e s , Haskins e t a l d i s c o v e r e d an apparent c o r r e l a t i o n between the frequency and l i n k a g e of c e r t a i n genes c o n t r o l l i n g c o l o u r p a t t e r n s of guppies (expressed p h e n o t y p i c a l l y o n l y i n the male) and the presence or absence of predaceous f i s h s p e c i e s . They suggested t h a t two opposing s e l e c t i v e f o r c e s were a t p l a y ( c r o s s - s e l e c t i o n ) , one, p o s s i b l y sexual s e l e c t i o n , d r i v i n g i n the d i r e c t i o n of more 3 conspicuous body markings and the o t h e r , p r e d a t i o n , s e l e c t i n g f o r a more c r y p t i c male phenotype. T h i s p r e d a t o r - p r e y i n t e r a c t i o n appeared very i n v i t i n g f o r a comparative e t h o l o g i c a l study. At the o u t s e t of t h i s r e s e a r c h I intended to study s e l e c t i o n by p r e d a t o r s i n r e l a t i o n t o geographic v a r i a t i o n i n c o l o r a t i o n , b e a r i n g i n mind t h a t a d d i t i o n a l m o r p h o l o g i c a l and b e h a v i o r a l v a r i a -t i o n might be d i s c o v e r e d . I t soon became obvious t h a t a wealth of microgeographic v a r i a t i o n i n behavior does e x i s t i n both w i l d f i s h and t h e i r l a b o r a t o r y - r e a r e d o f f s p r i n g . Because I f e l t t h i s d i s c o v e r y might g i v e some i n s i g h t i n t o the mechanisms of the e v o l u t i o n of b e h a v i o r , I devoted most of my time to the study of p o p u l a t i o n d i f f e r e n c e s i n behavior (Chapter 6). N e v e r t h e l e s s I do i n c l u d e i n t h i s t h e s i s the o r i g i n a l work concerned w i t h the "Haskins h y p o t h e s i s " (Chapter 4) as w e l l as an a n a l y s i s of the adaptive s i g n i f i c a n c e of geographic v a r i a t i o n i n body s i z e (Chapter 5). I hope to be able to demonstrate t h a t these three aspects are i n t e r r e l a t e d and can be c o n s i d e r e d under the c o l l e c t i v e term o f " a n t i p r e d a t o r a d a p t a t i o n s " . Main o b j e c t i v e of the study The main o b j e c t i v e of t h i s study was to assess the e v o l u t i o n a r y s i g n i f i c a n c e of n a t u r a l v a r i a t i o n i n popula-t i o n s of the guppy and to determine to what ext e n t t h i s v a r i a t i o n might be r e l a t e d t o the d i s t r i b u t i o n and abundance 4 of a q u a t i c p r e d a t o r s . Three aspects o f t h i s problem were co n s i d e r e d : 1. Sex r a t i o Though p o p u l a t i o n d i f f e r e n c e s i n the sex r a t i o s of f i s h are normally not g e n e t i c a l l y determined and hence do not p r e s e n t an e v o l u t i o n a r y problem per se, I s t u d i e d t h i s v a r i a t i o n i n some d e t a i l because sex r a t i o d i f f e r e n c e s c o u l d be v a l u a b l e c l u e s to the nature o f d i f f e r e n t i a l s e l e c t i o n by pr e d a t o r s or oth e r f a c t o r s on c o l o u r p a t t e r n s (which do have a g e n e t i c b a s i s ) . 2. Body s i z e There i s abundant evidence t h a t animal body s i z e and growth r a t e s are i n f l u e n c e d by both g e n e t i c and e n v i r o n -mental f a c t o r s . I wished to determine i f p r e d a t i o n or oth e r environmental f a c t o r s might be r e s p o n s i b l e f o r microgeo-g r a p h i c v a r i a t i o n i n the s i z e of a d u l t f i s h . 3. Behavior The t h i r d aspect concerns the a n t i p r e d a t o r behavior of guppies. I wanted t o d e s c r i b e the v a r i a t i o n among p o p u l a t i o n s i n nature and among l a b o r a t o r y - r e a r e d o f f s p r i n g of some of these n a t u r a l p o p u l a t i o n s . In p a r t i c u l a r I wished t o t e s t the s u r v i v a l value o f the behavior (how i s i t a d a p t i v e ? ) . My o v e r a l l h y p othesis was t h a t d i f f e r e n c e s i n the behavior of n a t u r a l p o p u l a t i o n s would be c o r r e l a t e d with the d i s t r i b u t i o n and abundance of the major p r e d a t o r s . More i m p o r t a n t l y , I p r e d i c t e d these d i f f e r e n c e s would be 5 h e r i t a b l e . To f a l s i f y t h i s h y p o t h e s i s i t would be necessary t o demonstrate t h a t the v a r i a t i o n observed i n nature does not p e r s i s t i n f i s h r a i s e d under constant conditions, i n the l a b o r a t o r y . T h i s assumes t h a t l a b o r a t o r y stocks are g e n e t i c a l l y r e p r e s e n t a t i v e of f i e l d samples and t h a t b e h a v i o r a l t e s t s are s u f f i c i e n t l y s e n s i t i v e to d e t e c t d i f f e r e n c e s , i f prese n t . I t i s f a i r to ask a t t h i s p o i n t i f such a study i s indeed necessary. There i s a p a u c i t y of comparative data of i n t r a s p e c i f i c v a r i a t i o n i n a n t i p r e d a t o r a d a p t a t i o n s . T h i s i s . s u r p r i s i n g because a n t i p r e d a t o r d e v i c e s occur i n v i r t u a l l y a l l animal (and many pl a n t ) groups and are v i t a l l y important to the s u r v i v a l of the s p e c i e s . There-f o r e such a study c o u l d have i m p l i c a t i o n s not onl y f o r the "pure" e v o l u t i o n a r y b i o l o g i s t who i s attempting t o assess the r e l a t i v e importance o f s e l e c t i o n and gene flow i n the d i f f e r e n t i a t i o n of l o c a l p o p u l a t i o n s (e.g. E h r l i c h and Raven, 1969), but a l s o the a p p l i e d b i o l o g i s t concerned wi t h p r e d i c t i n g the impact of predator i n t r o d u c t i o n s on the subsequent behavior of the prey p o p u l a t i o n . What'behavioral and m o r p h o l o g i c a l c h a r a c t e r s would be s e l e c t e d f o r under such c o n d i t i o n s ? How r a p i d l y c o u l d prey c o u n t e r a d a p t a t i o n s . evolve? These q u e s t i o n s are a l s o r e l e v a n t when man, i n 6 h a r v e s t i n g a n a t u r a l p o p u l a t i o n , a c t s as a s e l e c t i v e p r e d a t o r ( M i l l e r , 1957). More s p e c i f i c a l l y , an a p p r e c i a t i o n of p o p u l a t i o n v a r i a t i o n i n d e f e n s i v e b e h a v i o r c o u l d be important i n f i s h s t o c k i n g programs. R a l e i g h and Chapman (1971) r e f e r r i n g to t r o u t f r y m i g r a t i o n s s t a t e d , " f a i l u r e to match the i n n a t e behavior o f the donor p o p u l a t i o n t o the requirements of the r e c i p i e n t environment has l e d to f a i l u r e o f many f i s h t r a n s p l a n t e f f o r t s i n the past (p. 39)." As C a l a p r i c e (1969) has warned, the continuous s t o c k i n g of i l l - a d a p t e d f i s h to a n a t u r a l p o p u l a t i o n can decrease the mean p o p u l a t i o n f i t n e s s and p o s s i b l y culminate i n l o c a l e x t i n c t i o n s . My study i s a l s o r e l e v a n t to the s c i e n c e o f b e h a v i o r a l g e n e t i c s , a f i e l d devoted l a r g e l y to the measurement of b e h a v i o r a l d i f f e r e n c e s among i n b r e d s t r a i n s of mice, r a t s , and D r o s o p h i l a . B r u e l l (1967) has c r i t i c i z e d p a s t work i n t h i s f i e l d and made some u s e f u l suggestions f o r f u t u r e r e s e a r c h : The w i l d base p o p u l a t i o n s from which the anc e s t o r s o f our l a b o r a t o r y s t r a i n s were drawn are shrouded by the mis t of incomplete records and thus, i n most cases, we do not know anything about the n a t u r a l environmental c o n d i t i o n s under which the founders o f our s t r a i n s evolved. Work w i l l have to s t a r t w i t h proper samples of i n d i v i d u a l s drawn from l o c a l w i l d p o p u l a t i o n s which evolved under d i s t i n c t and well-known environmental c o n d i t i o n s . . I n v e s t i g a t i o n o f forms of behavior chosen on the b a s i s of e v o l u t i o n a r y c r i t e r i a i s probably the most c r i t i c a l requirement of f u t u r e b e h a v i o r - g e n e t i c a n a l y s e s . 7 To go beyond i n t u i t i o n and to understand the a d a p t i v e s i g n i f i c a n c e of a behavior, we must know the e n v i r o n -mental c o n d i t i o n s under which i t occurs and how i t v a r i e s as environmental c o n d i t i o n s vary. (pp. 284-286) In t h i s r e s e a r c h I s h a l l endeavour to implement some of B r u e l l ' s recommendations. C l e a r l y the problem of n a t u r a l v a r i a t i o n i n the morphology and behavior of animal s p e c i e s i s very complex, encompassing a g r e a t p o r t i o n of ecology, ethology, and e v o l u t i o n a r y b i o l o g y . My main aim i s to i d e n t i f y some of the n a t u r a l v a r i a t i o n i n T r i n i d a d p o p u l a t i o n s of the guppy and to assess i t s e v o l u t i o n a r y s i g n i f i c a n c e . The b i o l o g y of P o e c i l i a r e t i c u l a t a ; r e l e v a n t l i t e r a t u r e I t i s i r o n i c a l t h a t a s p e c i e s f a m i l i a r to a l l amateur a q u a r i s t s i s v i r t u a l l y unknown to the e c o l o g i s t . A very meagre l i t e r a t u r e e x i s t s f o r s t u d i e s on the guppy under n a t u r a l c o n d i t i o n s . Because the guppy i s s m a l l , has a s h o r t g e n e r a t i o n time (about 3 months), i s always i n good commercial supply, and adapts r e a d i l y to aquarium c u l t u r e , i t has been used as a convenient b i o a s s a y organism i n s t u d i e s ranging from t o x i c o l o g y and gerontology to t i s s u e t r a n s p l a n t a t i o n and mutation r e s e a r c h . In most of these s t u d i e s domestic s t r a i n s of the guppy were used. The guppy has been the s u b j e c t of s e v e r a l e t h o l o g i -c a l s t u d i e s . Aspects of sexual s e l e c t i o n have been i n v e s t i -gated by Noble (1938), Haskins and Haskins (1949, 1950), and 8 Haskins e t a l ( 1 9 6 1 ). L i l e y (1966) has examined how the guppy i s e t h o l o g i c a l l y i s o l a t e d from three, other sympatric p o e c i l i i d f i s h . The conspicuous c o u r t s h i p behavior of the male guppy has a t t r a c t e d the a t t e n t i o n of numerous workers ( B r e d e r a n d Coates, 1935; C l a r k and Aronson, 1951; Baerends e t a l , 1955; Rosen and Tucker, 1961). The p i o n e e r i n g g e n e t i c a l s t u d i e s of Winge (1922a, 1922b, 1927) and Winge and D i t l e v s e n ( 1 9 4 8 ) ( r e c e n t l y extended by Haskins e t a l , 1970) have e s t a b l i s h e d the p r i m a r i l y s e x - l i n k e d nature of c o l o u r p a t t e r n i n h e r i t a n c e i n l a b o r a t o r y s t r a i n s of the guppy. Winge's f i n d i n g s have subsequently been confirmed f o r n a t u r a l p o p u l a t i o n s by Haskins and Haskins (1951, 1954) and Haskins e t a l (1961). Lab o r a t o r y p o p u l a t i o n s of the guppy have been used as model systems f o r i n v e s t i g a t i n g s e v e r a l aspects o f p o p u l a t i o n r e g u l a t i o n (Breder and Coates, 1932; Shoemaker, 1944; Rose, 1959; S i l l i m a n and G u t s e l l , 1958; Geodakyan and K o s o b u t s k i i , 1972). However, no one has ever s t u d i e d the dynamics of n a t u r a l p o p u l a t i o n s . A d d i t i o n a l work of e c o l o g i c a l s i g n i f i c a n c e i n c l u d e s s t u d i e s of f e e d i n g (Hester, 1964; Davis, 1968), growth (Svardson, 1943, c i t e d i n Aim, 1959; B e r t a l a n f f y , 1938; Gibson and H i r s t , 1955), and l o n g e v i t y . (Comfort, 1961). U s e f u l data on v i s i o n are g i v e n i n Lang (1965, 1967), Waldman (1969), and Protasov (1970). The s y s t e m a t i c s and zoogeography of the f a m i l y P o e c i l i i d a e have been reviewed by Rosen and B a i l e y (1963). Avoidance behavior has been i n v e s t i g a t e d i n two l a b o r a t o r y s t u d i e s (Werboff and L l o y d , 196 3; R u s s e l l , 1967a 1967b). W i l l i a m s (1964) has b r i e f l y c o n s i d e r e d s c h o o l i n g behavior and Schutz (1956) touched upon the r e l a t i o n s h i p between pheromones and the " f r i g h t r e a c t i o n " . F i n a l l y t h e r e i s the study by B a l l i n (1973) of geographic v a r i a t i o n i n c o u r t s h i p and a g g r e s s i v e behavior o three p o p u l a t i o n s of T r i n i d a d guppies. T h i s i s a v a l u a b l e companion to my r e s e a r c h because the study was c a r r i e d out on s i m i l a r g e n e t i c m a t e r i a l but e x p l o r e d a d i f f e r e n t s e t of b e h a v i o r a l parameters. CHAPTER 2 MATERIALS AND METHODS I n t r o d u c t i o n T h i s r e s e a r c h i s a combination of f i e l d and l a b o r a -t o r y study. The f i e l d o b s e r v a t i o n s and c o l l e c t i o n s were made i n the r i v e r s of the Northern Range Mountain r e g i o n of the i s l a n d of T r i n i d a d , West Indies ( F i g . 1) and the bulk of the experimental work was conducted under c o n t r o l l e d c o n d i t i o n s i n the l a b o r a t o r y . The work i n T r i n i d a d was based on two f i e l d e x p e d i -t i o n s . The f i r s t of these (June-August, 1967) p r o v i d e d an i n t r o d u c t i o n to the problem of v a r i a t i o n i n T r i n i d a d guppies and the second (March-June, 1969) allowed me to make more d e t a i l e d c o l l e c t i o n s of f i s h , extend my 1967 o b s e r v a t i o n s , and conduct some experiments with w i l d f i s h . These c o l l e c t i o n p e r i o d s i n c l u d e p a r t s of the wet (June-October) and dry (November-May) seasons. C o l l e c t i o n techniques Guppies were c o l l e c t e d with a f i n e c i r c u l a r d i p n e t or a f i n e "one-man s e i n e " . A v a r i e t y of methods were used to c o l l e c t the pr e d a t o r s i n c l u d i n g d i p n e t s , s e i n e s , g i l l -n e t s , c a s t n e t , and hook-and-line. The c a s t n e t proved to be 10 11a FIGURE 1. Map of the n o r t h e r n h a l f o f the i s l a n d o f T r i n i d a d , West I n d i e s , showing the major r i v e r systems. C o l l e c t i o n s i t e s are i n d i c a t e d by numbers c o r r e s -ponding to the key below. Inset map i n d i c a t e s the p o s i t i o n o f the i s l a n d j u s t o f f the n o r t h -e a s t c o a s t of Venezuela. Standard Map no. Name of stream a b b r e v i a t i o n 1 S i e r r a Leone Road SLR 2 Blue Basin BB 3 Maracas V i l l a g e MV 4 Upper Curumpalo UCur 5 Lower Curumpalo LCur 6 Grande Curucaye GCur 7 P e t i t e Curucaye PCur 8 Santa Cruz SC 9 Ca r o n i Car 10 Guayamare Guay 11 Caparo Cap 12 . Lower T a c a r i g u a LTac 13 Upper T a c a r i g u a UTac 14 Upper Arouca UArouc 15 Yarra Yar 16 Marianne Mar 17 Lower P a r i a LPar 18 ' Upper P a r i a Par 19 Upper Guanapo UGuan 20 Upper A r i p o (Naranjo) UA(N) 20a Upper A r i p o (Crossing) UA(X) 21 Lower A r i p o LA 22 Oropuche Oro 23 Tompire T r i b u t a r y TT 12 the b e s t way to capture the l a r g e r f i s h p r e d a t o r s . C o l l e c t i o n s of guppies were made f o r s e v e r a l pur-poses. An i n s t a n t "dead" c o l l e c t i o n was made a t each s i t e o f i n t e r e s t by p l a c i n g the f i s h d i r e c t l y i n t o a 10% f o r m a l i n s o l u t i o n . T h i s c o l l e c t i o n was used to measure morphometric c h a r a c t e r s and determine the sex r a t i o . A " l i v e " c o l l e c t i o n was made a t c e r t a i n s i t e s t o o b t a i n : (1) f i s h f o r e x p e r i -ments i n T r i n i d a d , (2) founders f o r l a b o r a t o r y p o p u l a t i o n s at Vancouver, Canada and (3) males f o r the assessment of v a r i a t i o n i n c o l o u r p a t t e r n s ( L i l e y and Seghers, unpublished d a t a ) . Every e f f o r t was made to c o l l e c t a random, r e p r e -s e n t a t i v e sample f o r each stream. Because d e n s i t i e s f l u c t u a t e d from stream to stream, more time was r e q u i r e d a t some s i t e s to c o l l e c t s u f f i c i e n t numbers f o r s t a t i s t i c a l comparisons. Laboratory p o p u l a t i o n s a t Vancouver L i v e f i s h were shipped by a i r t o Vancouver i n both 1967 and 1969. These shipments i n c l u d e d s e v e r a l s p e c i e s of preda t o r s and samples of guppies taken from r e p r e s e n t a t i v e streams. Samples from d i f f e r e n t streams were kept i s o l a t e d . As many f i s h as p r a c t i c a l l y f e a s i b l e were shipped t o i n s u r e t h a t the l a b o r a t o r y p o p u l a t i o n s ( r e f e r r e d to h e r e a f t e r as "stocks") would be r e p r e s e n t a t i v e o f t h e i r r e s p e c t i v e n a t u r a l c o u n t e r p a r t s . 13 In a l l cases no fewer than 50 l a r g e g r a v i d females and 25 mature males were used t o s t a r t the l a b o r a t o r y c u l -t u r e s . In 1969 the m a j o r i t y of the stocks s t a r t e d i n 1967 were r e p l e n i s h e d or r e p l a c e d with f r e s h c o l l e c t i o n s . T h i s was done to a v o i d i n b r e e d i n g and s e l e c t i o n i n l a b o r a t o r y stock, i . e . to maintain as much of the n a t u r a l v a r i a b i l i t y as p o s s i b l e . A t o t a l of 2 3 s i t e s were sampled i n T r i n i d a d ( F i g . 1) and f i s h d e r i v e d from 10 of these were s e l e c t e d f o r b e h a v i o r a l s t u d i e s a t Vancouver. S e v e r a l stocks were used by o t h e r workers (Henderson, unpubl.; B a l l i n , 1973; M o r r e l l , unpubl.; L i l e y , unpubl.). I s e l e c t e d 5 stocks f o r the comparative study of a n t i p r e d a t o r behavior and used s e v e r a l o t h e r s , i n c l u d i n g one stock from Paramaribo, Surinam (Sommeldijske Kreek), f o r v a r i o u s o t h e r experiments. Maintenance of f i s h Standard procedures f o r the c u l t u r e of t r o p i c a l freshwater f i s h were used. Guppies were housed mostly i n 40 and 60 l i t e r g l a s s a q u a r i a maintained a t temperatures found i n nature (24-28°C). F i s h from each geographic l o c a l i t y were kept separate and bred i n mass c u l t u r e i n numerous a q u a r i a . Within each stock f i s h were p e r i o d i c a l l y mixed t o reduce i n b r e e d i n g and prevent g e n e t i c d r i f t . The p r e d a t o r s were housed i n g l a s s or wood-and-glass tanks of 40 to 400 l i t e r s ; i n T r i n i d a d the l a r g e r 14 i n d i v i d u a l s were kept i n concrete pools (indoor and outdoor) of 700 to 1600 l i t e r s . A l l a q u a r i a were f i t t e d with e i t h e r s u b - g r a v e l , out-s i d e , or i n s i d e f i l t e r s . The tank f l o o r s were covered w i t h approximately 4 cm of light-brown f i n e sand. Aq u a r i a not used f o r experiments c o n t a i n e d water p l a n t s ( C e r a t o p t e r i s sp.,-Lemna sp., S a g i t t a r i a s p . ) . I l l u m i n a t i o n was p r o v i d e d by c o o l white f l u o r e s c e n t tubes mounted 20-30 cm above the water s u r f a c e . Photoperiod was c o n t r o l l e d by I n t e r - m a t i c automatic time switches connected t o the l i g h t s ; the standard photoregime was 12 l i g h t — 1 2 ; dark. Guppies were fed d a i l y with f i n e l y - g r o u n d C l a r k ' s dry f i s h food supplemented s e v e r a l times per- week wit h chopped, l i v e T u b i f e x worms. The p r e d a t o r s were f e d on whole or chopped T u b i f e x supplemented i r r e g u l a r l y w i t h l i v e guppies. S p e c i a l f e e d i n g regimes were.used'for experiments and w i l l be d e s c r i b e d i n the a p p r o p r i a t e experimental s e c t i o n s . Observation and r e c o r d i n g methods T r i n i d a d . Under most c o n d i t i o n s i n nature, guppies are e a s i l y observed from the edge of a stream. S e v e r a l of the predator s p e c i e s may be observed i n t h i s way a l s o , or from a b r i d g e over a stream. U s u a l l y upon i n i t i a l approach, the p r e d a t o r s (and guppies a t c e r t a i n s i t e s ) w i l l f l e e and 15 move to deeper water or hide under b o u l d e r s , submerged l o g s , or l e a f d e t r i t u s . I found t h a t i f I s a t q u i e t l y f o r a few minutes the f i s h resumed' t h e i r normal a c t i v i t y . I a l s o made obs e r v a t i o n s underwater u s i n g a s n o r k e l and face mask; a few obs e r v a t i o n s were made a t n i g h t with a headlamp. B e h a v i o r a l i n f o r m a t i o n was d i r e c t l y recorded i n t o a f i e l d notebook or d i c t a t e d i n t o a p o r t a b l e t a p e - r e c o r d e r . The d u r a t i o n of behavior p a t t e r n s was' measured i n the f i e l d with a stopwatch.. Vancouver. More s o p h i s t i c a t e d equipment was a v a i l -able a t V a n c o u v e r — b o t h Rustrak (4-channel) and E s t e r l i n e -Angus (20-channel) event r e c o r d e r s were used t o r e c o r d the frequency and d u r a t i o n of behavior p a t t e r n s . By d e p r e s s i n g a key corresponding to a b e h a v i o r a l event (1 key per channel) a permanent r e c o r d was made on a c o n t i n u o u s l y moving c h a r t . S e v e r a l other methods were used t o study behavior. These w i l l be i n c o r p o r a t e d i n t o the s p e c i f i c experimental s e c t i o n s . E c o l o g i c a l measurements A d e t a i l e d . l i m n o l o g i c a l survey was not warranted f o r t h i s study but I d i d want q u a n t i t a t i v e measures of the most important v a r i a b l e s t h a t might a f f e c t the behavior of the f i s h . • (a) Stream dimensions. At each c o l l e c t i o n and' o b s e r v a t i o n s i t e I measured the average depth and width of 16 the stream and recorded changes caused by r a i n f a l l . (b) Water v e l o c i t y . V e l o c i t y was measured with a su r f a c e f l o a t p l a c e d i n the ce n t r e o f a r e p r e s e n t a t i v e s e c t i o n of stream. A separate measurement was made .for the p o r t i o n of the stream i n h a b i t e d by guppies ( m i c r o h a b i t a t ) . In each case the mode of 5 to 10 runs was used. . (c) Volume of flow ( d i s c h a r g e ) . T h i s was c a l c u l a t e d u s i n g the standard formula given by Needham and Needham (1962: 104). (d) Temperature. Temperature was measured to the near e s t 0.1°C with a mercury thermometer p l a c e d i n the mainstream and the m i c r o h a b i t a t s . (e) p_H. Fre s h Hydrion pH paper .was . used to measure r e l a t i v e a c i d i t y o r ' a l k a l i n i t y of the water.- The modal value o f s e v e r a l determinations was used. (f) DH. Hardness was measured wi t h a R i l a Water Hardness T e s t K i t and a l l determinations were r e p l i c a t e d . (g) T u r b i d i t y . In the few streams t h a t were • t u r b i d , an improvised S e c c h i d i s c was used to measure the. a t t e n u a t i o n of l i g h t with depth. (h) Shade (degree of overhead c o v e r ) . The amount of cover was estimated on a s e m i - q u a n t i t a t i v e 5-point s c a l e ranging from no shade (0) to complete shading (4). (i) S u b s t r a t e . Colour photographs were taken of the stream bottoms. (j) Other f i s h s p e c i e s . An attempt was made to c o l l e c t a l l f i s h s p e c i e s t h a t were sympatric w i t h the popu-l a t i o n s of the guppy. Stomach contents were analysed. (k) P o p u l a t i o n e s t i m a t e s . An a c c u r a t e census was not made f o r most streams although a " c a t c h - p e r - u n i t - e f f o r t " comparison gave a rough estimate of the r e l a t i v e abundance of guppies and other f i s h s p e c i e s . In one stream ( P e t i t e Curucaye) a 35 metre s e c t i o n was screened o f f and a l l f i s h were removed; i n another (Paria) a v i s u a l count of guppies was made. Measurements on guppies (a) Sex r a t i o . The guppy i s s e x u a l l y d i m o r p h i c — mature males are i d e n t i f i e d by the presence of a gonopodium (modified a n a l f i n ) and f u l l y developed c o l o u r markings. Immature males may l a c k c o l o r a t i o n but show a p a r t i a l l y o r f u l l y developed gonopodium. The sex of f i s h s m a l l e r than immature males i s u n c e r t a i n and they are c l a s s e d as immature. In t h i s t h e s i s I express the sex r a t i o as the number of males per one female. (b) Length. Two measurements of l e n g t h were made, standard l e n g t h ( t i p of snout t o p o s t e r i o r end of ca u d a l peduncle) and t o t a l l e n g t h (tip* o f snout t o t i p of t a i l ) . Some samples were measured w i t h s l i d i n g v e r n i e r c a l i p e r s t o the n e a r e s t 0.1 mm. Others were measured w i t h a r u l e r to the n e a r e s t 0.5 mm. L i v e f i s h used i n experiments were measured to the 18 ne a r e s t 0.5 mm t o t a l l e n g t h . The f i s h were p l a c e d d i r e c t l y from the water i n t o a dry p e t r i d i s h . When the f i s h stopped moving (few s e c ) , a r u l e r was passed under the d i s h t o measure the l e n g t h . T h i s caused no m o r t a l i t y and proved t o be the most e f f i c i e n t method to measure the thousands of l i v e f i s h t h a t were used d u r i n g t h i s study. A l l l e n g t h measurements of guppies and other f i s h g i v e n i n t h i s t h e s i s are t o t a l l e n g t h except where another measurement i s s p e c i f i c a l l y d e s i g n a t e d . (c) Weight. A few samples were weighed to the nea r e s t .001 g. The pr e s e r v e d f i s h were p l a c e d on a paper towel t o remove s u r f a c e f l u i d p r i o r t o weighing. (d) Colour p a t t e r n s . To r e c o r d i n d i v i d u a l and p o p u l a t i o n v a r i a t i o n i n the c o l o r a t i o n of males, a s c o r i n g method was developed ( L i l e y and Seghers, unpubl.) f o r b l a c k , red-orange-yellow, and i r i d e s c e n t (mainly blue and green) markings. Colour p a t t e r n s were assessed on w i l d -caught f i s h t h a t had been f r e s h l y k i l l e d by immersion i n ice-water. In a d d i t i o n c o l o u r photographic s l i d e s were made of the same i n d i v i d u a l s . CHAPTER 3 THE ENVIRONMENT I n t r o d u c t i o n Beebe (1952) has g i v e n a g e n e r a l account of the geography, c l i m a t i c c o n d i t i o n s , and p l a n t and animal assemblages of T r i n i d a d with s p e c i a l r e f e r e n c e t o the Arima V a l l e y . T h i s v a l l e y i s l o c a t e d i n the Northern Range Mountains near the centre o f my study area. Beebe d i d not de a l with the f i s h fauna nor was any a t t e n t i o n g i v e n t o the stream ecosystem. A study of the Maracas R. ( t r i b u t a r y of the St. Joseph R., see F i g . 1) by T h o r n h i l l e t a l (1966) i n d i c a t e d the main food webs i n the stream and d e s c r i b e d the micro-h a b i t a t d i s t r i b u t i o n of the f i s h s p e c i e s . A more g e n e r a l treatment of the ecology of streams i n the Northern Range wit h s p e c i a l r e f e r e n c e t o the guppy and i t s p r e d a t o r s was i n c l u d e d i n Haskins e t a l (1961). Boeseman (1960, 1964) has compiled a key to the sp e c i e s o f freshwater f i s h o f T r i n i d a d . T h i s monograph reviews most of the e a r l y c o l l e c t i o n s and taxonomy of T r i n i d a d f i s h but l a c k s e c o l o g i c a l and d e t a i l e d d i s t r i b u -t i o n a l d a ta. 19 20 In g e n e r a l the ecology of stream f i s h i n the t r o p i c s has been l i t t l e s t u d i e d (Gery, 1969;. A l l e n , 1969) and t h i s a p p l i e s to T r i n i d a d as w e l l . C l a s s i f i c a t i o n o f streams' Using the methods o u t l i n e d i n Chapter 2, i t was p o s s i b l e to e r e c t a c l a s s i f i c a t i o n of a r e p r e s e n t a t i v e sample of the streams and r i v e r s of the Northern Range r e g i o n . T h i s was an a b i o t i c c l a s s i f i c a t i o n based on the width, depth, and v e l o c i t y o f each stream i n the r e g i o n surrounding,the c o l l e c t i o n s i t e s (Table 1) . A more detailed.breakdown o f these measurements can. be found i n . the Appendix (Table 1) . Wherever p o s s i b l e I attempted to. make measurements•in dry (March, A p r i l , 1969) and wet (June, J u l y , 1969) seasons because s e v e r a l of the streams are g r e a t l y a f f e c t e d by the change i n r a i n f a l l ( i n c r e a s e i n volume of flow and t u r b i d i t y ) . Geographic d i s t r i b u t i o n of f i s h s p e c i e s As noted above, there i s l i t t l e p u b l i s h e d informa-t i o n r e g a r d i n g the d i s t r i b u t i o n o f f i s h i n T r i n i d a d . F i g u r e 2 summarizes the r e s u l t s of the 1967 and 1969 c o l l e c t i o n s . For s p e c i f i c names", c o n s u l t the Appendix (Table 2) . I t i s c l e a r t h a t the s i z e of the streams i s the main f a c t o r l i m i t i n g . t h e d i v e r s i t y o f . t h e f i s h fauna though streams i s o l a t e d by p h y s i c a l b a r r i e r s may have an impover-i s h e d f i s h fauna and many s p e c i e s which might otherwise be TABLE 1 . C l a s s i f i c a t i o n and physical features of streams i n the Northern Range (March - June, 1 9 6 9 ) . Stream type Width (m) Depth ( in) Velocity (m/sec) Volume of Flow (mVsec) Temp. (°C) Shade Turbidity Springwater PCur GCur UCur 0 . 5 0 - 1 . 0 TT 0 . 0 5 - 0 . 1 5 0 . 1 1 -0 . 2 9 0 . 0 0 8 5 -0 . 0 1 2 8 2 4 . 3 -2 6 . 2 3 - 4 0 Headstream UA(N) UTac UArouc UGuan 1 . 2 0 - 5 . 0 BB 0 . 0 6 - 0 . 1 5 0 . 3 2 -0 . 6 7 0 . 0 2 8 -0 . 2 6 7 2 4 . 6 -2 7 . 4 2 - 3 0 - 1 Yar Par LA LTac Midstream SC 3 . 0 - 8 . 0 0 . 1 3 - 0 . 2 0 0 . 4 2 - 0 . 1 5 0 - 2 4 . 3 - 2 1 Mar 1 . 1 8 1 . 1 2 9 3 0 . 0 Oro Lowland R. Guay Cap Car 2 . 0 - 2 5 . 0 1 . 5 - 3 . 0 0 . 3 3 -0 . 4 0 0 . 5 6 3 -2 2 . 5 0 2 6 . 9 -2 9 . 1 0 - 1 l 0 - no- shade 0 -1 - small amount of shade r e s t r i c t e d mainly to streambank 1 -2 - medium shade ( 5 0 % cover) 3 - medium to dense ( 7 5 % cover with few exposed parts) 2 T 4 - very dense cover with v i r t u a l l y complete shading always clear turbid only af t e r heavy rains turbid through-out year 22a FIGURE 2. The d i s t r i b u t i o n of f i s h s p e c i e s i n the Northern Range r e g i o n . For a l i s t of a b b r e v i a t i o n s , r e f e r t o F i g . 1. Streams o u t l i n e d by a box are the p r i n c i p a l ones t h a t were i n v e s t i g a t e d . 14 -cn 13-UJ 12-o 11 -LU 10-CL CO 9 -8 -o 7 -rr 6-5 -DD 4 -3 -2 -1 -POECILIA RIVULUS CICHL/DAE CHARACIDAE Q OTHERS Z3 o ZD Z3 O Q_ a: 3 —I > i — o W 2 h O SPR INGS o •3 < CD k. id «- O id «-3 QQ CL > < — \ c 3 ZD HEADSTREAMS J 2 J « O u MIDSTREAMS > a . i_ id < id rd —1 O o CD LOWLAND t o to a* 23 expected to occur are not p r e s e n t . The Northern Range has served as the major b a r r i e r (no i n t e r c o n n e c t i n g freshwater streams) to c h a r a c i d and c i c h l i d d i s p e r s a l from the C a r o n i R. system to the n o r t h -f l o w i n g P a r i a , Marianne and Y a r r a R i v e r s . In streams f l o w i n g south o f f the Northern Range, w a t e r f a l l s have prevented the access of c h a r a c i d s and c i c h l i d s to Blue Basin and Upper A r i p o R i v e r s . The s i t u a t i o n at the A r i p o R. i s extremely i n t e r -e s t i n g because a s i n g l e 5 metre w a t e r f a l l b l o c k s the passage of s e v e r a l of the l a r g e r s p e c i e s to the upper r e g i o n s . As I s h a l l demonstrate below, some of these are p r e d a t o r s of the guppy. Since guppies are d i s t r i b u t e d c o n t i n u o u s l y along t h i s r i v e r , i t i s p o s s i b l e to study p o p u l a t i o n s exposed to d i f f e r e n t p r e d a t i o n p r e s s u r e s over a very s h o r t d i s t a n c e . I t i s probable t h a t the Upper A r i p o R. has never co n t a i n e d c h a r a c i d or c i c h l i d f i s h , hence the guppy popula-t i o n s have been " p r o t e c t e d " from these s p e c i e s . I have summarized the i n f o r m a t i o n on the d i s t r i b u t i o n of the major s p e c i e s i n schematic form ( F i g . 3). Guppies are u b i q u i t o u s i n t h i s r e g i o n . Predators of the guppy I t would be expected a p r i o r i t h a t a s m a l l abundant f i s h such as the guppy would be s u b j e c t to c o n s i d e r a b l e p r e d a t i o n by l a r g e f i s h and p o s s i b l y f i s h - e a t i n g b i r d s , 24a FIGURE 3. A schematic r e p r e s e n t a t i o n o f the d i s t r i b u t i o n of the major f i s h s p e c i e s i n the northwest corner of T r i n i d a d . Major b a r r i e r s to f i s h d i s p e r s a l are shown. Small p o p u l a t i o n s of R i v u l u s do occur i n the v i c i n i t y o f streams c o n t a i n i n g c h a r a c i d s and c i c h l i d s but are c o n f i n e d to small pools or d i t c h e s . CARIBBEAN SEA GULF of PARIA WATERFALL BARRIER NORTHERN RANGE MOUNTAIN BARRIER (£>• U J CD — Z) t- CC Ul 3 0- O I- 0_ 'o I |2 -C A R O N I R I V E R i . . . . 11.. J : • . *. •.: .*.'• GUAYAMARE RIVER CAPARO RIVER I WATERFALL BARRIER MCHARACIDS4 UCICHLIDS l RIVULUS l-o *« a4 25 mammals, and i n s e c t s . D i r e c t evidence o f t h i s can be ob t a i n e d by o b s e r v i n g a t t a c k s i n nature o r by r e c o v e r i n g guppy remains i n the d i g e s t i v e t r a c t of the suspected p r e d a t o r . I t i s o f t e n d i f f i c u l t to o b t a i n e x t e n s i v e d i r e c t evidence o f p r e d a t i o n on a s p e c i f i c prey item (e.g. M c P h a i l , 1969) . Many, v e r t e b r a t e p r e d a t o r s are n o t o r i o u s l y s e c r e t i v e i n t h e i r h a b i t s . In a d d i t i o n , i f they are not s p e c i a l i s t f e e d e r s , p r e d a t i o n on a giv e n prey s p e c i e s w i l l f l u c t u a t e with the r e l a t i v e abundance and a v a i l a b i l i t y o f a l t e r n a t e prey. Even i n the s o - c a l l e d " s t a b l e " t r o p i c a l environment, seasonal d i e t changes are well-known f o r freshwater f i s h (Lowe-McConnell, 1964, 1969b; Zaret and Rand, 1971). Another problem i s t h a t p i s c i v o r o u s f i s h may go f o r long p e r i o d s without f e e d i n g , and when they do feed, d i g e s t i o n may be r a p i d . T h e r e f o r e a hig h p r o p o r t i o n of the stomachs are empty or the food may be d i g e s t e d beyond r e c o g n i t i o n . F i n a l l y , p r e d a t o r s occupying a hig h t r o p h i c l e v e l are n u m e r i c a l l y not abundant; sample s i z e s are bound t o be s m a l l . I encountered a l l these problems i n attempting t o assess the r e l a t i v e impact of each suspected p r e d a t o r on the guppy p o p u l a t i o n s . (a) Major p r e d a t o r s . Haskins e_t a l (1961) l i s t e d 5 s p e c i e s of f i s h p r e d a t o r s o f guppies i n the Northern 26 Range streams (Figure 4) i n c l u d i n g 4 termed " s e r i o u s " : Astyanax bimaculatus, Aeguidens l a t i f r o n s (= A. p u l c h e r ) , C r e n i c i c h l a s a x a t i l i s (probably C. a l t a , Boeseman, 1960) and H o p l i a s malabaricus. The f i f t h p r e d a t o r , R i v u l u s  h a r t i i , was termed " l e s s severe". Few data were given f o r the b a s i s of t h i s c l a s s i f i -c a t i o n . Presumably the body s i z e , r e l a t i v e abundance, mo r p h o l o g i c a l a d a p t a t i o n s , and wide d i s t r i b u t i o n of the. suspected p r e d a t o r s were used as the main c r i t e r i a , t o g e t h e r with f e e d i n g tendencies i n the l a b o r a t o r y . T h i s would be a good f i r s t approximation though my f i e l d evidence shows t h a t R i v u l u s i s a more severe guppy p r e d a t o r than Aeguidens. Aeguidens was never.observed to a t t a c k guppies and no guppies were recovered from the stomachs of j u v e n i l e s or a d u l t s of t h i s s p e c i e s (see Table 3 i n Appendix f o r the a n a l y s i s of stomach c o n t e n t s ) . In c o n t r a s t 10% of a l l R i v u l u s sampled had guppy remains i n t h e i r stomachs; a t t a c k s were a l s o observed i n nature. Before the p r e s e n t study, l i t t l e was known about the f e e d i n g h a b i t s of any of these 5 p o t e n t i a l p r e d a t o r s ( a l l occur on the South American mainland; H o p l i a s and Astyanax have a very wide d i s t r i b u t i o n ) . T h o r n h i l l e t 'al_ (1966) drew a food web showing H o p l i a s and C r e n i c i c h l a as p r e d a t o r s ( i n the Maracas R., T r i n i d a d ) of Hemibrycon, Astyanax and the guppy (no other data g i v e n ) . Lowe-McConnell (1969a) and Knoppel, 1970 ( c i t e d i n Roberts, 1972) found f i s h remains 27a FIGURE 4. Photograph of the major f i s h p r e d a t o r s the guppy. A S T Y A N A X H E M I B R Y C O N A E Q U I D E N S R I V U L U S 0 5 10 15 c m 28 i n the stomachs of H o p l i a s from Guyana and B r a z i l r e s p e c -t i v e l y . Sterba (1962) s t a t e d t h a t C r e n i c i c h l a s p e c i e s , "are t y p i c a l predatory f i s h e s which l i e i n w a i t ' f o r prey a f t e r the manner of the European P i k e , s e i z e i t with a l i g h t n i n g rush and swallow i t h e a d - f i r s t (p. 706)." Haskins e t a l (1961) claimed, " C r e n i c i c h l a i s a p i k e - l i k e c a r n i v o r e which i s an extremely s u c c e s s f u l — a n d q u i t e p o s s i b l y a s p e c i a l i z e d L e b i s t e s predator (p. 380)." ( L e b i s t e s r e t i c u l a t u s = P o e c i l i a r e t i c u l a t a , Rosen and B a i l e y , 1963). Although the m a j o r i t y of C r e n i c i c h l a i n my sample d i d have f i s h remains (guppies and other species) i n t h e i r stomachs, I a l s o recovered s n a i l s and i n s e c t l a r v a e from s e v e r a l i n d i v i d u a l s , suggesting t h a t t h i s s p e c i e s i s not a s t r i c t p i s c i v o r e (as Haskins e t al_ implied) . Beebe (1925) a l s o recovered i n v e r t e b r a t e s . f r o m C r e n i c i c h l a stomachs i n Guyana- (shrimps from C. a l t a ; ants and f i s h from C. l u g u b r i s ) . Though these data v i t i a t e the t h e s i s t h a t C r e n i c i c h l a i s a s p e c i a l i z e d guppy p r e d a t o r , an example from one of my experiments u n d e r l i n e s the p o t e n t i a l of t h i s p r e d a tor under i d e a l c o n d i t i o n s — 6 3 a d u l t male guppies were eaten by a sub-adult C r e n i c i c h l a over a 48-hour p e r i o d . H o p l i a s i s a l s o capable of a s i m i l a r f e a t . (b) Minor p r e d a t o r s . There are a number of o t h e r f i s h s p e c i e s which may feed on guppies o c c a s i o n a l l y but c o n s i d e r i n g e i t h e r t h e i r s m a l l body s i z e , l i m i t e d d i s t r i -b u t i o n , or low numbers, I have assumed they have a n e g l i g i b l e e f f e c t on the present-day s t r u c t u r e of the guppy po p u l a t i o n s i n the Northern Range r e g i o n . These i n c l u d e Roboides d a y i , Corynopoma r i i s e i , and Synbranchus marmoratus. I found a l l o f . t h e s e would eat guppies i n s m a l l a q u a r i a but only Synbranchus c o u l d a t t a c k and i n g e s t a d u l t guppies. T e r r e s t r i a l animals may a l s o feed on guppies although Haskins e t a l (1961) i n f e r r e d , on the b a s i s of s i g h t i n g o n l y two herons and no k i n g f i s h e r s over 13 years of e p i s o d i c o b s e r v a t i o n , t h a t t h i s source of p r e d a t i o n pressure.was unimportant. N e v e r t h e l e s s on June 22, 1969 (1300 hrs) and June 29, 1969 (1600 hrs) I observed a p a i r of L i t t l e Green K i n g f i s h e r s ( C h l o r o c e r y l e americana) s i t t i n g on an overhanging branch at the P a r i a R. c o l l e c t i o n s i t e . H e r k l o t s (1961) r e p o r t e d t h i s s p e c i e s a t P a r i a and i n the Gaura V a l l e y . (Tacarigua R. a r e a ) ; Beebe (1952) l i s t e d i t . with the fauna of the Arima V a l l e y . B e l c h e r and Smooker (1936) r e p o r t e d f i n d i n g s c a l e s of s m a l l f i s h i n a nest (tunnel), of t h i s s p e c i e s . They a l s o found a nest of the Great Green K i n g f i s h e r (C. amazona) on the Madamas R. (east of P a r i a R.). The Pygmy K i n g f i s h e r (C. aenae) has been seen by Junge and Mees (1961) near the mouth of a r i v e r (Burro R.?) at Maracas Bay and H e r k l o t s (1961) r e p o r t e d i t . f o r the Caura V a l l e y and the A r i p o road (Aripo R.) area. There have a l s o been s i g h t i n g s of the B e l t e d K i n g f i s h e r (Ceryle aIcyon), a winter migrant from the n o r t h , i n the r e g i o n of the 30 Northern Range. No d e t a i l s of f e e d i n g behavior or d i e t were given, f o r any of these r e p o r t s so the p o t e n t i a l impact on guppy po p u l a t i o n s of k i n g f i s h e r p r e d a t i o n remains c o n j e c t u r a l . The f i s h - e a t i n g , b a t N o c t i l i o l e p o r i n u s occurs i n T r i n i d a d . Most o b s e r v a t i o n s of i t s n a t u r a l f e e d i n g behavior have been made o f f the i s l a n d s of the northwest co a s t where the bats were f e e d i n g on s m a l l marine f i s h . Worth (1967: 224) s t a t e d t h a t T r i n i d a d N o c t i l i o use t h e i r elongated h i n d toes and'claws to scoop up guppies as they skim over the water s u r f a c e . U n f o r t u n a t e l y no f u r t h e r d e t a i l s were gi v e n . B l o e d e l (1955) s t u d i e d the hunting methods of N.. l e p o r i n u s i n Panama. He demonstrated t h a t one bat c o u l d capture 30 to 40 s m a l l f i s h ( i n c l u d i n g the p o e c i l i i d s Gambusia n i c a r a g u e n s i s and M o l l i e n i s i a sphenops) per n i g h t from an experimental p o o l maintained a t about four times the n a t u r a l prey d e n s i t y . Random d i p p i n g and g a f f i n g appeared to be the capture technique. Suthers (1965) concluded t h a t N o c t i l i o c o u l d not' use i t s sonar to l o c a t e f i s h below the water s u r f a c e because the a i r - t o - w a t e r . a c o u s t i c impedance match i s poor. However Sut h e r s 1 demonstration t h a t the bat c o u l d e c h o l o c a t e s u r f a c e r i p p l e s suggests t h a t i n nature f i s h might be caught i f they r e v e a l t h e i r presence by d i s t u r b i n g the water s u r f a c e . C e r t a i n l y i n smooth-flowing s e c t i o n s of stream and. i n p o o l s , guppies (and probably B l o e d e l ' s f i s h ) c r e a t e s u r f a c e 31 d i s t u r b a n c e s i n l o c a l i z e d areas t h a t might a i d the bat. Undoubtedly TSioctilio does eat guppies but no q u a n t i -t a t i v e data a r e ' a v a i l a b l e . There are a number of other p o t e n t i a l p r e d a t o r s of the guppy i n the Northern Range but I have no i n f o r m a t i o n o n . t h e i r f i s h - f e e d i n g habits." Many of the l a r g e r r e p t i l e s and mammals are u n f o r t u n a t e l y n e a r i n g e x t i n c t i o n on the i s l a n d due to hunting pressure and h a b i t a t d e s t r u c t i o n . Thus t h e i r impact ( i f any) on the guppy p o p u l a t i o n s i s presumably s t e a d i l y d e c r e a s i n g . I now wish to r e t u r n to the major pre d a t o r s and c o n s i d e r i n more d e t a i l , some aspects of t h e i r d i s t r i b u t i o n and n a t u r a l h i s t o r y . D i s t r i b u t i o n and n a t u r a l h i s t o r y  of the main pre d a t o r s I have summarized the important f e a t u r e s of the n a t u r a l h i s t o r y of the main pre d a t o r s i n Table 2.. A l l the b e h a v i o r a l i n f o r m a t i o n i s based on f i e l d o b s e r v a t i o n s except f o r the a t t a c k s t r a t e g y of Aequidens and H o p l i a s . These o b s e r v a t i o n s were made on wild-caught f i s h i n an aquarium and outdoor p o o l r e s p e c t i v e l y . I have a l r e a d y s t a t e d t h a t Aequidens i s probably the l e a s t important of the 5 p r e d a t o r s though I f e e l my data are not s t r o n g enough to exclude i t from the predator category a l t o g e t h e r . The n o c t u r n a l h a b i t s and low d e n s i t y ( i n c l e a r water) of H o p l i a s make f i e l d o b s e r v a t i o n s of i t s f e e d i n g behavior d i f f i c u l t . TABLE 2. N a t u r a l h i s t o r y of the main f i s h p r e d a t o r s . CICHLIDAE C r e n i c i c h l a Aequidens CHARACIDAE Hoplias Astyanax CYPRINODONTIDAE Rivulus Maximum length (mm)' P r i n c i p a l food P r i n c i p a l h a b i t a t Microhabitat S o c i a l o r g a n i z a t i o n A c t i v i t y p e r i o d b Attack strategy on guppies 200 (350) f i s h headstreams midstreams under logs, rocks; digs p i t s to hide s o l i t a r y or small groups of 2 to 5 i n d i v i d u a l s 148 (170) benthic i n v e r t . headstreams midstreams around l o g s , rocks, l e a f d e t r i t u s , shore vegetation loose aggre-gations or dispersed breeding p a i r s mainly d i u r n a l d i u r n a l ambush or s t a l k ; occa-s i o n a l con-tinuous p u r s u i t continuous p u r s u i t 392 (500) f i s h l a r g e t u r b i d r i v e r s open bottom at night; under l e a f d e t r i t u s , i n day s o l i t a r y 116 (150) aquatic i n s e c t larvae headstreams midstreams open midwater i n pools; i n r i f f l e s t i g h t schools mainly nocturnal s t a l k or continuous p u r s u i t d i u r n a l increase i n d i r e c t i o n and v e l o c i t y of normal swimming 105 (100) t e r r e s t r i a l i n s e c t s springwater ubiquitous i n pools dispersed through-out pool nocturnal and d i u r n a l continuous p u r s u i t aLengths i n brackets are the maxima given f o r the species by Sterba (1962); others are from the present T r i n i d a d c o l l e c t i o n s . D i u r n a l i n c l u d e s periods of dim l i g h t before s u n r i s e and a f t e r sunset. Hoplias i s p r i m a r i l y a f i s h o f the l a r g e r t u r b i d r i v e r s ; most of the i n d i v i d u a l s I caught came from the Car o n i and Guayamare R i v e r s . H o p l i a s a l s o occurs i n lower numbers i n a l l midstreams and headstreams not blocked by major w a t e r f a l l s . I t i s absent, from a l l streams f l o w i n g o f f the north f a c e . o f the Northern Range. C r e n i c i c h l a shares the o v e r a l l d i s t r i b u t i o n of Hoplia s but i s l e s s abundant than H o p l i a s i n lowland t u r b i d r i v e r s . In my samples, the r a t i o of - C r e n i c i c h l a : H o p l j a s was approximately 10:1 f o r headstreams and midstreams but the re v e r s e f o r l o w l a n d ' r i v e r s . As a very rough estimate of the numbers of a d u l t C r e n i c i c h l a i n a t y p i c a l midstream, I counted 50 of these p r e d a t o r s on a h a l f - d a y e x c u r s i o n along 3 km of the Lower A r i p o R. Astyanax and Aeguidens are d i s t r i b u t e d over the same range as the pr e v i o u s two s p e c i e s but some d i f f e r e n c e s do occur. Astyanax i s very prominent i n lowland r i v e r s and midstreams but tends t o decrease i n numbers i n the head-streams. In the s m a l l e r mountain streams a m o r p h o l o g i c a l l y s i m i l a r c h a r a c i d of the genus Hemibrycon (probably H. taen i u r u s ) appears to r e p l a c e Astyanax. Hemibrycon i s ' s m a l l e r than Astyanax but has s i m i l a r f e e d i n g behavior (see Appendix, Table 3 f o r d i e t ) ; one j u v e n i l e guppy was recovered from the stomach of a Hemibrycon. G e n e r a l l y , when I r e f e r to Astyanax i n t h i s t h e s i s , I am a l s o i n c l u d i n g i t s s m a l l e r c o u s i n . 34 Aeguidens has a s i m i l a r d i s t r i b u t i o n to C r e n i c i c h l a though I d i d not c o l l e c t i t i n the C a r o n l or Guayamare R i v e r s . I have s t a t e d b e f o r e t h a t R i v u l u s i s found i n springwaters or i n l a r g e r streams dev o i d of c h a r a c i d s and c i c h l i d s . I t appears t h a t i t s d i s t r i b u t i o n i s l i m i t e d by the presence of the l a r g e r p r e d a t o r s . In the l a b o r a t o r y the escape behavior of R i v u l u s is, p a r t i c u l a r l y i n e f f e c t i v e i n the face of the slow, s t a l k i n g approaches of Ho p l i a s or C r e n i c i c h l a . Small p o p u l a t i o n s o f R i v u l u s do occur i n the v i c i n i t y of the streams c o n t a i n i n g c h a r a c i d and c i c h l i d p r e d a t o r s but are c o n f i n e d t o smal l pools or d i t c h e s . T h i s may i n f a c t be a s m a l l e r "lowland r a c e " of R. h a r t i i ( J . S. Kenny, pe r s . comm.; see a l s o Boeseman, I960.: 120). The important p o i n t a r i s i n g out of t h i s d i s c u s s i o n of the d i s t r i b u t i o n of pre d a t o r s i s t h a t a l l p o p u l a t i o n s o f the guppy are not exposed t o the same p r e d a t o r s . Over most of i t s range i n northern T r i n i d a d , the guppy i s exposed to e i t h e r c h a r a c i d and c i c h l i d p r e d a t i o n or R i v u l u s p r e d a t i o n . Since i n the c h a r a c i d - c i c h l i d s i t u a t i o n there are pr e d a t o r s of a l l s i z e s t h a t can handle the s m a l l e s t or l a r g e s t guppy, I have termed the p r e d a t i o n pressure i n these environments as severe. Undoubtedly t h e r e are d i f f e r e n c e s i n p r e d a t i o n i n t e n s i t i e s between some of these l o c a t i o n s but I have i n s u f f i c i e n t data t o warrant s p l i t t i n g t h i s category. R i v u l u s on the other hand i s a smal l p r e d a t o r — i t s major impact f a l l s on s m a l l guppies. The s e v e r i t y of preda-t i o n i n t h i s case must depend on the abundance and s i z e d i s t r i b u t i o n of the predator (assuming a l t e r n a t e prey to be i n equal s u p p l y ) . Because R i v u l u s d e n s i t y i s more e a s i l y assessed than the d e n s i t y of c h a r a c i d s and c i c h l i d s , I have been able to d i s t i n g u i s h between p o p u l a t i o n s of guppies t h a t are exposed to i n t e n s e R i v u l u s p r e d a t i o n and those s u f f e r i n g o n l y moderate or weak p r e d a t i o n . In some streams (e.g. Paria) R i v u l u s d e n s i t y i s very low and few l a r g e i n d i v i d u a l s occur. P o p u l a t i o n s of guppies under these c o n d i t i o n s must enjoy v i r t u a l freedom from a q u a t i c p r e d a t o r s . For the d e t a i l e d study of a n t i p r e d a t o r behavior (Chapter 6) I have s e l e c t e d 5 main p o p u l a t i o n s of guppies to compare (Table 3). These p o p u l a t i o n s were chosen because they encompass the range of p r e d a t i o n p r e s s u r e s o c c u r r i n g i n the Northern Range area. In a d d i t i o n they are g e o g r a p h i c a l l y i s o l a t e d or semi-isolated--gene exchange must be very s m a l l among them. For these reasons, they are an e x c e l l e n t t e s t case f o r my hypothesis as o u t l i n e d i n . Chapter 1. M i g r a t i o n and d i s p e r s a l of guppies I have used the term " p o p u l a t i o n " r e l a t i v e l y l o o s e l y in- t h i s t h e s i s — i t should i d e a l l y be d e f i n e d by r a t e s of d i s p e r s a l or gene m i g r a t i o n . As Dobzhansky (1970: 240) p o i n t s out, a d e f i n i t i o n of geographic i s o l a t i o n must-36 TABLE 3. The p r i n c i p a l f i s h p r e d a t o r s o c c u r r i n g a t the main study areas. R i v e r C r e n i c i c h l a Aequidens H o p l i a s Astyanax R i v u l u s Guayamare + - ' +' +• Lower A r i p o + + + + -P. Curucaye - - - - +. Upper -Aripo - - - + P a r i a - - - - + Guayamare does have the m o r p h o l o g i c a l l y s i m i l a r Cichlasoma bimaculatum. b R i v u l u s i s not absent from t h i s geographic area but does not occur i n the r i v e r i t s e l f , hence guppies are not exposed to i t . 37 i n c o r p o r a t e both a b s o l u t e d i s t a n c e and the v a g i l i t y of the organism. Although I a s s o c i a t e a " p o p u l a t i o n " with a s p e c i f i c c o l l e c t i o n s i t e , there i s good evidence (at l e a s t f o r the 5 s i t e s i n Table 3) suggesting t h a t I am d e a l i n g with q u i t e d i s c r e t e u n i t s . Haskins e t a l (1961) found t h a t guppies show c o n s i d e r a b l e s i t e attachment; even d u r i n g p e r i o d s of sudden f l a s h f l o o d i n g the i n t e g r i t y of c e r t a i n subpopulations remained i n t a c t . They a l s o d i s c o v e r e d t h a t over a uniform s t r e t c h of the Arima R. the f r e q u e n c i e s of c e r t a i n c o l o u r p a t t e r n s (known to be g e n e t i c a l l y c o n t r o l l e d ) c o u l d change a b r u p t l y . However i n the same r i v e r they were able to e s t a b l i s h permanently (at l e a s t to about.36 generations) a marker gene up to 10 km from the p o i n t of i n t r o d u c t i o n . These o b s e r v a t i o n s suggest t h a t subpopulations w i t h i n a s i n g l e r i v e r may be q u i t e i s o l a t e d : but t h a t some gene exchange must be o c c u r r i n g . S i t e attachment was observed i n an experiment (Chapter 4) conducted i n the s c r e e n e d - o f f s e c t i o n of the P e t i t e Curucaye R. ( r e f e r t o F i g . 1, Appendix). The s e c t i o n was i n i t i a l l y c l e a r e d of a l l guppies and then 50 male and 50 female guppies were c o l l e c t e d from above and below the s e c t i o n and p l a c e d i n Pool 2 (Pools 1, 3, and 4 were now "vacant"). On subsequent days I recounted a l l the f i s h , noted t h e i r d i s t r i b u t i o n i n the s e c t i o n , and r e t u r n e d them to Pool 2. Although I had expected c o n s i d e r a b l e m i g r a t i o n 38 to Pools. 3 and. 4, t h i s was not observed (Table 4).. TABLE 4. S i t e attachment of Curucaye R. guppies i n the P e t i t e No. i n Pool 2 No. o u t s i d e Pool 2 S e c t i o n t o t a l Percent m i g r a t i o n June 14/69 84 2 86 2.3 16/69 80 5 85 5.9 18/69 79 2 81 2.5 20/69 76 3 79 3.8 22/69 79 0 79 0 26/69 70 5 75 6.7 J u l y 1/69 71 2 73 2.7 Mean 3.4 These data suggest t h a t . t h e f i s h i n each p o o l may be a r e l a t i v e l y i s o l a t e d b r e e d i n g u n i t (deme). Because . approximately 50 pools separate the P e t i t e Curucaye sampling s i t e from the Santa Cruz R. ( t o t a l d i s t a n c e of 1.6 km), gene flow i s probably s m a l l between them. However, the c o n t r i b u t i o n of j u v e n i l e f i s h to the t o t a l gene flow i s unknown. T h i s c o n d i t i o n i s r e m i n i s c e n t of. the u n i d i m e n s i o n a l "stepping stone" model envisaged by Kimura and Weiss (1964). Even i n the absence of major b a r r i e r s t o gene flow, i s o l a t i o n 39 by d i s t a n c e alone might f a c i l i t a t e the d i f f e r e n t i a t i o n of l o c a l p o p u l a t i o n s by g e n e t i c d r i f t (Rohlf and S c h n e l l , 1971) or d i f f e r e n t i a l s e l e c t i o n ( E h r l i c h and Raven, 1969). In a d d i t i o n to i s o l a t i o n by d i s t a n c e , two popula-t i o n s ( P a r i a and Upper Aripo) are a l s o i s o l a t e d by major p h y s i c a l b a r r i e r s ( r e f e r t o F i g . 3). Gene exchange with the C a r o n i R i v e r system i s probably r a r e though some f i s h undoubtedly' get swept over the A r i p o f a l l s (the p h e n o t y p e — s i z e and c o l o u r - - o f males c o l l e c t e d i n pools immediately below the f a l l s i s in t e r m e d i a t e between Upper A r i p o and Lower Aripo;. t h i s r e g i o n may be a narrow h y b r i d zone). D i s p e r s a l of f i s h to these i s o l a t e d p o p u l a t i o n s must be f o r t u i t o u s — t e r r e s t r i a l p r e d a t o r s (bats, k i n g f i s h e r s ) or tornadoes may a c c i d e n t a l l y drop l i v e f i s h above a b a r r i e r . In r e c e n t T r i n i d a d h i s t o r y a t l e a s t one tornado was observed to c a r r y small f i s h f o r a c o n s i d e r a b l e d i s t a n c e . Summary of Chapter 3 1. The limnology of T r i n i d a d streams i s r e l a t i v e l y unknown. 2. In the Northern Range area the streams may be c l a s s i f i e d i n t o f o u r i n t e r g r a d i n g types d i f f e r i n g i n a number of a b i o t i c and b i o t i c parameters. 3. To a c e r t a i n extent the d i s t r i b u t i o n of f i s h s p e c i e s i s c o r r e l a t e d with t h i s c l a s s i f i c a t i o n . D i v e r s i t y i s g r e a t e s t i n the lowland r i v e r s and l e a s t i n the s p r i n g -waters . 4. The d i s t r i b u t i o n o f f i s h i s a l s o g r e a t l y i n f l u -enced by the topography. The Northern Range has prevented c e r t a i n s p e c i e s i n the Ca r o n i R. system from c o l o n i z i n g the r i v e r s f l o w i n g northward i n t o the Caribbean Sea. In some streams w a t e r f a l l s have b l o c k e d the upward passage of f i s h . 5. I t i s concluded t h a t the most important p r e d a t o r s of the guppy are Ho p l i a s malabaricus, C r e n i c i c h l a  a l t a , Astyanax bimaculatus, and R i v u l u s h a r t i i . Guppies have been found i n the stomachs of the l a s t three s p e c i e s . 6. F i s h - e a t i n g a e r i a l p r e d a t o r s occur i n the regions under c o n s i d e r a t i o n but t h e i r impact on guppy p o p u l a t i o n s i s unknown. 7. The guppy i s u b i q u i t o u s i n i t s d i s t r i b u t i o n i n the r i v e r s of no r t h e r n T r i n i d a d but there are important d i s c o n t i n u i t i e s i n the d i s t r i b u t i o n of p r e d a t o r s . Some guppies are preyed upon by R i v u l u s ; others s u f f e r c h a r a c i d and c i c h l i d p r e d a t i o n . 8. Guppies show c o n s i d e r a b l e s i t e attachment to small p o r t i o n s of t h e i r home stream. Together w i t h i s o l a -t i o n by d i s t a n c e and p h y s i c a l b a r r i e r s , t h i s ..behavior probably reduces gene flow between p o p u l a t i o n s and f a c i l i t a t e s the d i f f e r e n t i a t i o n o f l o c a l p o p u l a t i o n s . CHAPTER- 4 GEOGRAPHIC VARIATION IN THE SEX RATIO I n t r o d u c t i o n In t h i s chapter I s h a l l d e s c r i b e p o p u l a t i o n d i f f e r e n c e s i n the sex r a t i o and attempt t o f i n d out why these d i f f e r e n c e s occur. The r e l a t i o n s h i p between p r e d a t i o n and the s e l e c t i v e l o s s of male, guppies from n a t u r a l p o p u l a t i o n s h a s . r e c e i v e d a gr e a t d e a l of a t t e n t i o n from Haskins e t a l (1961). These workers used the sex r a t i o as i n d i r e c t evidence f o r the hypothesis t h a t male c o l o r a t i o n i s determined by the con-' f l i c t i n g demands imposed by sexual s e l e c t i o n and p r e d a t i o n . Since I o r i g i n a l l y s e t out to study the behavior o f guppies of d i f f e r e n t c o l o u r s ( i . e . male c o l o u r s — f e m a l e s of a l l p o p u l a t i o n s are a c r y p t i c grey-brown c o l o u r ) , i t was important t o c r i t i c a l l y assess t h i s e a r l i e r study, f i r s t l y by e x t e n s i v e c o l l e c t i n g i n T r i n i d a d and secondly, by r e p e a t i n g some of the experiments of Haskins e t al_, but examining i n m o r e - d e t a i l the pred a t o r - p r e y b e h a v i o r a l i n t e r a c t i o n s . I reasoned t h a t i f conspicuous c o l o r a t i o n i s indeed a l i a b i l i t y w i t h r e s p e c t to p r e d a t i o n , geographic d i f f e r e n c e s i n the sex r a t i o should be c o r r e l a t e d with p r e d a t i o n p r e s s u r e ( i . e . a g r e a t e r l o s s of males r e l a t i v e 42 to females where p r e d a t i o n i s s e v e r e ) . During t h i s study of the sex r a t i o , a d d i t i o n a l geographic v a r i a t i o n was d i s c o v e r e d ; t h i s i s d e s c r i b e d i n the f o l l o w i n g two c h a p t e r s . Sex r a t i o s of n a t u r a l p o p u l a t i o n s Table 5 l i s t s the sex r a t i o s of the c o l l e c t i o n s made i n 1967 and 1969. The sex r a t i o i s always expressed as the r a t i o of males to one female. To determine the s t a b i l i t y of the d i f f e r e n c e s i n the sex r a t i o , 13 of the s i t e s sampled i n 1967 were resampled i n 1969. The c o l l e c t i o n s were separated i n time by a p p r o x i -mately 6 generations (18-26 months). I assume t h a t none of the a d u l t s sampled i n 1969 had been born a t the time of the 1967 c o l l e c t i o n s ( l o n g e v i t y i n nature i s probably l e s s than 1 year, Haskins e t a l , 1961). The c o r r e l a t i o n between years i n the sex r a t i o i s s i g n i f i c a n t ( F i g . 5) suggesting t h a t the sex r a t i o d i f f e r e n c e s among stream s i t e s are not simply random d e v i a t i o n s p e r t a i n i n g t o o n l y one c o l l e c t i o n year, season, or sample. Note t h a t Blue B a s i n was dropped from the a n a l y s i s f o r reasons g i v e n elsewhere. The sex r a t i o f o r a l l 1967 c o l l e c t i o n s , i s 0.77; f o r 1969 i t i s 0.73. The pooled r a t i o f o r both years i s 0.74 (n = 14245) and i s s i g n i f i c a n t l y d i f f e r e n t from.an expected 1 : 1 . r a t i o ( X 2 1 d f =305.47, p < .001). When the sex r a t i o s f o r a l l streams are pooled (and 43 TABLE 5A. The sex r a t i o s of n a t u r a l p o p u l a t i o n s (1967). C o l l e c t i o n Source date Males Females Immat. N Sex R a t i o Springwater Upper Curumpalo (1) 5/7/67 24 69 31 124 0 .35 Upper Curumpalo (2) 16/8/67 53 118 16 187 0 .45 t o t a l 77 187 47 311 0, .41 S i e r r a Leone Rd. (1) 17/7/67 115 243 19 377 0, .47 S i e r r a Leone Rd. (2) 16/8/67 210 335 446 991 0, .63 t o t a l 325 578 4.65 1368 0, .56 Maracas V i l l a g e (1) 25/7/67 179 219 37 435 0 , .82 Maracas V i l l a g e (2) 25/7/67 44 98 22 164 0, .45 t o t a l 223 317 59 599 0, ,70 Tompire T r i b u t a r y 22/7/67 50 114 10 174 0, .44 T o t a l S p r i n g s 675 1196 581 2452 0. .56 Mean sex r a t i o (n=4): 0.53 Headstream Upper A r i p o (1) 6/7/67 70 115 79 264 0. .61 Upper A r i p o (2) 6/7/67 83 127 122 332 0. .65 t o t a l 153 242 201 596 0. .63 Upper Blue B a s i n 19/8/67 95 213 12 320 0, .45 Upper P a r i a (1) 10/7/67 122 162 53 337 0, .75 Upper P a r i a (2) 24/8/67 85 150 150 385 0. .57 t o t a l 217 312 203 722 0. ,66 Y a r r a 10/7/67 65 57 3 125 1. .14 Upper T a c a r i g u a (1) 1/7/67 106 81 88 275 1. .31 Upper T a c a r i g u a (2) 17/8/67 261 161 234 656 1.62 t o t a l 367 242 322 931 1. ,52 T o t a l Headstream 887 1066 741 2694 0. ,83 Mean sex r a t i o (n=5): 0.88 Midstream Lower P a r i a 27/7/67 59 111 42 212 0. ,53 Marianne 24/7/67 97 67 6 170 1, .45 Santa Cruz 5/7/67 112 102 203 417 1. ,10 Lower T a c a r i g u a (1) 24/7/67 150 150 199 499 1. .00 Lower T a c a r i g u a (2) 7/8/67 268 276 849 1393 0. ,97 t o t a l 418 426 1048 1892 0. .98 Lower A r i p o (1) 12/7/67 81 118 116 315 0. .69 Lower A r i p o (2) 21/7/67 65 77 115 257 0, .84 t o t a l 146 195 231 572 0. ,75 Quebrada d.II. 3/7/67 73 71 80 224 1, .03 T o t a l Midstream 905 972 1610 3487 0. ,93 Mean sex r a t i o ( n - 6 ) : 0.97 Lowland R i v e r s Guayamare 19/7/67 105 122 70 297 0, .86 C a r o n i 12/7/67 92 112 351 555 0, .82 T o t a l Lowland R i v e r s 197 234 421 852 0. ,84 Mean sex r a t i o (n-2): 0.84 1967 T o t a l 2664 3468 3353 9485 0.77 44 TABLE 5B. The sex r a t i o s of n a t u r a l p o p u l a t i o n s (1969). Source C o l l e c t i o n d a te Males Females Immat. Sex r a t i o Sprlnqwater Upper Curumpalo (1) 13/3/69 30 73 19 122 0.41 Upper Curumpalo (2) t o t a l 13/3/69 40 70 70 143 8 27 118 240 0. 0. ,57 ,49 -P e t i t e Curucaye (1) P e t i t e Curucaye (2) t o t a l 13/6/69 26/6/69 84 43 127 251 134 385 129 • 85 214 464 262 726 0. 0. 0. .34 ,32 ,33 Grande Curucaye 22/4/69 263 432 381 1076 0. ,61 Torapire T r i b u t a r y 25/4/69 26 94 85 2 0 5 0. 2 8 T o t a l S p r i n g s Mean sex r a t i o (n-4): 0.43 486 1054 707 2247 0. .46 Headstream Upper A r i p o (1) Upper A r i p o (2) Upper A r i p o (3) t o t a l 29/3/69 13/5/69 13/5/69 55 86 111 252 89 164 275 528 365 87 113 565 509 337 499 1345 0. 0. 0. 0, ,62 52 ,40 ,48 Upper Blue B a s i n (1) Upper Blue B a s i n (2) t o t a l 6/5/69 29/5/69 224 194 418 261 164 425 417 305 722 902 663 1565 0. 1. 0. ,86 ,18 98 Upper P a r i a 18/4/69 292 256 212 760 1, ,14 Y a r r a 19/3/69 53 58 55 166 0. ,91 Upper T a c a r i g u a 15/4/69 286 293 347 926 0. 98 Upper Guanapo 2/5/69 331 449 585 1365 0. 74 Upper Arouca 2/5/69 158 184 164 506 0. 86 T o t a l Headstreams Mean sex r a t i o (n=7) : 0.87 1790 2193 2650 6633 0. 82 Midstream Marianne 19/3/69 68 85 81 234 0. 80 Santa Cruz 14/4/69 219 241 898 1358 0. 91 Lower T a c a r i g u a 5/4/69 317 350 528 1195 0. 91 Lower A r i p o 2/4/69 153 216 525 894 0. 71 Oropuche 25/4/69 138 275 324 737 0. 50 T o t a l Midstreams Mean sex r a t i o (n=5) -. 0.77 895 1167 2356 4418 0. ,77 Lowland R i v e r s Guayamare 16/3/69 110 120 85 315 0. 92 C a r o n i 7/4/69 84 114 58 256 0. ,74 Caparo 17/6/69 50 50 48 148 1. ,00 T o t a l Lowland R i v e r a Mean sex r a t i o (n-3) : 0.89 244 284 191 719 0. 86 1969 T o t a l 3415 4698 5904 14017 0. 73 T o t a l 1967 i. 1969 6079 8166 9257 23502 0. 74 45a FIGURE 5. A comparison of the sex r a t i o of 13 p o p u l a t i o n s of guppies sampled i n 1967 and resampled i n 1969. For sample s i z e s r e f e r to Table 5; f o r stream a b b r e v i a t i o n s r e f e r t o F i g . 1. 45b LU < UJ Lu CC UJ CL CO UJ _ l < C D c n x LU CO 15 -tt -15 -12 -1.1 -10 -09-0.8-0.7-0JB-a s -0.4-03-02--r s = • 052 t =192 df =10 p <.05 TT UA •UTac Mar »Car LA (Yar >SC > LTac •Guay Par B B UCur — i — i 1 1 1 — i 1 — i 1 1 02 03 OA 05 0.6 0.7 08 09 10 1.1 12 SEX RATIO 1969 (MALES PER FEMALE) 46 hence geographic d i f f e r e n c e s obscured) these data c o r r o b o r a t e the f i n d i n g s of Haskins e t a l (1961) f o r T r i n i d a d guppies (sex r a t i o = 0.61, n = 3994) and mainland South American guppies (F. F. Bond c o l l e c t i o n , sex r a t i o = 0.61, n = 7825). I t i s c l e a r t h a t on the average t h e r e i s a s i g n i f i c a n t excess of females i n n a t u r a l p o p u l a t i o n s . However as Table 5 r e v e a l s , a l l p o p u l a t i o n s are not e q u a l l y unbalanced and some (e.g. Lower Tacarigua) are remarkably w e l l balanced. T h e r e f o r e geographic v a r i a t i o n may be a good c l u e to the mechanisms i n v o l v e d i n the s e l e c t i v e l o s s of males, a p o i n t a p p a r e n t l y overlooked by Haskins et. aJL (1961). The presence of d i f f e r e n c e s i n the sex r a t i o r a i s e s the p o s s i b i l i t y t h a t i n some p o p u l a t i o n s the secondary sex r a t i o i s b i a s e d i n f a v o r of females because more female zygotes are produced (through abnormal mei o s i s or p a r t i a l "Y" sperm i n v i a b i l i t y ) or more males d i e before b i r t h (guppies are o v o v i v i p a r o u s and the embryos spend the f i r s t few weeks of l i f e i n s i d e the mother). I c o n s i d e r e d t h i s p o s s i b i l i t y i n my l a b o r a t o r y s t o c k s . • Sex r a t i o s of l a b o r a t o r y stocks To determine i f my stocks would produce an excess of females ( f o r g e n e t i c or other r e a s o n s ) , I r a i s e d a l a r g e number of guppies to sexual m a t u r i t y under c o n t r o l l e d p r e d a t o r - f r e e c o n d i t i o n s . I used stocks d e r i v e d from n a t u r a l p o p u l a t i o n s e x h i b i t i n g both balanced (e.g. Guayamare) and unbalanced (e.g. P e t i t e Curucaye) sex r a t i o s . The data were.obtained by (a) sexing the a d u l t s i n stocks t h a t had been s t a r t e d with immature f i s h (number and l e n g t h not recorded) and (b) running t e s t s t o check s p e c i f i c a l l y f o r v a r i a t i o n i n the sex r a t i o . In (b) I r a i s e d o n l y immature f i s h of l e s s than 12 mm, a s i z e w e l l below the p o i n t where secondary sexual c h a r a c t e r s become e v i d e n t . The sex r a t i o s o b t a i n e d i n these experiments (Table 6) d i d not d i f f e r s i g n i f i c a n t l y from the expected t h e o r e t i c a l 1:1 r a t i o though there was an o v e r a l l s l i g h t excess of females (sex r a t i o = 0.90, n = 1049). I conclude t h a t there i s no i n h e r e n t tendency f o r any of the stocks to produce an uneven sex r a t i o . I assume t h i s a p p l i e s to the r e s p e c t i v e n a t u r a l p o p u l a t i o n s as w e l l . My r e s u l t s support the f i n d i n g s of Haskins e t a l (1961). They o b t a i n e d a r a t i o of 0.94 when 3359 guppies from u n s p e c i f i e d r i v e r s i n T r i n i d a d were r a i s e d t o m a t u r i t y i n the l a b o r a t o r y . However Breder and Coates (1932) working with a stock d e r i v e d from a Jamaican p o p u l a t i o n found the sex r a t i o a t b i r t h to be 0.5. Shoemaker (1944) repeated these experiments with an " I l l i n o i s " s t r a i n of domestic guppies and concluded t h a t the r a t i o at b i r t h was indeed 1:1 but t h a t males were more s u s c e p t i b l e to h i g h temperatures and consequently d i e d at a younger age; t h i s r e s u l t e d i n very unbalanced sex r a t i o s . Comfort (1961) and Haskins e t a l (1961) working with domestic and w i l d stocks 48 TABLE 6. The sex r a t i o s of l a b o r a t o r y p o p u l a t i o n s . Sex Tank v o l . Orig.No. Stock Replic. Males Females Immat . r a t i o ( l i t e r s ) of f r y Lower Aripo 1 35 77 0 0.45 200 unknown 2 25 24 3 1.04 200 250 3 50 36 4 1.39 200 135 4 73 81 13 0.90 400 210 5 17 14 39 1.21 400 75 T o t a l 200 232 59 0.86 X 2 (males:females) = 2.22 ; p > 0. 10 Upper Aripo 1 41 45 17 0.91 400 unknown 2 25 37 28 0.68 400 165 3 24 15 22 1.60 40 unknown Tota l 90 97 67 0.93 X 2 = 0.19; p > 0.50 Guayamare 1 18 17 2 1.06 40 unknown 2 24 26 17 0.92 120 170 -T o t a l 42 43 19 0.98 X 2 = 0.00; p > .99 P e t i t e Curucaye 1 39 43 37 0.91 40 unknown 2 43 45 4 0.96 200 125 3 33 47 13 0.70 200 160 T o t a l 115 135 54 0.85 x 2 = 1.44; p > 0.20 P a r i a 1 25 20 33 1.25 40 unknown 2 24 26 19 0.92 400 unknown Tota l 49 46 52 1.07 x 2 = 0.04; p > 0.80 Grand T o t a l : males: 496 females: 553 sex r a t i o = 0.90 N = 1049 0 (adults) x 2 2.99; p > 0.05 r e s p e c t i v e l y , concluded t h a t i n the absence of p r e d a t i o n there was no d i f f e r e n c e i n l o n g e v i t y between the sexes. Geodakyan and K o s o b u t s k i i (1972) have shown e x p e r i -m e n t a l l y ( i n an u n s p e c i f i e d l a b o r a t o r y stock of guppies) t h a t , "the.sex r a t i o of n a t a l i t y . d e p e n d s i n t u r n on the sex r a t i o of the parent g e n e r a t i o n (p. 124)." I t i s not known how t h i s f eedback.is accomplished but v i s i o n appears to p l a y a r o l e . I t i s p o s s i b l e t h a t the g e n e t i c c o n t r o l of sex i s unstable and t h a t r a c i a l or s t r a i n d i f f e r e n c e s might account f o r t h e - c o n f l i c t i n g r e p o r t s from v a r i o u s l a b o r a -t o r i e s (Winge, 1934; Shoemaker, 1944; Kallman, 1965). For T r i n i d a d p o p u l a t i o n s the evidence f o r a s t a b l e XX-XY sex d e t e r m i n a t i o n system i s q u i t e c o n v i n c i n g . Why then i s there-such s t r i k i n g geographic v a r i a t i o n i n the sex r a t i o ? S e l e c t i v e p r e d a t i o n on males may be p a r t -of the answer. R e l a t i o n s h i p of p r e d a t i o n to  v a r i a t i o n i n the sex r a t i o When the sex r a t i o s of the f i e l d c o l l e c t i o n s are r e l a t e d t o the presence of f i s h p r e d a t o r s (Table 7) no c l e a r - c u t c o r r e l a t i o n i s d i s c e r n i b l e . In 1969 (the year f o r which the most complete data are a v a i l a b l e ) the sex r a t i o ranged from 0.28 to 1.14 f o r streams c o n t a i n i n g Rivulus and 0.50 to 1.00 f o r streams wit,h c h a r a c i d s and c i c h l i d s . 50 TABLE 7. The r e l a t i o n s h i p o f the sex r a t i o t o the presence of f i s h p r e d a t o r s . Sex r a t i o s are based on the 1969 c o l l e c t i o n s . RIVULUS CHARACIDS AND CICHLIDS Ri v e r Sex r a t i o R i v e r Sex r a t i o Tompire T r i b . 0 .28 Oropuche 0.50 P e t i t e Curucaye 0 .33 Lower A r i p o 0.71 Upper A r i p o 0 .48 Upper Guanapo 0.74 Upper Curumpalo 0 .49 Caroni 0.74 Grande Curucaye 0 .61 Upper Arouca 0.86 Marianne 0 .80 Santa Cruz 0.91 Ya r r a 0 .91 Lower T a c a r i g u a 0.91 Blue Basin 0 .98 Guayamare 0.92 P a r i a • 1 .14 Upper T a c a r i g u a 0.98 Caparo 1.00 51 I t i s important to r e c a l l t h a t i n g e n e r a l , streams with R i v u l u s are s m a l l e r , c o o l e r , and more h e a v i l y shaded, though where p h y s i c a l b a r r i e r s prevent the immigration of ch a r a c i d s and c i c h l i d s , R i v u l u s does occur under e n v i r o n -mental c o n d i t i o n s resembling some of the normal c h a r a c i d -c i c h l i d h a b i t a t s . Thus Upper A r i p o , P a r i a , and Ya r r a ( a l l with Rivulus) are p h y s i c a l l y s i m i l a r streams to Upper Ta c a r i g u a and Upper Arouca (both with c h a r a c i d s and c i c h l i d s ) . The r e l a t i o n s h i p between p r e d a t i o n and the sex r a t i o becomes more e v i d e n t when p r e d a t i o n i s assessed i n terms of the abundance,of p r e d a t o r s r a t h e r than o n l y t h e i r presence or absence ( F i g . 6 ) . T h i s a n a l y s i s c o u l d be made onl y f o r Ri v u l u s h a b i t a t s because the d e n s i t y estimates f o r c h a r a c i d s and c i c h l i d s were inadequate. With the ex c e p t i o n of Oropuche, the sex r a t i o s of a l l p o p u l a t i o n s exposed to ch a r a c i d s and c i c h l i d s tended.to be f a i r l y balanced with o n l y a s l i g h t preponderance of females. For these streams no r e l a t i o n s h i p was found between the sex r a t i o and the t u r b i d i t y , temperature, or s i z e of the stream. For streams c o n t a i n i n g R i v u l u s , the sex r a t i o was balanced when R i v u l u s d e n s i t y was low but unbalanced when high d e n s i t i e s were observed (low d e n s i t y = l e s s t h a n 10 ad u l t s caught i n 1 to 2 hours; high d e n s i t y = g r e a t e r than 50 a d u l t s caught with same e f f o r t ) . R i v u l u s d e n s i t y i s a l s o c o r r e l a t e d to some extent 52a FIGURE 6. The r e l a t i o n s h i p of the sex r a t i o t o the r e l a t i v e d e n s i t y of R i v u l u s . Data are f o r the 1969 c o l l e c t i o n s . 52b LU LU tr LU Q_ CO LU 1.2 H < 1.1 H cn UJ a . a . O o < tr x LU cn 1.0 0.9 0.8 0.7 0.6 0.5-0.4-0.3-Q2- I 1 1 1 NONE LOW MEDIUM HIGH RELATIVE DENSITY of RIVULUS 53 with the amount of cover and s i z e of the stream. The most extreme sex r a t i o s were observed i n the Tompire T r i b u t a r y and P e t i t e Curucaye R.; these streams a l s o had the s m a l l e s t volume of water flow and the l a r g e s t i n d i v i d u a l s , and densest p o p u l a t i o n s of R i v u l u s . There i s some a d d i t i o n a l comparative evidence r e l a t i n g R i v u l u s abundance to sex r a t i o s of guppies. At Blue Basin i n 1 9 6 9 , there was a conspicuous absence of l a r g e (> 50 mm) R i v u l u s compared to the 1967 l e v e l s . Although twice the e f f o r t was spent s e a r c h i n g f o r R i v u l u s i n 1969 ( c o l l e c t i n g gear was i d e n t i c a l and both c o l l e c t i o n s were i n the afternoon) very few R i v u l u s were caught or seen (Table 8 ) . The same between-years comparison with another stream (Tompire T r i b u t a r y ) r e v e a l e d no p a r a l l e l d i f f e r e n c e s suggesting the decrease was s p e c i f i c t o Blue B a s i n . TABLE 8 . R i v u l u s abundance i n two streams i n 1967 and 1 9 6 9 . Twice the e f f o r t . w a s spent, c o l l e c t i n g specimens a t Blue Basin i n 1969 compared.to 1 9 6 7 . The e f f o r t both years at Tompire T r i b u t a r y was about the same. BLUE BASIN TOMPIRE TRIBUTARY < 50 mm > 50 mm T o t a l < 50 mm > 50 mm T o t a l 1967 21 40 61 36 18 54 1969 16 6 22 41 15 56 T o t a l 37 46 83 . . . 77. . . . , 33 . 110 X 2 . - . 1 df 8 . 1 1 , p < . 0 0 5 x 2 x i df ; . 0 . 2 9 , p >. . 5 0 54 As w e l l as a s i g n i f i c a n t decrease i n the abundance of l a r g e R i v u l u s a t Blue Basin i n 1969, there was a l s o a s t r i k i n g change i n the s e x " r a t i o s of guppies (Table 9). TABLE 9. Sex r a t i o of guppies a t Blue Basin i n 1967 and 1969. Males Females T o t a l 1967 (Rivulus high) 95 213 308 1969 (Rivulus low) 418 425 843 T o t a l 513 638 1151 X 1 df 31.32, p < .001 The main e c o l o g i c a l d i f f e r e n c e between years appeared to be the l e v e l of p o l l u t i o n through the use of t h i s small mountain stream f o r the laundry of c l o t h e s . In 1967 there was l i t t l e human a c t i v i t y i n the headwater re g i o n s of Blue B a s i n , but i n 1969 I observed s e v e r a l groups of women washing c l o t h e s i n the stream. Pools a few hundred meters below the main laundry s i t e had accumulations of soap foam and empty detergent b o t t l e s . A few meters below the laundry s i t e I observed s e v e r a l dozen dead guppies and a dead Synbranchus marmpratus. Although I found no dead R i v u l u s , I put f o r t h the h y p o t h e s i s t h a t the decrease i n R i v u l u s numbers i n 1969 was caused d i r e c t l y or 55 i n d i r e c t l y by p o l l u t i o n . Because the number of guppies was s t i l l very high (perhaps above 1967 l e v e l s ) , i t i s p o s s i b l e t h a t they were not a f f e c t e d by the p o l l u t a n t s (except i n the immediate v i c i n i t y of the p o l l u t i o n s o u r c e ) . T h e r e f o r e the r e d u c t i o n of the R i v u l u s stock c o u l d have f a c i l i t a t e d the r e s t o r a t i o n of a balanced sex r a t i o i n 1969. (Of the 13 stream s i t e s t h a t were compared between 1967 and 1969 ( F i g . 5) Blue B a s i n was the o n l y one d i f f e r i n g n o t i c e a b l y i n i t s predator population;. For t h i s reason i t was dropped.from the c a l c u l a t i o n of the c o r r e l a -t i o n c o e f f i c i e n t . ) On the b a s i s of the comparative f i e l d data presented above, the hypothesis t h a t R i v u l u s s e l e c t i v e l y feeds on male guppies i s c e r t a i n l y c o mpelling; however i t s t i l l r e l i e s on c i r c u m s t a n t i a l evidence. Sex r a t i o s might a c t u a l l y be' determined by more s u b t l e c a u s a l f a c t o r s t h a t a l s o i n f l u e n c e (or a t l e a s t are c o r r e l a t e d with) R i v u l u s d e n s i t i e s . I attempted to get d i r e c t f i e l d evidence by examining stomach contents. U n f o r t u n a t e l y I found few guppies of r e c o g n i z a b l e sex i n predator stomachs. Of 58 guppies recovered from R i v u l u s there were-10 a d u l t s or sub-adults (6 males, 1 female-, 3 u n c e r t a i n ) . A l l guppies recovered from Astyanax and Hemibrycon were j u v e n i l e s (< 13 mm) but most of the. f i s h remains i n C r e n i c i c h l a stomachs (mostly s c a l e s and vertebrae) appeared to be of l a r g e r f i s h , probably a d u l t 56 guppies (sex unknown) and j u v e n i l e Aequidens or Astyanax. Therefore except f o r l i m i t e d data on R i v u l u s (6 males versus 1 female recovered) I have no d i r e c t f i e l d evidence demonstrating males are more l i a b l e to n a t u r a l p r e d a t i o n . For a d d i t i o n a l support of the h y p o t h e s i s , I adopted an experimental approach. T h i s seemed mandatory because, even a t t h i s p r e l i m i n a r y stage I had made s e v e r a l observa-t i o n s t h a t were a t v a r i a n c e with the "Haskins hypothesis'': (a) sex r a t i o s are r e l a t i v e l y balanced where the "more dangerous and s p e c i a l i z e d f i s h p r e d a t o r s " (characids and c i c h l i d s ) occur but are unbalanced (few males) where . good•numbers of R i v u l u s ("probably one of the l e s s severe f i s h p r edators of a d u l t L e b i s t e s " ) are found. (b) males are o f t e n g r e a t l y outnumbered by females y e t are s t i l l c o n s p i c u o u s l y c o l o u r e d (e.g. Upper.Aripo males are c h a r a c t e r i z e d by l a r g e b l a c k markings t h a t are o f t e n o c e l l a t e d with i r i d e s c e n t pigments; however the sex. r a t i o i n 1967 = 0.63; 1969 = 0.48). I f sex r a t i o s r e f l e c t the i n t e n s i t y of d i f f e r e n t i a l , s e l e c t i o n by p r e d a t o r s a g a i n s t males on the b a s i s of t h e i r c o l o u r , o b s e r v a t i o n s (a) and (b) are j u s t the o p p o s i t e of what would be expected. Furthermore, I found t h a t (c) sex r a t i o s are normally most unbalanced i n s m a l l , c l e a r streams (e.g. P e t i t e Curucaye, Tompire T r i b u t a r y ) , an environment ' where conspicuousness (based on e i t h e r hue or c o n t r a s t ) i s probably l e a s t important to v i s u a l p r e d a t o r s . 57 These c o n f l i c t i n g o b s e r v a t i o n s prompted experiments aimed at t e s t i n g a l t e r n a t e hypotheses t h a t might account f o r the observed v a r i a t i o n i n . t h e sex r a t i o . S i z e hypothesis I t i s w e l l known t h a t male guppies cease'growing a t about the time of sexual m a t u r i t y whereas females continue to grow u n t i l death. Consequently the sexes do not share the same s i z e d i s t r i b u t i o n . In the P e t i t e Curucaye R. f o r example, 73% of the females exceed the s i z e ( i n length) o f ' the l a r g e s t male.. T h i s i s shown v e r y ' c l e a r l y i n the. Appendix, F i g . 2). In the r e g i o n of o v e r l a p i n length,.no sex, d i f f e r e n c e i n .wet body weight was found. Since many pr e d a t o r s are s i z e - s e l e c t i v e i n t h e i r f e e d i n g ( I v l e v , '19.61; Jackson, 1961; Brooks and Dodson, 1965; Mason, 1965; G a l b r a i t h , 1967; Brooks, 1968; LeBrasseur, 1969; Parker, 1971), a l l s i z e c l a s s e s of prey may not be e q u a l l y v u l n e r a b l e to a given p r e d a t o r . S i z e - s e l e c t i v e p r e d a t i o n might p l a y an important r o l e i n the observed geographic trends i n the sex r a t i o . For example" I found no guppies above 23.0 mm i n the stomachs of R i v u l u s . For the P e t i t e Curucaye p o p u l a t i o n , t h i s upper l i m i t c o i n c i d e s with the mean s i z e of males (22,9 mm ± 1.41 SD) but f a l l s . w e l l below the mean s i z e of females (30.1 mm ± 5.63 SD). T h e r e f o r e . s i z e s e l e c t i o n alone might account f o r the. unbalanced sex r a t i o s observed i n p o p u l a t i o n s exposed to high d e n s i t i e s of R i v u l u s . 58 Conversely, i n p o p u l a t i o n s l i a b l e t o attack, by l a r g e p r e d a t o r s , the l a r g e r r e l a t i v e s i z e of a d u l t females might a c t u a l l y be a disadvantage i f i t i s t r u e t h a t , "Predators p r e f e r to d e v o u r ' v i c t i m s of the l a r g e s t p o s s i b l e s i z e " ( I v l e v , 1961: 85). In a q u a r i a and l a r g e outdoor p o o l s , half-grown C r e n i c i c h l a and H o p l i a s e a s i l y overwhelm even the l a r g e s t female guppy. However i n nature, the s e l e c t i v e impact of l a r g e . p i s c i v o r e s on females may be.balanced by (a) the g r e a t e r conspicuousness of males based on c o l o u r (as suggested by Haskins e t a l , 1961) and (b) s e l e c t i o n of males by s m a l l p r e d a t o r s (Astyanax and very s m a l l C r e n i c i c h l a or H o p l i a s ) . A balanced sex r a t i o might t h e r e f o r e r e f l e c t e i t h e r the absence of p r e d a t i o n a l t o g e t h e r , or p r e d a t i o n stemming from a community of p r e d a t o r s with a wide spectrum of s i z e s . Where "escape by growth" i s p o s s i b l e (no l a r g e predators) the s m a l l e r sex should be l e a s t abundant. T h i s model appears to f i t the observed sex r a t i o v a r i a t i o n i n the Northern Range. (For experimental demonstration of s i z e s e l e c t i o n by p r e d a t o r s , see Chapter 5.) Behavior hypothesis .. . ' Casual o b s e r v a t i o n s of guppies i n the f i e l d or l a b o r a t o r y q u i c k l y r e v e a l t h a t the sexes.do not behave i d e n t i c a l l y . A l a r g e p r o p o r t i o n of the male !s a c t i v i t y i s spent i n c o u r t s h i p while the female shows l i t t l e s e xual 59 behavior and appears to be r a t h e r wary. I t has been p o s t u l a t e d f o r other v e r t e b r a t e s t h a t sex d i f f e r e n c e s i n behavior might c o n t r i b u t e t o . a g r e a t e r m o r t a l i t y of one sex, o f t e n . t h e male (Lack, 1954; Thompson, 1955, c i t e d i n Maher, 1970; Aim, 1959; Selander, 1965; Estes and Goddard, 1967; Holcomb and Twiest, 1970) and o c c a s i o n a l l y the female (Olson, 1965). Although d i r e c t f i e l d c o n f i r m a t i o n i s g e n e r a l l y l a c k i n g , i t i s suspected, t h a t i n some sp e c i e s males are more v u l n e r a b l e because they: (a) have l a r g e r home ranges, (b) are.more a g g r e s s i v e , (c) are more a c t i v e , (d) are l e s s " t i m i d " , (e) defend, t e r r i t o r i e s , (f) have conspicuous c o u r t s h i p d i s p l a y s . These male t r a i t s are presumed to a i d the p r e d a t o r i n e i t h e r d e t e c t i n g the prey or f a c i l i t a t i n g a c l o s e approach to w i t h i n a t t a c k i n g d i s t a n c e . These disadvantages are t o a c e r t a i n extent by-products of otherwise v i t a l male f u n c t i o n s . There remains the p o s s i b i l i t y t h a t sex d i f f e r e n c e s i n escape behavior per se might be p r e s e n t . I s h a l l c o n s i d e r some of the p o s s i b l e sex d i f f e r -ences i n behavior l a t e r i n t h i s chapter. R e l a t i v e s u r v i v a l of the sexes  i n p r e d a t i o n experiments As a f i r s t - step i n understanding how p r e d a t o r s might a l t e r the sex r a t i o , I conducted s e v e r a l p r e d a t i o n e x p e r i -ments with wild-caught and l a b o r a t o r y - r e a r e d guppies. To e l i m i n a t e the " s i z e h y p o t h e s i s " as a p o s s i b l e e x p l a n a t i o n 60 f o r any s u r v i v a l d i f f e r e n c e s , I c o n t r o l l e d - s i z e as s t r i c t l y as p o s s i b l e by matching male and female s i z e s . A. Experiments with wild-caught f i s h Experiment 4.1 P r e d a t i o n by R i v u l u s i n the l a b o r a t o r y . Methods A l a r g e sample of guppies and R i v u l u s was c o l l e c t e d from above and below the experimental s e c t i o n of P e t i t e Curucaye R., brought to the laboratory' and measured f o r s i z e . ' The f o l l o w i n g day 2 female R i v u l u s (80 and 82 mm) were s e l e c t e d and p l a c e d i n a 40 l i t e r aquarium c o n t a i n i n g n a t u r a l P e t i t e Curucaye s u b s t r a t e (deep brown d e t r i t u s and decaying c i t r u s l e a v e s ) . The tank was maintained under a n a t u r a l photoperiod w i t h . n a t u r a l s u n r i s e and sunset. Ten male and 10 female guppies ( i n i t i a l prey popu-l a t i o n , sexes same s i z e ) were then p l a c e d with the pr e d a t o r s . The experimental design c a l l e d f o r the a d d i t i o n of new f i s h (from the same c o l l e c t i o n as the i n i t i a l 20 f i s h ) to r e p l a c e those eaten. To av o i d l a r g e ' f l u c t u a t i o n s i n d e n s i t y and sex r a t i o (prey r i s k ) these a d d i t i o n s were made a t frequent i n t e r v a l s d u r i n g the day. T h e . f i s h added were matched f o r s i z e (x f o r males = ,22.6 mm, range 20-26; x f o r females = 22.9, range 21-25). R e s u l t s A t o t a l of 61 guppies were added ( i . e . eaten by the predators) over a 12-day p e r i o d (Table 10). T h i s was composed of 44 males and 17 females showing c l e a r l y t h a t 2 males were taken s i g n i f i c a n t l y more o f t e n (X ^  d f = 11.08, p < .001). TABLE 10. P r e d a t i o n by R i v u l u s on male and female guppies i n ' a 40 l i t e r aquarium. Predator and prey were wild-caught. NUMBER ADDED Day. Males Females 1 6 2 2 3 1 3 1 4 4 5 3 5 4 1 6 2 1 7 9 2 8 2 2 9 0 0 10 2 0 11 7 0 12 .3 1. T o t a l 44 17 61 Experiment 4.2' Pr e d a t i o n by Ri v u l u s i n the f i e l d . Methods T h i s experiment was conducted i n the c l e a r e d s e c t i o n of the P e t i t e Curucaye R. ( r e f e r to map and f i g u r e s i n the Appendix f o r d e t a i l s on t h i s s i t e and. the. a b s o l u t e number of pred a t o r s and prey p r e s e n t p r i o r t o c l e a r i n g ) . F o l l o w i n g the removal of a l l guppies and over 90% of the R i v u l u s , I c o l l e c t e d f i s h from above and below the 62 screened-off s e c t i o n . E i g h t R i v u l u s (x = 73.6 mm;' range 63-89) were added to the s e c t i o n t o supplement about 6 others of s i m i l a r s i z e t h a t had not been caught i n the o r i g i n a l census. The guppies used i n t h i s experiment were matched f o r s i z e (x f o r males = 21.5 mm, range 19-25; x f o r females = 21.7, range 19-25) . Then 50 of each sex were p l a c e d i n the s e c t i o n ( r e l e a s e d , a t 1600 hr i n Pool 2). Thus a " s y n t h e t i c " p o p u l a t i o n of prey with a 1:1 sex r a t i o was c r e a t e d . I monitored the sex r a t i o a t frequent i n t e r v a l s f o r 22 days. Because the n a t u r a l sex r a t i o of t h i s stream i s 0.33, I p r e d i c t e d t h a t males would disappear more q u i c k l y . Results-By Day 22 the sex r a t i o of the s e c t i o n had s t a b i l i s e d a t 6.74 ( F i g . 7). The number of f i s h ' d i s -2 appearing (8 females and 19 males, X ^ ^ = 3.70) j u s t f a i l s t o reach the .05 l e v e l of p r o b a b i l i t y . Since i t was not f e a s i b l e to run a c o n t r o l s e c t i o n without p r e d a t o r s , i t i s not c e r t a i n t h a t R i v u l u s p r e d a t i o n accounted f o r a l l the l o s s e s . However a l a r g e group of guppies c o l l e c t e d at P e t i t e Curucaye a t about'the same time as.the experimental sample and p l a c e d i n l a r g e , p r e d a t o r -f r e e outdoor pools showed no m o r t a l i t y . (Handling m o r t a l i t y caused b y capture, t r a n s p o r t , and measurement i s normally about 1%.) A l s o there was no evidence t h a t the guppies c o u l d escape from the s e c t i o n under' or through the f i n e 63a FIGURE 7. R e l a t i v e s u r v i v a l of male and female guppies i n the experimental s e c t i o n of the P e t i t e Curucaye R i v e r . tt 1 0 -7D 0 H 1 1 1 r—i 1 1 1 1 1 1 r 0 2 4 6 8 10 12 14 16 18 20 22 24 DAYS 64 screen. As I i n d i c a t e d i n Chapter 3, about 95% of the guppies remained i n Pool 2 d u r i n g the course of t h e . e x e r p i -ment. Since guppies had been found p r e v i o u s l y i n the stomachs of R i v u l u s from t h i s s e c t i o n and numerous approaches and o c c a s i o n a l a t t a c k s were observed d u r i n g the experiment, I conclude t h a t the g r e a t e r m o r t a l i t y o f males was caused by s e l e c t i v e p r e d a t i o n . T h i s f i e l d t e s t then confirms the l a b o r a t o r y r e s u l t s (Exp. 4.1). B. Experiments w i t h l a b o r a t o r y stocks In a d d i t i o n t o these 2 experiments with w i l d f i s h , I exposed l a b o r a t o r y stocks ( i . e . p r e d a t o r - n a i v e ) to C r e n i c i c h l a or R i v u l u s . The methods used i n these e x p e r i -ments s h a l l be o u t l i n e d i n c o n s i d e r a b l e d e t a i l because the same procedure was used i n experiments i n subsequent chap t e r s . I s h a l l r e f e r t o i t as the " s t a n d a r d ' s u r v i v a l t e s t " . Procedure f o r standard s u r v i v a l t e s t In a l l t e s t s of t h i s type a 200 l i t e r g l a s s and s t a i n l e s s s t e e l frame aquarium was used (dimensions:-L = 92 cm, W = 48 cm, D = 46.5-cm). Four i d e n t i c a l a q u a r i a were s e t up i n the same room so t h a t s e v e r a l t e s t s c o u l d be conducted s i m u l t a n e o u s l y . These tanks were covered on the back and s i d e s w i t h b l a c k p l a s t i c ; the f r o n t pane was covered w i t h a moveable b l a c k c u r t a i n . The depth. was kept at 30 cm to allow space beneath the cover g l a s s f o r guppies to "surface jump" when a t t a c k e d . A l a r g e flower pot-was p l a c e d i n one s i d e of the tank t o serve as a refuge f o r the p r e d a t o r ( s ) . The s u b s t r a t e c o n s i s t e d of l i g h t brown sand mixed with f i n e pebbles and some l a r g e r stones to 10 cm i n diameter. The tank was devo i d of a q u a t i c p l a n t s s i n c e no submerged v a s c u l a r p l a n t s occur i n most Northern Range streams (sedges and grasses near the shore do get covered p e r i o d i c a l l y i n the wet season). Normally 50 guppies of type "a" and 50 of type "b" were s e l e c t e d a t random from stock a q u a r i a (e.g. a = males, b = females). These were measured 24 to 48 hours b e f o r e the s t a r t of the experiment. The predator was u s u a l l y the r e s i d e n t i n the t e s t tank and the guppies were p l a c e d with i t ( a f t e r a 1 hour p e r i o d f l o a t i n g i n a screen basket),. normally i n the l a t e a f t e r n o o n . T h i s was termed. "Day 0'1. S u r v i v a l was measured simply by removing the guppies w i t h a l a r g e net and counting the sexes, s i z e s , s t o c k s , e t c . i n qu e s t i o n . They were immediately r e t u r n e d to the t e s t tank. T h i s procedure d i d not appear t o d i s t u r b e i t h e r p r e d ator or prey. When approximately 50% of the p o p u l a t i o n had been eaten, a l l remaining f i s h were measured again to t e s t f o r p o s s i b l e s i z e s e l e c t i v i t y by the predator (Chapter 5, F i g . 16). When d e n s i t i e s reached l e s s than 15, i t ' was p o s s i b l e to count f i s h without removing them. The t e s t was 66 terminated when numbers e i t h e r reached or approached zero. When the predator was C r e n i c i c h l a , o n l y one i n d i v i d u a l was used f o r 3 reasons': (a) C r e n i c i c h l a i s a very a g g r e s s i v e s p e c i e s and dominance by one f i s h c o u l d l e a d to i n j u r y or death of subdominant f i s h , (b) f i e l d observa-t i o n s r e v e a l e d t h a t a d u l t C r e n i c i c h l a are s o l i t a r y p r e d a t o r s , and (c) C r e n i c i c h l a o f even j u v e n i l e s i z e are able t o con-sume- a l a r g e number of a d u l t guppies; to d e t e c t d i f f e r e n c e s i n s u r v i v a l with an i n i t i a l d e n s i t y of 100 guppies per 135 l i t e r s of water, o v e r a l l m o r t a l i t y should not exceed a mean r a t e of about 10 per day. When the p r e d a t o r was R i v u l u s , u s u a l l y more than one i n d i v i d u a l was used so t h a t the p r e d a t i o n r a t e s would be comparable to t h a t of a s i n g l e C r e n i c i c h l a . A l s o , s o c i a l f e e d i n g appears to be p a r t of t h e . n a t u r a l b e h a v i o r a l . r e p e r t o i r e of t h i s s p e c i e s ; a t P e t i t e Curucaye s e v e r a l p r e d a t o r s were observed to pursue a single.guppy. In a l l experiments p r e d a t o r s were f e d an excess of T u b i f e x the day be f o r e the s t a r t o f the experiment; t h i s was to prevent a p r e d a t o r " o v e r k i l l " on Day 1 as w e l l as t o c o n t r o l the i n i t i a l hunger l e v e l ( m o t i v a t i o n a l s t a t e ) . During the experiment prey were fed f i n e d r i e d food.which the p r e d a t o r s d i d not eat. Four out of the 5 C r e n i c i c h l a used i n experiments a t Vancouver were wild-caught as j u v e n i l e s a n d ' r a i s e d on. a v a r i e t y of foods (see Chapter 2). R i v u l u s were a l l 67 aquarium bred. Both s p e c i e s had had c o n s i d e r a b l e p r i o r experience f e e d i n g on male and female guppies.. Because I was u n s u c c e s s f u l i n breeding C r e n i c i c h l a , I had to use the same i n d i v i d u a l s i n many of the experiments. As a p r e -c a u t i o n a g a i n s t b i a s r e s u l t i n g from i n d i v i d u a l d i f f e r e n c e s , I have r e t a i n e d t h e i r i n d i v i d u a l i d e n t i t y i n p r e s e n t i n g experimental r e s u l t s (Table 11). With R i v u l u s i n d i v i d u a l d i f f e r e n c e s are l e s s l i k e l y to b i a s r e s u l t s because i n a l l standard s u r v i v a l t e s t s , the r e s u l t s are the c o l l e c t i v e a c t i o n of 6 p r e d a t o r s . A l s o s e v e r a l "6-man teams" were . used. TABLE 11. The o r i g i n , sex, and s i z e of i n d i v i d u a l C r e n i c i c h l a used i n experiments at Vancouver. Code Home T o t a l l e n g t h (mm) Source no. Sex tank no. when caught max. reached Lower A r i p o C - l m T51 75-100 205 Lower A r i p o C-2 f T52 75-100 185 Lower A r i p o C-3 f T53 75-100 173 Lower A r i p o C-4 m T52 75-100 187 Vancouver s u p p l i e r C-5 m 15A 250 68 Experiment 4.3 P r e d a t i o n by C r e n i c i c h l a on Caparo s t o c k y Methods Using the standard s u r v i v a l test,. I exposed Caparo stock males and females to C r e n i c i c h l a C - l and C-2 (2 r e p l i -c a tes per p r e d a t o r ) . For one r e p l i c a t e w i t h each predator the s i z e of the sexes was matched as c l o s e l y as p o s s i b l e and f o r the other a random sample of a d u l t s was taken ( i . e . a sample with greater' v a r i a n c e , females g e n e r a l l y l a r g e r than males). The mean s i z e s and s i z e ranges used are g i v e n below ( r e f e r a l s o t o F i g . 8). Test Sex, Mean l e n g t h (mm) Range a m , 19.8 1 6 . 0 - 2 5 . 0 f 21.8 17.0' - 32.0 b m 20.3 1 6 . 5 - 2 4 . 0 f 20.1 17.0 - 24-.0' c m 20.2 18.0 - 23.5 f 20.1 16.0 - 24.5 d m 19.5 15.5 - 24.0 f 24.2 18.5 - 32.5 R e s u l t s I t i s c l e a r ( F i g . 8) t h a t females had l i t t l e i f any advantage over males under my experimental c o n d i t i o n s . T h i s i s i n sharp c o n t r a s t to the r e s u l t s o b t a i n e d i n Exp. 4.1 and 4.2 with wild-caught R i v u l u s and P e t i t e Curucaye guppies. Even more s u r p r i s i n g i s the d i f f e r e n c e between these r e s u l t s and those p u b l i s h e d by Haskins e t a l (1961). In t e s t s i n a q u a r i a and outdoor pools they showed a dramatic sex 69a FIGURE 8. R e l a t i v e s u r v i v a l of male and female Caparo stock guppies exposed to C r e n i c i c h l a . M A L E S © F E M A L E S • i i i i i i i i 0 2 4 6 8 10 12 PREDATOR C-1 2 4 6 8 10 12 DAYS 0 2 4 6 8 10 12 14 25 DAYS * cr 70 d i f f e r e n c e i n s u r v i v a l r a t e — i n 2 t e s t s w i t h C r e n i c i c h l a males were over 5 times more v u l n e r a b l e than females. U n f o r t u n a t e l y too few d e t a i l s were gi v e n as.to the.methods used, t h e r e f o r e I am unable t o formulate a. p l a u s i b l e reason f o r the d i f f e r e n c e i n o u r - r e s u l t s . Experiment 4.4 P r e d a t i o n by R i v u l u s on Caparo a n d ' P e t i t e Curucaye stock. Methods . The methods used were the same as i n Exp. 4.3 except t h a t body s i z e was r i g i d l y c o n t r o l l e d i n a l l . 4 r e p l i c a t e s . Two r e p l i c a t e s were performed with Caparo stocks and 2 with P e t i t e Curucaye stock. A l l 4 t e s t s were conducted w i t h the same 6 R i v u l u s (75, 75, 85, 73, 73, 85 mm) i n the same aquarium. S i z e s of prey are gi v e n below. Tes t Sex Mean l e n g t h (mm) Range a m 19.9 1 7 . 0 - 2 3 . 0 f 19.9 1 6 . 0 - 2 3 . 5 b m 19.2 1 5 . 0 - 2 2 . 5 f 19.8 1 6 . 5 - 2 3 . 0 c m 20.4 18.5 - 23.5 f 20.4 17.5 - 23.5 d m 20.1 1 7 . 5 - 2 3 . 0 f , 2 0 . 5 17.5 - 23.5 Results, Again i t i s obvious t h a t males and females were taken approximately e q u a l l y by the pre d a t o r s ( F i g . 9 ) . In 71a FIGURE 9. R e l a t i v e s u r v i v a l of male and female guppies of two stocks exposed t o R i v u l u s . NUMBER of SURVIVORS 72 one of the r e p l i c a t e s w i t h P e t i t e Curucaye stock, the' females showed b e t t e r s u r v i v a l but t h i s c o u l d have been'due to a chance d e v i a t i o n (no. eaten by Day 4 - 2 1 females, 3 4 males; 2 expected no. by n o n - s e l e c t i v e p r e d a t i o n = 2 7 . 5 , X ^ d f = 2 . 6 2 , . 2 5 > p > . 1 0 ) . D i s c u s s i o n of - s u r v i v a l ' experiments Taken tog e t h e r , the r e s u l t s of the p revious 4 e x p e r i m e n t s i l l u s t r a t e t h a t when body s i z e i s a c o n t r o l l e d v a r i a b l e , the male sex i s not always more v u l n e r a b l e to p r e d a t o r s . However'Exp. 4 * 1 and 4 . 2 show t h a t under c e r -t a i n circumstances.males are taken more o f t e n . S e v e r a l reasons c o u l d be suggested f o r . t h e s e c o n f l i c t i n g r e s u l t s : 1 . The d i f f e r e n c e c o u l d have been an a r t i f a c t caused by.a d i f f e r e n c e i n experimental methods. (a) d i f f e r e n c e i n apparatus, l i g h t regime, micro-environment, e t c . (b) d i f f e r e n c e i n measurement of r e l a t i v e v u l n e r a b i l i t y — E x p . 4 . 1 by the "replacement method" and Exp. 4 . 3 and. 4 . 4 by the "standard s u r v i v a l t e s t " . (c) d i f f e r e n c e . i n body s i z e o f . p r e y - - f i s h used i n Exp. 4 . 1 were l a r g e r . (d) d i f f e r e n c e i n e x p e r i e n c e — f i s h used i n Exp. 4 . 1 and 4 . 2 were wild-caught; those i n Exp. 4 . 3 and 4 . 4 were l a b o r a t o r y - r e a r e d . • . 2 . P o s s i b l y there i s a stock d i f f e r e n c e i n the 73 degree bf sexual dimorphism (colour or b e h a v i o r ) . R e l a t i v e to t h e i r r e s p e c t i v e females, Caparo males may be l e s s v u l n e r a b l e than P e t i t e Curucaye males.-3. Subtle s e x . d i f f e r e n c e s might be i n c o n s e q u e n t i a l to a l a r g e p i s c i v o r e such as C r e n i c i c h l a (Exp. 4.3). S e v e r a l of these p o i n t s r e q u i r e f u r t h e r e l a b o r a t i o n . L i g h t i s an important e c o l o g i c a l f a c t o r i n most predator-prey systems. My own o b s e r v a t i o n s of R i v u l u s i n nature r e v e a l e d t h a t the g r e a t e s t p r o p o r t i o n of f e e d i n g a c t i v i t y o c c u r r e d under dim or dark l i g h t c o n d i t i o n s . T h i s was a l s o observed i n Exp. 4.1: of the 61 guppies added, only 12 were added between 900 and 18 00 hr ( s u n r i s e i n . T r i n i d a d approximately 600 h r s ; sunset, 1800 h r s ) . Hyatt (unpubl.) - has a l s o observed t h a t R i v u l u s has remarkable s c o t o p i c v i s i o n . , Since i n Exp. 4.4 there.was a more sudden t r a n s i t i o n from dark to l i g h t (and v i c e - v e r s a ) than i n nature (Exp. 4.2) or Exp. 4.1, R i v u l u s may have been e x h i b i t i n g an a t y p i c a l f e e d i n g p a t t e r n . I t i s .possible t h a t i n nature the d i f f e r e n c e i n conspicuoushess between the sexes i s most accentuated under dim i l l u m i n a t i o n ; i n . t w i l i g h t the i r i d e s c e n t pigments of the male may a l r e a d y be r e f l e c t i n g l i g h t w h i le the female i s s t i l l r e l a t i v e l y i n v i s i b l e . . Under these c o n d i t i o n s a v i s u a l p r e d a t o r would probably a t t a c k and capture more males. I have no evidence t h a t the "replacement method" 74 used i n Exp. 4.1 c o u l d have b i a s e d the t e s t i n favour, of females. From s l i g h t . d i f f e r e n c e s in. the s i z e and morphology of the f i s h , I was able to r u l e out the p o s s i b i l i t y t h a t the newly-added f i s h were more v u l n e r a b l e than the o r i g i n a l r e s i d e n t s (as Beyerle and W i l l i a m s (1968) found i n a study of food s e l e c t i v i t y by p i k e ) . The problem of prey s i z e d i f f e r e n c e s . may be r e l e v a n t because i n Exp. 4.1 the mean s i z e of guppies was c l o s e r to the upper h a n d l i n g l i m i t f o r R i v u l u s . I f a sex d i f f e r e n c e i n escape behavior does e x i s t , i t i s p o s s i b l e t h a t i t . o n l y becomes r e a l l y important as the p r e d a t o r encounters g r e a t e r d i f f i c u l t y i n subduing the prey. As I suggested above, t h i s c o n d i t i o n may not b e . a p p l i c a b l e to l a r g e C r e n i c i c h l a . F i n a l l y t here i s the q u e s t i o n of p r i o r - e x p e r i e n c e . I t i s p o s s i b l e t h a t the wild-caught R i v u l u s used i n Exp. 4.1 and 4.2. had been c o n d i t i o n e d to a t t a c k male guppies. T h i s c o n d i t i o n i n g might occur i f , (a) males are e a s i e r to capture than females of a s i m i l a r s i z e , or.(b). R i v u l u s has d i f f i c u l t y d i s t i n g u i s h i n g female guppies from j u v e n i l e s of i t s own s p e c i e s (immature R i v u l u s and female guppies are roughly the same s i z e , shape, and c o l o u r , - b u t d i f f e r c o n s i d e r a b l y i n t h e i r locomotory behavior; i n e x p e r i e n c e d a q u a r i s t s o f t e n mistake one s p e c i e s f o r the o t h e r ) . A l t e r n a t i v e (b) i m p l i e s t h a t female guppies enjoy an advantage through a f o r t u i t o u s form of B a t e s i a n mimicry. C e r t a i n l y there i s no evidence of c a n n i b a l i s m i n n a t u r a l 75 p o p u l a t i o n s of R i v u l u s — I examined over 300. stomachs of t h i s s p e c i e s and found no.trace of p r e d a t i o n on c o n s p e c i f i c s . A t t a c k s on c o n s p e c i f i c s have been observed in. the l a b o r a t o r y but o f t e n t h e . f i s h were egested unharmed, suggesting preda-t i o n may be i n h i b i t e d by a t a s t e f a c t o r ( i f s p e c i e s r e c o g n i -t i o n f a i l s a t the v i s u a l l e v e l ) . I"made a p r e l i m i n a r y attempt to t e s t t h i s mimicry h y p o t h e s i s . In 2 p i l o t experiments concerned p r i m a r i l y w i t h the avoidance'.conditioning of guppies to a d u l t R i v u l u s , I purposely added 15 j u v e n i l e R i v u l u s of .16 to 37 mm ( s i z e of female guppies) to a c t as p o t e n t i a l n e gative reinforcement a g a i n s t a t t a c k s on female guppies ( i n i t i a l prey p o p u l a t i o n 50 males and 50 females). However i n both t e s t s the s u r v i v a l r a t e of males and females was v i r t u a l l y i d e n t i c a l suggesting the presence of "models" d i d not decrease the r e l a t i v e r a t e of p r e d a t i o n on females. P o s s i b l y I used too few "models" i n these experiments ( c f . Brower, 1960). Tes t s of the behavior hypothesis The r e s u l t s of the p r e v i o u s experiments have shown t h a t under c e r t a i n circumstances male guppies are more v u l n e r a b l e to R i v u l u s predation.. T h i s suggests t h a t (a) R i v u l u s hunts non-randomly or (b) R i v u l u s hunts randomly but males are l e s s able to a v o i d capture.. In the absence of d i r e c t o b s e r v a t i o n s of predatory b e h a v i o r , i t i s i m p o s s i b l e to decide which mechanism i s o p e r a t i v e . I s h a l l 76 now t e s t the assumption ( i m p l i c i t i n the study of Haskins et a l , 1961) t h a t there are no sexual d i f f e r e n c e s i n escape behavior. Experiment 4.5 Predator behavior o f R i v u l u s and escape behavior o f male and female guppies. Methods Six R i v u l u s (60 t o 70 mm) wit h p r i o r experience i n f e e d i n g on male and female guppies were p l a c e d i n a 4 0 l i t e r aquarium. A f t e r s t a r v i n g the pr e d a t o r s f o r 24 hours (approximately the time r e q u i r e d -to completely d i g e s t a meal of 3 gup p i e s ) , 10 male and 10 female guppies were added t o t h i s aquarium. They, were f i r s t p l a c e d i n a screen basket f o r 20 min and then r e l e a s e d . These guppies were a l l f i r s t and second g e n e r a t i o n f i s h of the Paramaribo stock. They had never been' exposed t o pre d a t o r s b e f o r e and w e r e ' a l l matched c l o s e l y f o r s i z e (range-—21 to 24 mm), As soon as the fish'were r e l e a s e d , the predatory behavior o f R i v u l u s was recorded on a Rustrak event r e c o r d e r f o r 60 min. A f t e r t h i s the s u r v i v o r s were removed and d i s c a r d e d . T h i s procedure was r e p l i c a t e d 10 times with the same 6 R i v u l u s . The f o l l o w i n g behavior p a t t e r n s were recorded: (a) A p p r o a c h — t h e movement of a predator towards a guppy. T h i s may vary i n speed from a slow movement to a r a p i d d a r t . No attempt was made to separate these. The p u r s u i t of a guppy by R i v u l u s can be separated i n t o phases of a c c e l e r a t i o n and d e c e l e r a t i o n — e a c h renewed a c c e l e r a t i o n r e c e i v e d a s i n g l e "approach" scor e . (b) A t t a c k — a s u c c e s s f u l approach.. The p r e d a t o r makes c o n t a c t w i t h the prey. (c) C a p t u r e — a s u c c e s s f u l a t t a c k . The prey i s normally swallowed immediately ( i . e . capture = k i l l ) . R e s u l t s The r e s u l t s ( F i g . 10) r e v e a l t h a t females and not males were a c t u a l l y approached and a t t a c k e d more o f t e n (Wilcoxon t e s t ; approach: T = 4 , n = 10, p < .02; a t t a c k : T = 4, n = 8, p < .05). The p r o b a b i l i t y l e v e l s are f o r a t w o - t a i l e d t e s t (no p r e d i c t i o n made). The. most i n t e r e s t i n g r e s u l t however, was t h a t more males than females were k i l l e d i n d i c a t i n g . i t took l e s s e f f o r t to capture males s u c c e s s f u l l y ( k i l l s f o r matched p a i r s : T = 0, n = 9, p < .01; t o t a l k i l l : X 2 1 d f = 7.51, .01 > p > .005). When the p r e d a t o r y attempts are expressed as a r a t i o per s i n g l e k i l l (predatory " e f f o r t " ) , t here i s no o v e r l a p between the sexes f o r any of the 10 r e p l i c a t e s ( F i g . 11). Because the prey were a l l of s i m i l a r ' l e n g t h and weight, i t i s c l e a r t h a t females were, more' adept at a v o i d i n g capture. Even when con t a c t e d by the p r e d a t o r they were- able to d a r t away to s a f e t y more o f t e n . Males however, were u s u a l l y captured i n 3 or l e s s a t t a c k s under' 78a FIGURE 10. Predatory behavior of R i v u l u s exposed to male and female guppies of the Paramaribo stock. KILL (no.) PER CENT ATTACK PER CENT APPROACH TOTAL KILL (no.) K > C D C D o o o o o j ^ n ^ ^ i i i i i i i BBB > m 79a FIGURE 11. The r a t i o of p r e d a t i o n attempts to s u c c e s s f u l captures f o r R i v u l u s f e e d i n g on male.and female guppies. —J 160 140 a} 120 LJJ 80 X O 60 < O 40 cr CL 20 CL < 0 MALES o FEMALES • 20 -18 -1 16 -_J 14 -12 -a> Q_ 10 -CO 8 -dm o 6 -TA 4 -»— < 2 -" l — I — I — i — i — i — I — i — i — r 1 2 3 4 5 6 7 8 9 10 i — i — i — i — i — i — i — i — i — T 1 2 3 4 5 6 7 8 9 10 TEST 80 these experimental c o n d i t i o n s . The g r e a t e r number of approaches d i r e c t e d towards females should not be taken to mean t h a t the p r e d a t o r s p r e f e r r e d females. To some extent t h i s g r e a t e r score r e s u l t s from my i n a b i l i t y to d i s t i n g u i s h between an i n i t i a l approach s t a r t i n g a bout of c h a s i n g ( i . e . a c h o i c e of prey) and approaches w i t h i n a bout. In c o n c l u s i o n t h i s experiment demonstrates t h a t there are sex d i f f e r e n c e s i n escape b e h a v i o r , a t l e a s t f o r Paramaribo stock guppies. Evidence from o t h e r experiments f o r sex  d i f f e r e n c e s i n a n t i p r e d a t o r b ehavior Although Exp. 4.5 demonstrates c l e a r l y t h a t females are more adept a t a v o i d i n g R i v u l u s p r e d a t i o n , the nature of t h i s sex d i f f e r e n c e was observed on only a q u a l i t a t i v e l e v e l . Because of the f l u r r y of a c t i v i t y d u r i n g the e x p e r i -ment, I c o n c e n t r a t e d on the behavior of the p r e d a t o r s ; i t was not f e a s i b l e t o s i m u l t a n e o u s l y measure the escape behavior of the guppies. However, i n experiments designed to measure b e h a v i o r a l d i f f e r e n c e s among stocks (Chapter 6 ) , I o f t e n used males and females and q u a n t i f i e d t h e i r b e havior s e p a r a t e l y . Without going i n t o d e t a i l here, I found no sex d i f f e r e n c e i n (a) the form and frequency of escape. motor p a t t e r n s , (b) the tendency to swim at a p a r t i c u l a r depth or remain i n a shore refuge, and (c) s c h o o l i n g 81 beh a v i o r . I d i d f i n d a d i f f e r e n c e i n r e a c t i o n d i s t a n c e to a p o t e n t i a l p r e d a t o r . The d e t a i l s o f the methods used i n measuring r e a c t i o n d i s t a n c e s are g i v e n i n Chapter 6. B r i e f l y , what I measured was the d i s t a n c e at which i n d i v i d u a l guppies showed avoidance behavior to a p o t e n t i a l p r e d a t o r , i n t h i s case a dead C r e n i c i c h l a . For a l l 5 stocks t e s t e d (Table 12) the mean r e a c t i o n d i s t a n c e of the females was g r e a t e r than the males although these d i f f e r e n c e s were very s l i g h t f o r some of the s t o c k s . Though the o v e r a l l d i f f e r e n c e between the sexes f a i l e d t o reach s t a t i s t i c a l s i g n i f i c a n c e (p > .05), a t r e n d does seem to be apparent. TABLE 12. Reaction d i s t a n c e of naive guppies to a dead C r e n i c i c h l a . Stock Mean r e a c t i o n d i s t a n c e (cm) ± standard e r r o r Males Females n Lower A r i p o 14.0 •± 0.64 15.6 + 0.98 25 per sex Guayamare 11.7 + 1.29 11.9 ± 1.11 25 Upper A r i p o 12.3 + 0.72 13.0 ± 1.12 25 P e t i t e Curucaye 6.8 ± 1.09 10.2 ± 1.09 25 P a r i a 7.8 + 0.94 10.4 ± 1.12 25 T o t a l 10.53 + 0.49 12.20+ 0.51 125 per sex 82 D i s c u s s i o n of sex d i f f e r e n c e s i n behavior I t i s noteworthy t h a t the g r e a t e s t sex d i f f e r e n c e i n r e a c t i o n d i s t a n c e i s i n P e t i t e Curucaye stock, a stock d e r i v e d from a n a t u r a l p o p u l a t i o n w i t h a v e r y unbalanced sex r a t i o i n favour of females. U n f o r t u n a t e l y I do not have comparable data on the r e a c t i o n d i s t a n c e s of Caparo guppies but there i s no reason to suspect t h a t these would be very d i f f e r e n t from Guayamare stock (Table 12), i . e . no sex d i f f e r e n c e . E a r l i e r I drew a t t e n t i o n to the p o s s i b i l i t y of stock d i f f e r e n c e s i n the degree of sexual dimorphism and h i n t e d t h a t t h i s might have p l a y e d a r o l e i n the s u r v i v a l experiments ( P e t i t e Curucaye and Caparo stocks were used). These data suggest t h a t the balanced sex r a t i o s observed i n p o p u l a t i o n s exposed to severe c h a r a c i d - c i c h l i d p r e d a t i o n may i n p a r t be a r e s u l t of the g r e a t e r s i m i l a r i t y i n the a n t i p r e d a t o r behavior of the sexes. However, i n p o p u l a t i o n s exposed to R i v u l u s p r e d a t i o n , the sexes are a p p a r e n t l y more dimorphic and consequently a g r e a t e r percentage of males are l o s t each g e n e r a t i o n . For i n d i v i d u a l males to be able to " a f f o r d " t h i s i n c r e a s e d r i s k , v i z . p r e d a t i o n , some s u r v i v a l b e n e f i t s must accrue from the r e t e n t i o n of sexual dimorphism. Strong sexual s e l e c t i o n i s u s u a l l y i n f e r r e d t o be the mechanism t h a t p l a c e s a premium on c e r t a i n male t r a i t s t h a t would otherwise be a l i a b i l i t y (Selander, 1965). I suggest escape behavior may be one "concession" made to 83 sexual s e l e c t i o n i n p o p u l a t i o n s where characid, and c i c h l i d p r e d a t i o n i s absent. More work needs to.be done to c o n f i r m t h i s . A s e r i e s of experiments s i m i l a r t o Exp. 4.5 but w i t h P a r i a and Upper A r i p o males and females might be p r o f i t a b l e . T h i s s u g g e s t i o n does not v i t i a t e the " s i z e hypo-t h e s i s " — b o t h aspects of d i f f e r e n t i a l m o r t a l i t y may be o p e r a t i v e . For example, i n the P e t i t e Curucaye R. a male guppy may become a t a r g e t of R i v u l u s because i t i s s m a l l , or i t allows the p r e d a t o r to approach too c l o s e l y . The p a t t e r n of geographic v a r i a t i o n i n the sex r a t i o i s c o n s i s t e n t w i t h both these i d e a s . Although the d i s c o v e r y of a b e h a v i o r a l d i f f e r e n c e i n the escape a b i l i t y of the sexes (Exp. 4.5) may not be a p p l i c a b l e t o a l l p o p u l a t i o n s of guppies, i t c e r t a i n l y does u n d e r l i n e the need f o r a c a u t i o u s i n t e r p r e t a t i o n of the r e s u l t s of simple s u r v i v a l experiments ( c f . Haskins 'et a l , 1961). Colour i s but one of many a t t r i b u t e s of a prey t h a t determines i t s r e l a t i v e v u l n e r a b i l i t y . C o n c l u s i o n and g e n e r a l d i s c u s s i o n of  geographic v a r i a t i o n i n the sex r a t i o In t h i s chapter I have d e s c r i b e d the p a t t e r n of geographic v a r i a t i o n i n the sex r a t i o and have used a comparative and experimental approach to d i s c o v e r why these d i f f e r e n c e s occur. The main c o n c l u s i o n t h a t can be drawn •from t h i s work i s t h a t p o p u l a t i o n d i f f e r e n c e s i n the sex r a t i o are r e l a t e d to d i f f e r e n c e s i n p r e d a t i o n . However, 84 t h i s r e l a t i o n s h i p i s not simply a f u n c t i o n o f sexual dimorphism i n c o l o u r . I have demonstrated t h a t s e v e r a l o t h e r sex d i f f e r e n c e s are r e l e v a n t to d i f f e r e n t i a l p r e d a t i o n , e s p e c i a l l y i n environments i n f e s t e d w i t h R i v u l u s . Few comparable s t u d i e s appear to have been conducted on the r e l a t i o n s h i p between the sex r a t i o and p r e d a t i o n . N e v e r t h e l e s s there has been c o n s i d e r a b l e debate about the t h e o r e t i c a l aspects of p o p u l a t i o n v a r i a t i o n i n the sex r a t i o . One view, which might be termed " a n t i - s e l e c t i o n i s t " , i s t h a t , "Changes i n the q u a n t i t y and the q u a l i t y of the food supply are the main causes of a l t e r a t i o n s i n the sex r a t i o ( N i k o l s k i i , 1969: 130)." From a review of f i s h p o p u l a t i o n s t u d i e s , N i k o l s k i i (1969) a s s e r t s t h a t females predominate when d e n s i t y i s low and food i s abundant while the o p p o s i t e c o n d i t i o n s are conducive t o an excess produc-t i o n of males. These "adaptive" a l t e r a t i o n s i n sex r a t i o are not seen as h e r e d i t a r y changes but occur v i a u n s p e c i f i e d metabolic and hormonal r o u t e s . The opposing s c h o o l adheres more c l o s e l y t o F i s h e r ' s theory ( F i s h e r , 1958: 162): "the a c t i o n o f N a t u r a l S e l e c t i o n w i l l tend t o e q u a l i z e the p a r e n t a l expenditure devoted to the p r o d u c t i o n of the two sexes . . ," Wi l l i a m s (1966: 152) concluded t h a t , ''there i s no evidence from data on sex r a t i o s to support the concept of b i o t i c a d a p t a t i o n . " In a broad review (mostly of i n s e c t s ) , Anderson (1961) found no good evidence f o r a feedback from the sex r a t i o to p o p u l a t i o n d e n s i t y . In two more r e c e n t s t u d i e s o f i n s e c t s however, extreme crowding and food shortage d i d r e s u l t i n a l t e r e d sex r a t i o s ; i n one case i t was caused by an i n c r e a s e d m o r t a l i t y of females (Dingle, 1966) and i n the other (Feinberg and Pimentel, 1966), a ge n e t i c change i n the primary sex r a t i o "evolved". In both these examples the c o m p e t i t i o n was probably f a r more severe than N i k o l s k i i (1969) had i m p l i e d was necessary f o r sex r a t i o changes i n f i s h p o p u l a t i o n s . Although I have very l i t t l e q u a n t i t a t i v e data on the food supply and r e l a t i v e d e n s i t i e s of p o p u l a t i o n s of the guppy, there appears to be no obvious c o r r e l a t i o n of these f a c t o r s w i t h the sex r a t i o . In 1969 very dense p o p u l a t i o n s o c c u r r e d a t P a r i a and P e t i t e Curucaye R i v e r s , y e t P a r i a had a balanced sex r a t i o (1.14) and P e t i t e Curucaye d i d not (0.33). By the same token very low d e n s i t i e s were observed a t Caparo, Guayamare, and Marianne R i v e r s , and very h i g h d e n s i t i e s a t Santa Cruz and Lower Ta c a r i g u a R i v e r s ; however a l l p o p u l a t i o n s showed a balanced sex r a t i o (1.00, 0.92, 0.80, and 0.91, a.91 r e s p e c t i v e l y ) . In summary, my r e s u l t s do not appear to support N i k o l s k i i ' s (1969) hypothesis t h a t p o p u l a t i o n d i f f e r e n c e s i n the sex r a t i o are governed by food supply or d e n s i t y . I concur with Haskins e t ajL (1961) t h a t an excess of females i n any g i v e n p o p u l a t i o n i s probably caused by a 86 g r e a t e r v u l n e r a b i l i t y of males to p r e d a t o r s , but I d i f f e r i n my i n t e r p r e t a t i o n as to how p r e d a t o r s e x e r t t h i s s e l e c t i v e e f f e c t . In a search f o r a l t e r n a t e mechanisms to c o l o u r I d i s c o v e r e d t h a t s i z e s e l e c t i o n was important; a l s o males were found to be l e s s adept a t e s c a p i n g R i v u l u s p r e d a t i o n , though t h i s may not be t r u e f o r a l l p o p u l a t i o n s . Whether c o u r t s h i p a c t i v i t y i n c r e a s e s the v u l n e r a b i l i t y of males i s unknown. I t seems i n t u i t i v e l y obvious t h a t when a male i s i n f u l l c o u r t s h i p d i s p l a y , he i s a t t e n d i n g to s t i m u l i from the female. Not o n l y may h i s d i s p l a y j e o p a r d i z e h i s camouflage but h i s c o n c e n t r a t i o n on the female may a l l o w a p r e d a t o r to approach more c l o s e l y . I t should be p o s s i b l e to measure the r e a c t i o n d i s t a n c e to p r e d a t o r s of d i s p l a y i n g and n o n - d i s p l a y i n g males i n nature. Although I was unable to t e s t the "conspicuousness h y p o t h e s i s " d i r e c t l y , the c i r c u m s t a n t i a l evidence suggests t h a t the c o l o u r ' o f the males may be o n l y a minor f a c t o r i n the s t r i c t sense of camouflage. There remains, however, the p o s s i b i l i t y t h a t p r e d a t o r s can become c o n d i t i o n e d to s e l e c t i v e l y a t t a c k the males because t h i s sex i s l e s s p r o f i c i e n t a t e s c a p i n g . Where l a r g e c h a r a c i d and c i c h l i d p r e d a t o r s abound, t h i s disadvantage may be counterbalanced by the g r e a t e r energy r e t u r n to be o b t a i n e d from the l a r g e r females. I t would be of i n t e r e s t t o see i f my f i n d i n g s apply-to o t h e r s p e c i e s of s e x u a l l y dimorphic f i s h . U n f o r t u n a t e l y 87 I c o u l d f i n d few s t u d i e s where both e x t e n s i v e data on sex r a t i o s and p r e d a t o r s were a v a i l a b l e . Krumholz's (1963) o b s e r v a t i o n t h a t sex r a t i o s i n the p o e c i l i i d Gambusia manni tended to be more balanced i n the absence of p r e d a t i o n support my f i n d i n g s although I observed no s e n i l e " t h i n and gaunt" or "emaciated" males and females under such c o n d i -t i o n s . Where p r e d a t i o n was thought t o be severe, Krumholz always found more females than males but i t i s not known why males had h i g h e r death r a t e s . (Sexual dimorphism i n s i z e and c o l o u r i s c o n s i d e r a b l y l e s s i n G. manni than i n the guppy.) In G. p a t r u e l i s , George (1960.) found t h a t females were more r e s p o n s i v e to p r e d a t o r models than males (c f . my Table 12). U n f o r t u n a t e l y , when he presented the Gambusia to a r e a l p r e d a t o r (Esox americanus), the sex of the s u r v i v o r s was not recorded. In the Diamond K i l l i f i s h , Hastings and Yerger (1971) noted t h a t on the average males outnumbered females about 2:1 (n = 2127). Although they d i d not s p e c u l a t e why t h i s unusual sex r a t i o o c c u r r e d , i t i s noteworthy t h a t t h i s s p e c i e s i n h a b i t s weedy areas i n very shallow water, the male i s s l i g h t l y l a r g e r than the female, and c o l o u r dimorphism i s s e a s o n a l . P o s s i b l y a s m a l l f i s h p r e d a t o r t h a t can p e n e t r a t e shallow water s e l e c t i v e l y feeds on s m a l l f i s h ( c f . R i v u l u s ) . Females would be moire abundant i n t h i s f r a c t i o n of the p o p u l a t i o n , o p p o s i t e to the s i t u a t i o n i n the guppy. 88 L i l e y (1966) found t h a t a t s e v e r a l s i t e s i n Georgetown, Guyana, the sex r a t i o of 3 s p e c i e s of p o e c i l i i d s ( i n c l u d i n g the guppy) appeared to be r e l a t e d to the c l a r i t y o f the water. C o l l e c t i o n s from green and cloudy water showed a roughly balanced r a t i o w h i l e those from c l e a r water c o n t a i n e d fewer males. L i l e y suggested t h a t i n c l e a r water the males are more v u l n e r a b l e to v i s u a l p r e d a t o r s . However i t i s i n t e r e s t i n g t o note t h a t the most extreme r a t i o was found f o r P. p i c t a , even though P. p i c t a males are l e s s c o l o u r e d than P. r e t i c u l a t a males (females of both s p e c i e s are n e a r l y i d e n t i c a l ) . Perhaps sex d i f f e r e n c e s o t h e r than c o l o u r p l a y a more important r o l e i n determining v u l n e r a b i l i t y . With r e s p e c t to water c l a r i t y and T r i n i d a d popula-t i o n s of P. r e t i c u l a t a , I have a l r e a d y p o i n t e d out t h a t where the guppy i s exposed to the major p r e d a t o r s , i t tends to have a balanced sex r a t i o i n e i t h e r t u r b i d (Caparo, Guayamare) or c l e a r (Upper T a c a r i g u a , Upper Arouca) water. The o u t s t a n d i n g e x c e p t i o n to t h i s g e n e r a l i z a t i o n i s the Oropuche R.; i t has c h a r a c i d and c i c h l i d p r e d a t o r s and i s c l e a r f o r most of the year, y e t the sex r a t i o i s 0.50 (n = 413). Although t h i s r i v e r was sampled o n l y once, I do not t h i n k the r a t i o r e s u l t e d from sampling e r r o r . Thus I am unable to e x p l a i n t h i s r a t i o from my experience w i t h other s i m i l a r environments. Perhaps t h i s i s one case where the "Haskins h y p o t h e s i s " i s v a l i d , i . e . severe p r e d a t i o n by 89 C r e n i c i c h l a , H o p l i a s , e t c . r e s u l t i n g i n a very unbalanced sex r a t i o i n favour of females. Summary of Chapter 4 1. C o l l e c t i o n s of guppies from T r i n i d a d i n 1967 and 1969 y i e l d e d a t o t a l o f 23502 f i s h ; o f these 6079 are males, 8166 females, and 9257 immature. The sex r a t i o i s 0.74 and i s s i g n i f i c a n t l y d i f f e r e n t (p < .001) from ah expected Mendelian r a t i o of 1.00. 2. There are l a r g e i n t e r p o p u l a t i o n d i f f e r e n c e s i n the sex r a t i o . The r a t i o s found i n 1967 are p o s i t i v e l y c o r r e l a t e d (p < .05) with those observed i n 1969. 3. Sex r a t i o s approaching u n i t y were found i n p r e d a t o r - f r e e l a b o r a t o r y stocks of guppies s t a r t e d from immature f i s h . T h i s i s evidence a g a i n s t the h y p o t h e s i s t h a t geographic v a r i a t i o n i n the sex r a t i o i s caused by g e n e t i c d i f f e r e n c e s i n the sex d e t e r m i n a t i o n system. 4. Sex r a t i o s are r e l a t i v e l y balanced i n popula-t i o n s exposed to e i t h e r c h a r a c i d and c i c h l i d p r e d a t o r s , or low d e n s i t i e s of R i v u l u s h a r t i i . Very unbalanced sex r a t i o s i n favour of females occur where R i v u l u s d e n s i t y i s h i g h . 5. In one stream a change i n R i v u l u s d e n s i t y between 1967 (high) and 1969 (low) was c o r r e l a t e d w i t h a s i g n i f i c a n t (p < .001) r e s t o r a t i o n of balance i n the sex r a t i o . 90 6. Sex r a t i o s do not appear to be c o r r e l a t e d w i t h geographic v a r i a t i o n i n male c o l o r a t i o n . 7. A model i n c o r p o r a t i n g the s i z e d i s t r i b u t i o n of the sexes and the s i z e - s e l e c t i v e f e e d i n g behavior of the p r e d a t o r s appears to f i t the observed sex r a t i o v a r i a t i o n i n the Northern Range streams. 8. Experiments w i t h wild-caught guppies and R i v u l u s conducted i n the f i e l d and l a b o r a t o r y demonstrated t h a t males were more v u l n e r a b l e t o p r e d a t o r s even when body s i z e was a c o n t r o l l e d v a r i a b l e . However i n experiments u s i n g l a b o r a t o r y stocks of guppies as the prey and e i t h e r a s i n g l e C r e n i c i c h l a or s i x R i v u l u s as the p r e d a t o r ( s ) , no sex d i f f e r e n c e i n s u r v i v a l was apparent. I sought an e x p l a n a t i o n f o r these c o n f l i c t i n g r e s u l t s i n terms of d i f f e r e n c e s i n experimental d e s i g n and p r i o r experience of the p r e d a t o r s . 9. In a separate experiment of R i v u l u s p r e d a t i o n on a l a b o r a t o r y stock of guppies, males were not s e l e c t i v e l y a t t a c k e d but were l e s s adept than females a t a v o i d i n g c a p t u r e . Consequently more males were k i l l e d than females (p < .005) . 10. The r e a c t i o n d i s t a n c e of female guppies to a p o t e n t i a l p r e d a t o r i s l a r g e r than t h a t of males, however there appears to be geographic v a r i a t i o n i n the degree of sexual d i s s i m i l a r i t y i n t h i s t r a i t . 11. In c o n c l u s i o n , the geographic v a r i a t i o n i n the 91 sex r a t i o of guppies i n the Northern Range r e g i o n of T r i n i d a d appears to be caused by d i f f e r e n t i a l p r e d a t i o n on males. Other b i o t i c and a b i o t i c f a c t o r s seem to be i n v o l v e d o n l y to the e x t e n t t h a t they govern the d i s t r i b u t i o n and abundance of p r e d a t o r s . The r e l a t i v e v u l n e r a b i l i t y of the male sex i s dependent upon the i n t e r a c t i o n of the s i z e s and hunting t a c t i c s of the p r e d a t o r s , and the sex d i f f e r e n c e s i n the s i z e and behavior of the prey. Although i t seems reasonable t h a t conspicuous c o l o r a t i o n i s a l i a b i l i t y , t h i s has y e t to be demonstrated u n e q u i v o c a l l y . CHAPTER 5 GEOGRAPHIC VARIATION IN BODY SIZE I n t r o d u c t i o n P o p u l a t i o n d i f f e r e n c e s i n body s i z e are a conspicuous aspect of geographic v a r i a t i o n i n T r i n i d a d guppies. In t h i s chapter I e x p l o r e some of the environmental f a c t o r s and s e l e c t i o n mechanisms t h a t might be r e s p o n s i b l e f o r the observed s i z e t r e n d s . In p a r t i c u l a r I wish to ev a l u a t e the hypo t h e s i s t h a t d i f f e r e n c e s i n body s i z e are t o some extent a d a p t a t i o n s to s i z e - s e l e c t i v e p r e d a t i o n . In s p i t e of the voluminous l i t e r a t u r e devoted to problems of f i s h growth and body s i z e (see Aim, 1959 f o r review) there i s very l i t t l e known about the g e n e t i c component of s i z e v a r i a t i o n i n n a t u r a l p o p u l a t i o n s . In g e n e r a l , f i s h e r i e s models view body s i z e i n terms of a phenotypic response to a giv e n temperature and food supply. T h i s i s reasonable because u n l i k e most ot h e r animals, f i s h g e n e r a l l y have extremely f l e x i b l e growth and m a t u r i t y r a t e s t h a t are very s e n s i t i v e to environmental c o n d i t i o n s (Weatherley, 1966). Although i t i s well-known t h a t d i f f e r e n c e s i n body s i z e are h e r i t a b l e ( C a l a p r i c e , 1969) and t h a t f i s h i n g m o r t a l i t y may be s i z e - s e l e c t i v e ( N i k o l s k i i , 1969; R i c k e r , 93 1969) , the r e s u l t a n t growth and s i z e responses to s e l e c t i v e f i s h i n g are u s u a l l y i n t e r p r e t e d as a change i n age s t r u c t u r e or c o m p e t i t i o n f o r food. M i l l e r (1957) reviewed the e a r l y l i t e r a t u r e and concluded, "There i s no c l e a r - c u t evidence of e x p l o i t a t i o n having caused any h e r i t a b l e changes i n f i s h e s (p. 803)." With t h i s p a u c i t y of data on s e l e c t i v e m o r t a l i t y i n managed f i s h p o p u l a t i o n s , i t i s not s u r p r i s i n g t h a t con-s i d e r a b l y l e s s i s known about n a t u r a l s e l e c t i v e m o r t a l i t y , i n p a r t i c u l a r the response to s i z e - s e l e c t i v e p r e d a t i o n . The b e s t examples f o r t h i s phenomenon are s t u d i e s on i n v e r t e b r a t e prey organisms (Mason, 1965; Brooks, 1968, 1971). Two s t u d i e s of s i z e - s e l e c t i v e p r e d a t i o n on f i s h by p i s c i v o r e s have suggested t h a t h e r i t a b l e changes i n growth r a t e s and body s i z e may have o c c u r r e d i n the e v o l u t i o n a r y h i s t o r y of the s p e c i e s . Jackson (1961, 1965) argued t h a t the impact of the t i g e r - f i s h , Hydrocynus v i t t a t u s on s m a l l f i s h ( l e s s than 20 cm) has s e l e c t e d f o r f i s h s p e c i e s t h a t are l a r g e as a d u l t s (but see F r y e r , 1965 f o r c r i t i q u e ) . More r e c e n t l y Parker (1971) has s p e c u l a t e d t h a t i n " t h e B e l l a Coola R., chum salmon f r y have "evolved a s t r a t e g y " to outgrow a s m a l l , s i z e - s e l e c t i v e p r e d a t o r (coho salmon p a r r ) . For e v o l u t i o n i n the o p p o s i t e d i r e c t i o n , Roberts (1972: 134) has commented t h a t minute f i s h i n the Amazon and Congo R. systems may be l e s s v u l n e r a b l e to predaceous 94 f i s h because they are, "below the s i z e t h r e s h o l d f o r While p r e d a t i o n may be an important s e l e c t i o n mechanism i n some cases, body s i z e must i n e v i t a b l y be a compromise between numerous complementary and c o n f l i c t i n g s e l e c t i v e f o r c e s . Hamilton (1961) has t a b u l a t e d 7 s e l e c -t i v e f o r c e s which might i n t e r a c t to cause i n t r a s p e c i f i c s i z e trends i n b i r d s ; most of these are p e c u l i a r to home-otherms or concern problems of f l i g h t . For numerous animals n a t u r a l s e l e c t i o n may favour an o p t i m a l body s i z e f o r : (1) f o o d - g e t t i n g (Brooks and Dodson, 1965; Es t e s and Goddard, 1967); (2) r e s i s t i n g a b i o t i c s t r e s s e s such as h i g h water v e l o c i t y (Hubbs, 1940; Hartman, 1969), or wave a c t i o n (Struhsaker, 1968; Berry and C r o t h e r s , 1968); (3) a v o i d i n g non-predatory i n t e r s p e c i f i c i n t e r a c t i o n s (Hamilton, 1961; Soule, 1966); (4) s e c u r i n g some form of mating advantage (Hanson and Smith, 1967; Hartman, 1969). With r e s p e c t to t h i s l a s t s e l e c t i v e f o r c e , i t i s important t o note t h a t Henderson (unpubl.) found t h a t aggres-s i v e male guppies were able t o " o r i e n t " ( i . e . court) more to females than d o c i l e males, y e t B a l l i n (1973) c o u l d d e t e c t no c l e a r - c u t c o r r e l a t i o n of a g g r e s s i v e behavior and body s i z e . T h i s i m p l i e s t h a t mating success may not be r e l a t e d to s i z e per se. Presumably male s i z e , c o l o u r , c o u r t s h i p i n t e n s i t y , and a g g r e s s i v e behavior a l l i n t e r a c t i n s e xual s e l e c t i o n . With t h i s b r i e f i n t r o d u c t i o n to the problem of the adaptive s i g n i f i c a n c e o f i n t r a s p e c i f i c s i z e d i f f e r e n c e s , I s h a l l now document the s i z e v a r i a t i o n o f T r i n i d a d guppies and search f o r e c o l o g i c a l c o r r e l a t e s o f t h i s v a r i a t i o n . Body s i z e v a r i a t i o n i n n a t u r a l p o p u l a t i o n s F i g u r e 12 g i v e s the mean s i z e and the v a r i a t i o n about the mean f o r the 20 p o p u l a t i o n s sampled i n 1969 (rank o r d e r ) . The s i z e s of mature males are used i n t h i s com-p a r a t i v e a n a l y s i s because once the male has a t t a i n e d f u l l c o l o r a t i o n and complete gohopodium development, s i z e i s not a f u n c t i o n o f age. Hence the sample v a r i a n c e i s s m a l l e r and the o v e r a l l p o p u l a t i o n trends are e a s i e r to d i s c e r n . N e v e r t h e l e s s , a l b e i t t h a t female body s i z e i s in d e t e r m i n a t e , there i s a s i g n i f i c a n t p o s i t i v e c o r r e l a t i o n between male and female t o t a l l e n g t h (9 p o p u l a t i o n s were compared: Spearman rank c o r r e l a t i o n , r g =•+ 0.72, t = 2.74, df = 7, p < .05) . F i g u r e 12 r e v e a l s t h a t there i s c o n s i d e r a b l e v a r i a t i o n among p o p u l a t i o n s r a n g i n g i n mean s i z e from 25.3 to 17.9 mm. T h i s may not appear to be a l a r g e d i f f e r e n c e i n a b s o l u t e terms, but f o r a s m a l l f i s h such as the guppy i t r e p r e s e n t s a t h r e e - f o l d d i f f e r e n c e i n wet body weight (150 mg versus 50 mg). By a r r a n g i n g the s i z e s i n rank o r d e r , the s i z e 96a FIGURE 12. The body s i z e of a d u l t male guppies c o l l e c t e d a t 20 s i t e s i n 1969. V e r t i c a l l i n e i n d i c a t e s the mean, h o r i z o n t a l l i n e the t o t a l range, b l a c k r e c t a n g l e i 2 SE, open r e c t a n g l e i 1 SD. The sample s i z e i s shown at the upper end of the t o t a l range. 16 GRANDE CURUCAYE UPPER ARIPO (N) UPPER CURUMPALO BLUE BASIN UPPER ARIPO (X) YARRA SANTA CRUZ PETITE CURUCAYE PARIA UPPER AROUCA MARIANNE UPPER GUANAPO LOWER TACARIGUA UPPER TACARIGUA TOMPIRE TRIBUTARY LOWER ARIPO GUAYAMARE OROPUCHE CARONI CAPARO 18 20 22 24 26 28 30 •100 -48 •100 T— i—i— i—i— i—'—i— 1 —r 22 24 26 28 30 ~ i — i — i —1 — r 16 18 20 TOTAL LENGTH (mm) 97 trends appear to f o l l o w a smooth, gra d u a l c l i n e perhaps i n d i c a t i n g a phenotypic response to an environmental g r a d i e n t r a t h e r than a g e n e t i c response to a g e o g r a p h i c a l l y v a r i a b l e s e l e c t i v e f a c t o r . But when these s i z e trends are viewed i n terms of a c t u a l geographic d i s t a n c e s , the c l i n e s are o f t e n very steep s u g g e s t i n g s e l e c t i v e f a c t o r s may be important. For example, the change i n mean body l e n g t h from Upper A r i p o (X) to Lower A r i p o , a d i s t a n c e of only 5 km, r e v e a l s a s i z e d i f f e r e n c e o f n e a r l y h a l f the maximum mean d i f f e r -ence f o r a l l the T r i n i d a d p o p u l a t i o n s . To determine i f these s i z e d i f f e r e n c e s are a s t a b l e f e a t u r e o f the p o p u l a t i o n s , I compared the 1969 measurements with those taken approximately 6 genera t i o n s b e f o r e i n 1967 (Figure 13). Though o n l y 13 p o p u l a t i o n s are comparable between y e a r s , the c o r r e l a t i o n i s h i g h l y s i g n i f i c a n t ( r g = + 0.90, t = 6.69, p < .001). T h i s c o n s i s t e n c y i s indeed remarkable, e s p e c i a l l y f o r the lowland r i v e r s and midstreams, which are uns t a b l e environments wi t h r e s p e c t to seasonal changes i n temperature, volume of flow, and perhaps food supply f o r f i s h . R e l a t i o n s h i p of body s i z e and temperature Temperature i s known to have a c o n s i d e r a b l e e f f e c t on the growth, a d u l t body s i z e , and m e r i s t i c s of f i s h p o p u l a t i o n s (reviews: Brown, 1957; Barlow, 1961; Shontz, 1962; Paloheimo and D i c k i e , 1966). The r e l a t i o n s h i p between 98a FIGURE 13. A comparison of the body s i z e of a d u l t male guppies from 13 p o p u l a t i o n s sampled.in 1967 and resampled i n 1969. For stream a b b r e v i a -t i o n s r e f e r to F i g u r e 1. e E 2 5 -^ 2 4 - ' S C.UCur'UA 2 .UTac X •» _ 2 3 - r a r 0 2 2 - • Mar Z ^ 2 1 - .TT • LA • LTac < 2 0 " »Car rs=*a90 o«« t = 6£9 P19 - «6uay df = 11 P < .001 18 H 1 1 1 1 1 1 1 1 — 18 19 20 21 22 23 24 25 26 TOTAL LENGTH 1969 (mm) 99 temperature and body s i z e i s not a simple one, e s p e c i a l l y i f food supply i s a l s o a v a r i a b l e ( B r e t t e t a l , 1969). In the m a j o r i t y of f i s h s t u d i e d , warm temperatures hasten growth and sexual m a t u r i t y ; s i n c e growth decreases or stops a t m a t u r i t y , p o p u l a t i o n s i n warm water u s u a l l y c o n s i s t of s m a l l e r i n d i v i d u a l s (Gunter, 1950). T h i s has been demon-s t r a t e d e x p e r i m e n t a l l y f o r the t r o p i c a l freshwater c y p r i n o -d o n t i d , C y n o l e b i a s a d l o f f i ( L i u and Walford, 1966): males r a i s e d a t 16 C were 19% longer and 76.3% h e a v i e r than those kept at 22 C; i n a d d i t i o n the l i f e - s p a n was doubled a t the c o o l e r temperature. In a .study of p r e - a d u l t growth of domestic guppies, Gibson and H i r s t (1955) noted t h a t f a s t e r growth and m a t u r i t y o c c u r r e d at 2 3 and 25 C than at high e r or lower temperatures. A l s o , l a r g e r females were found a t the c o o l e r temperatures (20, 23, 25 C). Even under co n s t a n t temperature (24 C) and food regime, B e r t a l a n f f y (1938) noted c o n s i d e r a b l e d i f f e r e n c e s i n the growth r a t e and asymptotic weight of 3 "breeds" of domestic guppies. He concluded these d i f f e r e n c e s were g e n e t i c . L i l e y (unpubl. data) a l s o p r o v i d e d experimental evidence t h a t g e n e t i c d i f f e r e n c e s i n body s i z e were p r e s e n t i n s e v e r a l stocks of T r i n i d a d guppies. His r e s u l t s showed t h a t temperature had a s i g n i f i c a n t e f f e c t ( f i s h a t 23 C grew t o a l a r g e r s i z e than those a t 28 C; c f . L i u and 100 Waiford, 1966) but more i m p o r t a n t l y , a t e i t h e r temperature, the f i s h descended from p o p u l a t i o n s which had t y p i c a l l y l a r g e - s i z e d i n d i v i d u a l s grew l a r g e r than those d e r i v e d from s m a l l - s i z e d w i l d founders. My own data (Figure 14) r e v e a l t h a t temperature i s a t l e a s t i n d i r e c t l y i n v o l v e d w i t h the observed s i z e trends i l l u s t r a t e d i n F i g u r e 12 ( r e f e r t o Table 1, Appendix f o r temperature v a r i a t i o n , d a i l y changes, e t c . ) . Samples of a d u l t males taken from p o p u l a t i o n s l i v i n g i n c o o l water were s i g n i f i c a n t l y l a r g e r than those taken from warmer . water ( r g = (-) 0.64, t = 3.48, p < .01), i n compliance with Bergmann's Rule (Ray, 1960). T h i s r e l a t i o n s h i p need not be a c a u s a l one because o t h e r environmental f a c t o r s (e.g. food supply, p r e d a t o r pressure) may be c o r r e l a t e d w i t h temperature and c o u l d p o s s i b l y a f f e c t body s i z e . There i s some c o n v i n c i n g f i e l d evidence however, t h a t both temperature and p r e d a t i o n are important f a c t o r s . In the Upper A r i p o R. I sampled two subpopulations of guppies and d i s c o v e r e d t h a t the males of each d i f f e r e d by 2.0 mm i n mean l e n g t h (UA(N) = 25.1 mm, n = 4 8; UA(X) = 23.1 mm, n = 85). Since these subpopulations are c l o s e to each other and the e c o l o g i c a l c o n d i t i o n s ( i n 1969) were v i r t u a l l y i d e n t i c a l (roughly equal d e n s i t i e s of guppies and R i v u l u s were found), t h i s s i z e d i f f e r e n c e seemed p u z z l i n g a t f i r s t . However I soon d i s c o v e r e d t h a t UA(X) had a much more v a r i a b l e (and g e n e r a l l y warmer) temperature 101a FIGURE 14. The r e l a t i o n s h i p of mean body s i z e of a d u l t males to stream temperature. TOTAL LENGTH (mm) co ro ro CO ro cn ro m ~o m > m o O ro ro cn ro cn ro ^ 3 ro co ro co co o -| -a P- ~ A II II n o co co p cn co ^ <IT0T 102 than UA(N). The reason f o r t h i s d i f f e r e n c e was t h a t the water of the stream below UA(N) was d i v e r t e d through a s e r i e s of d i t c h e s used to i r r i g a t e water c r e s s beds. These c r e s s beds were very exposed (no cover) hence on a sunny day the temperature of the water e n t e r i n g the c r e s s beds area was up to 4.0 C c o o l e r than the water l e a v i n g i t . T h i s warmer water subsequently flowed to the UA(X) s i t e and was probably i n s t r u m e n t a l i n producing . p h e h o t y p i c a l l y s m a l l e r f i s h ( c f . L i l e y ' s l a b o r a t o r y experiments). The important p o i n t , however, i s t h a t i n t h i s same stream, the mean l e n g t h dropped another.3.3 mm i n samples taken about 5 km downstream. Since the temperature a t t h i s lower s i t e was approximately the same as a t UA(X), i t i s u n l i k e l y t h a t the s i z e decrease c o u l d be a d i r e c t e f f e c t of temperature. The most conspicuous d i f f e r e n c e between these two s i t e s i s p r e d a t i o n . The upper l o c a t i o n i s above a w a t e r f a l l s e r v i n g as a b a r r i e r to c h a r a c i d and c i c h l i d d i s p e r s a l from below ( d e t a i l s i n Chapter 3). Because the d i s t r i b u t i o n of p r e d a t o r s i s such t h a t p o p u l a t i o n s of guppies i n c o o l water are i p s o f a c t o exposed to R i v u l u s p r e d a t i o n , i t i s not p o s s i b l e to f i n d o t h e r good " n a t u r a l experiments" where guppies have been exposed to l a r g e p r e d a t o r s but c o o l water ( i . e . < 26 C). I t i s of i n t e r e s t to note t h a t where R i v u l u s d e n s i t y i s low and the water i s c o o l ( P a r i a , Y a r r a , Marianne), the guppies are not as l a r g e as a t s i t e s having s i m i l a r temperatures 103 but good numbers of R i v u l u s (Grande Curucaye, Upper A r i p o (N)). But another confounding v a r i a b l e may be i n v o l v e d : a c c o r d i n g . t o Hynes (1970: 340) there are some data t o show t h a t t r o u t grow f a s t e r i n harder water. My data on t h i s are l i m i t e d (see Table 1, Appendix), but i n a d d i t i o n to having a lower d e n s i t y of R i v u l u s , P a r i a , Y a r r a , and Marianne R i v e r s a l s o have s o f t e r water. R e l a t i o n s h i p o f body s i z e and p r e d a t i o n I f i t i s assumed t h a t the phenotypic v a r i a t i o n i n body s i z e of f i s h (same age and sex) maintained under a constant temperature and food regime r e p r e s e n t s g e n e t i c v a r i a t i o n , then n a t u r a l s e l e c t i o n f o r body s i z e c o u l d be d i r e c t i o n a l , s t a b i l i z i n g , o r d i s r u p t i v e . In an uns t a b l e environment there c o u l d be a s h i f t from one mode of s e l e c -t i o n t o another. When these simple models of q u a n t i t a t i v e g e n e t i c s are viewed with r e g a r d t o the s e l e c t i v e impact of a number of p r e d a t o r s o f d i f f e r e n t s i z e s , the s i t u a t i o n i s immediately very complex. At any g i v e n p o i n t i n time and space, a s i n g l e prey may be exposed to p o t e n t i a l p r e d a t o r s of d i f f e r e n t (1) s p e c i e s , (2) s i z e s , (3) age c l a s s e s , and (4) m o t i v a t i o n a l s t a t e s (e.g. hunger). A l l o f these f a c t o r s are known to i n f l u e n c e the p r o b a b i l i t y t h a t a prey o f a gi v e n s i z e w i l l be a t t a c k e d and captured, not to mention the host o f s i z e - s p e c i f i c a n t i p r e d a t o r a d a p t a t i o n s the prey 104 might have (swimming speed, p r o x i m i t y to cover, v i s u a l a c u i t y , e t c . ) . For a d i v e r s e p r e d a t o r community (e.g. Lower A r i p o and Guayamare Rivers) the number of combinations of these f a c t o r s would be enormous. To study o n l y a few of these s i z e - s p e c i f i c i n t e r a c t i o n s would be a separate p r o j e c t i n i t s e l f and I d i d not attempt t h i s here. In the a n a l y s i s t h a t f o l l o w s , I wish to determine i f s i z e d i f f e r e n c e s of the magnitude observed i n n a t u r a l popu-l a t i o n s (see F i g u r e 12) are of any consequence to the d e t e c -t i o n and capture e f f i c i e n c y of a v e r a g e - s i z e d R i v u l u s (a s m a l l predator) and c h a r a c i d s and c i c h l i d s (small to very l a r g e p r e d a t o r s ) . Evidence from the f i e l d f o r s i z e s e l e c t i o n Except f o r R i v u l u s and Astyanax I have l i t t l e d i r e c t evidence t h a t p r e d a t o r s are i n f a c t s i z e - s e l e c t i v e i n t h e i r p r e d a t i o n on guppies. The r e s u l t s of the stomach analyses f o r R i v u l u s r e v e a l ' a p o s i t i v e c u r v i l i n e a r r e l a t i o n s h i p of p r e d a t o r s i z e to maximum prey s i z e (Figure 15). In my samples the c u t - o f f p o i n t beyond which even very l a r g e R i v u l u s d i d not capture guppies i s 23.0 mm. The s i g n i f i -cance of t h i s f o r sex r a t i o v a r i a t i o n was o u t l i n e d i n Chapter 4. For Astyanax the l a r g e s t of 18 guppies recovered was a 13.0 mm j u v e n i l e . I should s t r e s s t h a t t h i s i s not a good estimate of the s i z e c a p a b i l i t i e s bf the Astyanax p o p u l a t i o n 105a FIGURE 15. The r e l a t i o n s h i p of R i v u l u s body s i z e to the s i z e of guppies taken as prey. The data are taken from s e v e r a l n a t u r a l p o p u l a t i o n s . E E > C L C L 3 O *o I— o UJ —J £ o 2 3 -21 -19 -17 -15 -13 -11 -9 -7 -5 -30 • • • • • • 40 60 70 80 90 TOTAL LENGTH of RIVULUS (mm) -r~  loo 106 as a whole—my sampling was very b i a s e d towards small i n d i v i d u a l s because the methods were inadequate to capture many of the very s w i f t l a r g e r f i s h . The samples i n c l u d e d o n l y 3 i n d i v i d u a l s l a r g e r than 100 mm (fork length) though many i n d i v i d u a l s g r e a t e r than 100 mm were seen i n l a r g e , r o v i n g s c hools of approximately 20 t o 100 f i s h each. Presumably these f i s h can normally handle a d u l t guppies of a l l s i z e s but sampling by e l e c t r o f i s h i n g or p o i s o n i n g would be r e q u i r e d t o c o n f i r m t h i s . The f i e l d d ata f o r the oth e r p r e d a t o r s are even l e s s s a t i s f a c t o r y because e i t h e r no guppies were found i n the stomachs or the remains were too d i g e s t e d t o get s i z e measurements. In one C r e n i c i c h l a the remains were of f i s h probably l a r g e r than 25 mm. In a d d i t i o n , a t the Oropuche R. c o l l e c t i o n s i t e (at 1500 hr) I observed an u n s u c c e s s f u l a t t a c k on a l a r g e (> 30 mm) female guppy by a C r e n i c i c h l a of about 200 mm. On the b a s i s of s i z e alone, H o p l i a s would be expected to s e l e c t out even l a r g e r prey than C r e n i c i c h l a . I t i s p o s s i b l e t h a t a d u l t H o p l i a s prey mainly on s p e c i e s l a r g e r than the guppy. At Lower A r i p o , one j u v e n i l e C r e n i c i c h l a of 65 mm was recovered from the stomach of a 205 mm H o p l i a s s u p p o r t i n g t h i s i d e a . The c h a r a c t e r i s t i c t o o t h puncture marks of Ho p l i a s were a l s o found on a moribund Astyanax (> 90 mm f o r k length) a t the Oropuche R. and on s e v e r a l o c c a s i o n s a t C a r o n i and Guayamare R i v e r s I observed s m a l l 107 c h a r a c i d s of unknown s p e c i e s jump out of the water i n response to p u r s u i t from a very l a r g e p r e d a t o r (probably H o p l i a s ) . In summary, the f i e l d o b s e r v a t i o n s i n d i c a t e t h a t R i v u l u s and s m a l l to medium-sized Astyanax prey s e l e c t i v e l y on s m a l l guppies ( i . e . f i s h on the lower t a i l o f the s i z e d i s t r i b u t i o n ) ; scanty f i e l d d ata on a d u l t C r e n i c i c h l a and H o p l i a s suggest these p r e d a t o r s probably prey more h e a v i l y on l a r g e guppies. Experimental a n a l y s i s of s i z e s e l e c t i o n Two b a s i c q u e s t i o n s were asked i n t h i s s e c t i o n : can s i z e s e l e c t i o n be demonstrated e x p e r i m e n t a l l y , and i f so, what are the mechanisms? My p r e d i c t i o n was t h a t i n a sample of guppies d i f f e r i n g i n body s i z e , the l a r g e preda-t o r s would s e l e c t out l a r g e f i s h f i r s t w h i l e the s m a l l p r e d a t o r s would do the r e v e r s e . I t should be s t r e s s e d t h a t the prey o f f e r e d i n sub-sequent experiments d i f f e r i n s i z e but a l s o i n age; by c o i n c i d e n c e some of the f i s h w i l l be of the same age and d i f f e r i n s i z e due to h e r e d i t y but t h i s i s i m p o s s i b l e to a s s e s s . I s h a l l argue t h a t i f s i z e s e l e c t i o n can be demonstrated i n a s y n t h e t i c experimental p o p u l a t i o n , i t might a l s o have an e v o l u t i o n a r y impact on n a t u r a l popula-t i o n s , assuming body s i z e t o be a h e r i t a b l e t r a i t and t h a t " a v e r a g e - s i z e d " p r e d a t o r s e x e r t the g r e a t e s t s e l e c t i v e f o r c e . 108 To answer the f i r s t q u e s t i o n , I exposed l a r g e groups of guppies d i f f e r i n g i n body s i z e to p r e d a t o r s . As p r e d a t i o n proceeded I noted i f a s h i f t o c c u r r e d i n the mean s i z e of s u r v i v i n g prey. Because t h i s d i d not r e q u i r e c l o s e observa-t i o n of be h a v i o r , I used the l a r g e s t p o s s i b l e arena f o r these t e s t s so t h a t p r e d a t o r and prey were not unduly crowded. For the d e t a i l e d o b s e r v a t i o n s o f predatory behavior I had to s a c r i f i c e r e a l i t y to o b t a i n p r e c i s i o n i n measure-ment. Thus s m a l l e r a q u a r i a were used and fewer prey, and the p r e d a t o r s were s t a r v e d s u f f i c i e n t l y to i n s u r e t h a t they would a t t a c k d u r i n g the o b s e r v a t i o n s e s s i o n s . Experiment 5.1 P r e d a t i o n by s e v e r a l s p e c i e s on guppies o f d i f f e r e n t s i z e s . T h i s experiment i s a c t u a l l y a summary of 18 separate experiments. Some of these were designed e s p e c i a l l y f o r t e s t i n g s i z e s e l e c t i v i t y ; o t h e r s were mainly f o r t e s t s d e a l i n g with a n t i p r e d a t o r behavior (Chapter 6 ) . A l l the experiments had one common f e a t u r e : there was v a r i a b i l i t y i n the s i z e s of guppies presented t o the p r e d a t o r s . Methods Table 13 l i s t s the s t o c k s , p r e d a t o r s , e t c . used i n these t e s t s . The guppies were measured the day be f o r e the s t a r t . o f each t e s t and re-measured a t about the 50% m o r t a l i t y p o i n t . The p r e d a t o r s were not f e d any other food 109 TABLE 13. Predators and prey used i n Exp. 5.1. Du r a t i o n Mean s i z e of prey(mm) Te s t (days) Stock(s) Sex Predators before a f t e r change a 4 PCur m+f R i v u l u s 20.43 20.92 +0.49 b 3 PCur m+f R i v u l u s ^ 20.30 20.81 +0.51 c 2 Cap m+f Ri v u l u s 19,89 20,50 +0.61 d 3 Cap m+f R i v u l u s ^ 19.52 20.05 +0.53 e 4 Cap m+f • c Ri v u l u s 18.42 19.90 +1.48 f 3 SC f H o p l i a s ^ 27.8 26.4 -1.4 . g 4 LA+PCur f C - l e 21.81 22.60 + 0.79 h 2 LA+Par f C - l 21.88 21.86 -0.02 i 4 LA+UA f C - l 22.31 22.24 -0.07 j 3 LA+Guay m C - l 20.75 20.65 -0.10 k 3 LA+Par m C - l 20.70 20.62 -0.08 1 2 LA+UA m C-2 21.35 21.17 -0.18 m 3 LA+Par f C-4 21.44 20.89 -0.55 n 2 LA+Guay m C-4 18.93 18.99 +0.06 o 3 Cap m+f C - l 20.18 20.13 -0.05 P 3 Cap m+f C-2 20.18 20. 30 +0.12 q 4 Cap m+f C - l .20.81 20.68 -0.13 r 2 Cap m+f C-2 21.83 21.78 -0.05 I n i t i a l number f o r each stock or sex equal (e.g. 50 LA + 50 PCur). Three males and 3 females (75, 75, 73, 73, 85, 85 mm). c T h r e e 'males *arid?3.,-females (75, -75 , >82 ,'- 85.',. 88 , 90 mm). Two wild-caught specimens (175 and 2 05 mm). e C = C r e n i c i c h l a ( r e f e r t o Table 11). "^For sample s i z e s , r e f e r t o F i g . 16. 110 d u r i n g the t e s t s . A l l t e s t s except Exp. 5.1 (f) were con^-ducted i n 200 l i t e r tanks s e t up as d e s c r i b e d b e f o r e (standard s u r v i v a l t e s t ) . Exp. 5.1 (f) was conducted i n a 1600 l i t e r outdoor c o n c r e t e p o o l w i t h a depth o f 50 cm; the Hop l i a s and guppies used i n t h i s t e s t were wild-caught and had been i n c a p t i v i t y f o r onl y a few weeks. R e s u l t s I have presented the r e s u l t s as frequency histograms of the i n i t i a l . s i z e s and the s i z e s a t about 50% m o r t a l i t y (Figure 16). The mean "before" and " a f t e r " , s i z e s are gi v e n i n Table 13. To r u l e out s i z e changes caused by growth, o n l y the r e s u l t s of experiments completed i n l e s s than 5 days are g i v e n . The s i z e changes are analysed s t a t i s t i c a l l y as simply the d i r e c t i o n of the change i n the mean body s i z e f o l l o w i n g p r e d a t i o n (Table 14). Although the s i z e changes f o r most of the t e s t s w i t h C r e n i c i c h l a were s m a l l , 9 out of 12 were i n the p r e d i c t e d d i r e c t i o n ; o v e r a l l the r e s u l t s cannot be accounted f o r by chance. T h i s i n d i c a t e s some s e l e c t i o n must have been o c c u r r i n g s i n c e i t was known t h a t the p r e d a t o r s c o u l d handle a l l the s i z e s of prey t h a t were o f f e r e d . 111a FIGURE 16. S i z e s e l e c t i o n by p r e d a t o r s on 18 experimental p o p u l a t i o n s of guppies. Upper histogram b e f o r e s e l e c t i o n , lower histogram a f t e r . Sample s i z e s are i n d i c a t e d and the d i r e c t i o n of mean body s i z e change f o l l o w i n g p r e d a t i o n . Refer a l s o to Table 13. 111b 16 22 28 34 '5 18 21 24 15 18 21 24 15 18 21 24 16 22 28 34 16 22 28 34 TOTAL LENGTH (mm) 112 TABLE 14. D i r e c t i o n o f change of mean body s i z e i n 18 s i z e s e l e c t i o n t e s t s D i r e c t i o n of change i n mean body s i z e  Predator + - t o t a l C r e n i c i c h l a & H o p l i a s 3 10 13 Ri v u l u s 5 0 5 T o t a l 8 10 18 p = .01 ( F i s h e r Exact P r o b a b i l i t y T e s t , o n e - t a i l e d ) Mechanisms of p r e d a t o r s e l e c t i o n  f o r s i z e d i f f e r e n c e s i n prey The p r e v i o u s experiments have shown t h a t l a r g e and sm a l l p r e d a t o r s e x e r t d i r e c t i o n a l s e l e c t i o n ( i n o p p o s i t e d i r e c t i o n s ) when p r e y i n g upon l a b o r a t o r y p o p u l a t i o n s o f guppies which e x h i b i t a range of body s i z e s comparable to those of n a t u r a l p o p u l a t i o n s . T h i s s e l e c t i o n was much more ev i d e n t i n t e s t s w i t h R i v u l u s and Ho p l i a s than t e s t s w i t h C r e n i c i c h l a . S e v e r a l mechanisms might have been o p e r a t i v e i n these experiments: (a) R e l a t i v e conspicuousness of prey. V i s u a l p r e d a t o r s u s u a l l y can d e t e c t l a r g e prey a t g r e a t e r d i s t a n c e s than sm a l l prey; a t a giv e n d i s t a n c e , the former subtend a l a r g e r v i s u a l angle on the p r e d a t o r ' s r e t i n a , In the few f i s h s p e c i e s t h a t have been s t u d i e d (cod, Brawn, 1969; 113 mackerel and m u l l e t , Protasov, 1970; t r o u t , Ware, 1971), the r e a c t i o n d i s t a n c e to moving prey (or o t h e r o b j e c t s ) i n c r e a s e s as a l i n e a r or c u r v i l i n e a r f u n c t i o n of prey s i z e . (b) A c c e s s i b i l i t y of prey. Prey of d i f f e r e n t s i z e s are o f t e n found i n d i f f e r e n t m i c r o h a b i t a t s , e.g. l a r g e guppies are normally found f u r t h e r from shore and over deeper water; consequently they are more a c c e s s i b l e to l a r g e p r e d a t o r s . (c) Escape behavior of prey. Because swimming speed i s p o s i t i v e l y c o r r e l a t e d w i t h body l e n g t h (Bainbridge, 1960) , a l a r g e f i s h i s l e s s l i k e l y t o be overtaken by a p r e d a t o r than a s m a l l f i s h of the same s p e c i e s . A l s o a l a r g e f i s h may d e t e c t a p r e d a t o r a t a g r e a t e r d i s t a n c e because i t has b e t t e r v i s u a l a c u i t y . I f I apply Protasov's (1970: 81-82) data on guppies to my s t o c k s , i t i s e v i d e n t t h a t the l a r g e s t guppies used i n Exp. 5.1 had almost twice the v i s u a l a c u i t y of the s m a l l e s t (measured as the minimal angle of r e s o l u t i o n ) . (d) Handling e f f i c i e n c y of p r e d a t o r . The prey-c a p t u r i n g apparatus of most p r e d a t o r s f u n c t i o n s o p t i m a l l y over a l i m i t e d s i z e range of prey. Prey above or below t h i s range are handled ( i . e . grasped and swallowed) w i t h g r e a t e r d i f f i c u l t y and prey escape i s more probable. (e) C o n d i t i o n i n g of p r e d a t o r . Although anatomical c o n s t r a i n t s w i l l p l a c e l i m i t s on what s i z e of prey can be handled, a p r e d a t o r may a c t u a l l y s e l e c t prey over a much 114 narrower range, presumably determined by the most f a v o r a b l e schedule of r e i n f o r c e m e n t a t t a i n a b l e from a given s i z e spectrum of prey. For example, i f j u v e n i l e Aequidens (40-50 mm) are added to an aquarium c o n t a i n i n g guppies and a p r e d a t o r ( C r e n i c i c h l a or H o p l i a s ) , the p r e d a t o r q u i c k l y s h i f t s h i s a t t a c k to Aequidens. However, these l a r g e r prey can o n l y be swallowed with g r e a t d i f f i c u l t y ( i f a t a l l ) . A f t e r s e v e r a l encounters w i t h Aequidens, the predator l e a r n s to ignore them and resumes f e e d i n g on guppies. (f) Hunger l e v e l of p r e d a t o r . As the hunger l e v e l of a p r e d a t o r r i s e s , the s i z e range of a c c e p t a b l e prey i n c r e a s e s towards the l i m i t s of h a n d l i n g a b i l i t y (and o c c a s i o n a l l y beyond). Starved C r e n i c i c h l a and H o p l i a s w i l l a t t a c k newborn guppies (5 mm); s t a r v e d (and naive) R i v u l u s w i l l a t t a c k female guppies (> 30 mm) too l a r g e to swallow. Though a l l of these f a c t o r s probably i n t e r a c t i n nature to determine the r e l a t i v e v u l n e r a b i l i t y of a guppy of a c e r t a i n s i z e , mechanisms (a), ( c ) , (d), and (e) seem to be the most l i k e l y f a c t o r s t h a t a f f e c t e d s e l e c t i o n i n Exp. 5.1; (b) and (f) can be r u l e d out because no refuge f o r s m a l l f i s h was p r o v i d e d and the p r e d a t o r s c o u l d feed ad  l i b i t u m . I a l s o have some evidence t h a t s i z e - r e l a t e d d i f f e r -ences i n v i s i o n (mechanism (c) ) may have been unimportant. In Chapter 6 I measured the r e a c t i o n d i s t a n c e of 5 stocks 115 of guppies t o a dead 190 mm C r e n i c i c h l a ; i n one t e s t the "predator" was m o t i o n l e s s and i n the other i t was ''animated". Though s i g n i f i c a n t stock d i f f e r e n c e s were found ( d e t a i l s i n Chapter 6) , a non-parametric t e s t of the a s s o c i a t i o n or independence of body s i z e and r e a c t i o n d i s t a n c e (Table 15) r e v e a l e d these 2 v a r i a b l e s were independent i n a l l 10 t e s t s (5 s t o c k s x 2 t r e a t m e n t s ) . Thus a l a r g e guppy i s j u s t as l i k e l y to show avoidance at a l a r g e d i s t a n c e as a s m a l l guppy, and v i c e - v e r s a . TABLE 15. The r e l a t i o n s h i p o f body s i z e and r e a c t i o n d i s t a n c e to a p r e d a t o r . For each stock and treatment/ the sample was dichotomized a t the median body l e n g t h and r e a c t i o n d i s t a n c e ; scores f a i l i n g above (+) or below (*-) the medians were pooled i n a 2 x 2 contingency t a b l e . Independence was t e s t e d by c h i square. (n = 125 females; scores f a l l i n g on a median were omitted) Body l e n g t h Predator not moving Predator moving Reaction d i s t a n c e Reaction d i s t a n c e ~ F 1 (+1 ~ F 1 FT" (-) 28 25 30 25 (+) 29 29 25 31 X 2 = 0.089, .80 > p > .70 X 2 = 1.09, .30 > p > .20 However, I do not know i f experience w i t h , or v i s u a l cues emanating from, a l i v e C r e n i c i c h l a might a f f e c t 116 l a r g e and s m a l l guppies d i f f e r e n t l y . Perhaps the g r e a t e r v i s u a l a c u i t y of l a r g e guppies o n l y becomes s i g n i f i c a n t when the f i s h have been c o n d i t i o n e d t o a v o i d a p r e d a t o r from a g r e a t e r d i s t a n c e ( i . e . > 40 cm). With exper i e n c e , the. r e a c t i o n d i s t a n c e to l i v e R i v u l u s i n c r e a s e s almost 50%; f o r C r e n i c i c h l a t h i s i s over 100% (casual o b s e r v a t i o n s d u r i n g standard s u r v i v a l t e s t s ) . To examine mechanism ( d ) — h a n d l i n g e f f i c i e n c y — m o r e c l o s e l y , I conducted 3 s h o r t p i l o t experiments where I recorded d i r e c t l y the behavior of pr e d a t o r s exposed t o l a r g e and s m a l l guppies. Experiment 5.2 Handling e f f i c i e n c y o f C r e n i c i c h l a . Methods F i v e l a r g e (24-27 mm, x = 26.1) and 5 s m a l l (18-20.5 mm, x = 19.4) female guppies of the same stock were p l a c e d i n a 400 l i t e r aquarium wi t h C r e n i c i c h l a C-5. The p r e d a t o r had not been f e d f o r 24 hr ( i . e . i t was moderately hungry). Approach, a t t a c k , and capture scores were recorded on a Rustrak event r e c o r d e r f o r 60 min a f t e r the i n t r o d u c t i o n of the prey. Though I a l r e a d y knew t h a t t h i s p r e d a t o r c o u l d handle guppies l a r g e r and s m a l l e r than those o f f e r e d , the o b j e c t of t h i s experiment was to see i f a mean s i z e d i f f e r e n c e o f 6.7 mm ( t y p i c a l of a d u l t f i s h i n nature) might a f f e c t the h a n d l i n g e f f i c i e n c y o f a l a r g e p r e d a t o r . 117 R e s u l t s Although t h i s t e s t was not r e p l i c a t e d , i t i s c l e a r t h a t i t took many more approaches ( p u r s u i t s without prey contact) to capture a s m a l l guppy than i t d i d to capture a l a r g e one (Table 16). Even when the s m a l l f i s h were a t t a c k e d , they c o u l d sometimes escape the p r e d a t o r ' s attempted grasp. A t t a c k s on l a r g e f i s h were 100% s u c c e s s f u l under these experimental c o n d i t i o n s (no prey refuge, very c l e a r water, b r i g h t i l l u m i n a t i o n ) . t TABLE 16. P r e d a t i o n e f f i c i e n c y of C r e n i c i c h l a on l a r g e and s m a l l guppies. Frequency/hr Small guppies Large guppies Approach 28 13 A t t a c k 5 4 Capture 2 4 Approach:Capture 14 :1 3.25:1 Attack:Capture 2.5:1 1:1 Although t h i s experiment was conducted i n a l a r g e aquarium, the s i z e d i f f e r e n c e s d i d not appear t o a f f e c t the d e t e c t i o n of the prey from a d i s t a n c e (the p r e d a t o r r e a c t e d to even the s m a l l e s t guppy from the maximum a v a i l a b l e d i s t a n c e — 1 . 2 m), but i t i s p o s s i b l e t h a t the s m a l l e r 118 guppies were l e s s e a s i l y t r a c k e d d u r i n g the c r u c i a l m i l l i -seconds p r i o r t o the opening of the jaw. T h i s might have r e s u l t e d i n an e r r o r i n the d i r e c t i o n of a t t a c k . With h i g h -speed cinematography i t might be p o s s i b l e t o a s c e r t a i n the p r e c i s e mechanism t h a t g i v e s s m a l l guppies an edge over l a r g e guppies i n d e a l i n g w i t h a C r e n i c i c h l a . In largemouth bass most prey-capture f a i l u r e s occur when the mouth i s opened too soon (Nyberg, 1971). Experiment 5.3 Handling e f f i c i e n c y o f Astyanax. Methods The same i n f o r m a t i o n as i n the p r e v i o u s experiment was d e s i r e d , however a s m a l l e r p r e d a t o r and s m a l l e r aquarium were used. Two Astyanax of 65 mm (fork l e n g t h ) , one per 40 l i t e r aquarium, were p l a c e d w i t h female guppies of 2 s i z e c l a s s e s (5 per c l a s s ) d i f f e r i n g by a mean l e n g t h of 7.3 mm (x l a r g e = 24.8; x s m a l l = 17.5). Before each t e s t the pre d a t o r was s t a r v e d f o r 24 hr. The t e s t was repeated once f o r each p r e d a t o r . R e s u l t s O v e r a l l the r e s u l t s r e v e a l t h a t l a r g e guppies were approached and a t t a c k e d more o f t e n than sm a l l ones, but the t o t a l capture success was about the same (Table 17). T h i s means t h a t the p r e d a t o r s were more e f f i c i e n t i n h a n d l i n g s m a l l f i s h but spent more time p u r s u i n g the l a r g e prey. I t i s i n t e r e s t i n g t h a t there were i n d i v i d u a l d i f f e r e n c e s i n the 119 TABLE 17. P r e d a t i o n e f f i c i e n c y of Astyanax on l a r g e and s m a l l guppies. Small guppies Large Guppies Frequency/hr T e s t 1 T e s t 2 T o t a l T e s t 1 T e s t 2 T o t a l Astyanax 1 Approach 10 9 19 29 22 51 A t t a c k .3 1 4 16 10 26 Capture 1 0 1 2 2 4 Astyanax 2 Approach 13 27 40 37 63 100 A t t a c k 2 6 8 4 9 13 Capture 1 4 5 0 0 0 Pooled Approach 59 151 A t t a c k 12 39 Capture 6 4 120 a b i l i t y t o handle l a r g e guppies. Astyanax 1 was s u c c e s s f u l on 4 of 26 a t t a c k s w h i l e Astyanax 2 was u n s u c c e s s f u l on a l l 13 a t t a c k s . The reason(s) f o r t h i s d i f f e r e n c e i s unknown; the p r e d a t o r s were the same s i z e , had had an i d e n t i c a l f e e d i n g h i s t o r y i n the l a b o r a t o r y , and were probably s i b l i n g s . The r e s u l t s of t h i s experiment support the hypothesis t h a t a s m a l l p r e d a t o r s e l e c t s out s m a l l prey; h a n d l i n g e f f i c i e n c y may be one mechanism f a v o r i n g the escape of l a r g e guppies. The f i e l d evidence (stomach samples) i n d i c a t e s t h a t i n nature, the s e l e c t i o n of Astyanax i s even more b i a s e d towards s m a l l guppies. Presumably a c c e s s i b i l i t y and escape behavior (swimming speed) p l a y a more important r o l e under n a t u r a l c o n d i t i o n s . Wild Astyanax might a l s o be c o n d i t i o n e d t o a t t a c k o n l y s m a l l guppies because, as I have demonstrated, they are r e l a t i v e l y e a s i e r to handle. F i n a l l y , i t i s p o s s i b l e t h a t the Astyanax i n the experiment were h u n g r i e r than t h e i r w i l d c o u n t e r p a r t s and hence a t t a c k e d more l a r g e guppies; Astyanax caught i n nature ( i n the day-time) normally have over o n e - h a l f of t h e i r stomachs f u l l of food. Experiment 5.4 Handling e f f i c i e n c y of R i v u l u s . The f i e l d (Figure 15) and l a b o r a t o r y (Figure 16) evidence has a l r e a d y i n d i c a t e d t h a t R i v u l u s preys mostly . on s m a l l guppies; the upper l i m i t f o r the f i e l d i s about 121 23.0 mm and f o r the l a b o r a t o r y i t i s about 26.0 mm. Thus R i v u l u s c o u l d not take the m a j o r i t y of guppies i n the " l a r g e " category i n Exp. 5.2 and 5.3. However i t i s s t i l l of i n t e r e s t t o q u a n t i f y the p r e d a t o r y behavior of R i v u l u s a t t a c k i n g guppies near the upper l i m i t because t h i s i s the s i z e range of the m a j o r i t y of male guppies. I t i s important to know how much e f f e c t a few m i l l i m e t r e s l e n g t h d i f f e r e n c e i n mature males has on the capture e f f i c i e n c y . I p r e d i c t e d t h a t even s l i g h t l y l a r g e r f i s h would be more d i f f i c u l t to c a p t u r e . Methods For t h i s experiment i t was important to o b t a i n 2 s i z e c l a s s e s of males d i f f e r i n g by o n l y a few mm, each with a s m a l l s i z e v a r i a n c e , and of the same stock. By good fo r t u n e I was a b l e t o use mature males of the Guayamare stock which had been r a i s e d a t e i t h e r a low or h i g h temperature by N. R. L i l e y i n h i s experiments on the e f f e c t s of temperature on body s i z e . The f i s h from the " c o o l " treatment were a l l l a r g e (x = 22.8 mm, range 22.0-24.0) and the ones from the "warm" treatment were a l l sma l l (x = 18.1, range 16.5-19.5). My t e s t s were c a r r i e d out a t a constant i n t e r m e d i a t e temperature. Four R i v u l u s (2 of each sex) of g r e a t e r than 70 mm were used as the p r e d a t o r s . The experiment was performed i n a 40 l i t e r aquarium. For each t e s t , 2 male guppies of the 2 s i z e groups were p l a c e d i n the aquarium with the 121 a p r e d a t o r ( a f t e r the guppies had had a 15 min "calming down" p e r i o d behind an opaque p a r t i t i o n i n one corner of the t a n k ) . The same b e h a v i o r a l measures were taken as f o r the p r e v i o u s t e s t s w i t h C r e n i c i c h l a and Astyanax. Since, there were fewer prey i n t h i s experiment, a l l prey were consumed before a t e s t was terminated. A t e s t was conducted on 10 c o n s e c u t i v e days. In most t e s t s each of the p r e d a t o r s con-sumed 1 prey; thus they were a l l at comparable hunger l e v e l s a t the s t a r t of each new t e s t . R e s u l t s Small guppies were caught with g r e a t e r ease than l a r g e ones (Table 18). Both the approach and a t t a c k scores are s i g n i f i c a n t l y d i f f e r e n t between the s i z e groups (Wilcoxon Signed Ranks T e s t , approach: T = 5, n = 10, p < .01; a t t a c k : T = 4 , n = 10, p < .01; both t e s t s one-t a i l e d ) . There was no evidence t h a t the p r e d a t o r s were a t t r a c t e d t o e i t h e r s i z e c l a s s (the order of captures was random), but once an a t t a c k sequence had been i n i t i a t e d , i t took more e f f o r t (approaches and a t t a c k s ) to capture the l a r g e prey. I t would be expected t h a t w i t h i n c r e a s i n g experience the p r e d a t o r s would p r e f e r e n t i a l l y a t t a c k the s m a l l guppies but t h i s d i s c r i m i n a t i o n was not observed over the 1 0 - t r i a l p e r i o d of t h i s experiment. There was an o v e r a l l s l i g h t improvement i n the a b i l i t y to handle guppies (compare approaches and a t t a c k s , t e s t s 1-5 vs. 6-10), but TABLE 18. P r e d a t i o n e f f i c i e n c y o f Ri v u l u s on l a r g e and s m a l l guppies. T o t a l l e n g t h of prey (mm) S u r v i v a l Approach A t t a c k Capture T e s t s m a l l l a r g e time (sec) small l a r g e s m a l l l a r g e sequence 1 18.0 19.0 23.5 22.0 235 27 22 11 10 LLSS 2 19.0 17.0 23.0 22.5 155 14 32 5 17 SLSL 3 16.5 18.5 23.0 23.5 115 13 27 7 13 SLSL 4 18.0 17.0 22.0 22.0 65 16 24 4 9 SLLS 5 18.5 19.0 22.0 24.0 65 15 19 6 5 SSLL 6 18.5 18.5 22.0 24.0 55 17 16 6 7 LLSS 7 19.0 19.5 24.0 22.0 90 15 26 4 8 SSLL 8 17.0 18.5 22.5 23.0 100 16 34 4 12 LSSL 9 16.5 18.0 22.0 22.0 60 9 18 2 10 SSLL 10 17.0 19.0 23.0 23.0 35 10 14 2 4 SLSL Mean 18. 1 22. 8 97.5 15.2 23.2 5.1 9.5 123 t h i s was not size-dependent. The s u r v i v a l time (time to consume a l l 4 prey) a l s o decreased over the course of the experiment, due to (a) the i n c r e a s e d p r o f i c i e n c y i n h a n d l i n g guppies and (b) a decrease i n the l a t e n t p e r i o d f o l l o w i n g the r e l e a s e of the prey from behind the p a r t i t i o n . C o n c l u s i o n and g e n e r a l d i s c u s s i o n of  geographic v a r i a t i o n i n body s i z e " In g e n e r a l , the p a t t e r n of geographic v a r i a t i o n i n body s i z e (Figure 14) conforms to Bergmann's Rule as a p p l i e d to p o i k i l o t h e r m s by Ray (1960), v i z . f i s h taken from popu-l a t i o n s l i v i n g i n c o o l water are l a r g e r than those r e s i d i n g i n warmer water. The p h y s i o l o g i c a l mechanisms r e s p o n s i b l e f o r the apparent s i z e a d a p t a t i o n s to temperature are unknown. However, t h i s i s not the primary concern here. Superimposed upon the o v e r a l l temperature-size t r e n d i s geographic v a r i a t i o n which cannot r e a d i l y be e x p l a i n e d by g e n e t i c or phenotypic responses to temperature, i . e . the f i s h appear to be too l a r g e or s m a l l f o r a p a r t i c u l a r temperature regime (e.g. the p o p u l a t i o n s f a l l i n g near the upper and lower- extremes i n F i g u r e 12) . Though i t i s p o s s i b l e t h a t d i f f e r e n c e s i n food a v a i l a b i l i t y might r e s u l t i n r e t a r d e d or a c c e l e r a t e d growth r a t e s , thereby determining the s i z e of males a t m a t u r i t y ( c f . Svardson, 1943, c i t e d i n Aim, .1959: 97), I f e e l t h i s i s important i n o n l y 2 streams, Lower T a c a r i g u a and Santa Cruz. Both of these have been e n r i c h e d through 124 human a c t i v i t y . The r e s u l t a n t i n c r e a s e i n primary produc-t i v i t y appears to have favored the growth of guppies to a l a r g e r s i z e than i n comparable " n a t u r a l " streams. N e v e r t h e l e s s , L i l e y ' s experiments with Guayaniare and Upper A r i p o stocks ( n u t r i t i o n c o n t r o l l e d ) have p r o v i d e d evidence t h a t g e n e t i c d i f f e r e n c e s i n body s i z e are pr e s e n t . With a d d i t i o n a l experimental work, i t i s probable t h a t g e n e t i c d i f f e r e n c e s w i l l be confirmed among other p o p u l a t i o n s . The q u e s t i o n now a r i s e s as to why n a t u r a l s e l e c t i o n has favored l a r g e - b o d i e d animals i n some environments and small-bodied ones i n o t h e r s . Temperature ( v i a the Baldwin e f f e c t ? ) i s but one of a mu l t i t u d e o f s e l e c t i o n f a c t o r s t h a t might c o n c e i v a b l y a f f e c t body s i z e . I have mentioned some of the other f a c t o r s i n -the i n t r o d u c t i o n ; Ray (1960) l i s t s over 16 b i o t i c and a b i o t i c f a c t o r s t h a t might be i n v o l v e d but many of these are not a p p l i c a b l e to the guppy. For some of the more p l a u s i b l e mechanisms, -information i s e i t h e r u n a v a i l a b l e (feeding s p e c i a l i z a t i o n , i n t e r s p e c i f i c competition) or i n c o n c l u s i v e (sexual s e l e c t i o n , see B a l l i n , 1973). C l e a r l y the problem warrants an i n t e n s i v e study. In t h i s chapter I have made a p r e l i m i n a r y attempt to t e s t one of the many a l t e r n a t e e x p l a n a t i o n s f o r the s i z e t r e n d s - - t h e hypothesis t h a t v a r i a t i o n i n body s i z e may r e f l e c t an a d a p t a t i o n t o s i z e - s e l e c t i v e predation.. T h i s i d e a arose out of the o b s e r v a t i o n t h a t guppies i n the Lower A r i p o R. are much s m a l l e r than i n the Upper A r i p o R., though 125 a p a r t from p r e d a t i o n , the e c o l o g i c a l c o n d i t i o n s ( i n c l u d i n g temperature) are very s i m i l a r . For l a r g e s i z e , t o be an advantage, a f i s h should be able to "outgrow" a p r e d a t o r , and c o n v e r s e l y , f o r smal l s i z e to be adaptive i t should reduce the v u l n e r a b i l i t y of a f i s h to a predator t h a t r e q u i r e s l a r g e prey i n i t s d i e t . Cer-t a i n l y i n the A r i p o R., the r e q u i s i t e . p r e d a t o r s are present and I have assembled f i e l d and l a b o r a t o r y evidence'to show t h a t l a r g e guppies do enjoy an advantage with r e s p e c t to Ri v u l u s p r e d a t i o n , but are more, v u l n e r a b l e to C r e n i c i c h l a or H o p l i a s . P r e d a t i o n might a l s o a c t on body s i z e i n d i r e c t l y . Where the l a r g e p r e d a t o r s a r e . p r e s e n t , guppies are found i n very shallow water at the stream edge (see Chapter 6 ) . T h i s r e s t r i c t e d environment presumably exposes the f i s h popula-t i o n to new s e l e c t i o n p r e s s u r e s f a v o r i n g e f f i c i e n t f e e d i n g , r e p r o d u c t i o n , e t c . i n very c o n f i n e d surroundings. Thus an i n i t i a l b e h a v i o r a l response to escape c h a r a c i d and c i c h l i d p r e d a t i o n may have i n d i r e c t l y r e s u l t e d i n the e v o l u t i o n o f small f i s h . Parents producing l a r g e o f f s p r i n g w i l l be l e s s f i t because t h e i r progeny are (a) more a t t r a c t i v e t o l a r g e p r e d a t o r s and (b) l e s s able t o cope wi t h l i f e i n shallow water. Both disadvantages may reduce the r e p r o d u c t i v e p o t e n t i a l of the o f f s p r i n g ; hence genes f a v o r i n g l a r g e body s i z e should be g r a d u a l l y e l i m i n a t e d from the p o p u l a t i o n . 126 I f p r e d a t i o n i s p a r t l y r e s p o n s i b l e f o r the s i z e t r e n d s , there should be other examples of a predator-body, s i z e c o r r e l a t i o n . O v e r a l l , of the 10 p o p u l a t i o n s above the median mean s i z e o f males, 8 are exposed to" R i v u l u s and 2 to c h a r a c i d s and c i c h l i d s ; f o r the 10 p o p u l a t i o n s below the median, on l y 2 are exposed to' R i v u l u s and 8 to c h a r a c i d s and c i c h l i d s . .Thus there i s a good a s s o c i a t i o n (p = .05, F i s h e r E x a c t - P r o b a b i l i t y T e s t , 2 - t a i l e d ) of small body s i z e w i t h c h a r a c i d s and c i c h l i d s , and l a r g e body s i z e w i t h R i v u l u s . However, I have a l r e a d y c a u t i o n e d t h a t temperature and predator d i s t r i b u t i o n are a l s o a s s o c i a t e d (p < .01, F i s h e r Exact P r o b a b i l i t y T e s t , 2 - t a i l e d ) , so i t i s i m p o s s i b l e , except i n i s o l a t e d cases as the A r i p o R., to determine which mechanism i s o p e r a t i v e . Perhaps c a r e f u l s e a r c h i n g i n the Northern Range might uncover other " n a t u r a l experiments" where p r e d a t i o n and temperature are not confounded. Though the a s s o c i a t i o n of. l a r g e body s i z e with s i z e -s e l e c t i v e p r e d a t i o n by Ri v u l u s i s an a t t r a c t i v e h y p o t h e s i s , there are s e v e r a l anomalies i n t h e . f i e l d c o l l e c t i o n s (Figure..1-2) t h a t m i l i t a t e a g a i n s t t h i s simple" idea, as a s a t i s f a c t o r y e x p l a n a t i o n f o r the s i z e t r e n d s . If Ri v u l u s does e x e r t d i r e c t i o n a l s e l e c t i o n on body s i z e , the l a r g e s t guppies should be i n p o p u l a t i o n s exposed t o the densest p o p u l a t i o n s of R i v u l u s . T h i s c l e a r l y i s not always the case s i n c e the mean body s i z e of males a t Tompire T r i b u t a r y 127 and P e t i t e Curucaye (both w i t h dense R i v u l u s p o p u l a t i o n s ) i s w e l l below the maximum of 25.3 mm (Grande Curucaye). For Tompire T r i b u t a r y , the s m a l l . s i z e may be p a r t l y a t t r i b u t a b l e to warm temperature (26.2 C), sampling e r r o r (n = 21), and a d i f f e r e n t g e n e t i c h i s t o r y ( i t i s i s o l a t e d from the Caroni system). But t h i s does not account f o r the sma l l males a t P e t i t e Curucaye. P e t i t e Curucaye i s a t r i b u t a r y o f Grande Curucaye and i s i d e n t i c a l to i t i n temperature, pH, hardness, s u b s t r a t e , e t c . ; both f i s h p o p u l a t i o n s i n these streams are undoubtedly c l o s e l y r e l a t e d h i s t o r i c a l l y and are probably connected a t pr e s e n t by some gene flow (male c o l o u r p a t t e r n s are very s i m i l a r ) . However, there are 2 conspicuous d i f f e r e n c e s between them: P. Curucaye R. i s s m a l l e r and has a g r e a t e r abundance of R i v u l u s . Thus i t would appear t h a t the s i z e (and hence the v e l o c i t y ) o f the stream might be an important s e l e c t i v e f a c t o r . There are a t l e a s t 2 ways t h i s c o u l d operate. The f i r s t i s the mechanism I a l l u d e d to e a r l i e r f o r popula-t i o n s of guppies " f o r c e d " t o l i v e i n ,a very s m a l l e n v i r o n -m e n t — s m a l l s i z e may have d e f i n i t e advantages, p a r t l y c o u n t e r a c t i n g the g r e a t e r v u l n e r a b i l i t y to R i v u l u s . The second f a c t o r i s water v e l o c i t y . Though guppies g e n e r a l l y s e l e c t microenvironments where water v e l o c i t y i s w e l l below the stream maximum, l a r g e body s i z e (with i t s concomitant e f f e c t on swimming speed) might a l l o w a f i s h 128 to maneuver more e a s i l y up and down or across a f a s t - f l o w i n g stream. T h i s may have advantages i n i n t r a s p e c i f i c behavior and es.cape from R i v u l u s and t e r r e s t r i a l p r e d a t o r s . T h i s might e x p l a i n the l a r g e body s i z e of Grande Curucaye males. I suggest t h a t i n the P e t i t e Curucaye R. there i s l i t e r a l l y no p l a c e f o r the f i s h to go. T h i s p r e s e n t s unique s u r v i v a l problems which cannot be s o l v e d by l a r g e body s i z e and swimming speed alone. I e x p l o r e the a n t i -p r e d a t o r b e h a v i o r a l adaptations o f P e t i t e Curucaye guppies i n the' f o l l o w i n g chapter. P o s s i b l y s i z e - s e l e c t i v e p r e d a t i o n per se has no b e a r i n g whatsoever on geographic v a r i a t i o n i n body s i z e . I f t h i s i s t r u e , the d i s c o v e r y t h a t the body s i z e of the m a j o r i t y o f adult, males exposed to R i v u l u s f a l l s near or above t h i s p r e d a t o r ' s upper h a n d l i n g l i m i t must be c o i n c i d e n t a l . As with c h a r a c i d and c i c h l i d p r e d a t i o n , body s i z e must be i n t e r p r e t e d not onl y i n i t s d i r e c t e f f e c t s on the h a n d l i n g e f f i c i e n c y and p r e f e r e n c e o f ' t h e p r e d a t o r but a l s o on the oth e r s e l e c t i v e f a c t o r s t h a t p r e d a t o r avoidance might impose upon the prey. F i n a l l y , i t should be r e c a l l e d t h a t many of the p o p u l a t i o n s i n the Ca r o n i system are i s o l a t e d o n l y by d i s t a n c e . Geographic trends may simply r e f l e c t the degree to which l o c a l p o p u l a t i o n s are ab l e to preserve adaptive f e a t u r e s i n the face of gene flow from other p o p u l a t i o n s . 129 Summary o f Chapter 5 1. P o p u l a t i o n s of guppies sampled i n 1967 and 1969 show c o n s i s t e n t d i f f e r e n c e s i n a d u l t body s i z e . D i f f e r e n c e s of over 41% in- body l e n g t h and 200% i n weight (mean measurements on a d u l t males) have been found. 2. In compliance with Bergmann's Rule there i s a s i g n i f i c a n t n e g a t i v e c o r r e l a t i o n (p < .01) between stream temperature and the body s i z e of guppies. F i e l d and l a b o r a t o r y o b s e r v a t i o n s i n d i c a t e t h a t a s t r i c t l y phenotypic response to environmental temperature i s inadequate to account f o r the s i z e t r e n d s , i . e . much of the geographic v a r i a t i o n appears to have a g e n e t i c b a s i s . 3. A r e l a t i o n s h i p e x i s t s between the d i s t r i b u t i o n of f i s h p r e d a t o r s and the body s i z e of guppies. Small guppies occur where t h e r e are p r i m a r i l y l a r g e p r e d a t o r s and l a r g e guppies predominate i n streams i n f e s t e d w i t h a s m a l l p r e d a t o r . In most streams the p o t e n t i a l s e l e c t i v e impact of temperature and p r e d a t i o n i s d i f f i c u l t t o assess because these 2 environmental v a r i a b l e s are confounded. 4. L a b o r a t o r y experiments demonstrate t h a t l a r g e and s m a l l p r e d a t o r s e x e r t d i r e c t i o n a l s e l e c t i o n ( i n o p p o s i t e d i r e c t i o n s ) when p r e y i n g upon l a b o r a t o r y p o p u l a t i o n s of guppies which e x h i b i t a range of body s i z e s comparable t o those of n a t u r a l p o p u l a t i o n s . 5. One aspect of s i z e - s e l e c t i v e p r e d a t i o n , the h a n d l i n g e f f i c i e n c y of the p r e d a t o r , i s shown to be an important factor determining the relative vulnerability of guppies of different body sizes. 6 . In conclusion, size-selective predation is a plausible mechanism for the evolution of body size differences in guppies. However, i t does not adequately explain a l l the geographic variation—other selective factors must be involved, including water velocity, the size of stream, and the indirect effects of antipredator behavior. The adaptive significance of geographic variation in body size can only be understood in terms of the action and counteraction of numerous selective factors, coupled with an appreciation of the magnitude of gene flow. CHAPTER 6 GEOGRAPHIC VARIATION IN BEHAVIOR I n t r o d u c t i o n In t h i s chapter I s h a l l d e s c r i b e geographic v a r i a -t i o n i n the b ehavior of guppies and endeavor to c o r r e l a t e t h i s v a r i a t i o n w i t h f e a t u r e s of the h a b i t a t . In p a r t i c u l a r , I am i n t e r e s t e d i n a s c e r t a i n i n g the s i g n i f i c a n c e of the d i f f e r e n c e s . Are these d i f f e r e n c e s h e r i t a b l e ? Do they make f u n c t i o n a l sense i n terms of the major c h a l l e n g e s . t o s u r v i v a l ? The emphasis i s on p o p u l a t i o n d i f f e r e n c e s i n a n t i -p r e d a t o r b e h a v i o r — t h e b e h a v i o r a l components e n a b l i n g an animal to share an environment with i t s p r e d a t o r s . I have a l r e a d y attempted to j u s t i f y t h i s r e s e a r c h on s e v e r a l grounds (see Chapter 1); most important i s the p o t e n t i a l i n s i g h t to be gained i n t o the mechanisms a f f e c t i n g the e v o l u t i o n of b e h a v i o r . The working h y p o t h e s i s was e r e c t e d f o l l o w i n g the d i s c o v e r y of b e h a v i o r a l d i f f e r e n c e s i n s e v e r a l s i t u a t i o n s i n n a t u r e . To examine these d i f f e r e n c e s more c l o s e l y and to c o n t r o l f o r the e f f e c t s of environmental d i f f e r e n c e s , I s t u d i e d b ehavior i n the l a b o r a t o r y , u s i n g mostly the 131 132 o f f s p r i n g of samples taken from 5 r e p r e s e n t a t i v e p o p u l a t i o n s (Lower A r i p o , Guayamare, Upper A r i p o , P e t i t e Curucaye, and P a r i a ) . These r i v e r s i t e s were chosen because they r e p r e -sent the range of e c o l o g i c a l c o n d i t i o n s where guppies are commonly found (at l e a s t f o r Northern T r i n i d a d , see Chapter 3). They a l s o encompass the range of p r e d a t i o n p r e s s u r e o c c u r r i n g i n t h i s r e g i o n of T r i n i d a d (Chapter 3, Table 3). T h e r e f o r e , i f b e h a v i o r a l v a r i a t i o n does r e p r e s e n t (to some degree) a m i c r o e v o l u t i o n a r y phenomenon, i t should become apparent i n a comparative study of these 5 p o p u l a t i o n s , e s p e c i a l l y the p o p u l a t i o n s exposed to e i t h e r extreme of p r e d a t i o n p r e s s u r e . F i e l d o b s e r v a t i o n s of behavior My o b s e r v a t i o n s are based on the study of guppies i n most'of the streams sampled f o r body s i z e and sex r a t i o v a r i a t i o n ( f o r methods, r e f e r t o Chapter 2). In a d d i t i o n , c a s u a l o b s e r v a t i o n s were made on p o p u l a t i o n s i n a stream at Paramaribo, Surinam, and i n pools and d i t c h e s i n George-town, Guyana (both are mainland'South American p o p u l a t i o n s ) . I d i d not make a d e t a i l e d q u a n t i t a t i v e comparison of the b e h a v i o r a l d i f f e r e n c e s i n nature. T h i s s e c t i o n i s then mostly d e s c r i p t i v e . D i s t r i b u t i o n of guppies i n the stream environment The d i s t r i b u t i o n of f i s h a cross and along a stream i s seldom random. In the streams I examined, d i f f e r e n c e s i n 133 depth occurrence and p r o x i m i t y to the s h o r e l i n e were n o t i c e -a b l e . In some p o p u l a t i o n s f i s h were t i g h t l y bunched near each s i d e o f the stream while i n others they were spread q u i t e u n i f o r m l y a c r o s s the stream. S e v e r a l reasons c o u l d be suggested f o r these d i f f e r e n c e s . O bviously a b i o t i c f a c t o r s . c a n i n f l u e n c e d i s t r i b u t i o n . Since guppies are s m a l l f i s h and not p a r t i c u -l a r l y s t r o n g swimmers, they do not occupy water f l o w i n g f a s t e r than 0.3 m/sec. In s e c t i o n s of streams with a hig h water v e l o c i t y , guppies are found i n back-eddys or at the extreme edge of the stream " c l i n g i n g " t o the s h o r e l i n e ; the water v e l o c i t y u s u a l l y approaches zero m/sec i n these r e g i o n s . Even along f a s t - f l o w i n g streams however, th e r e are reg i o n s w i t h pools and wider, smooth-flowing s e c t i o n s of lower v e l o c i t y . I f water v e l o c i t y i s the only f a c t o r r e s t r i c t i n g the movement of guppies, the f i s h should spread out i n these s e c t i o n s . T h i s i s not observed i n a l l popula-t i o n s , s u g g e s t i n g t h a t i n some cases other f a c t o r s are p l a y i n g a r o l e , p o s s i b l y the d i s t r i b u t i o n of food or the t h r e a t from l a r g e p r e d a t o r s . (a) The e f f e c t of pr e d a t o r s There i s good f i e l d evidence tha,t p r e d a t o r s may have an e f f e c t on the d i s t r i b u t i o n of guppies. As mentioned b e f o r e , the A r i p o R. i s d i v i d e d i n t o an "upper" 134 and "lower" r e g i o n by a s e r i e s of f a l l s (limestone l e d g e s ) . One of these w a t e r f a l l s i s 5 m h i g h , completely b l o c k i n g the upward passage of c h a r a c i d s and c i c h l i d s . The v e l o c i t y and depth of water above and below the f a l l s are comparable but the d i s t r i b u t i o n of guppies d i f f e r s markedly. To c o l l e c t samples of f i s h from below the f a l l s , i t was necessary to sample the very edge of the stream and the s i d e - p o o l s ; o n l y a s m a l l number of f i s h were seen near the c e n t r e of the stream (mostly l a r g e females). Above the f a l l s , the s i t u a t i o n was s t r i k i n g l y d i f f e r e n t — b o t h males and females were d i s t r i b u t e d a cross the stream. T h i s i s i n d i r e c t evidence t h a t the s h o r e l i n e o f f e r s some p r o t e c t i o n i n a v o i d i n g c h a r a c i d and c i c h l i d p r e d a t o r s . To t e s t t h i s i d e a , I took some guppies from a shore refuge a t Lower A r i p o and p l a c e d them 1-2 m from shore over deeper water. Almost immediately I observed s e v e r a l Astyanax p u r s u i n g the guppies as they d a r t e d back to shore. The Astyanax d i s c o n t i n u e d t h e i r p u r s u i t once the guppies reached the shallower water. The depth a t which guppies o c c u r r e d was a l s o v a r i a b l e . Where c h a r a c i d and c i c h l i d p r e d a t o r s were p r e s e n t , the few f i s h t h a t d i d venture from shore swam very c l o s e to the s u r f a c e of the water. Where these p r e d a t o r s were absent, the guppies were d i s t r i b u t e d throughout the water column. In a few streams ( P a r i a , Y a r r a , Upper Aripo) I had the impression t h a t most of the f i s h were i n the bottom 135 o n e - t h i r d of the water column. (b) The e f f e c t of water v e l o c i t y To determine i f water v e l o c i t y has an e f f e c t on the d i s t r i b u t i o n of guppies i n a stream v i r t u a l l y devoid of a q u a t i c p r e d a t o r s , I made a census of f i s h along a 4-meter t r a n s e c t a c r o s s the P a r i a R. (Figure 17). T h i s t r a n s e c t was 2 d i v i d e d i n t o f o u r 1.0 m s e c t i o n s w i t h s e c t i o n s 1 and 4 r e p r e s e n t i n g the r e g i o n s on e i t h e r s i d e of the- stream and 2 and 3 as the two i n n e r s e c t i o n s . The important d i f f e r e n c e among these s e c t i o n s was t h a t 2 of them were p r o t e c t e d from the f u l l stream v e l o c i t y by l a r g e boulders immediately upstream. T h i s meant t h a t most of the water passed through the o t h e r 2 s e c t i o n s w i t h an i n c r e a s e d v e l o c i t y . One of the p r o t e c t e d s e c t i o n s was a t the shore, the other near the c e n t e r of the stream. Thus i t was p o s s i b l e to separate the e f f e c t of v e l o c i t y on the d i s t r i b u t i o n of guppies from the a f f i n i t y f o r the s h o r e l i n e . To estimate the number of f i s h i n each s e c t i o n , I proceeded from s e c t i o n 1 to 4 i n sequence and counted very r a p i d l y a l l the f i s h i n each s e c t i o n . T h i s was r e p l i c a t e d 16 times w i t h a 4-min i n t e r v a l between census p e r i o d s . The f i s h moved r a p i d l y i n and out of these s e c t i o n s so there was c o n s i d e r a b l e v a r i a b i l i t y i n the numbers seen at any one time. However the data i n F i g u r e 17 demonstrate q u i t e c l e a r l y t h a t fewer f i s h o c c u r r e d i n the s e c t i o n s w i t h a h i g h 136a FIGURE 17. The d i s t r i b u t i o n of a d u l t guppies a c r o s s a s e c t i o n of the P a r i a R i v e r . Arrows i n d i c a t e the d i r e c t i o n of water flow. Refer to t e x t f o r census methods. CrtOSS SECTION SECTION No. MAX. WATER DEPTH = 30 cm PARIA RIVER (JUNE 29.1969) r- 1 t r 137 water v e l o c i t y , r e g a r d l e s s o f the p r o x i m i t y to the shore. T h i s suggests t h a t i n the absence of p r e d a t i o n by l a r g e f i s h , guppies d i s t r i b u t e themselves throughout the a v a i l a b l e environment; however, water v e l o c i t y can l i m i t the time f i s h spend i n any p a r t i c u l a r a r ea. There i s some a d d i t i o n a l evidence t h a t v e l o c i t y i s not the o n l y f a c t o r l i m i t i n g the d i s t r i b u t i o n where p r e d a t i o n i s severe. At c e r t a i n , times of the year a s m a l l " l a k e " i s formed behind the Guayamare i r r i g a t i o n dam ( l o c a t e d 0.5 km below the c o l l e c t i o n s i t e on t h i s stream). Although the water v e l o c i t y i n t h i s l a k e was e s s e n t i a l l y zero, the guppies were n e v e r t h e l e s s d i s t r i b u t e d around the p e r i p h e r y . The m a j o r i t y were w i t h i n 1 m of the shore. Since l a r g e H o p l i a s were caught i n t h i s l a k e , the s u r v i v a l value of t h i s behavior i s probably l i n k e d to p r e d a t o r avoidance. Table 19 summarizes the o b s e r v a t i o n s on d i s t r i b u t i o n f o r the 5 p r i n c i p a l p o p u l a t i o n s under study. I t should be s t r e s s e d t h a t t h i s i n f o r m a t i o n i s only f o r a t y p i c a l s e c t i o n of stream having both a depth and v e l o c i t y g r a d i e n t . Obviously t h e r e w i l l be s e c t i o n s o f even the P a r i a R. t h a t are too shallow f o r guppies to be f a r from the s u r f a c e , or too f a s t t o al l o w the f i s h t o be spread a c r o s s the stream. S c h o o l i n g behavior Besides d i f f e r e n c e s i n the gross d i s t r i b u t i o n of f i s h a cross a stream, I found d i f f e r e n c e s i n s c h o o l i n g TABLE 19. The d i s t r i b u t i o n of 5 p o p u l a t i o n s of guppies i n r e l a t i o n to water v e l o c i t y , depth, and d i s t a n c e from the shore. Water v e l o c i t y (m/sec) D i s t r i b u t i o n of guppies Stream Mainstream M i c r o h a b i t a t of guppies P o s i t i o n i n water column P o s i t i o n a c r o s s stream L. A r i p o 0.61 0 - 0.20 near s u r f a c e near shore Guayamare 0.40 approx. 0 near s u r f a c e . near shore P. Curucaye 0.29 0 - 0.20 u b i q u i t o u s no p r e f e r e n c e Upper A r i p o 0.55 0 - 0.20 near bottom no p r e f e r e n c e P a r i a 0.16 0 - 0.16 near bottom no p r e f e r e n c e to CO 139 behavior. I use Keenleyside's (1955) d e f i n i t i o n : "any f i s h a ggregation can be c o n s i d e r e d a s c h o o l , p r o v i d e d the f i s h are together because they are r e a c t i n g to each o t h e r , and not because of s i m i l a r r e a c t i o n s by each i n d i v i d u a l to a common e x t e r n a l stimulus (p. 183)." Schools of guppies may have s e v e r a l c o n f i g u r a t i o n s . In fast-moving water the f i s h are a l r e a d y c o n f i n e d to the stream edge so i t i s d i f f i c u l t to separate the aggregating e f f e c t of the e x t e r n a l environment.from the s o c i a l f o r c e of s c h o o l i n g . In such r e g i o n s , however, i t is. common to see groups of 5 to 50 f i s h , i n c l u d i n g both males and females, moving together up or down the.stream, p a r a l l e l and c l o s e to the shore. These are " l i n e a r " s c h ools because the.depth and width of the s c h o o l i s very s m a l l — t h e f i s h appear to f o l l o w each other i n " s i n g l e . v f i l e " f a s h i o n . T h i s type of s c h o o l was commonly seen i n some p o r t i o n s of the Lower A r i p o R. I f water v e l o c i t y i s l e s s r e s t r i c t i v e to movement, l a r g e r s c h o o l s may be observed. S e v e r a l l a r g e e l l i p t i c a l or n e a r l y s p h e r i c a l s c hools were observed i n smooth-flowing s e c t i o n s of the Lower T a c a r i g u a R. Counts f o r these l a r g e schools ranged from 75 to over 200 a d u l t f i s h . E l l i p t i c a l s c h o o ls are o r i e n t e d w i t h the l a r g e s t diameter p a r a l l e l to the d i r e c t i o n of flow. When l a r g e schools are d i s t u r b e d by a p r e d a t o r ( f i s h or human), they may break up i n t o sub-schools and reform 140 i n t o a s i n g l e u n i t a f t e r a few seconds. T h i s s p l i t t i n g v a r i e s w i t h the i n t e n s i t y o f the d i s t u r b a n c e : a slow, gra d u a l approach w i l l e l i c i t o n l y a c l e a v i n g of the sch o o l around the t h r e a t e n i n g o b j e c t (e.g. l a r g e Aequidens) f o l l o w e d by immediate r e f o r m a t i o n . Sudden approaches ( e s p e c i a l l y by a p o t e n t i a l preda-t o r h i d i n g under rocks or i n the shore ve g e t a t i o n ) w i l l e l i c i t a r a p i d f a n - l i k e s c a t t e r o f the f i s h , each i n d i v i d u a l moving i n h i s own p a r t i c u l a r d i r e c t i o n away from the stimulus source. A t Lower T a c a r i g u a R. I was able to f o l l o w the m a j o r i t y o f such d i s p e r s e d i n d i v i d u a l s and they a l l appeared to r e - u n i t e . S c h o o l i n g i s n o t i c e a b l y p o o r l y developed i n some streams. F i s h i n the P a r i a R. behave very much as i n d i v i d u a l s ; the most common s o c i a l groups i n v o l v e f i s h engaged i n c o u r t s h i p a c t i v i t y (e.g. 3 males f o l l o w i n g a female). These groups do not p e r s i s t f o r more than a few seconds as the males move o f f to c o u r t other females. A number of streams have f i s h p o p u l a t i o n s e x h i b i t i n g an i n t e r m e d i a t e development of the s c h o o l i n g response. The cohesion may be q u i t e loose and d i s t a n c e s between f i s h l a r g e , i . e . the f i s h do move together but i n a l e s s o r g a n i z e d manner. I f these f i s h are threatened, they o f t e n escape i n groups but q u i c k l y d i s s o c i a t e once the emergency has passed. T h i s i s f o l l o w e d by a r a p i d resumption of f e e d i n g and c o u r t s h i p b e h a v i o r . 141 When the tendency t o s c h o o l i s r e l a t e d to the preda-t o r s i n the surrounding environment, the t r e n d i s t h a t where c h a r a c i d and c i c h l i d p r e d a t o r s occur, s c h o o l i n g behavior i s more p r e v a l e n t (Table 20). A l s o , i n streams with R i v u l u s , s c h o o l i n g i s b e t t e r developed where t h i s p r e d a t o r i s very abundant (e.g. P. Curucaye). TABLE 20. The development of s c h o o l i n g behavior i n 5 p o p u l a t i o n s o f guppies Stream Pr e d a t o r ( s ) S c h o o l i n g behavior Guayamare c h a r a c i d s and c i c h l i d s w e l l -developed L. A r i p o c h a r a c i d s and c i c h l i d s w e l l - d e v e l o p e d P. Curucaye R i v u l u s i n t e r m e d i a t e D. A r i p o R i v u l u s p o o r l y - d e v e l o p e d P a r i a R i v u l u s absent Reaction d i s t a n c e to p o t e n t i a l p r e d a t o r s R e a c t i o n (= r e a c t i v e ) d i s t a n c e i s d e f i n e d here as the d i s t a n c e from an e x t e r n a l stimulus a t which a f i s h responds wi t h an o v e r t change i n beh a v i o r . In the context of a n t i p r e d a t o r b e h a v i o r , t h i s u s u a l l y e n t a i l s a change i n the d i r e c t i o n , v e l o c i t y , or form of swimming behavior. The term i s thus roughly e q u i v a l e n t t o " f l i g h t d i s t a n c e " (Walther, 1969, f o r Thomson's g a z e l l e ) , "approach d i s t a n c e " 142 (Heatwole, 1968, and Johnson, 1970, f o r l i z a r d s ) , and "Feindabstand" (Curio, 1969, f o r Darwin's f i n c h e s ) . Some-times a d i s t i n c t i o n i s made between the d i s t a n c e a t which a prey w i l l approach a p r e d a t o r ( a v o i d i n g d i s t a n c e ) and the d i s t a n c e a t which a prey f l e e s from the approach of a preda-t o r ( f l i g h t d i s t a n c e ) . I t i s r e g r e t a b l e t h a t I d i d not have the f o r e s i g h t to a c c u r a t e l y measure t h i s important b e h a v i o r a l parameter i n s i t u . However, I d i d note d i f f e r e n c e s i n r e a c t i o n d i s t a n c e on a more gross q u a l i t a t i v e s c a l e . When I approached streams c o n t a i n i n g the l a r g e r p r e d a t o r s , the guppies were o b v i o u s l y very r e a c t i v e . They avoided the net at a c o n s i d e r a b l e d i s t a n c e ; although I c o u l d see many f i s h a t any given time, these were very d i f f i c u l t t o capt u r e . They u s u a l l y spread out a t r a p i d speed, moved to deeper water, or f u r t h e r from shore. T h i s was e s p e c i a l l y t r u e a t C a r o n i , Guayamare, Lower A r i p o , Caparo, and Oropuche R i v e r s . In some streams i t was much e a s i e r t o approach the f i s h w i t h a d i p n e t . The extreme s i t u a t i o n was a t P a r i a where the. f i s h c o u l d be caught by hand. These f i s h would a l s o p i c k a t my l e g s even when these l a r g e " o b j e c t s " were moved. T h i s s h o r t ' r e a c t i o n d i s t a n c e to a p o t e n t i a l p r e d a t o r i s probably r e l a t e d to the v i r t u a l absence of a l l a q u a t i c p r e d a t o r s from the P a r i a R. 14 3 Motor p a t t e r n s used i n a n t i p r e d a t o r behavior A number o f d i s t i n c t motor p a t t e r n s (= f i x e d a c t i o n p a t t e r n s ) were observed d u r i n g encounters between guppies and t h e i r p r e d a t o r s i n nature. These w i l l be i n t r o d u c e d here but s t u d i e d i n g r e a t e r d e t a i l i n a l a t e r s e c t i o n where they have been q u a n t i f i e d f o r the l a b o r a t o r y s t o c k s . I have been ab l e to d i s t i n g u i s h 6 main escape motor p a t t e r n s t h a t are used by i n d i v i d u a l guppies. These are d i s t i n c t from group responses such as s c h o o l i n g but may a l s o be e l i c i t e d i n a number of animals s i m u l t a n e o u s l y , should a group suddenly encounter a p r e d a t o r . Here I s h a l l c o n s i d e r the case f o r a 1 p r e d a t o r — 1 prey system. 1. Weak, "avoidance d r i f t T h i s behavior p a t t e r n i s seen most commonly when a p o t e n t i a l f i s h p r e d a t o r or strange o b j e c t has appeared i n the guppy's v i s u a l range at a c o n s i d e r a b l e d i s t a n c e . On p e r c e i v i n g the p r e d a t o r , the guppy may e i t h e r stop swimming momentarily, or a c t u a l l y move c l o s e r to the p r e d a t o r . At a c e r t a i n c r i t i c a l d i s t a n c e , the guppy turns i t s body l a t e r a l to the p r e d a t o r and moves some d i s t a n c e one way, and then t u r n s around and repeats i t i n the o p p o s i t e d i r e c t i o n . At a l l times the eyes are kept on the p r e d a t o r (with b i n o c u l a r f i x a t i o n , head h e l d s l i g h t l y downwards), as i f the f i s h was s u r v e y i n g i t from d i f f e r e n t angles. The f i s h remains roughly e q u i d i s t a n t from the p r e d a t o r d u r i n g t h i s " i n s p e c t i o n " . 144 There appears t o be a c o n f l i c t of approach-with-drawal m o t i v a t i o n i n v o l v e d i n t h i s b ehavior because the f i s h may a l t e r n a t e l y move c l o s e r t o , and then away from the pr e d a t o r . Should the p r e d a t o r move, e s p e c i a l l y i n the d i r e c t i o n o f the guppy, the behavior r e v e r t s t o one of the other motor p a t t e r n s d e s c r i b e d below. In some r e s p e c t s , the body c o n f i g u r a t i o n i n weak avoidance d r i f t and avoidance d r i f t (below) i s r e m i n i s c e n t of the weak sigmoid d i s p l a y of the male guppy d u r i n g c o u r t -s h i p . I use the term " d r i f t " because the f i s h appears t o be s c u l l i n g mainly w i t h i t s p e c t o r a l f i n s and the body i s h e l d r a t h e r r i g i d l y . 2. Avoidance d r i f t T h i s p a t t e r n i s a more s t e r e o t y p e d form of the pr e v i o u s one. The body i s h e l d more r i g i d l y and a d e f i n i t e sigmoid p o s i t i o n i s assumed. The d o r s a l and caudal f i n s are u s u a l l y spread and h e l d r i g i d l y . There i s l e s s move-ment towards or away from the pr e d a t o r than i n the pr e v i o u s p a t t e r n ; turns i n d i r e c t i o n are smooth and r a p i d . 3. Turn around T h i s p a t t e r n i s simply a sudden c e s s a t i o n o f swimming a c t i v i t y towards a p o t e n t i a l p r e d a t o r coupled w i t h a r a p i d "about t u r n " , f o l l o w e d by swimming i n the o p p o s i t e d i r e c t i o n . 145 4. Rapid d a r t T h i s motor p a t t e r n occurs most o f t e n when a p r e d a t o r has a c t u a l l y moved towards a guppy, or has appeared suddenly at a c l o s e range (e.g. ambush). To d e s c r i b e a l l the d e t a i l s of t h i s p a t t e r n would r e q u i r e slow-motion cinematographic a n a l y s i s because the p a t t e r n l a s t s o n l y a f r a c t i o n of a second. During t h i s time the f i s h may have moved as much as 20-30 cm. I t appears t h a t the body i s h e l d i n a r i g i d p o s i t i o n and t h r u s t i s a p p l i e d by the p e c t o r a l and caudal f i n s i n a s h o r t b u r s t , p r o p e l l i n g the f i s h u s u a l l y upwards i n the water column and o c c a s i o n a l l y out of the water. I t i s p o s s i b l e f o r the guppy to m a i ntain the same p o s i t i o n i t had b e f o r e the d a r t was i n i t i a t e d , i . e . i t seems to swim backwards. In t h i s way, the b i n o c u l a r f i x a t i o n on the p r e d a t o r i s u n i n t e r r u p t e d but the guppy i s a t a s a f e r d i s t a n c e . A r a p i d d a r t may i n t e r g r a d e w i t h a t u r n around and be f o l l o w e d by a bout of r a p i d j e r k y swimming movements (zigzag s , etc.) t h a t are very i r r e g u l a r and d i f f i c u l t to d e s c r i b e . These- l a t t e r movements appear to f a l l i n t o a c l a s s of behavior termed "protean", i . e . , "behavior which i s s u f f i c i e n t l y unsystematic to prevent a r e a c t o r p r e d i c t i n g i n d e t a i l the p o s i t i o n or a c t i o n s of the a c t o r (Humphries and D r i v e r , 1970, p. 286)." 146 5. Surface skim A s u r f a c e skim i s e l i c i t e d when an a t t a c k sequence i s i n progress or has j u s t ended u n s u c c e s s f u l l y . The f i s h swims r a p i d l y away from the pr e d a t o r , j u s t below the s u r f a c e water f i l m o f t e n making a r i p p l e on the s u r f a c e . T h i s i s commonly seen i n nature when a s c h o o l i s approached suddenly by a p r e d a t o r and i s o f t e n accompanied by one or more s u r f a c e jumps. 6. Surface jump In t h i s b ehavior p a t t e r n , the f i s h a c t u a l l y leaves the water once or maybe a dozen times i n s u c c e s s i o n . I t occurs normally o n l y when a pr e d a t o r has s t r u c k or i s about to s t r i k e . However, i t can a l s o be e l i c i t e d by ta p p i n g on an aquarium c o n t a i n i n g wary guppies t h a t have r e c e n t l y been a t t a c k e d by a p r e d a t o r . Surface jumps are a conspicuous f e a t u r e of schools of guppies, numerous f i s h l e a v i n g the water s i m u l t a n e o u s l y . I t i s a l s o d i f f i c u l t t o see the d e t a i l s of the sub-components of t h i s behavior because of i t s s h o r t d u r a t i o n . I t appears t h a t i n c e r t a i n cases ( s i n g l e jumps) the guppy re v e r s e s i t s p o s i t i o n i n m i d - a i r and r e t u r n s t o the water wit h i t s head f a c i n g towards the p o i n t o f e x i t from the water. In m u l t i p l e s u r f a c e jumps the o r i e n t a t i o n i s forward and the f i s h l i t e r a l l y s k i p s along the water s u r f a c e . The l a s t form was seen when a l a r g e female guppy was pursued by 147 a C r e n i c i c h l a (Oropuche R.); about a dozen separate jumps were counted b e f o r e the guppy f i n a l l y reached s a f e t y . A very s i m i l a r response ("surface breaking") has been d e s c r i b e d f o r the p o e c i l i i d Gambusia p a t r u e l i s d u r i n g encounters w i t h p i c k e r e l (George, 1960). The 6 p a t t e r n s d e s c r i b e d above, though viewed here as separate e n t i t i e s , may be p a r t of a s i n g l e attack-escape sequence. As a p r e d a t o r moves i n the v i c i n i t y o f a guppy, the guppy may show weak avoidance d r i f t a t f i r s t but s h i f t to a r a p i d d a r t i f the pr e d a t o r approaches too c l o s e l y . I f an a t t a c k o c c u r s , s u r f a c e skims and even s u r f a c e jumps may be e l i c i t e d . Thus the motor p a t t e r n s appear to be arranged on a s c a l e o f r e a c t i v i t y r a nging from " p r e c a u t i o n a r y " behavior (avoidance d r i f t , t u r n around) t o "emergency" behavior ( r a p i d d a r t , s u r f a c e skim and jump). Presumably the sensory-motor c o o r d i n a t i o n c e n t r e s operate on a system of r i s i n g t h r e s h o l d s with l e v e l s appro-p r i a t e f o r each motor p a t t e r n . These l e v e l s do not appear to be f i x e d . T h i s can be r e a d i l y demonstrated by moving a model of a pr e d a t o r a t a con s t a n t v e l o c i t y towards a group of p r e d a t o r - n a i v e and a group of experienced guppies. The f i r s t group w i l l show avoidance d r i f t b e havior while the second w i l l respond w i t h r a p i d d a r t or p o s s i b l y s u r f a c e jump, i . e . the t h r e s h o l d f o r the r e l e a s e of emergency p a t t e r n s i s lowered w i t h experience. T h i s t h r e s h o l d change i s not o n l y s p e c i f i c to v i s u a l s t i m u l i s i n c e the 148 responsiveness to mechanical d i s t u r b a n c e s (water v i b r a t i o n s ) i s a l s o i n f l u e n c e d by experience w i t h p r e d a t o r s . Though the a c o u s t i c o - l a t e r a l i s and o l f a c t o r y systems may f u n c t i o n i n pr e d a t o r avoidance, the primary sensory m o d a l i t y appears t o be v i s i o n . A l l the escape motor p a t t e r n s can be e l i c i t e d when a pr e d a t o r " a t t a c k s " a group of guppies i s o l a t e d i n a c l e a r g l a s s j a r . I d i d not study chemical communication between guppies, or between guppies and preda-t o r s , but o b s e r v a t i o n s d u r i n g a c t u a l p r e d a t i o n r e v e a l e d no b e h a v i o r a l changes of the type a t t r i b u t e d t o " S c h r e c k s t o f f " (Schutz, 1956; P f e i f f e r , 1962). S i m i l a r l y , I observed no o v e r t responses t o samples of water taken from a q u a r i a c o n t a i n i n g p r e d a t o r s ( c f . Goz, 1941; George, 1960; Reed, 1969; Ruppell and Gosswein, 1972). Disturbances do not always a r i s e from w i t h i n the a q u a t i c medium; t e r r e s t r i a l p r e d a t o r s a l s o evoke responses from guppies. Objects moved over a stream e l i c i t a "down-d a r t " response. The f i s h moves very q u i c k l y t o a deeper p o r t i o n of the water column; i n shallow streams i t may even come to r e s t on the s u b s t r a t e . P o p u l a t i o n d i f f e r e n c e s i n escape motor p a t t e r n s Although a l l the p a t t e r n s d e s c r i b e d above can be e l i c i t e d i n a l l the p o p u l a t i o n s o f guppies i t was e v i d e n t t h a t q u a n t i t a t i v e d i f f e r e n c e s were p r e s e n t under n a t u r a l c o n d i t i o n s . I saw v i r t u a l l y a l l the s u r f a c e skim and 149 s u r f a c e jump behavior i n streams c o n t a i n i n g c h a r a c i d and c i c h l i d p r e d a t o r s , even when the "predator" was the end of the d i p n e t p o l e . Using the same s t i m u l u s , - i t was d i f f i c u l t to e l i c i t these responses from guppies who were not exposed to c h a r a c i d s and c i c h l i d s . I t appears, t h e r e f o r e , t h a t there may be geographic v a r i a t i o n i n the degree.of s t i m u l a -t i o n necessary to e l i c i t c e r t a i n escape motor p a t t e r n s . The down-dart response was a l s o g e o g r a p h i c a l l y v a r i a b l e . T h i s q u a n t i t a t i v e . d i f f e r e n c e c o u l d be demon-s t r a t e d when an o b j e c t was passed with a. uniform v e l o c i t y over a group of•newly-caught guppies from P a r i a and Guayamare'Rivers t h a t had been p l a c e d i n , a d j a c e n t opaque water p a i l s . The Guayamare f i s h s c a t t e r e d r a p i d l y downwards i n a "panic" r e a c t i o n but q u i c k l y r e - s u r f a c e d . The P a r i a guppies on the other hand moved s l o w l y and d i r e c t l y down-wards (no v i o l e n t s c a t t e r ) to the bottom of the p a i l . Often they remained motionless on the bottom f o r over 10 sec; by t h i s time a l l the Guayamare f i s h had f u l l y recovered and were swimming normally near the s u r f a c e again. Thus i n a d d i t i o n to a p o p u l a t i o n d i f f e r e n c e i n the i n t e n s i t y of the response and the r a t e of recovery, t h e r e a l s o seems to be a q u a l i t a t i v e d i f f e r e n c e i n the form of the behavior. I c o u l d never e l i c i t a " P a r i a response" from Guayamare guppies, i n f a c t the o n l y other p o p u l a t i o n showing t h i s response was Y a r r a . 150 Summary of the f i e l d o b s e r v a t i o n s of behavior The p r e v i o u s s e c t i o n has been mainly a q u a l i t a t i v e review of some.of the d i f f e r e n c e s i n the m i c r o d i s t r i b u t i o n and a n t i p r e d a t o r behavior of guppies i n streams with d i f f e r e n t p r e d a t o r s . The d i s t r i b u t i o n of f i s h i n terms of depth i n the water column and p r o x i m i t y to the shore i s a f f e c t e d i n p a r t by a b i o t i c f a c t o r s i n c l u d i n g the streambed morphology and water v e l o c i t y . However, I put f o r t h the hypothesis t h a t the m i c r o d i s t r i b u t i o n of f i s h i s a l s o a s e n s i t i v e i n d i c a t o r of d i f f e r e n c e s i n the q u a l i t y of p r e d a t i o n . Where l a r g e p r e d a t o r s occur, the l i t t o r a l r e s t r i c t i o n of guppies may i n i t s e l f be an a n t i p r e d a t o r mechanism. •Large d i f f e r e n c e s were observed i n the development of s c h o o l i n g behavior. S c h o o l i n g i s a conspicuous f e a t u r e of guppy behavior i n streams with c h a r a c i d and c i c h l i d p r e d a t o r s but,tends to be l e s s prominent i n streams with only R i v u l u s . Gross estimates of r e a c t i o n d i s t a n c e s were made i n the f i e l d and these were a l s o c o r r e l a t e d with the d i s t r i b u -t i o n and abundance of p r e d a t o r s . In the absence of s i g n i f i -cant a q u a t i c p r e d a t i o n (Paria) the r e a c t i o n d i s t a n c e to a t h r e a t e n i n g stimulus was very s m a l l , whereas i t was l a r g e i n streams w i t h c h a r a c i d and c i c h l i d p r e d a t o r s . The guppy may use at l e a s t s i x f a i r l y d i s t i n c t motor 151 p a t t e r n s i n response to p o t e n t i a l p r e d a t o r s . These p a t t e r n s appear to be o r g a n i z e d i n a ranked s c a l e with the lowest p a t t e r n appearing under c o n d i t i o n s of a m i l d change i n the a n i m a l 1 s immediate environment and the h i g h e s t under d i r e c t a t t a c k c o n d i t i o n s . No q u a l i t a t i v e d i f f e r e n c e s i n these p a t t e r n s were seen among p o p u l a t i o n s exposed to d i f f e r e n t p r e d a t o r s , but q u a n t i t a t i v e d i f f e r e n c e s were found. I t appears t h a t the h i g h e r l e v e l s of escape p a t t e r n s are e l i c i t e d by a l e s s severe t h r e a t i n streams wi t h c h a r a c i d and c i c h l i d p r e d a t o r s than streams with only R i v u l u s . T h e r e f o r e , f i s h exposed to d i f f e r e n t p r e d a t o r s have d i f f e r e n t alarm t h r e s -h o l d s . The response to an o b j e c t passed over the water s u r f a c e i s a s t e r e o t y p e d downward " f r e e z e " a t P a r i a while i t i s a more sudden random s c a t t e r at Guayamare. T h i s might be r e l a t e d to the r e l a t i v e importance of a e r i a l and a q u a t i c p r e d a t o r s i n each stream. S u r v i v a l value of b e h a v i o r a l d i f f e r e n c e s  i n wild-caught f i s h I f my h y p o t h e s i s i s c o r r e c t t h a t much of the v a r i a -t i o n i n behavior observed i n nature i s r e l a t e d to d i f f e r e n c e s i n the impact of p r e d a t o r s , i t should be p o s s i b l e to t e s t f i s h taken from "low p r e d a t i o n " and "high p r e d a t i o n " r e g i o n s with the n a t u r a l p r e d a t o r s . I t would be p r e d i c t e d t h a t guppies taken from p o p u l a t i o n s which showed (1) long, r e a c t i o n d i s t a n c e s to p o t e n t i a l p r e d a t o r s , (2) well-developed 152 s c h o o l i n g b e h a v i o r , (3) e a s i l y - e l i c i t e d escape motor p a t t e r n s and (4) well-developed h a b i t a t s e l e c t i o n (close t o s u r f a c e and shore) would have b e t t e r s u r v i v a l when p l a c e d e x p e r i m e n t a l l y with the n a t u r a l p r e d a t o r s . T h i s p r e d i c t i o n makes no assumptions r e g a r d i n g the c a u s a t i o n of the phenotypic d i f f e r e n c e s i n b e h a v i o r — t h e y c o u l d be the r e s u l t of experience w i t h , or n a t u r a l s e l e c t i o n by p r e d a t o r s , or an i n t e r a c t i o n of both e f f e c t s . Here I am only concerned with demonstrating whether or not the d i f f e r e n c e s have s u r v i v a l v a l u e . The s e p a r a t i o n of e x p e r i e n t i a l and g e n e t i c f a c t o r s w i l l be d e a l t w i t h l a t e r . My o r i g i n a l p l a n had been t o t e s t the d i f f e r e n t p o p u l a t i o n s i n " f i e l d experiments" where f i s h from a low pre d a t o r stream (showing concomitant b e h a v i o r a l d i f f e r e n c e s ) would be t e s t e d f o r s u r v i v a l i n a p r e d a t o r - i n f e s t e d stream. Samples from the l a t t e r stream would be handled i d e n t i c a l l y and serve as a c o n t r o l . A l s o I wanted t o see i f perhaps a well - d e v e l o p e d a n t i - c h a r a c i d and c i c h l i d response might i n f a c t be disadvantageous i n a stream i n f e s t e d with R i v u l u s . Such t r a n s p l a n t experiments are u s u a l l y viewed by e t h o l o g i s t s as the b e s t way to t e s t the s u r v i v a l value of behavior. For numerous reasons I had to abandon my f i e l d t e s t s and perform the experiments i n a q u a r i a and outdoor p o o l s . The main problems w i t h the f i e l d t e s t s centered on m a i n t a i n i n g "guppy-proof" screen e n c l o s u r e s (I d i d not want to " p o l l u t e " a stream by the a c c i d e n t a l r e l e a s e of f o r e i g n 153 genotypes). During the r a i n y season the h i g h l y v a r i a b l e water l e v e l and the abundance of suspended macrophytic m a t e r i a l and d e t r i t u s i n the water made t h i s work very d i f f i c u l t . A secondary problem r e s u l t e d from human i n t e r -f e r ence with m a t e r i a l s l e f t at f i e l d s i t e s . Experiment 6.1 R e l a t i v e s u r v i v a l of wild-caught guppies. Methods An "indoor" and "outdoor" s e r i e s of experiments were designed to t e s t f o r s u r v i v a l d i f f e r e n c e s among samples of guppies taken from 4 p o p u l a t i o n s . The f i r s t s e r i e s was done u s i n g e i t h e r i n d o o r a q u a r i a of 80 l i t e r c a p a c i t y or an indoor c o n c r e t e p o o l of 700 l i t e r s (depth 50 cm). These were maintained at ambient temperature and n a t u r a l photo-p e r i o d . The second s e r i e s was done i n 2 a d j a c e n t outdoor concrete pools of 1600 l i t e r s (depth 50 cm). The water i n the indoor experiments was completely c l e a r w h ile the water i n the outdoor experiments was very t u r b i d w i t h phyto-plankton (Secchi d i s c d isappeared a t 35 cm) . In the indoor t e s t s , prey were added a t frequent i n t e r v a l s d u r i n g the day t o r e p l a c e those eaten (replacement method). The r e l a t i v e number of 2 or more prey types added over a p e r i o d of 5 or more days of p r e d a t i o n was used as the index of v u l n e r a b i l i t y . In the outdoor t e s t s , r e l a t i v e s u r v i v a l was simply 154 measured as the number of f i s h remaining from an i n i t i a l 50 i n d i v i d u a l s of each p o p u l a t i o n ; p e r i o d i c a l censuses were made with a "one-man s e i n e " . In a l l experiments the p r e d a t o r s and prey had been i n c a p t i v i t y f o r o n l y a s h o r t p e r i o d of time. Prey s i z e s were matched between p o p u l a t i o n s as a c o n t r o l f o r p o t e n t i a l s i z e s e l e c t i v i t y by the p r e d a t o r s . Because my knowledge of the d i s c r i m i n a t i o n l e a r n i n g o f • t h e predators was minimal, I u s u a l l y used female guppies f o r these t e s t s . Females are a l l m o r p h o l o g i c a l l y s i m i l a r , r e g a r d l e s s of t h e i r p o p u l a t i o n of o r i g i n ; t h i s was a con-v e n i e n t c o n t r o l f o r the p o s s i b i l i t y t h a t p r e d a t o r s might l e a r n to a s s o c i a t e the p o p u l a t i o n - s p e c i f i c c o l o u r p a t t e r n s of the males with t h e i r ease of c a p t u r e . In a d d i t i o n i t would r u l e out s e l e c t i o n based on conspicuousness per se (though-, as I have p o i n t e d out i n Chapter 4, good evidence f o r t h i s i s s t i l l l a c k i n g ) . To d i s t i n g u i s h among the female types, those of one p o p u l a t i o n were marked on e i t h e r the lower or upper caudal peduncle with a #30 hypodermic needle t h a t had been dipped i n a concentrated Trypan Blue ( v i t a l s t a i n ) s o l u t i o n . F i s h were not p l a c e d with predators f o r a t l e a s t 24 hr a f t e r marking. The m o r t a l i t y d u r i n g t h i s recovery p e r i o d was l e s s than 1%. The mark faded n o t i c e a b l y a f t e r the f i r s t day but remained v i s i b l e on c l o s e i n s p e c t i o n f o r a t l e a s t a week. Where prey s u r v i v e d l o n g e r than t h i s , they were re-marked 155 as soon as the mark became almost i m p e r c e p t i b l e . In a c o n t r o l experiment, I p l a c e d 50 marked and 50 unmarked females of the same p o p u l a t i o n with a j u v e n i l e C r e n i c i c h l a . The s u r v i v a l r a t e of each group was i d e n t i c a l s u g g e s t i n g the marking procedure d i d not a f f e c t the v u l n e r -a b i l i t y of the f i s h to p r e d a t o r s . R e s u l t s Table 21 shows the r e s u l t s of the indoor experiments (replacement method) g i v i n g the r e l a t i v e number of f i s h 2 added each day over the course of the experiment. X values were c a l c u l a t e d to t e s t the n u l l h y p othesis t h a t the d i f f e r e n c e between the t o t a l observed and expected number of f i s h eaten c o u l d be e x p l a i n e d simply by sampling f l u c t u a t i o n s ( f o r 1 df Yates's c o r r e c t i o n f o r c o n t i n u i t y was a p p l i e d ) . I t i s c l e a r t h a t the n u l l h y p othesis c o u l d be r e j e c t e d i n most experiments. In t e s t s w i t h a j u v e n i l e C r e n i c i c h l a (Table 21 A, B, and C) s i g n i f i c a n t l y more f i s h of the "low p r e d a t i o n " p o p u l a t i o n s ( P a r i a , Upper A r i p o , P e t i t e Curucaye) were taken than the "high p r e d a t i o n " p o p u l a t i o n (Lower A r i p o ) . The r e s u l t s f o r a r e p l i c a t e of one of these t e s t s w i t h a j u v e n i l e H o p l i a s (Table 21 D) were l e s s s i g n i f i c a n t but i n the same d i r e c t i o n . The r e s u l t s with an a d u l t C r e n i c i c h l a p r e y i n g on males from 3 p o p u l a t i o n s i n d i c a t e d t h a t P a r i a was most TABLE 21. R e l a t i v e m o r t a l i t y of wild-caught guppies exposed to p r e d a t o r s . Tests A, B,C-—95 mm C r e n i c i c h l a , 80 l i t e r aquarium; Te s t D — 95 mm H o p l i a s , 80 l i t e r aquarium; Te s t E — 2 0 0 mm C r e n i c i c h l a , 700 l i t e r indoor p o o l . TEST A TEST B TEST ' C TEST D TEST E Lower Lower Upper Lower P e t i t e Lower Upper Lower Upper Day A r i p o P a r i a A r i p o A r i p o A r i p o Curucaye A r i p o A r i p o A r i p o A r i p o P a r i a 1 0 0 2 2 0 0 0 1 0 0 0 2 0 0 3 10 5 8 . 9 11 - - -3 1 7 5 15 7 11 4 10 3 6 9 4 2 9 6 7 0 3 5 12 - - -5 3 9 5 4 2 9 2 2 - - - -6 1 6 2 8 7 4 12 12 20 7 13 11 8 2 7 9 6 15 10 3 4 11 7 8 12 7 9 13 10 7 14 4 5 T o t a l 7 31 23 46 14 31 79 106 15 18 29 X 2 X (1 1 3 df) 1 J # 92 7. 01 5. 69 3. 65 x 2 X (2 df) 5.25 P < . 001 01 02 .10 > p • > .05 .10 > p > .05 157 v u l n e r a b l e (Table 21 E ) . On the average i t would appear t h a t about twice as many guppies taken from R i v u l u s h a b i t a t s were eaten than those from c h a r a c i d - c i c h l i d h a b i t a t s . U n f o r t u n a t e l y a t the time these experiments were done I was unable to c o l l e c t enough Guayamare a d u l t s to compare t h e i r s u r v i v a l with Lower A r i p o f i s h . T h i s would have been a n i c e c o n t r o l experiment f o r the " p r e d a t i o n h y p o t h e s i s " because these streams d i f f e r i n s e v e r a l a b i o t i c f e a t u r e s (temperature, water v e l o c i t y , depth, shade, t u r b i d i t y , s u b s t r a t e composition) and are separated by over 30 km; however, both have c h a r a c i d and c i c h l i d p r e d a t o r s . T h e i r s u r v i v a l r a t e s probably would have been more s i m i l a r than Lower A r i p o vs. P a r i a e t c . ( l a t e r i n t h i s chapter I do t e s t l a b o r a t o r y stocks of Guayamare a g a i n s t Lower A r i p o ) . Besides s u r v i v a l experiments between p o p u l a t i o n s exposed to d i f f e r e n t p r e d a t o r s p e c i e s (Table 21), i t i s of i n t e r e s t t o determine i f there are d i f f e r e n c e s between p o p u l a t i o n s exposed to d i f f e r e n t d e n s i t i e s of the same pr e d a t o r s p e c i e s . The 2 outdoor experiments were designed f o r t h i s purpose, P e t i t e Curucaye (high R i v u l u s d e n s i t y ) versus Upper A r i p o (medium R i v u l u s d e n s i t y ) . In one t e s t a l a r g e C r e n i c i c h l a was the p r e d a t o r , and i n the o t h e r 2 l a r g e H o p l i a s were used. The r e s u l t s (Figure 18) of the 2 t e s t s were q u i t e s i m i l a r : a p a r a l l e l decrease f o r the f i r s t 4 to 5 days 158a FIGURE 18. R e l a t i v e s u r v i v a l of wild-caught female guppies of two p o p u l a t i o n s exposed t o e i t h e r a s i n g l e C r e n i c i c h l a or two H o p l i a s . T e s t s were c a r r i e d out i n adjacent 1600 l i t e r outdoor p o o l s . 158b PETITE CURUCAYE O I DAYS 159 f o l l o w e d by a r e d u c t i o n i n m o r t a l i t y of P e t i t e Curucaye guppies. The t e s t was more d e c i s i v e when C r e n i c i c h l a was used. I t should be r e c a l l e d , however, t h a t Upper A r i p o guppies occur above a major b a r r i e r t o gene flow whereas P e t i t e Curucaye f i s h are i s o l a t e d o n l y by d i s t a n c e . The s u p e r i o r s u r v i v a l of the l a t t e r might thus r e f l e c t the i n f l u x of genes from the Santa Cruz R. where c h a r a c i d s and c i c h l i d s are abundant. On the o t h e r hand, the p h i l o p a t r y d e s c r i b e d i n Chapter 3 might f a v o r l o c a l a d a p t a t i o n i f s e l e c t i o n i s i n t e n s e . But t h i s i s p r e j u d g i n g the i s s u e — I have thus f a r presented no data to show t h a t g e n e t i c s has anything to do with these d i f f e r e n c e s i n s u r v i v a l . C o n c l u s i o n f o r s u r v i v a l experiments  with wild-caught guppies The o v e r a l l r e s u l t s of these experiments, which used a v a r i e t y of p r e d a t o r s , d i f f e r e n t methods, and d i f f e r e n t s i z e s of experimental environments, a l l p o i n t to the c o n c l u s i o n t h a t f i s h taken from p o p u l a t i o n s exposed to a p p a r e n t l y heavy p r e d a t i o n p r e s s u r e enjoy b e t t e r s u r v i v a l when t e s t e d c o m p e t i t i v e l y a g a i n s t samples taken from popu-l a t i o n s exposed to o n l y moderate or l i g h t p r e d a t i o n (or a d i f f e r e n t type of p r e d a t i o n — t h e r e i s no reason to assume a p r i o r i t h a t R i v u l u s p r e d a t i o n d i f f e r s from c h a r a c i d -c i c h l i d p r e d a t i o n o n l y i n a q u a n t i t a t i v e manner). The q u e s t i o n t h a t immediately a r i s e s from these 160 r e s u l t s i s why s u r v i v a l should d i f f e r among p o p u l a t i o n s , e s p e c i a l l y i n experiments t h a t c o n t r o l f o r m o r p h o l o g i c a l d i f f e r e n c e s among p o p u l a t i o n s ( s i z e and c o l o u r ) . The obvious f a c t o r appears to be beh a v i o r . I have s t r e s s e d e a r l i e r t h a t under n a t u r a l c o n d i t i o n s guppies exposed t o d i f f e r e n t p r e d a t o r s show s e v e r a l conspicuous d i f f e r e n c e s i n behavior t h a t appear to make f u n c t i o n a l sense i f viewed as a n t i p r e d a t o r d e v i c e s . I t seems reasonable t h a t some of these d i f f e r e n c e s may have been o p e r a t i v e i n the aquarium and p o o l s u r v i v a l experiments a l b e i t t h a t the environment was more homogeneous than i n nature (no s h o r e l i n e depth g r a d i e n t , l i t t l e s u b s t r a t e v a r i a t i o n , no water c u r r e n t , f i n i t e volume f o r escape, e t c . ) . I t may be concluded then t h a t some of the b e h a v i o r a l d i f f e r e n c e s o c c u r r i n g i n n a t u r a l p o p u l a t i o n s do have s u r v i v a l v a l u e , even i n a " s i m p l i f i e d " p r e d a t o r - p r e y encounter s i t u a t i o n . The c r u c i a l q u e s t i o n I asked a t t h i s p o i n t was: Are the d i f f e r e n c e s i n s u r v i v a l a f u n c t i o n o f phenotypic d i f f e r e n c e s i n behavior r e s u l t i n g from d i f f e r e n t i a l exposure to p r e d a t o r s (avoidance c o n d i t i o n i n g ) , o r are they a r e f l e c -t i o n of -m^groevolutionary ( i . e . g e n e t i c ) d i f f e r e n c e s i n behavior. I have a l r e a d y remarked upon the '. l a b i l i t y o f avoidance t h r e s h o l d s ; f o r t h i s reason i t was necessary to c o n t r o l f o r e x p e r i e n t i a l f a c t o r s . 161 Experiment 6.2 R e l a t i v e s u r v i v a l of l a b o r a t o r y s t o c k s . I n t r o d u c t i o n The study of b e h a v i o r a l g e n e t i c s has r e v e a l e d t h a t the m a j o r i t y o f h e r i t a b l e behavior p a t t e r n s (or d i f f e r e n c e s between s i m i l a r p a t t e r n s ) are c o n t r o l l e d by complex p o l y g e n i c systems ( C a s p a r i , 1967). Only r a r e l y have adap t i v e or p o t e n t i a l l y a d a p t i v e p a t t e r n s been found to be under simple (one or two f a c t o r ) g e n e t i c c o n t r o l (e.g. Bastock, 1956; Rothenbuhler, 1964). T h i s does not mean t h a t d i r e c t g e n e t i c evidence f o r a p o l y g e n i c system cannot be o b t a i n e d — i t i s j u s t more d i f f i c u l t (Franck, 1969; see Lindzey e t a l , 1971, f o r a review o f r e s e a r c h methodology). In the pr e s e n t study, I was not d i r e c t l y concerned wit h o b t a i n i n g an accurate h e r i t a b i l i t y estimate of some b e h a v i o r a l t r a i t (DeFries, 1967; Van Oortmerssen, 1970); my c h i e f aim was to determine i f phenotypic d i f f e r e n c e s observed in, nature (and r e f l e c t e d i n the s u r v i v a l t e s t s w i t h w i l d f i s h ) would p e r s i s t i n animals r a i s e d under i d e n t i c a l c o n d i t i o n s i n the l a b o r a t o r y . I f such d i f f e r e n c e s s t i l l p e r s i s t ( e s p e c i a l l y a f t e r s e v e r a l g e n e r a t i o n s of l a b o r a t o r y breeding) then i t i s safe to assume t h a t g e n e t i c d i f f e r e n c e s are pr e s e n t . T h i s i s termed an in n a t e d i f f e r e n c e by the m a j o r i t y o f European e t h o l o g i s t s (e.g. Tinbergen, 1959; Hinde, 1959); d e f i n e d as above, t h i s term i s a l s o a c c e p t a b l e t o American experimental p s y c h o l o g i s t s (Lehrman, 1970). 162 When g e n e t i c d i f f e r e n c e s are found i n n a t u r a l popu-l a t i o n s , they are normally assumed to have a r i s e n i n response to s e l e c t i v e p r e s s u r e s (adaptive g e n e t i c d i f f e r e n c e ) , though a t the molecular l e v e l t h e r e i s much debate about the p o s s i b i l i t y o f n e u t r a l genes and "non-Darwinian" e v o l u t i o n . Methods The procedures f o r the esta b l i s h m e n t o f the l a b o r a -t o r y stocks have a l r e a d y been o u t l i n e d (Chapter 2 ) . For the 5 stocks I s t u d i e d i n d e t a i l , e s p e c i a l l y l a r g e c u l t u r e s were maintained. In order to r e p l i c a t e p r e d a t i o n experiments where s t r i c t c o n t r o l i s p l a c e d on the s i z e , sex, expe r i e n c e , and geographic o r i g i n of the f i s h , one must r a i s e l i t e r a l l y thousands of a d u l t animals. The experiments were performed u s i n g the "standard s u r v i v a l t e s t " procedure as o u t l i n e d i n Chapter 4. I n d i v i d u a l C r e n i c i c h l a ( C - l , C-2, C-3, C-4) were used i n 3 adjacent a q u a r i a f i t t e d as d e s c r i b e d b e f o r e (12 l i g h t - 1 2 dark photoregime). The guppies were a t l e a s t second g e n e r a t i o n l a b o r a t o r y stock and the m a j o r i t y were probably f o u r t h and f i f t h g e n e r a t i o n f i s h . A l l had been maintained under i d e n t i c a l l a b o r a t o r y c o n d i t i o n s and were p r e d a t o r -naive ( e x c l u d i n g attempted c a n n i b a l i s m which was r a r e l y observed). Two p a r a l l e l s e t s of experiments were run. One used on l y females, the other males. A l l t e s t s were of the com-p e t i t i v e type w i t h Lower A r i p o as the r e f e r e n c e stock 163 a g a i n s t which the other 4 were t e s t e d , one a t a time. In order to r u l e out p o s s i b l e b i a s r e s u l t i n g from s i z e - s e l e c -t i v e p r e d a t i o n , each p a i r of stocks t e s t e d was matched as c l o s e l y as p r a c t i c a l l y f e a s i b l e w i t h r e s p e c t to the mean, range, and v a r i a n c e of body s i z e s ( f o r r e l e v a n t s t a t i s t i c s , r e f e r t o Appendix, Table 4). Stocks were re c o g n i z e d by Trypan Blue marks on the caudal peduncle. T h i s 4 x 2 design (stocks x sex) was a l s o repeated under a d i f f e r e n t l i g h t regime. P r e l i m i n a r y o b s e r v a t i o n s on C r e n i c i c h l a p r e d a t i o n i n ad l i b i t u m t e s t s (predator not s t a r v e d e x p e r i m e n t a l l y ) r e v e a l e d t h a t the m a j o r i t y of p r e d a t i o n o c c u r r e d i n dim or dark l i g h t . T h e r e f o r e , to i n s u r e t h a t the prey were ab l e to see the p r e d a t o r , I mounted a 7-watt incandescent bulb 65 cm over the water s u r f a c e . T h i s p r o v i d e d dim, even i l l u m i n a t i o n d u r i n g the p e r i o d when the main l i g h t s were o f f . In each experiment the i n i t i a l d e n s i t y was always 100, 50 of each stock. S u r v i v a l was f o l l o w e d i n i t i a l l y at 24-hour i n t e r v a l s and then a t longer i n t e r v a l s u n t i l the d e n s i t y approached or reached zero. The t o t a l number of f i s h used i n the e n t i r e experiment was 1600, i . e . 800 per l i g h t regime. To f a c i l i t a t e s t a t i s t i c a l comparisons, I compared the r e l a t i v e numbers of each stock t h a t had been eaten a t the census p e r i o d c l o s e s t to 50% s u r v i v a l (50 out of 100 f i s h remain). The observed m o r t a l i t y and t h a t expected 164 under the n u l l h y p o thesis (random preda t i o n ) were t e s t e d f o r 2 s i g n i f i c a n c e u s i n g the X s t a t i s t i c ( c o r r e c t e d f o r c o n t i n u i t y ) . R e s u l t s In the experiments w i t h a 12 l i g h t - 1 2 dark photo-regime, m o r t a l i t y d i d not d i f f e r s i g n i f i c a n t l y from chance l e v e l s except f o r P e t i t e Curucaye females (Table 22 A ) . S u r p r i s i n g l y , P e t i t e Curucaye guppies showed b e t t e r s u r v i v a l than Lower A r i p o (the t r e n d f o r P e t i t e Curucaye males was i n the same d i r e c t i o n as the females but not s i g n i f i c a n t ) . In the 12 l i g h t - 1 2 dim photoregime, t h e r e were s e v e r a l n o t a b l e d e v i a t i o n s from random p r e d a t i o n (Table 22 B). Lower A r i p o f i s h showed l e s s m o r t a l i t y than P a r i a o r Upper A r i p o , c o n f i r m i n g the r e s u l t s w i t h wild-caught f i s h (Table 21 A and 21 B r e s p e c t i v e l y ) . The d i f f e r e n c e s between Lower A r i p o vs. Guayamare, and Lower A r i p o vs. P e t i t e Curucaye were not s i g n i f i c a n t , though i n the l a t t e r case, the t r e n d was s t i l l i n favour o f the P e t i t e Curucaye stock. When the r e s u l t s of the Lower A r i p o v s . P e t i t e Curucaye t e s t s with wild-caught f i s h (Table 21 C) are com-pared i n a 2 x 2 contingency t a b l e w i t h the r e s u l t s o b t a i n e d f o r the same t e s t w i t h l a b o r a t o r y stock (sexes p o o l e d ) , the d i f f e r e n c e i s h i g h l y s i g n i f i c a n t (wild vs. l a b o r a t o r y 12 l i g h t - 1 2 dark: X 2 ( 1 d f ) = 12.61, p < .001; w i l d vs. l a b o r a t o r y 12 l i g h t - 1 2 dim: X 2 , f. = 7.88, p < .01); when 165 TABLE 22. R e l a t i v e m o r t a l i t y of naive l a b o r a t o r y stocks of guppies exposed t o C r e n i c i c h l a . A = 12 l i g h t — 1 2 dark photoregime; B = 12 l i g h t — 1 2 dim l i g h t photoregime; C = males and females pooled (NS = not s i g n i f i c a n t ; S = s i g n i f i c a n t a t 5% l e v e l or l e s s ) . A. Test Predator Number eaten at approx. 50% m o r t a l i t y T o t a l X p (females) 1 C-3 Lower Aripo 26 Guayamare 17 43 1, .49 > .20 2 C- l Lower Aripo 31 P. Curucaye 14 45 5 , .69 < .02 3 C - l Lower Aripo 26 Paria 17 43 1, .49 > .20 4 C - l Lower Aripo 28 Upper Aripo 29 57 0. .00 -T o t a l 111 77 188 (males) 1 C- l Lower Aripo 18 Guayamare 25 43 0. ,84 > . 30 2 C-2 Lower Aripo 27 P. Curucaye 17 44 1. .84 > .10 3 C - l Lower Aripo 23 Paria 21 44 0. ,023 > .80 4 C-2 Lower Aripo 26 Upper Aripo 22 48 0. .188 > .50 T o t a l 94 85 179 B. (females) 1 C - l Lower Aripo 30 Guayamare 20 50 1, .62 > .20 2 C-4 Lower Aripo 31 P. Curucaye 26 57 0. .281 > .50 3 C-4 Lower Aripo 16 Paria 32 48 4 . 69 < .05 4 C - l Lower Aripo 16 Upper Aripo 34 50 5. .78 < .02 T o t a l 93 112 205 (males) 1 C-4 Lower Aripo 26 Guayamare 37 63 1. .59 > .20 2 C - l Lower Aripo 36 P. Curucaye 24 60 2 . 02 > .10 3 C-2 Lower Aripo 15 Paria 33 48 6 , .02 < .02 4 C-4 Lower Aripo 18 Upper Aripo 31 49 2 . 94 < .10 T o t a l 95 125 220 12 light-12 dark X 2 p Lower Aripo X Guayamare 0 .012 > 0. .90 (NS) P e t i t e Curucaye 7 .59 < 0. .01 (S) Upper Aripo 0 .038 > 0. .80 (NS) Pa r i a 1 .15 > 0. .20 (NS) li g h t --12 dim l i g h t Lower Aripo X Guayamare 0 .00 > 0. .90 (NS) P e t i t e Curucaye 2 .19 > 0, .10 (NS) Upper Aripo 9 .09 < 0. .005 (S) Paria 11 .34 < 0. .001 (S) 166 o n l y the l i g h t treatments f o r l a b o r a t o r y stock are compared 2 there i s no s i g n i f i c a n t d i f f e r e n c e (X ^ = 1.01, p > .30) . D i s c u s s i o n of r e s u l t s of s u r v i v a l experiments Some c a s u a l n i g h t o b s e r v a t i o n s of C r e n i c i c h l a f e e d i n g behavior d u r i n g Exp. 6.2 should be brought to bear on the d i f f e r e n c e i n the r e s u l t s o b t a i n e d w i t h and without supplementary l i g h t ( i . e . Table 22 A vs. 22 B). From numerous o b s e r v a t i o n s i t was c l e a r t h a t the C r e n i c i c h l a were not f e e d i n g d u r i n g the day. Daytime a c t i v i t y c o n s i s t e d almost e n t i r e l y of " r e s t i n g " i n a p i t under the flower pot t h a t had been p r o v i d e d . Though v i s u a l l y i s o l a t e d i n adjacent a q u a r i a , a l l 4 C r e n i c i c h l a dug p i t s i n e x a c t l y the same l o c a t i o n between the f r o n t g l a s s and the underside of the flower pot. T h i s seemed p u z z l i n g a t f i r s t but I soon r e a l i z e d t h a t t h i s l o c a t i o n p r o v i d e d the b e s t concealment from the prey. When the l i g h t s a u t o m a t i c a l l y turned o f f a t n i g h t , the p r e d a t o r s would become a c t i v e , f i r s t moving i n t o the flower pot and then s t r i k i n g a t guppies i n a l l r e g i o n s of the tank. I t was obvious t h a t the guppies were more " h e l p l e s s " under c o n d i t i o n s of n o c t u r n a l p r e d a t i o n ; e v a s i v e a c t i o n was o f t e n taken when the p r e d a t o r had almost e n g u l f e d the prey. T h i s suggests t h a t C r e n i c i c h l a has a v i s u a l advantage under low l i g h t i n t e n s i t y . Since under my 167 experimental c o n d i t i o n s the guppies had no ref u g e , the i n t e r -stock d i f f e r e n c e s i n escape behavior (except P e t i t e Curucaye) may have been i n c o n s e q u e n t i a l when the l i g h t l e v e l s a t n i g h t were very low. When l i g h t was added though, d i f f e r e n c e s d i d appear i n the p r e d i c t e d d i r e c t i o n and c l o s e l y resembled the r e s u l t s o b t a i n e d with wild-caught f i s h . Although no dim l i g h t was p r o v i d e d a t n i g h t i n t e s t s w i t h wild-caught guppies, t h i s probably would not have a f f e c t e d the r e s u l t s because the C r e n i c i c h l a used i n these t e s t s were d a y - a c t i v e ; a t n i g h t they were q u i e s c e n t . Since i n nature I have seen C r e n i c i c h l a a t t a c k guppies a t midday, i t i s p o s s i b l e t h a t the n o c t u r n a l h a b i t s observed i n Exp. 6.2 were u n n a t u r a l . The most reasonable e x p l a n a t i o n i s t h a t the C r e n i c i c h l a l e a r n e d t h a t guppies are much e a s i e r to capture i n darkness. Night o b s e r v a t i o n s i n nature have r e v e a l e d t h a t i n streams w i t h c h a r a c i d s and c i c h l i d s , guppies are d i s t r i b u t e d i n a narrow r i b b o n along the stream edge (shallow water). T h i s might f u n c t i o n to reduce the a c c e s s i b i l i t y o f guppies to n o c t u r n a l p r e d a t o r s (e.g. H o p l i a s ) . T h i s h a b i t a t s e l e c t i o n may be e f f e c t i v e enough t o f o r c e C r e n i c i c h l a t o s t a l k o r ambush unwary f i s h d u r i n g the day when guppies t y p i c a l l y swim f u r t h e r from shore. Apparently t h i s e x t r a "work" was not r e q u i r e d i n Exp. 6.2 (dark n i g h t s ) so the pre d a t o r s s h i f t e d t o n o c t u r n a l p r e d a t i o n . The a d d i t i o n of a dim l i g h t a t n i g h t , however, 168 allowed the guppies to use t h e i r v i s i o n more e f f e c t i v e l y ( i n c r e a s e i n r e a c t i o n d i s t a n c e ) and the experimental r e s u l t s o b t a i n e d are probably more r e p r e s e n t a t i v e of n a t u r a l p r e d a t i o n . The C r e n i c i c h l a used i n Exp. 6.1 (day-active) a p p a r e n t l y had not been i n c a p t i v i t y l o n g enough to l e a r n t h a t i n the absence of a shallow r e f u g e , guppies are more a c c e s s i b l e a t n i g h t . T h i s r a i s e s the q u e s t i o n of why l a b o r a t o r y stocks of P e t i t e Curucaye guppies were the l e a s t v u l n e r a b l e under con-d i t i o n s of n o c t u r n a l p r e d a t i o n . F i r s t i t i s important to r e c a l l than i n nature, the founders of t h i s stock s u f f e r from severe R i v u l u s p r e d a t i o n . A l s o , i n the P e t i t e Curucaye R., R i v u l u s normally feeds under dim l i g h t or dark c o n d i t i o n s ; u n l e s s c o n d i t i o n e d otherwise, i t does the same i n the l a b o r a t o r y . Since i t i s a s m a l l p r e d a t o r , the shore o f f e r s no refuge f o r guppies. Thus h a b i t a t s e l e c t i o n i s i n e f f e c t u a l f o r a v o i d i n g R i v u l u s a t n i g h t and other s u r v i v a l s t r a t e g i e s are r e q u i r e d . Though I am not c e r t a i n what these are, the evidence from Exp. 6.2 suggests P e t i t e Curucaye guppies may have s u p e r i o r s c o t o p i c v i s i o n . T h i s c o u l d be a f u n c t i o n of s e v e r a l f a c t o r s i n c l u d i n g an i n c r e a s e i n the l e n s diameter or number of rod c e l l s . T h i s might be a t the expense of v i s u a l a c u i t y (cone v i s i o n ) but i n a very s m a l l stream such as P e t i t e Curucaye, t h i s may not be too c r i t i c a l . As i n o t h e r n o c t u r n a l f i s h , the a c o u s t i c o - l a t e r a l i s system may a l s o be b e t t e r developed. 169 The h y p o t h e s i s t h a t i n t e n s e R i v u l u s p r e d a t i o n i n a c o n f i n e d environment has been r e s p o n s i b l e f o r the micro-e v o l u t i o n of s p e c i a l i z e d b e h a v i o r a l and sensory a d a p t a t i o n s demands a d d i t i o n a l experimental t e s t i n g . I p l a n t o pursue t h i s i n the f u t u r e , l o o k i n g i n d e t a i l a t the p h o t o b e h a v i o r a l and anatomical f e a t u r e s of t h i s p o p u l a t i o n . Returning to t h e , o v e r a l l r e s u l t s of the s u r v i v a l experiments with l a b o r a t o r y s t o c k s , i t may be concluded t h a t when the v i s i o n of guppies i s not impaired by very low l i g h t l e v e l s , Lower A r i p o and Guayamare stocks are l e s s v u l n e r a b l e to C r e n i c i c h l a than Upper A r i p o and P a r i a s t o c k s . Since these d i f f e r e n c e s p e r s i s t e d i n p r e d a t o r - n a i v e stocks t h a t had been bred f o r s e v e r a l g e n e r a t i o n s under i d e n t i c a l l a b o r a t o r y c o n d i t i o n s , they are almost c e r t a i n l y g e n e t i c a l l y determined. Geographic v a r i a t i o n i n the a n t i p r e d a t o r  behavior of l a b o r a t o r y s t o c k s The o v e r a l l q u e s t i o n i n t h i s s e c t i o n was prompted by 4 p r e v i o u s f i n d i n g s : 1. B e h a v i o r a l d i f f e r e n c e s i n a n t i p r e d a t o r behavior occur i n n a t u r a l p o p u l a t i o n s ( o b s e r v a t i o n a l e v i d e n c e ) . 2. D i f f e r e n c e s i n v u l n e r a b i l i t y to p r e d a t o r s occur among samples taken from n a t u r a l p o p u l a t i o n s (Exp. 6.1). 3. D i f f e r e n c e s i n v u l n e r a b i l i t y to p r e d a t o r s occur among samples taken from l a b o r a t o r y stocks (Exp. 6.2). 4. The r e s u l t s of (2) and (3) are p o s i t i v e l y 170 c o r r e l a t e d (except f o r P e t i t e Curucaye). These r e s u l t s r a i s e the q u e s t i o n : What b e h a v i o r a l d i f f e r e n c e s occur among l a b o r a t o r y stocks? In e f f e c t , t h i s i s a s k i n g what d i f f e r e n c e s have been i n h e r i t e d from the wild-caught founders of the l a b o r a t o r y s t o c k s . To answer t h i s q u e s t i o n I made d e t a i l e d measurements of the b e h a v i o r a l parameters t h a t showed geographic v a r i a -t i o n i n na t u r e : (a) m i c r o h a b i t a t s e l e c t i o n , (b) r e a c t i o n d i s t a n c e t o p r e d a t o r s , (c) escape motor p a t t e r n s of i n d i v i d u a l f i s h , (d) s c h o o l i n g behavior. (a) M i c r o h a b i t a t s e l e c t i o n The f i e l d o b s e r v a t i o n s have shown t h a t p o p u l a t i o n s of guppies t h a t are exposed to heavy p r e d a t i o n by c h a r a c i d and c i c h l i d f i s h are more r e s t r i c t e d t o the s h o r e l i n e . In s e c t i o n s o f stream with deep water and low to moderate v e l o c i t y , the f i s h who do leave the s h o r e l i n e occur near the s u r f a c e . I wished t o t e s t i f th e r e are a l s o d i f f e r e n c e s among the 5 l a b o r a t o r y stocks i n t h e i r tendency t o remain c l o s e to the shore, or near the s u r f a c e , or both. Since movement up a shore g r a d i e n t i s a l s o movement towards the water s u r f a c e , i t was necessary to separate these 2 f a c t o r s i n separate experiments, one wit h and one without a shore depth g r a d i e n t . Based on the f i e l d o b s e r v a t i o n s (Table 19), I expected t h a t P a r i a , Upper A r i p o , and P e t i t e Curucaye would 171 spend l e s s time near the shore edge and more time i n deeper water than e i t h e r Lower A r i p o or Guayamare. Since i n nature, Upper A r i p o and P e t i t e Curucaye showed l e s s d i s t i n c t s preading a c r o s s the stream and l e s s adhesion to the sub-stratum, I p r e d i c t e d these stocks would f a l l i n t e r m e d i a t e between P a r i a and Lower Aripo/Guayamare. Experiment 6•3 Depth p r e f e r e n c e of 5 l a b o r a t o r y stocks of guppies. Methods In t h i s experiment I measured the depth of spon-taneous swimming and e x p l o r a t o r y b ehavior. A 200 l i t e r aquarium (bare except f o r a sand s u b s t r a t e ) was f i l l e d to a depth of 35 cm. The water column was d i v i d e d i n t o 5 depth s t r a t a of 7 cm each by a g r i d on the f r o n t pane of the tank. Observations were made from 1 m i n f r o n t of the tank w i t h the viewing l e v e l a cross from the middle stratum ( e r r o r due to p a r a l l a x was s m a l l ) . The procedure c o n s i s t e d of r e c o r d i n g the p o s i t i o n of each f i s h w i t h r e f e r e n c e t o the 5 s t r a t a . Groups of 10 f i s h per stock (5 males and 5 females of uniform s i z e ) were used a t a time. They were s e l e c t e d a t random from stock a q u a r i a , p l a c e d i n a p l a s t i c basket i n the t e s t tank f o r 10 min and then r e l e a s e d ( p r i o r to s e l e c t i o n a l l s u b j e c t s were f e d ) . A f t e r an a d d i t i o n a l 5 min of f r e e swimming i n the tank, the p o s i t i o n of each animal was recorded a t 1-minute i n t e r v a l s 172 f o r a t o t a l of 30 min. The order of t e s t i n g f o r each stock was randomized. F i v e r e p l i c a t e s were completed. Thus 250 f i s h (5 stocks x 5 r e p l i c a t e s per stock x 10 f i s h per r e p l i -cate) were used i n t h i s experiment w i t h 30 o b s e r v a t i o n s per f i s h . R e s u l t s The r e s u l t s are shown as a depth p r o f i l e from the s u r f a c e stratum to the bottom (Figure 19). Even wi t h c a s u a l a n a l y s i s i t i s c l e a r t h a t the 5 stocks d i d not spend equal time a t the same depth. The data were t r e a t e d s t a t i s t i c a l l y by comparing the frequency of occurrence of f i s h i n e i t h e r the top or bottom depth stratum. Each stock was compared wi t h the other 4 u s i n g the Mann-Whitney U T e s t ; the r e s u l t s are shown as a matrix of p r o b a b i l i t i e s (Table 23). Where p a i r s of stocks were not p r e d i c t e d to d i f f e r (Lower A r i p o vs. Guayamare; P e t i t e Curucaye vs. Upper A r i p o ) , t w o - t a i l e d p r o b a b i l i t i e s are g i v e n ; a l l other t e s t s are o n e - t a i l e d . The r e s u l t s r e v e a l t h a t P a r i a had a d e f i n i t e p r e f e r -ence f o r the bottom (deep water); t h i s was s i g n i f i c a n t l y d i f f e r e n t from the other 4 s t o c k s . Lower A r i p o spent s i g n i f i c a n t l y more time near the s u r f a c e than Upper A r i p o but no d i f f e r e n c e was observed f o r the bottom stratum. P e t i t e Curucaye spent s i g n i f i c a n t l y more time near the bottom than e i t h e r Lower or Upper A r i p o . A l l other compari-sons showed d i f f e r e n c e s t h a t c o u l d have o c c u r r e d by chance. 173a FIGURE 19. Depth p r o f i l e of spontaneous swimming behavior of 5 l a b o r a t o r y stocks of guppies. Refer t o Table 23 f o r s t a t i s t i c a l a n a l y s i s . 173 b 0 20 40 60 80 100 120 140 160 180 0 20 40 60 80 100 120 140 160 180 MEAN FREQUENCY/30 min. 174 TABLE 23. A comparison of the frequency of occurrence of 5 stocks o f guppies i n r e l a t i o n t o water depth. Values i n the matrix are p r o b a b i l i t i e s f o r each p a i r o f comparisons (Mann-Whitney U T e s t ) . A. Top depth stratum H 1:LA/Guay >PCur/UA>Par Mean frequency LA Guay PCur UA Par Lower A r i p o 81.4 -Guayamare 63.6 . 42 -P. Curucaye 46.2 .075 .345 Upper A r i p o 41.8 .048 .421 1.00 P a r i a 10.8 .004 .004 .004 .048 -B. Bottom depth stratum H,:LA/Guay<PCur/UA<Par Mean frequency LA Guay PCur UA Par Lower A r i p o 77.8 -Guayamare 123.2 .15 -P. Curucaye 133.2 .048 .345 -Upper A r i p o 96.2 .500 .210 .008 -P a r i a 183.2 .004 .016 .016 .004 -175 When both the upper and lower stratum comparisons are viewed t o g e t h e r , i t i s c l e a r the P a r i a i s d i s t i n c t from the o t h e r 4 s t o c k s . Lower A r i p o and Guayamare do move f r e e l y i n a l l depth s t r a t a but spend a c o n s i d e r a b l e time near the s u r f a c e . The other stocks f a l l i n t o an i n t e r m e d i a t e category. These r e s u l t s support the p r e d i c t i o n s r a t h e r w e l l and suggest t h a t even i n the absence of a shore g r a d i e n t , water c u r r e n t , or p r e d a t o r s , some of the d i f f e r e n c e s observed i n nature p e r s i s t i n l a b o r a t o r y s t o c k s . A s i m i l a r depth measurement made 24 hr a f t e r the i n t r o d u c t i o n of the f i s h to the t e s t tank showed no stock d i f f e r e n c e s . Most f i s h were near the bottom f e e d i n g on the s u b s t r a t e . I t appears t h e r e f o r e , t h a t the depth p r e f e r e n c e s are important when f i s h are i n a "wary s t a t e " , as was the case when they were taken from t h e i r f a m i l i a r h o l d i n g tanks and i n t r o d u c e d i n t o the t e s t tank. Since a l l 5 stocks were handled i d e n t i c a l l y , the d i f f e r e n c e s observed i n the f i r s t 30 min i n the new environment c o u l d not have been due to d i f f e r e n t i a l s t r e s s a p p l i e d by the experimenter, but must have r e f l e c t e d a r e a l d i f f e r e n c e i n the p o r t i o n of the h a b i t a t t h a t was e x p l o r e d . Presumably once no r e a l "danger" was found, the responses waned and the main determinant of h a b i t a t c h o i c e became the r e g i o n of food abundance. No f e e d i n g behavior was observed d u r i n g the f i r s t 30-minute t e s t f o r any of the 250 f i s h . A l s o there was no d i f f e r e n c e between the sexes i n depth p r e f e r e n c e . 176 Experiment 6.4 Shore p r e f e r e n c e of 5 l a b o r a t o r y stocks of guppies. In t h i s experiment I wanted to determine i f there were any stock d i f f e r e n c e s i n the choice of h a b i t a t , g i v e n a g r a d i e n t from deep water to dry l a n d . Because f i s h are p l a c e d i n a wary c o n d i t i o n when they are t r a n s f e r r e d to a new (and p o s s i b l y dangerous) environment, i t might be expected t h a t stocks d e r i v e d from p o p u l a t i o n s t h a t are found r e s t r i c t e d t o the stream edge (see Table 19) may show a h a b i t a t p r e f e r e n c e even i n the absence of experience w i t h p r e d a t o r s . ( P r e l i m i n a r y t e s t s had shown t h a t guppies q u i c k l y l e a r n t o remain i n s a f e r e g i o n s of an aquarium when exposed to predators.) Methods I t e s t e d the 5 stocks i n a s i m u l a t e d c r o s s - s e c t i o n of a stream (tank dimensions: L = 240, W = 58, D = 30 cm). One s i d e of the tank had a stream "bank" made of r o c k s , g r a v e l , and sand. T h i s bank was formed to p r o v i d e a smooth depth g r a d i e n t from 2 0 to 0 cm. T h i s g r a d i e n t s e c t i o n was d i v i d e d from the r e s t of the aquarium by a t i g h t - f i t t i n g c l e a r g l a s s p a r t i t i o n . T e s t s w i t h guppies were done on the "shore" s i d e of the p a r t i t i o n w h i l e the deep s i d e had a predator complement of 2 C r e n i c i c h l a , 4 Aequidens, and 3 Astyanax. An opaque cover was p l a c e d over the c l e a r g l a s s p a r t i t i o n to v i s u a l l y i s o l a t e the p r e d a t o r s from the prey. On the prey s i d e of the p a r t i t i o n , a r e f e r e n c e g r i d 177 was marked on the f r o n t pane; f o u r 20 cm s e c t i o n s were d e l i n e a t e d with s e c t i o n 1 as the deep zone next to the p a r t i t i o n (and a l s o the p r e d a t o r s ) , s e c t i o n s 2 and 3 as the t r a n s i t i o n a l zones from deep to shallow water, and s e c t i o n 4 as the s h a l l o w e s t s e c t i o n t e r m i n a t i n g a t the s h o r e l i n e . For each t e s t , 5 males and 5 females were s e l e c t e d a t random from stock tanks. I attempted to keep the s i z e s of the f i s h i n each group as s i m i l a r as p o s s i b l e . The f i s h were f i r s t p l a c e d i n a f l o a t i n g basket f o r 10 min and then r e l e a s e d i n t o s e c t i o n 1 (deep p o r t i o n ) . The f i s h were allowed another 5 min to move about the tank; f o l l o w i n g t h i s t h e i r p o s i t i o n was recorded every minute f o r 30 min (Test A ) . Another 30-minute r e c o r d i n g was made 6 hr l a t e r (Test B). A f t e r t h i s the opaque p a r t i t i o n was removed so t h a t the pr e d a t o r s and guppies were i n v i s u a l communication. The f i s h were allowed a 5-minute " s e t t l i n g down" p e r i o d and then another 30-minute r e c o r d i n g was made (Test C ). A f i n a l 30-minute o b s e r v a t i o n was made 24 hr a f t e r the removal of the cover (Test D). Th e r e f o r e , 1 hour of data was c o l l e c t e d on the p o s i t i o n s of the guppies i n i s o l a t i o n and 1 hour w i t h o n l y a c l e a r g l a s s p a r t i t i o n s e p a r a t i n g predator and prey. A l l sp e c i e s of pr e d a t o r s made numerous approaches to the p a r t i -t i o n once the cover was removed. Thus s e c t i o n s 1 and 2 were c l o s e s t t o the pr e d a t o r s and guppies i n these s e c t i o n s encountered a v i s u a l t h r e a t from the p r e d a t o r s . 178 A l l 5 stocks were t e s t e d i n the same manner with 2 r e p l i c a t e s per stock ( t o t a l o f 100 f i s h used). R e s u l t s The r e s u l t s are presented as the mean frequency d i s t r i b u t i o n along the depth g r a d i e n t f o r each stock d u r i n g each of the 4 t e s t s (Figure 20). Although the r e p l i c a t i o n was i n s u f f i c i e n t to permit a s t a t i s t i c a l comparison among the 5 s t o c k s , some noteworthy trends were e v i d e n t . In T e s t A ( i n i t i a l h a b i t a t c h o i c e ) , P a r i a and Lower A r i p o showed o p p o s i t e responses: P a r i a f i s h remained mainly i n the deep s e c t i o n (1) whereas Lower A r i p o f i s h spent v i r t u -a l l y a l l t h e i r time i n the extreme shallow zone (4). The o t h e r 3 s t o c k s showed i n t e r m e d i a t e responses with l e s s c l e a r - c u t p r e f e r e n c e . By T e s t B the f i s h had a l r e a d y spent 6 hr e x p l o r i n g the new environment and were now l e s s wary. There was an o v e r a l l s h i f t away from the shore by a l l s t o c k s . T h i s was most e v i d e n t f o r Lower A r i p o . T e s t s C and D i n c o r p o r a t e d a second component i n t o the h a b i t a t s e l e c t i o n of guppies: the v i s u a l presence of p r e d a t o r s . Though a l l guppies t h a t were used had never been exposed t o p r e d a t o r s b e f o r e , they a l l showed a major s h i f t t o shallower water ( i . e . away from the p r e d a t o r s ) . Of the 10 groups of f i s h t e s t e d (2 per s t o c k ) , 8 moved to shallower water when the opaque cover was removed. Two groups (1 Lower A r i p o and 1 P e t i t e Curucaye) moved to 179a FIGURE 20. H a b i t a t s e l e c t i o n of 5 l a b o r a t o r y stocks of guppies p l a c e d i n a depth g r a d i e n t . Refer to t e x t f o r d e t a i l s . 179 b 180 s l i g h t l y deeper water but these f i s h were a l r e a d y r e s t r i c t e d to s e c t i o n s 3 and 4 bef o r e the pr e d a t o r s were made v i s i b l e , i . e . they were a l r e a d y i n a " s a f e " zone. I t i s noteworthy t h a t although Upper A r i p o and P a r i a f i s h d i d e x h i b i t a s h i f t towards the shore once the preda-t o r s were exposed, they n e v e r t h e l e s s spent c o n s i d e r a b l e time i n s e c t i o n s 1 and 2 (mean o b s e r v a t i o n s per f i s h i n s e c t i o n s 1 and 2: Upper A r i p o = 12.9; P a r i a = 12.4). T h i s i s i n sharp c o n t r a s t to the other 3 stocks (Lower A r i p o = 1.2; Guayamare = 1.8; P e t i t e Curucaye = 0 . 9 ) . By T e s t D (24 hr of exposure t o p r e d a t o r s ) , 8 out of the 10 groups s h i f t e d to deeper water again. Both Upper A r i p o r e p l i c a t e s , however, moved t o shallower water. T h i s cannot be e x p l a i n e d by d i f f e r e n c e s i n pre d a t o r a c t i v i t y s i n c e I a l s o had a complete r e c o r d o f the p o s i t i o n o f each p r e d a t o r r e l a t i v e to the p a r t i t i o n (these were made every 30 sec a f t e r r e c o r d i n g the guppies' p o s i t i o n s ) . The pred a t o r r e c o r d s i n d i c a t e a normal number of v i s i t s to the area w i t h i n 30 cm of the p a r t i t i o n . Since the Upper A r i p o d i f f e r e n c e between T e s t C and D was s m a l l , i t i s safe t o conclude t h a t a f t e r 24 hr exposure t o pr e d a t o r s without n e g a t i v e r e i n f o r c e m e n t ( e x c e p t . o c c a s i o n a l a t t a c k s by C r e n i c i c h l a which terminated when the pre d a t o r s t r u c k the g l a s s p a r t i t i o n ) , the guppies became h a b i t u a t e d to the pre d a t o r s and were l e s s r e s t r i c t e d t o the s h o r e l i n e . How-ever, they d i d remain c l o s e r t o the shore than b e f o r e the 181 p r e d a t o r s were v i s i b l e (Test B). T h i s suggests t h a t the presence of l a r g e moving o b j e c t s e l i c i t s an avoidance response t h a t i s not c o n t i n g e n t upon a c t u a l a t t a c k s . Some c a s u a l o b s e r v a t i o n s made up to 1 week a f t e r exposure to pr e d a t o r s r e v e a l e d t h a t the h a b i t a t d i s t r i b u t i o n observed a f t e r 24 hr of exposure was s t a b l e . Even the P a r i a f i s h moved onl y s l i g h t l y c l o s e r t o the p a r t i t i o n . For a l l s t o c k s , the d i s t r i b u t i o n observed a t T e s t B d i d not reappear. The main c o n c l u s i o n from t h i s experiment i s t h a t Lower A r i p o and P a r i a guppies have d i f f e r e n t h a b i t a t p r e f e r -ences. The shore adhesion of Lower A r i p o i s very r e m i n i s c e n t of the behavior seen under n a t u r a l c o n d i t i o n s , suggesting again t h a t g e n e t i c f a c t o r s may be important i n determining these d i f f e r e n c e s . The absence of a w e l l - d e f i n e d h a b i t a t response i n Guayamare guppies i s c u r i o u s s i n c e i n nature these f i s h are a l s o found very c l o s e to shore. The Guayamare h a b i t a t does d i f f e r from Lower A r i p o i n 2 main ways: the water i s t u r b i d and i n most p l a c e s there i s no g e n t l e g r a d i e n t t o deep w a t e r — t h e d r o p - o f f i s sudden. With the appearance of pr e d a t o r s (Tests C and D), other b e h a v i o r a l components come i n t o p l a y to modify the i n i t i a l h a b i t a t c h o i c e ( r e a c t i o n d i s t a n c e and h a b i t u a t i o n to p r e d a t o r s ) . The responses of the d i f f e r e n t stocks t o the appearance o f the pr e d a t o r s g i v e s the f i r s t h i n t t h a t the presence of pr e d a t o r s does not have the same impact on the behavior of a l l stocks of guppies. In t h i s experimental 182 d e s i g n , however, there were too many v a r i a b l e s and not enough r e p l i c a t e s to be c e r t a i n of t h i s d i f f e r e n c e . The f o l l o w i n g experiment was designed to more a c c u r a t e l y measure the b e h a v i o r a l responses to a p r e d a t o r . (b) Reaction d i s t a n c e to p r e d a t o r s  Experiment 6.5 Reaction d i s t a n c e of 5 l a b o r a t o r y stocks of guppies to a p r e d a t o r . I t has been p o i n t e d out t h a t geographic v a r i a t i o n i n the r e a c t i o n d i s t a n c e to t h r e a t e n i n g s t i m u l i i s c o r r e l a t e d w i t h p r e d a t i o n . F i s h from streams which have few p r e d a t o r s respond a t a s m a l l e r d i s t a n c e than those exposed to i n t e n s e p r e d a t i o n . The aim of t h i s experiment was to determine i f these d i f f e r e n c e s are r e t a i n e d i n p r e d a t o r - n a i v e l a b o r a t o r y s t o c k s . I f d i f f e r e n c e s observed i n the f i e l d are the product of d i f f e r e n t i a l experience w i t h p r e d a t o r s , no d i f f e r e n c e s should appear i n these t e s t s ( n u l l h y p o t h e s i s ) . I f g e n e t i c f a c t o r s are i n v o l v e d , the l a b o r a t o r y stocks should resemble to a c e r t a i n e x t e n t the responsiveness of the w i l d founders, i . e . r e a c t i o n d i s t a n c e of Lower A r i p o / Guayamare>Petite Curucaye/Upper A r i p o > P a r i a . ( R e c a l l the l a t t e r c o u l d be caught by hand i n the f i e l d . ) Methods The experiment was conducted i n a long shallow tank f i l l e d to a depth of 8.0 cm (tank dimensions same as i n Exp. 6.4). The bottom was covered with l i g h t brown sand. I l l u m i n a t i o n was even over the e n t i r e l e n g t h of the tank 183 (40-watt c o o l white f l u o r e s c e n t tubes mounted 50 cm above the water). In one end of the tank I p l a c e d a s m a l l 21 l i t e r g l a s s aquarium c o n t a i n i n g a preserved 190 mm C r e n i c i c h l a (same i n d i v i d u a l as i n photograph, F i g u r e 4). T h i s "predator" was suspended by monofilament so t h a t i t hung i n midwater. T h i s was arranged on a p u l l e y system which made i t p o s s i b l e to e i t h e r suspend the f i s h m o t i o n l e s s i n midwater, or to g i v e i t an up-and-down move-ment manually (amplitude = 2 cm, 1 c y c l e per s e c ) . At the o t h e r end of the long o u t e r tank, a bottomless white p l a s t i c p a i l was connected to a separate second p u l l e y system. T h i s p a i l was pushed i n t o the sand and h e l d the t e s t f i s h b efore i t was r e l e a s e d i n t o the tank f o r an "encounter" w i t h the p r e d a t o r . The d i s t a n c e of r e l e a s e was 200 cm d i r e c t l y i n f r o n t of the p r e d a t o r . With these 2 p u l l e y systems, t h e r e f o r e , i t was p o s s i b l e t o r e l e a s e the prey, measure the r e a c t i o n d i s t a n c e , and "animate" the p r e d a t o r , a l l from a p o s i t i o n d i r e c t l y i n f r o n t of the p r e d a t o r . I t should be noted t h a t the p r e d a t o r was p r e s e r v e d i n f o r m a l i n but r e t a i n e d most of the markings and c o l o u r of a l i v e f i s h . T e s t s w i t h a l i v e p r e d a t o r proved u n f e a s i b l e because of the h i g h degree of v a r i a b i l i t y i n the b e h a v i o r of the p r e d a t o r and i t s d i s t a n c e from the prey. For t h i s reason I r e s o r t e d to the use of a p r e s e r v e d preda-t o r because i t s "behavior" and d i s t a n c e to the prey c o u l d be 184 r i g i d l y c o n t r o l l e d . To i n s u r e t h a t a l l f i s h had i d e n t i c a l p r e - t e s t experience, the f o l l o w i n g procedure was s t r i c t l y e n f o r c e d . Twenty-four hours before a t e s t , l o t s were drawn to d e t e r -mine which of the 5 s t o c k s would be t e s t e d . Ten f i s h (5 males and 5 females) o f t h i s stock were subsequently s e l e c t e d a t random, measured, and p l a c e d i n i n d i v i d u a l , numbered g l a s s beakers with 300 ml of water. A s m a l l f l o a t i n g C e r a t o p t e r i s p l a n t was added to each beaker. For the t e s t , each i n d i v i d u a l i n t u r n was p l a c e d i n the opaque p r e - r e l e a s e p a i l f o r 5 min. A f t e r t h i s I moved to the o b s e r v a t i o n area i n f r o n t of the p r e d a t o r and g e n t l y p u l l e d the p a i l up and out of the water with the p u l l e y , thereby r e l e a s i n g the t e s t f i s h . In the m a j o r i t y of cases, the f i s h immediately began e x p l o r i n g the l a r g e tank ( e x p l o r a t i o n area = 56 x 216 cm). During t h i s e x p l o r a t o r y p e r i o d , the f i s h would e v e n t u a l l y swim to the other end of the tank where the p r e d a t o r was l o c a t e d . In most cases, the f i s h p e r c e i v e d the p r e d a t o r and gave an avoidance response (one of the 6 types d e s c r i b e d p r e v i o u s l y ) . The d i s t a n c e from the p r e d a t o r (to n e a r e s t 1 cm), b e h a v i o r , and time s i n c e r e l e a s e were recorded f o r t h i s response. As soon as the response had been e l i c i t e d , the f i s h was removed and r e t u r n e d to i t s beaker and the next f i s h was t e s t e d i n the same manner. C e r t a i n f i s h d i d not respond to the p r e d a t o r but 185 swam r i g h t up to the g l a s s and moved along or up and down i n f r o n t of the p r e d a t o r ; t h i s was recorded as "no response". A few f i s h f a i l e d to swim c l o s e enough to the p r e d a t o r i n the maximum time a l l o t m e n t of 15 min. These f i s h were r e t e s t e d 24 hr l a t e r . Each group of 10 f i s h c o n s t i t u t e d a " s e t " f o r t h a t t e s t day. A t o t a l of 5 s e t s were completed f o r each of the 5 s t o c k s ( t o t a l of 250 f i s h used). Two t e s t s were conducted on each f i s h : the f i r s t w ith the p r e d a t o r m o t i o n l e s s and the second w i t h the p r e d a t o r moving i n the f a s h i o n d e s c r i b e d above. The f i r s t t e s t (A) was completed f o r 1 s e t of each of the 5 stocks b e f o r e they were r e t e s t e d with the moving p r e d a t o r . The same order of t e s t i n g was f o l l o w e d i n t e s t B as i n A, thus a s s u r i n g t h a t each f i s h had equal r e s i d e n c y i n the beakers between t e s t s . Each i n d i v i d u a l t h e r e f o r e , was t e s t e d t w i c e , once at t e s t A and once at t e s t B. A f t e r t h i s they were d i s c a r d e d . R e s u l t s The r e s u l t s of the experiment i n terms of the r e a c t i o n d i s t a n c e to the p r e d a t o r are g i v e n i n F i g u r e 21. Non-overlap of the 95% confidence l i m i t s i n d i c a t e s a s i g n i -f i c a n t d i f f e r e n c e between any 2 p a i r s compared (see Eberhardt, 1968 f o r a u s e f u l d i s c u s s i o n of t h i s p r o c e d u r e ) . In t e s t A (predator s t i l l ) t h ere was a s i g n i f i c a n t d i f f e r e n c e between Lower A r i p o and each o f : Guayamare, P a r i a , and P e t i t e Curucaye. Upper A r i p o showed a s i g n i f i c a n t l y l a r g e r 186a FIGURE 21. Reaction d i s t a n c e of 5 l a b o r a t o r y stocks of guppies t o a dead C r e n i c i c h l a . (a) p r e d a t o r not.moving; (b) p r e d a t o r moving. V e r t i c a l l i n e shows the mean, h o r i z o n t a l l i n e the t o t a l range, b l a c k r e c t a n g l e the 95% con f i d e n c e l i m i t s . LOWER ARIPO UPPER ARIPO GUAYAMARE PARIA PETITE CURUCAYE LOWER ARIPO UPPER ARIPO GUAYAMARE PARIA PETITE CURUCAYE a. i — i — i — i — i — i — i — i — i — i — i — i — i — i — i — i 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 b. I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 REACTION DISTANCE TO PREDATOR (cm) 187 r e a c t i o n d i s t a n c e than P e t i t e Curucaye and P a r i a . When the predator was moving ( t e s t B) the same rank order p e r s i s t e d ( p e r f e c t rank c o r r e l a t i o n : r c = +1.00, p = .01, o n e - t a i l e d t e s t ) . The r e a c t i o n d i s t a n c e s were approximately doubled w i t h the a d d i t i o n of movement. (A non-parametric t e s t o f r e l i a b i l i t y f o r each of the 250 f i s h i n t e s t A and B r e v e a l e d t h a t i n s p i t e of the i n d i v i d u a l v a r i a b i l i t y i n r e a c t i o n d i s t a n c e , the method of measurement was a r e l i a b l e i n d i c a t o r of an i n d i v i d u a l ' s r e a c t i v i t y 2 (X ^ d ^ = 4.07, p < .05), i . e . an i n d i v i d u a l t h a t responded at a d i s t a n c e g r e a t e r than the median ( f o r i t s r e s p e c t i v e stock) a t t e s t A, was l i k e l y to respond s i m i l a r l y when r e t e s t e d s e v e r a l days l a t e r at t e s t B.) In t e s t B, Lower A r i p o f i s h were s i g n i f i c a n t l y d i f f e r e n t from a l l the ot h e r s t o c k s . Upper A r i p o , Guayamare, and P a r i a d i d not d i f f e r s i g n i f i c a n t l y from each o t h e r but Upper A r i p o and Guayamare were s i g n i f i c a n t l y d i f f e r e n t from P e t i t e Curucaye. With the e x c e p t i o n of Lower A r i p o and P a r i a , these r e s u l t s do not f i t the p r e d i c t i o n s too c l o s e l y (Guayamare and P e t i t e Curucaye had s h o r t e r r e a c t i o n d i s t a n c e s than expected and Upper A r i p o had a g r e a t e r r e a c t i o n d i s t a n c e ) . I t i s u n l i k e l y t h a t t h i s was due to experimental e r r o r because the sample s i z e was l a r g e and the d i s t a n c e measure-ments were a c c u r a t e . I t i s more probable t h a t experience w i t h p r e d a t o r s enhances the r e a c t i o n d i s t a n c e of Guayamare 188 f i s h r e l a t i v e l y more than Upper A r i p o . I t would be i n t e r -e s t i n g to measure the r e a c t i o n d i s t a n c e of Guayamare vs. Upper A r i p o a f t e r prolonged exposure to a l i v e C r e n i c i c h l a . R e l a t i v e to Lower A r i p o , the s h o r t e r r e a c t i o n d i s -tances of P e t i t e Curucaye and Guayamare might r e f l e c t h a b i t a t d i f f e r e n c e s ; the s m a l l s i z e of the P e t i t e Curucaye R. and the t u r b i d i t y of the Guayamare R. would presumably make long r e a c t i o n d i s t a n c e s s u p e r f l u o u s . F i n a l l y , r e a c t i o n d i s t a n c e i s an unweighted measure of an animal's avoidance b e h a v i o r . One animal may respond v i g o r o u s l y a t a s h o r t d i s t a n c e whereas another may show m i l d avoidance a t a l a r g e d i s t a n c e . The r e s u l t s of Exp. 6.2 suggest t h a t although Upper A r i p o has a r e a c t i o n d i s t a n c e equal to Guayamare, Guayamare i s l e s s v u l n e r a b l e when exposed to a r e a l C r e n i c i c h l a . I t i s probably s a f e s t to use s e v e r a l measures of a n t i p r e d a t o r b e h a v i o r i n a r r i v i n g at a p r e d i c t i o n of r e l a t i v e v u l n e r a b i l i t y . (c) Escape motor p a t t e r n s of i n d i v i d u a l f i s h Under n a t u r a l c o n d i t i o n s , guppies use a v a r i e t y of d i f f e r e n t escape motor p a t t e r n s to a v o i d p r e d a t o r s . I have s t a t e d b e f o r e t h a t the stimulus s t r e n g t h r e q u i r e d to e l i c i t c e r t a i n of these p a t t e r n s tends to vary with the exposure to p r e d a t o r s . Thus, under the same stimulus c o n d i t i o n s , Lower A r i p o and Guayamare q u i c k l y r e v e r t to r a p i d d a r t s and even s u r f a c e jumps whereas P a r i a guppies may o n l y show a m i l d response. T h i s d i f f e r e n c e was not measured a c c u r a t e l y 189 i n the f i e l d ; f o r t h i s reason i t i s of g r e a t i n t e r e s t t o know i f naive f i s h show d i f f e r e n t responses to the same st i m u l u s . I f d i f f e r e n c e s do occur i n the p r e d i c t e d d i r e c t i o n , i t g i v e s c r e d i b i l i t y t o the hypothesis t h a t m i c r o e v o l u t i o n a r y d i f f e r e n c e s i n behavior occur among n a t u r a l p o p u l a t i o n s . Experiment 6.6 Escape motor p a t t e r n s of 5 l a b o r a t o r y stocks o f guppies exposed to a pre s e r v e d C r e n i c i c h l a . The b e h a v i o r a l responses of the 5 stock s of guppies to a p r e s e r v e d m o t i o n l e s s and moving C r e n i c i c h l a were q u a n t i f i e d a t the same time as the r e a c t i o n d i s t a n c e s were measured (see Exp. 6.5 f o r methods). R e s u l t s The responses of the 250 f i s h under each treatment are t a b u l a t e d i n Table 24. Surface skim was not observed i n any of the 500 t e s t s suggesting t h a t i t i s e l i c i t e d under c o n d i t i o n s of r e a l p u r s u i t . For the r e s t of the p a t t e r n s , i t i s c l e a r t h a t there i s c o n s i d e r a b l e v a r i a t i o n w i t h i n each stock. For t e s t A (predator not moving) the modal response f o r Lower A r i p o , Guayamare, and Upper A r i p o was avoidance d r i f t (72%, 48%, and 76% r e s p e c t i v e l y ) ; P a r i a and P e t i t e Curucaye most o f t e n showed weak avoidance d r i f t (46% and 42% r e s p e c t i v e l y ) . P o o l i n g the r e s u l t s f o r a l l 5 stocks i n t e s t A, e x a c t l y 50% (125/250) of the animals showed avoidance d r i f t . TABLE 24. Escape motor p a t t e r n s of 5 l a b o r a t o r y stocks of guppies exposed to a dead C r e n i c i c h l a . (n = 50 f i s h per stock) STOCK _^ • Behavior p a t t e r n L. A r i p o Guayamare U. A r i p o P a r i a P. Curucaye T o t a l A. Predator not moving no response 0 2 2 6 10 20 weak avoidance d r i f t 6 14 5 23 21 69 avoidance d r i f t 36 24 38 13 14 125 t u r n around 1 4 2 2 1 10 r a p i d d a r t 7 6 2 6 2 23 s u r f a c e jump 0 0 1 0 . 2 . . . .3 T o t a l 50 50 50 50 50 250 B. Predator moving no response 0 0 0 0 0 0 weak avoidance d r i f t 8 12 11 21 24 76 avoidance d r i f t 17 13 . 18 18 15 81 t u r n around 20 17 16 . 11 7 71 r a p i d d a r t 5 5 5 0 3 18 s u r f a c e jump 0 3 0 0 1 4 T o t a l 50 50 .50 50 50 . 250 191 I t i s noteworthy t h a t 8% of the f i s h d i d not respond to the p r e d a t o r a t a l l when i t was m o t i o n l e s s . A l a c k of response was most common i n P e t i t e Curucaye and P a r i a where t h i s group made up 20% and 12% of the sample r e s p e c t i v e l y . In c o n t r a s t , a l l Lower A r i p o guppies responded to the m o tionless p r e d a t o r . In t e s t A no i n t e r - s t o c k trends were e v i d e n t f o r the p a t t e r n s t u r n around, r a p i d d a r t , and s u r f a c e jump because they were e l i c i t e d so i n f r e q u e n t l y . The main i n t e r - s t o c k d i f f e r e n c e was i n the r e l a t i v e p r o p o r t i o n s of animals showing e i t h e r a weak avoidance d r i f t or avoidance d r i f t . In t e s t B (predator moving), a l l animals showed a response and there was a g e n e r a l s h i f t to a more r e a c t i v e type of behavior p a t t e r n . The main e f f e c t of moving the p r e d a t o r was to decrease the number of f i s h showing avoidance d r i f t and i n c r e a s e the number showing the next "higher" category of r e a c t i o n : t u r n around (turn around t e s t A = 10; t e s t B.= 71). There was no d i s c e r n i b l e change i n the r a p i d d a r t and s u r f a c e jump c a t e g o r i e s , s u g g e s t i n g again t h a t a c t u a l p u r s u i t , and not movement alone, t r i g g e r these responses. A t u r n around i n t h i s experiment was s u f f i c i e n t to "escape" from the p r e d a t o r . For Lower A r i p o and Guayamare f i s h , t u r n around was the most common response e l i c i t e d by the moving p r e d a t o r (40% and 34% r e s p e c t i v e l y ) w h ile weak avoidance d r i f t remained the modal p a t t e r n f o r P a r i a and P e t i t e Curucaye (42% and 48% r e s p e c t i v e l y ) . T h e r e f o r e , the onl y e f f e c t of moving the pr e d a t o r on the l a s t 2 stocks was to double the r e a c t i o n d i s t a n c e ; the motor p a t t e r n s remained v i r t u a l l y unchanged. Experiment 6.7 Escape motor p a t t e r n s of 5 l a b o r a t o r y stocks o f guppies exposed to R i v u l u s . I n t r o d u c t i o n Exp. 6.5 and 6.6 have suggested t h a t r e a c t i o n d i s t a n c e and the p r o p o r t i o n of f i s h showing c e r t a i n escape motor p a t t e r n s may be s i g n i f i c a n t measures f o r comparing s t o c k s , even when the f i s h are not a c t i v e l y a t t a c k e d . How-ever, under these c o n d i t i o n s most of the a n t i p r e d a t o r b e h a v i o r i s of an e x p l o r a t i v e or m i l d l y a v o i d i n g nature. These p a t t e r n s c l e a r l y are important as a f i r s t l i n e of defense but would not be.very e f f e c t i v e under r e a l a t t a c k . T h e r e f o r e , I made simultaneous q u a n t i t a t i v e obser-v a t i o n s on the a t t a c k behavior o f R i v u l u s and the a n t i -p r e d a t o r behavior o f the 5 s t o c k s . R i v u l u s i s an i d e a l p r e d a t o r f o r t h i s type of t e s t because i t i s r e l a t i v e l y s mall and a f t e r a p e r i o d of h a n d l i n g and s t a r v a t i o n ( t r a i n i n g period) w i l l a t t a c k prey r e p e a t e d l y u n t i l they are s u c c e s s f u l l y captured. I t should be emphasized t h a t t h i s experiment was an attempt to make comparative measurements of the prey's behavior. T h e r e f o r e , I had to c o n t r o l the behavior o f the pr e d a t o r as much as p o s s i b l e ; hence o n l y a s i n g l e female 193 R i v u l u s (75 mm) was used i n a l l t e s t s . Methods A l l t e s t s were conducted between 1500 and 1945 hr i n a 60 l i t e r g l a s s aquarium f i l l e d t o a depth of 20 cm (tank dimensions: L = 61 cm, W = 32, D = 31). Except f o r a l i g h t - b r o w n sand s u b s t r a t e and a " h i d i n g " rock f o r the p r e d a t o r , the tank was bare. I l l u m i n a t i o n (12 l i g h t - 1 2 dark) was p r o v i d e d by a 40-watt c o o l white f l u o r e s c e n t tube mounted 35 cm over the water s u r f a c e . S i x t y - f i v e male guppies, 13 o f each stock were used. They were a l l p r e d a t o r - n a i v e and ranged from 19.0 to 23.5 mm. Since i t was known from Exp. 5.4 t h a t prey body s i z e i s an important v a r i a b l e i n the capture success of R i v u l u s , g r e a t care was taken to match the stocks f o r body s i z e (x ± SD: Lower A r i p o = 21.7 + 1.07; Guayamare = 21.0 + 1.36; P e t i t e Curucaye = 20.8 ± 1.52; Upper A r i p o = 21.9 + 1.13; P a r i a = 20.8 ± 1.44). The procedure c o n s i s t e d of p l a c i n g a s i n g l e guppy with the p r e d a t o r and r e c o r d i n g a l l the i n t e r a c t i o n s between prey r e l e a s e and prey k i l l . An E s t e r l i n e - A n g u s event r e c o r d e r (see Chapter 2) was used to q u a n t i f y the f o l l o w i n g b e h a v i o r a l measures. Predator behavior Prey behavior 1. t o t a l time, prey r e l e a s e 1. avoidance d r i f t frequency to k i l l 2. r a p i d . d a r t frequency 2. p r e d a t o r o r i e n t a t i o n time 3. s u r f a c e skim frequency 3. approach frequency 4. s u r f a c e jump frequency 4. a t t a c k frequency 5. h i d i n g d u r a t i o n 5. capture 194 Except "predator o r i e n t a t i o n time", a l l these p a t t e r n s have been d e s c r i b e d b e f o r e or are s e l f - e x p l a n a t o r y . Predator o r i e n t a t i o n time i s the t o t a l time spent by the p r e d a t o r o r i e n t i n g and chasing the prey, i ^ e . t o t a l time, prey r e l e a s e to k i l l , minus p e r i o d s of p r e d a t o r i n a c t i v i t y ( r e s t pause, e t c . ) . I t i s a good index of the escape a b i l i t y o f the prey. Before the prey was r e l e a s e d , i t was p l a c e d behind an opaque p a r t i t i o n i n one corner of the aquarium f o r 5 min. Since the p r e d a t o r had been c o n d i t i o n e d to r e c e i v e guppies near the p a r t i t i o n , i t was u s u a l l y swimming i n t h i s v i c i n i t y p r i o r t o prey r e l e a s e . To "give the prey a chance' 1 I c o n f i n e d the pr e d a t o r to i t s h i d i n g p l a c e l o c a t e d 50 cm from the p a r t i t i o n . T h i s i n s u r e d t h a t a l l the guppies had equal time t o r e a c t t o the f i r s t few approaches by the pr e d a t o r . The guppies were presented to the p r e d a t o r a t the r a t e o f 1 per 24 hr; no other food was p r o v i d e d so t h a t the hunger l e v e l was always the same p r i o r t o each t e s t . The sequence of p r e s e n t a t i o n was arranged so t h a t a f i s h of the same stock would be t e s t e d every f i f t h day (e.g. Day 1: UA; Day 2: Par; Day 3: Guay; Day 4: LA; Day 5: PCur; Day 6: UA; e t c . ) . T h i s d e s i g n c a l l e d f o r 65 conse-c u t i v e days of prey p r e s e n t a t i o n to meet the 24-hour i n t e r v a l c r i t e r i o n . U n f o r t u n a t e l y I was unable to conduct a t e s t on 6 days i n t e r s p e r s e d throughout the 65-day 195 schedule. Thus on 6 t e s t s (3 P a r i a , 2 Lower A r i p o , 1 Upper Aripo) the p r e d a t o r had not r e c e i v e d a guppy f o r 48 h r . T h i s d i d not appear to b i a s the r e s u l t s . In 4 t e s t s (3 Lower A r i p o , 1 Guayamare) the p r e d a t o r was unable .to capture the prey even though i t made numerous attempts. On these o c c a s i o n s the r e c o r d i n g s were terminated a t 15 min and the prey d i s c a r d e d . To keep the hunger l e v e l the same f o r the t e s t the f o l l o w i n g day, a s m a l l amount of T u b i f e x was f e d to the p r e d a t o r . R e s u l t s The r e s u l t s of the experiments showed t h a t s e v e r a l of the behavior p a t t e r n s o c c u r r e d so i n f r e q u e n t l y t h a t they were not amenable to s t a t i s t i c a l a n a l y s i s (avoidance d r i f t , s u r f a c e skim, prey h i d i n g ) . The low occurrence of avoidance d r i f t supports the i d e a t h a t i t serves mainly an e x p l o r a t i v e f u n c t i o n . Since the p r e d a t o r d i d not remain motionless or permit the c l o s e approach of the prey, t h i s b e h a v i o r was observed i n only 3 out of the 65 f i s h . F i v e of the measures were analyzed f o r d i f f e r e n c e s between a l l 10 p o s s i b l e p a i r s of stocks (Table 25). The Mann-Whitney U T e s t was used to t e s t the hypothesis t h a t f o r each measure: Lower Aripo/Guayamare>Petite Curucaye/Upper A r i p o > P a r i a . Comparisons among these 3 groups used the o n e - t a i l e d p r o b a b i l i t i e s w h ile comparisons w i t h i n groups (LA vs. Guay; PCur v s . UA) used the t w o - t a i l e d t e s t because no p r e d i c t i o n was made. 196 TABLE 25. Comparison of predatory success and prey escape behavior f o r 5 l a b o r a t o r y stocks of guppies. Values i n the matrix are the s i g n i f i c a n c e l e v e l s f o r each p a i r of comparisons (Mann-Whitney U T e s t ) ; NS = p > .05. Mean frequency LA Guay PCur UA Par A. Frequency of p r e d a t o r approaches, to prey Lower A r i p o 17.8 _ Guayamare 10.9 .05 P. Curucaye 9.7 .001 NS -Upper A r i p o 9.7 .001 NS NS P a r i a 8.7 .001 NS NS NS B. Frequency of p r e d a t o r a t t a c k s a t prey Lower A r i p o 5.7 Guayamare 3.5 .02 P.. .Curucaye 3.1 .001 .05 -Upper A r i p o 2.9 .001 NS NS P a r i a 2.8 .001 .05 NS NS C. Mean pr e d a t o r o r i e n t a t i o n time (sec) Lower A r i p o 64.6 Guayamare 37.7 NS P. Curucaye 27.9 .05 NS -Upper A r i p o 27.1 .025 NS NS P a r i a 20.2 .001 .01 .025 05 D. Frequency of r a p i d d a r t s away from p r e d a t o r Lower A r i p o 13.4 Guayamare 8.7 NS P. Curucaye 7.9 .01 NS Upper A r i p o 7.6 .01 NS NS P a r i a 5.5 .001 .01 .01 01 E. Frequency of s u r f a c e jumps away from p r e d a t o r Lower A r i p o 5.1 _ Guayamare 2.9 NS P. Curucaye 2.2 .025 .05 -Upper A r i p o 2.6 .025 NS NS P a r i a 1.5 .001 .001 .05 01 197 The 4 f i s h t h a t were not caught by the p r e d a t o r were dropped from the a n a l y s i s of the a t t a c k frequency, thus f o r t h i s measure the sample s i z e f o r Lower A r i p o was 10, f o r Guayamare, 12. For a l l other comparisons the U s t a t i s t i c was c a l c u l a t e d f o r n^ = 13, n 2 = 13. The r e s u l t s of t h i s a n a l y s i s r e v e a l t h a t the p r e -d i c t i o n s f i t the observed data r a t h e r c l o s e l y . The main d e v i a t i o n from the expected r e l a t i o n s h i p among the stocks was the d i f f e r e n c e between Guayamare and Lower A r i p o . By a l l measures, Lower A r i p o f i s h showed a s u p e r i o r a b i l i t y t o escape p r e d a t i o n and i n "predator e f f o r t per k i l l " (approach and a t t a c k frequency) were s t a t i s t i c a l l y d i s t i n c t from Guayamare. Though the mean pre d a t o r o r i e n t a t i o n times between Lower A r i p o and Guayamare were not s i g n i f i c a n t , a t r e n d i s apparent (64.6 vs. 37.7 sec r e s p e c t i v e l y ) . No s i g n i f i c a n t d i f f e r e n c e was found between Lower A r i p o and Guayamare f o r the 2 motor p a t t e r n s used i n escape. The d i f f e r e n c e between the means suggests t h a t a s t a t i s t i -c a l l y s i g n i f i c a n t d i f f e r e n c e might have been d e t e c t e d i f a l a r g e r sample s i z e had been used. For the other comparisons, my hypothesis p r e d i c t e d the outcome more c l o s e l y . No d i f f e r e n c e c o u l d be d e t e c t e d between P e t i t e Curucaye and Upper A r i p o f o r any of the 5 m e a s u r e s — t h e mean scores were v i r t u a l l y i d e n t i c a l ( i n sharp c o n t r a s t to the r e a c t i o n d i s t a n c e to the preserved C r e n i c i c h l a , see F i g . 21). 198 As expected, P a r i a guppies showed the p o o r e s t escape a b i l i t y of the 5 s t o c k s ; the p r e d a t o r o r i e n t a t i o n time d i f f e r e d s i g n i f i c a n t l y from the other 4 s t o c k s . T h i s d i f f e r e n c e was r e f l e c t e d i n the low frequency of r a p i d d a r t s and s u r f a c e jumps. The P a r i a f i s h were u s u a l l y approached by the p r e d a t o r to w i t h i n easy s t r i k i n g range. On the average, i t took twice as many a t t a c k s ( s t r i k e s ) t o capture a Lower A r i p o guppy than a P a r i a . The P a r i a f i s h d i d not appear to d a r t away as r a p i d l y once s t r u c k . The Lower A r i p o guppies would thus appear to be more f i t i n s e v e r a l ways: r e a c t i o n d i s t a n c e , escape motor p a t t e r n s , and o v e r a l l endurance. Three Lower A r i p o guppies i n f a c t completely exhausted the p r e d a t o r and were not caught while a f o u r t h endured 14 a t t a c k s b e f o r e i t was captured. As an a i d to v i s u a l i z i n g the d i f f e r e n c e s between the 2 extreme stocks (Lower A r i p o and P a r i a ) , I have p l o t t e d the frequency of r a p i d d a r t s and s u r f a c e jumps a g a i n s t the p r e d a t o r o r i e n t a t i o n time (= s u r v i v a l time, F i g u r e 22). I f I can assume t h a t the behavior of the R i v u l u s was the same f o r these t e s t s (I have no evidence to the c o n t r a r y ) , then the s u r v i v a l value of the behavior i s obvious. These r e s u l t s suggest t h a t p o p u l a t i o n s of guppies t h a t have been s u b j e c t to i n t e n s e s e l e c t i o n by c h a r a c i d and c i c h l i d p r e d a t o r s are a l s o s u p e r i o r i n a v o i d i n g R i v u l u s p r e d a t i o n . Whether t h i s a p p l i e s under n a t u r a l c o n d i t i o n s can o n l y be decided by a t r a n s p l a n t experiment. Conceivably 199a FIGURE 22. R e l a t i o n s h i p of escape motor p a t t e r n s t o s u r v i v a l time i n two l a b o r a t o r y stocks of guppies exposed to R i v u l u s . 199b LOWER ARIPO 301 PARIA O 20H LU 3 < 9 1 Ql O > 0 < LU CD 15 H > °- 1 O § 1 0 -LU O LU I T * 5 CO % 10 2 IT ~ i — i — i — i — i — i — i — r ^ ^ r 30 40 50 60 70 80 90 100 340 0» C O • O O C O O » i—r 10 20 30 40 50 60 70 80 90 100 340 SURVIVAL TIME (sec) 200 the h y p e r r e a c t i v i t y of Lower A r i p o f i s h may a c t u a l l y be a disadvantage i n a R i v u l u s - i n f e s t e d stream (e.g. P e t i t e Curucaye). In c o n t r a s t to the Lower A r i p o s i t u a t i o n , good escape behavior i n R i v u l u s - i n f e s t e d streams does not guarantee immunity simply by a l l o w i n g an animal to reach a r e f u g e — R i v u l u s i s q u i t e capable of pouncing on a guppy i n o n l y a few mm of water (and even on dry l a n d ! ) . C r e n i c i c h l a , H o p l i a s , or Astyanax on the other hand, do not continue to pursue guppies t h a t have eluded them and reached shallow water. Experiment 6.8 Responses of Guayamare and P a r i a guppies to a simulated a e r i a l p r e d a t o r . I n t r o d u c t i o n The behavior p a t t e r n "down-dart" has a l r e a d y been d e s c r i b e d f o r w i l d guppies. T h i s response i s e l i c i t e d by a d i s t u r b a n c e above the water s u r f a c e and may be evoked by e i t h e r the passage of an o b j e c t or i t s shadow over a f i s h . The adaptive value of such behavior i s probably l i n k e d t o the r i s k of p r e d a t i o n from non-aquatic p r e d a t o r s such as b i r d s , b a t s , and perhaps t e r r e s t r i a l animals " f i s h i n g " from shore. I t i s o f t e n s t a t e d (but r a r e l y demonstrated) t h a t the a n t i p r e d a t o r behavior o f an animal i s adapted to the hunting s t r a t e g y of i t s most common pre d a t o r (e.g. K e e n l e y s i d e , 1955). For any g i v e n s p e c i e s or p o p u l a t i o n of prey, the main t h r e a t may stem from one or a few 201 p r e d a t o r s (Larson, 1960; C u r i o , 1970a, 1970b, 1970c) but more commonly a host of p r e d a t o r s with a v a r i e t y of s t r a -t e g i e s (Crane, 1952; Thorpe, 196 3:69; Kruuk, 1964, 1972; Walther, 1969; G h i s e l i n and R i c k l e f s , 1970; Snyder and Snyder, 1971). As an hypothesis i t would be p r e d i c t e d t h a t i f a wide v a r i e t y of p r e d a t o r s (with d i s s i m i l a r h u nting methods) have had a s e l e c t i v e impact oh a prey p o p u l a t i o n , the a n t i -p r e d a t o r response might be q u i t e n o n - s p e c i f i c and a p p l i c a b l e to a l l strange moving o b j e c t s of a p p r o p r i a t e s i z e ( " g e n e r a l i s t " ) . C o n t r a r i w i s e , a response might be very s p e c i f i c i f o n l y one s p e c i e s of p r e d a t o r , or group of s p e c i e s (with s i m i l a r h u nting methods) must be avoided ( " s p e c i a l i s t " ) . The v i r t u a l absence of a q u a t i c p r e d a t o r s from the P a r i a R. p r e s e n t s a unique o p p o r t u n i t y to t e s t f o r the responsiveness t o a e r i a l p r e d a t o r s s i n c e these are presumably the most important i n t h i s environment. I have a l r e a d y d e s c r i b e d d i f f e r e n c e s between w i l d P a r i a and Guayamare guppies i n t h e i r response to an overhead o b j e c t ; I now wish to compare the responses of l a b o r a t o r y stocks of P a r i a and Guayamare f i s h . Methods The experiment was conducted w i t h the apparatus i l l u s t r a t e d i n F i g u r e 23. The aim of the experiment was to q u a n t i f y the response of s i n g l e , i s o l a t e d , n aive f i s h to a 202a FIGURE 23. T e s t apparatus used f o r measuring the responsiveness of l a b o r a t o r y stocks of Guayamare and P a r i a guppies t o a simulated a e r i a l p r e d a t o r . A l l measurements i n cm. FRONT VIEW 6 0 W LIGH T B U L B W A T E R L I N E A B T D E P T H G R I D O N / ONE-WAY A L U M P A P E R TOP VIEW T E S T T A N K ' O U T E R T A N K R E L E A S E L A T C H O P A Q U E D I V I D E R ' J O B S E R V A T I O N ' P O I N T R E L E A S E C A B L E 203 standard a e r i a l , escape-evoking s t i m u l u s . The a e r i a l "predator" was a b l a c k cardboard, bird-shaped model which was a t t a c h e d . t o a c l e a r monofilament l i n e t h a t ran over the e n t i r e l e n g t h of the outer tank a t an angle of 12° (no s p e c i a l s i g n i f i c a n c e i s a t t a c h e d t o the shape of the model). T h i s angle was s u f f i c i e n t to a l l o w the p r e d a t o r t o s l i d e down the l i n e w i t h a uniform v e l o c i t y over the i n n e r tank ( t e s t tank;:—refer to f i g u r e ) . As the p r e d a t o r passed over the t e s t tank, i t produced a shadow about twice i t s s i z e . The d u r a t i o n of the s timulus was 0.5 sec f o r each h a l f o f the t e s t tank ( v e l o c i t y a t t h i s p o i n t was 40 cm/sec). The t e s t tank was e n c l o s e d on the s i d e s and r e a r w i t h b l a c k p l e x i g l a s and was d i v i d e d i n the middle by a b l a c k p l e x i g l a s p a r t i t i o n . The f r o n t of the tank was covered w i t h one-way Alum paper; thus the f i s h were ab l e to see o u t s i d e the t e s t tank o n l y d i r e c t l y upwards. A depth g r i d was marked o f f on the Alum paper i n f i f t e e n 1 cm i n t e r v a l s c o r r e s p o n d i n g to the depth s t r a t a from zero t o 15 cm. A l l t e s t s were conducted i n the evening i n a darkened room. Only a 60 watt incandescent bulb d i r e c t l y over the mid-point of the t e s t tank p r o v i d e d i l l u m i n a t i o n . Water temperature was h e l d constant a t 25.0 C. Twenty male Guayamare and P a r i a guppies were s e l e c t e d randomly from stock a q u a r i a and assigned to 204 i n d i v i d u a l beakers c o n t a i n i n g 300 ml of water and a s m a l l S a g i t t a r i a p l a n t . A l l f i s h were measured to assure s i z e homogeneity (Guayamare: x = 17.7 mm ± 0.95 SD; P a r i a : x = 18.0 mm ± 0.72 SD). F i s h were t e s t e d i n a p a i r e d design which was randomized w i t h r e s p e c t to the i n d i v i d u a l of P a r i a t h a t was p a i r e d w i t h the i n d i v i d u a l of Guayamare. The compartment (A or B) was d e c i d e d by f l i p p i n g a c o i n . P a r i a was i n A nine times, Guayamare 11 times. F i v e p a i r s were t e s t e d on each n i g h t f o r 4 c o n s e c u t i v e n i g h t s ( t o t a l of 20 p a i r s ) . Each f i s h was p l a c e d i n i t s r e s p e c t i v e compartment (A or B) f o r 15 min; by t h i s time the guppies were always a c t i v e l y swimming about the compartment. A t r i a l was s t a r t e d as soon as both f i s h ( v i s u a l l y i s o l a t e d from each other) were above the 10 cm g r i d mark ( i . e . i n the top two-t h i r d s of the water column). The p r e d a t o r was r e l e a s e d a t t h i s time by a c a b l e which I h e l d while viewing the f i s h d i r e c t l y i n f r o n t o f , and i n m i d l i n e of the g r i d on the t e s t tank. The moment the " b i r d flew" over the r i g h t - h a n d corner of compartment B, the p o s i t i o n of both f i s h was noted (d^). The maximum downward p o s i t i o n of movement of the f i s h was noted as soon as the p r e d a t o r had passed over both compartments ( d 2 ) . The d i f f e r e n c e ( d 2 - d^) i s the d i s t a n c e moved i n the down-dart. At the same time as the d, depths were recorded, 205 2 stopwatches were s t a r t e d to measure the d u r a t i o n of " f r e e z i n g " ( i . e . the swimming i n h i b i t i o n of each f i s h ; I d e f i n e d t h i s p e r i o d o p e r a t i o n a l l y as the l e n g t h o f time each f i s h remained mo t i o n l e s s i n i t s d 2 p o s i t i o n ) . To reduce v a r i a t i o n a r i s i n g from d i f f e r e n c e s i n the p o s i t i o n of the f i s h a t the moment of stimulus p r e s e n t a t i o n , I repeated the t e s t 5 times f o r each p a i r ; a 5-minute i n t e r -v a l was a l l o t e d between s u c c e s s i v e t e s t s . Thus f o r each f i s h , the d i s t a n c e of the down-dart and the d u r a t i o n o f swimming i n h i b i t i o n are based on the means of these 5 sub-samples . R e s u l t s To t e s t f o r the p o s s i b i l i t y of b i a s r e s u l t i n g from a stock depth d i f f e r e n c e a t d^, I c a l c u l a t e d the means and 95% c o n f i d e n c e l i m i t s f o r these p r e - s t i m u l u s depths. The means d i f f e r e d by onl y 0.7 cm ( P a r i a : 9.0 cm, Guayamare: 8.3 cm) and there was complete o v e r l a p i n the 95% conf i d e n c e i n t e r v a l s suggesting t h a t the p r e - s t i m u l u s c o n d i t i o n s were i d e n t i c a l f o r both s t o c k s . A l s o , a Sign T e s t was used t o t e s t i f there was an e f f e c t o f the s e l e c t i o n of the compartment on the swimming i n h i b i t i o n time (the water s u r f a c e o f compartment A i s c l o s e r to the predator) but t h i s a l s o proved to be untrue (p = .504). The o n l y s i g n i f i c a n t d i f f e r e n c e s t h a t c o u l d be d e t e c t e d were the d i f f e r e n c e s between stocks (Table 26). 206 TABLE 26. Responses of naive Guayamare and P a r i a guppies to a simulated a e r i a l p r e d a t o r . The data f o r each p a i r o f f i s h are the means f o r 5 separate t r i a l s . D u r a t i o n o f swimming Down-dart d i s t a n c e (cm) i n h i b i t i o n (sec) P a i r P a r i a Guayamare P a r i a Guayamare 1 3.4 1.0 10.6 7.4 2 3.8 4.2 8.2 3.8 3 2.6 1.0 9.8 5.0 4 3.2 3.2 6.8 6.4 5 3.4 2.8 9.2 9.4 6 5.2 6.2 7.6 13.4 7 2.2 2.2 8.0 4.0 8 3.4 3.8 8.4 3.4 9 4.6 2.0 8.6 3.6 10 5.8 5.4 10.4 8.0 11 3.0 0.8 5.8 2.8 12 3.8 4.2 14.8 3.0 13 4.2 3.0 8.0 6.2 14 4.2 2.0 6.0 4.2 15 4.0 3.0 3.2 3.6 16 3.8 5.2 7.4 2.6 17 . 8.-6 5.8 5.8 8.6 18 4.8 7.8 9.8 12.0 19 4.6 4.2 5.4 3.0 20 4.2 2.2 2.6 0.2 Mean 4.14 3.50 7.82 5.53 Wilcoxon T N P 41.5 18 < .05 38.5 20 < .01 207 A Wilcoxon matched-pairs signed-ranks t e s t was used to t e s t the h ypothesis t h a t P a r i a would have a g r e a t e r down-dart d i s t a n c e and g r e a t e r swimming i n h i b i t i o n d u r a t i o n than Guayamare ( i n conformity w i t h the f i e l d o b s e r v a t i o n s ) . T h i s h y p o t h e s i s c o u l d not be f a l s i f i e d f o r e i t h e r b e h a v i o r a l measure (down-dart d i s t a n c e : T = 41.5, p < .05; swimming i n h i b i t i o n time: T = 38.5, .01 > p > .005). These r e s u l t s suggest t h a t i n the absence of s i g n i -f i c a n t a q u a t i c p r e d a t i o n , guppies who, i n response to an a e r i a l t h r e a t , move deeper i n the water column and remain m o t i o n l e s s l o n g e r are favored by n a t u r a l s e l e c t i o n . To v a l i d a t e t h i s h y p o thesis f u r t h e r i t would be necessary to t e s t the 2 stocks w i t h r e a l a e r i a l p r e d a t o r s but I d i d not have the time or f a c i l i t i e s t o do t h i s . The h ypothesis assumes t h a t moving i n t o deeper water and remaining m o t i o n l e s s decreases the l i k e l i h o o d of detec-t i o n or capture, or both, by a e r i a l p r e d a t o r s . From what l i t t l e i s known about the f e e d i n g h a b i t s of f i s h - e a t i n g bats (see d i s c u s s i o n i n Chapter 3) and k i n g f i s h e r s , these assumptions are not unreasonable. Though the f i s h i n g h a b i t s of T r i n i d a d k i n g f i s h e r s have not been s t u d i e d , o b s e r v a t i o n s on the European (Eastman, 1969) and B e l t e d (White, 1936; S a l y e r and L a g l e r , 1949; Hyatt, p e r s . comm.) k i n g f i s h e r i n d i c a t e t h a t these p r e d a t o r s are most e f f i c i e n t i n c l e a r shallow water. However, Eastman (1969) has shown t h a t when the European k i n g f i s h e r launches an a t t a c k from a perch, 208 the aim a t the perch i s d e c i s i v e because the eyes are c l o s e d underwater. The " P a r i a response" would probably be l e s s e f f e c t i v e than a r a p i d d a r t i n c o u n t e r i n g t h i s s t r a t e g y . N e v e r t h e l e s s , the response would appear to be admirably s u i t e d f o r the k i n g f i s h e r ' s a l t e r n a t e f i s h i n g s t r a t e g y — t h e h o v e r i n g a t t a c k . (d) S c h o o l i n g behavior D i f f e r e n c e s among the w i l d p o p u l a t i o n s of guppies i n the tendency to form cohesive groups have been d e s c r i b e d p r e v i o u s l y i n a g e n e r a l q u a l i t a t i v e manner (Table 20). I suggested t h a t a c o r r e l a t i o n e x i s t s between the i n t e n s i t y of a q u a t i c p r e d a t i o n and the development of s c h o o l i n g behavior. Since s c h o o l i n g behavior i s o f t e n thought to have evolved as an a n t i p r e d a t o r s t r a t e g y ( f o r reviews see Breder, 1967; Shaw, 1970), I f e l t i t was v i t a l to determine i f there were d i f f e r e n c e s i n the cohesive p r o p e r t i e s of groups of naive guppies o r i g i n a t i n g from n a t u r a l p o p u l a t i o n s t h a t s u f f e r from d i f f e r e n t p r e d a t i o n p r e s s u r e s . Experiments of t h i s type have not been c a r r i e d out p r e v i o u s l y and are necessary to understanding the adaptive s i g n i f i c a n c e of s c h o o l i n g behavior. Experiment 6.9 S c h o o l i n g behavior of 5 l a b o r a t o r y stocks of guppies. Methods The t e s t s were conducted i n a bare, blue-green 209 wooden tank (48 x 110 cm) f i l l e d t o a depth o f 3 cm with aged tap water maintained a t 25 t 2 C. I l l u m i n a t i o n was h e l d constant w i t h a 40-watt c o o l white f l u o r e s c e n t tube mounted 60 cm above the water s u r f a c e . The shallow water f l a t t e n e d the s c h o o l to a depth of one or two f i s h . A 2 r e f e r e n c e g r i d of ten 52 8 cm squares was marked on the bottom o f the tank t o r e c o r d the p o s i t i o n of each f i s h . O bservations were made by l o o k i n g i n t o a m i r r o r (mounted On the r e a r w a l l o f the tank 35 cm above the water and h e l d a t a 45° angle to the water) through a small hole i n a b l i n d c o v e r i n g the top of the tank. In t h i s way I c o u l d observe f i s h without d i s t u r b i n g them (disturbance i n c r e a s e s c o h e s i o n ) . A l l r e c o r d i n g s were made on groups of 10 mature f i s h , 5 o f each sex chosen randomly from stock a q u a r i a . The s i z e d i s t r i b u t i o n s of a l l t e s t f i s h were the same. They were fed p r i o r t o , and d i s c a r d e d a f t e r t e s t i n g . The t e s t i n g schedule was randomized w i t h r e s p e c t to the day and time o f day each stock was t e s t e d (845-1745 h r ) . Ten r e p l i -c a tes f o r each o f the 5 stocks were completed f o r a t o t a l of 50 groups ( t o t a l of 500 f i s h used). Each group was p l a c e d i n the t e s t tank f o r 10 min. A f t e r t h i s the p o s i t i o n of each o f the 10 f i s h was recorded at 1-minute i n t e r v a l s f o r 30 min ( t e s t A ) . T h i s was repeated 5 hr l a t e r a f t e r the f i s h had ex p l o r e d t h e i r new environment ( t e s t B). For each 30-minute t e s t I c a l c u l a t e d 210 an "index of cohesion" i n the f o l l o w i n g way: f o r each 1-minute o b s e r v a t i o n the maximum d e n s i t y f o r any o f the 10 g r i d squares was recorded. The index i s the mean maximum d e n s i t y f o r 30 of these o b s e r v a t i o n s . The index has a t h e o r e t i c a l minimum of 1 (1 f i s h i n each of the 10 squares) and a maximum of 10 ( e n t i r e group i n the area o f 1 square or l e s s ) . Although t h i s method of q u a n t i f y i n g s c h o o l i n g behavior i s l e s s p r e c i s e than.those used by other workers (e.g. W i l l i a m s , 1964; C u l l e n e t a l , 1965; Hunter, 1966; Symons, 1971), i t appeared t o be adequate f o r the compara-t i v e purposes d e s i r e d here. R e s u l t s W i t h i n each stock I found a s i g n i f i c a n t decrease i n the index from t e s t A t o t e s t B (Figure 24); a l l stocks were l e s s cohesive once they had e x p l o r e d the tank. However, the degree o f d i s s o c i a t i o n (school spread, or break-up) was g r e a t e r f o r P e t i t e Curucaye, Upper A r i p o , and P a r i a than e i t h e r Lower A r i p o or Guayamare. More noteworthy were the d i f f e r e n c e s among the 5 stocks a t both t e s t p e r i o d s . The t r e n d i n the mean index score from a maximum a t Lower A r i p o t o a minimum a t P a r i a c l o s e l y p a r a l l e l s the v a r i a t i o n I observed i n nature (Table 20). The stocks d e r i v e d from r i v e r s where p r e d a t i o n i s i n t e n s e have a s i g n i f i c a n t l y h i g h e r score than those o b t a i n e d from r i v e r s with low p r e d a t i o n . S t a t i s t i c a l l y , P e t i t e Curucaye i s i n s e p a r a b l e from Guayamare o r Lower 211a FIGURE 24. Mean index of cohesion f o r f i v e l a b o r a t o r y s t o c k s of guppies. V e r t i c a l l i n e shows the mean, h o r i z o n t a l l i n e the t o t a l range, b l a c k r e c t a n g l e s the 95% conf i d e n c e l i m i t s , (a) = T e s t A; (b) = T e s t B 3.0 40 I 5.0 _ L 6.0 7.0 ao _ L 9.0 mo _ L _ LOWER ARIPO GUAYAMARE PETITE CURUCAYE UPPER ARIPO PARIA a. LOWER ARIPO GUAYAMARE P CURUCAYE UPPER ARIPO PARIA 6J0 b. n — 1 — r 9.0 10.0 T ao 40 7.0 5.0 6.0 INDEX of C O H E S I O N 212 A r i p o , though i n t e s t B, there i s g r e a t e r o v e r l a p with Upper A r i p o and P a r i a . P e t i t e Curucaye thus appears to be a t r a n s i t o r y s c h o o l e r , very cohesive i n strange surroundings but quick to spread out once the new environment has been thoroughly e x p l o r e d (< 5 h r ) . Guayamare and Lower A r i p o on the o t h e r hand are s t i l l q u i t e cohesive a f t e r 5 hr. I have no evidence t h a t the method of measuring s c h o o l i n g r e s u l t e d i n any s t o c k - s p e c i f i c b i a s . Though a l l stocks d i d spend more time i n the 4 corner g r i d s than i n the o t h e r s , i n s p e c t i o n of i n d i v i d u a l r e c o r d i n g s r e v e a l e d no tendency f o r any stock to be more a t t r a c t e d to one r e g i o n of the t e s t tank than the o t h e r . In a d d i t i o n , l i g h t i n t e n -s i t y and f e e d i n g m o t i v a t i o n (2 v a r i a b l e s known t o a f f e c t cohesion i n the guppy) were c o n t r o l l e d and the e n t i r e t e s t i n g schedule was randomized ( c o n t r o l l i n g f o r p o t e n t i a l c i r c a d i a n d i f f e r e n c e s ) . I conclude t h a t the methods d i d not i n t r o d u c e a systematic b i a s f a v o r i n g g r e a t e r or l e s s e r cohesiveness i n any one stock. O v e r a l l , the r e s u l t s of t h i s experiment c o n f i r m the hypothesis t h a t d i f f e r e n c e s i n the tendency to s c h o o l are c o n t r o l l e d by u n d e r l y i n g g e n e t i c f a c t o r s . But t h i s i s o n l y a p a r t i a l answer to the q u e s t i o n of the a d a p t i v e s i g n i f i -cance of d i f f e r e n c e s i n s c h o o l i n g i n n a t u r a l p o p u l a t i o n s of guppies. Although the c o r r e l a t i o n w i t h p r e d a t i o n may be a c a u s a l one, a t the i n t e r s p e c i f i c l e v e l i t has o f t e n been warned t h a t , "we are making a s e r i o u s e r r o r i n f o r c i n g 213 o u r s e l v e s to f i n d a s i n g l e adaptive f e a t u r e f o r s c h o o l i n g (Shaw, 1970: 471)." However, I have some f i e l d and l a b o r a t o r y evidence t h a t suggests s c h o o l i n g may reduce p r e d a t i o n . On s e v e r a l o c c a s i o n s I observed an e l l i p t i c a l s c h o o l of 20 to 50 guppies moving upstream towards a p o t e n t i a l p r e d a t o r ( l a r g e Aequidens) concealed near the shore. The guppies a t the f r o n t of the s c h o o l p e r c e i v e d the p r e d a t o r and t h e i r e v a s i v e movements ( r a p i d d a r t s and s u r f a c e skim away from the shore) were t r a n s m i t t e d t o the r e a r of the s c h o o l . Thus the advance warning of p o t e n t i a l danger was advantageous to the f i s h who d i d not see the p r e d a t o r . A l s o , i n the f i e l d I have observed p r e d a t o r s (Rivulus) s w i t c h i n g t h e i r a t t a c k course behind a f l e e i n g s c h o o l of guppies; the same phenomenon has been seen i n outdoor pools c o n t a i n i n g guppies and H o p l i a s or C r e n i c i c h l a . These p r e d a t o r s do appear to have l e s s d i f f i c u l t y a t t a c k i n g an i s o l a t e d f i s h than one i n a s c h o o l ("confusion e f f e c t " ) . I t i s p a r t i c u l a r l y r e v e a l i n g to watch the eye movements of a C r e n i c i c h l a l u r k i n g near a s c h o o l of guppies. Guppies t h a t d a r t away from the s c h o o l p e r i p h e r y are q u i c k l y f i x a t e d and i f they wander too f a r are o f t e n a t t a c k e d . The p r e d a t o r appears to be i n h i b i t e d from d a r t i n g i n t o the c e n t r e of a s c h o o l . In the l a b o r a t o r y a hungry C r e n i c i c h l a may wait f o r hours before an opportune moment a r i s e s to s t r i k e an unwary guppy. T h i s a n e c d o t a l evidence suggests 214 t h a t s c h o o l i n g behavior f u n c t i o n s to i n c r e a s e the p e r c e p t u a l awareness of a group of guppies and a l s o serves to thwart the decision-making apparatus of the p r e d a t o r . Experiment 6.91 Predatory behavior of R i v u l u s exposed t o Lower A r i p o and P a r i a guppies. I n t r o d u c t i o n I f a wel l - d e v e l o p e d s c h o o l i n g response does reduce the r i s k of p r e d a t i o n , then P a r i a f i s h should s u f f e r g r e a t e r r e l a t i v e m o r t a l i t y than Lower A r i p o f i s h when groups of both stocks are exposed t o a p r e d a t o r . T h i s experiment has al r e a d y been done f o r wild-caught guppies (Table 21 A) and l a b o r a t o r y stocks (Table 22 C) but i n both experiments no behavior was recorded so i t i s not known why P a r i a s u f f e r e d g r e a t e r m o r t a l i t y . As subsequent experiments have r e v e a l e d , Lower A r i p o and P a r i a stocks d i f f e r i n many ways other than s c h o o l i n g b e h a v i o r . Even when s c h o o l i n g i s im p o s s i b l e (Exp. 6.7), Lower A r i p o guppies are l e s s v u l n e r a b l e than P a r i a . Thus s c h o o l i n g behavior per se may not have been r e s p o n s i b l e f o r the s u r v i v a l trends observed in.the- o r i g i n a l s e r i e s of experiments. To check t h i s I made d i r e c t o b s e r v a t i o n s o f R i v u l u s a t t a c k i n g mixed groups of P a r i a and Lower A r i p o guppies. Methods Two R i v u l u s of approximately 70 mm were p l a c e d i n a 40 l i t e r aquarium and s t a r v e d f o r 24 hr. F i v e P a r i a and 215 5 Lower A r i p o male guppies of equal s i z e were added to t h i s tank and the frequency o f approaches, a t t a c k s , and captures were recorded i n the standard manner ( t e s t d u r a t i o n 1 hour). T h i s was r e p l i c a t e d 5 times, each t e s t preceded by a 24 hr s t a r v a t i o n p e r i o d . A l l guppies were p r e d a t o r - n a i v e and second or t h i r d g e n e r a t i o n l a b o r a t o r y stock. R e s u l t s When the 2 stocks of guppies were f i r s t added to the p r e d a t o r s ' tank, they formed a loose c o l l e c t i v e s c h o o l i n the c e n t r e . At f i r s t the hungry R i v u l u s were observed to dash w i l d l y i n t o the s c h o o l u s u a l l y without success. The "c o n f u s i o n e f f e c t " was c l e a r l y seen as the pr e d a t o r s s h i f t e d t h e i r approaches from one prey t o the next. Sometimes a R i v u l u s d i d appear to get a " f i x " on one guppy and would chase i t i n c e s s a n t l y , q u i t e o b l i v i o u s of the oth e r guppies t h a t would appear i n i t s path. E v e n t u a l l y the R i v u l u s ceased d a r t i n g i n t o the sch o o l but remained some d i s t a n c e away near the bottom. The f i s h t h a t were approached and a t t a c k e d were u s u a l l y the ones t h a t dropped out of the s c h o o l ; i n most cases these were P a r i a guppies. T h i s i s r e f l e c t e d i n the g r e a t e r number of approaches and a t t a c k s d i r e c t e d towards P a r i a f i s h (Figure 25). Because the stock d i f f e r e n c e i n the percentage of a t t a c k s (contacts) i s g r e a t e r than i n approaches, i t suggests Lower A r i p o guppies were a l s o b e t t e r a t e l u d i n g the p r e d a t o r s ( c f . Exp. 6.7). 216a FIGURE 25. Predatory behavior of R i v u l u s exposed t o equal numbers of P a r i a and Lower A r i p o guppies ( l a b o r a t o r y s t o c k ) . Values above each histogram are the a c t u a l s c o r e s . 2 1 6 b PARIA 1 0 Q _ LOWER ARIPo| 100-90-80-70-60-o 50-< 40-30-< 20-10 -0 -2 1 7 I t i s concluded t h a t s c h o o l i n g behavior reduces the v u l n e r a b i l i t y of guppies to f i s h p r e d a t o r s . A p r e c i s e measure of i t s e f f i c a c y i s d i f f i c u l t to o b t a i n because the tendency t o s c h o o l i s l i n k e d to other b e h a v i o r a l t r a i t s t h a t a l s o serve to reduce p r e d a t i o n . C o n c l u s i o n and g e n e r a l d i s c u s s i o n of  geographic v a r i a t i o n i n behavior In t h i s chapter I have attempted to determine the s i g n i f i c a n c e of n a t u r a l v a r i a t i o n i n the escape responses of c e r t a i n p o p u l a t i o n s of the guppy. I have a l s o e v a l u a t e d the adaptive r o l e of p o p u l a t i o n d i f f e r e n c e s i n h a b i t a t s e l e c t i o n and s c h o o l i n g b e h a v i o r . The f i e l d o b s e r v a t i o n s revealed, a c l o s e c o r r e l a t i o n between the development of presumed a n t i p r e d a t o r behavior and the d i s t r i b u t i o n and abundance of predaceous f i s h . P r e d a t i o n experiments with wild-caught f i s h demonstrated, t h a t samples of guppies taken from a stream i n f e s t e d with c h a r a c i d and c i c h l i d p r e d a t o r s (Lower Aripo) had a s i g n i -f i c a n t s u r v i v a l advantage when t e s t e d a g a i n s t one of s e v e r a l p o p u l a t i o n s exposed to only R i v u l u s h a r t i i ( P e t i t e Curucaye, Upper A r i p o , P a r i a ) . Since most of these experiments were conducted w i t h female guppies of comparable body s i z e , r e l a t i v e v u l n e r a b i l i t y was c e r t a i n l y based upon b e h a v i o r a l d i f f e r e n c e s . In a d d i t i o n , t e s t s with P e t i t e Curucaye and Upper A r i p o females suggested t h a t d i f f e r e n c e s i n r e l a t i v e 218 v u l n e r a b i l i t y e x i s t even between p o p u l a t i o n s exposed to the same p r e d a t o r . T h i s seems to be r e l a t e d t o the abundance of p r e d a t o r s (and hence presumed p r e d a t i o n i n t e n s i t y ) w i t h i n a given stream. Because.the r a t e of gene flow between P e t i t e Curucaye and Santa Cruz R i v e r s ( i s o l a t i o n by d i s t a n c e only) i s probably g r e a t e r than between Upper and Lower A r i p o (major w a t e r f a l l b a r r i e r ) , these r e s u l t s may not have e n t i r e l y r e f l e c t e d l o c a l a d a p t a t i o n s to R i v u l u s p r e d a t i o n . T e s t s on a p o p u l a t i o n exposed to i n t e n s e R i v u l u s p r e d a t i o n but completely i s o l a t e d from streams with c h a r a c i d s and c i c h l i d s w i l l be necessary to e s t a b l i s h i f R i v u l u s p r e d a t i o n alone can account f o r the b e h a v i o r a l c h a r a c t e r i s t i c s of the P e t i t e Curucaye p o p u l a t i o n . The most s i g n i f i c a n t f i n d i n g i n t h i s chapter was t h a t samples of l a b o r a t o r y p o p u l a t i o n s of guppies t h a t had been bred and r e a r e d under i d e n t i c a l c o n d i t i o n s were taken non-randomly by p r e d a t o r s . Because there was good agreement between the t e s t s with wild-caught and naive l a b o r a t o r y s t o c k s , the h y p o t h e s i s t h a t s e l e c t i v e p r e d a t i o n has been r e s p o n s i b l e f o r the m i c r o e v o l u t i o n of behavior i s supported. These experiments a l s o r e v e a l e d the importance of l i g h t i n p r e d a t o r avoidance. With the n o t a b l e e x c e p t i o n of the P e t i t e Curucaye sto c k , s e l e c t i v e p r e d a t i o n c o u l d not be demonstrated under c o n d i t i o n s of near t o t a l darkness. When dim l i g h t was p r o v i d e d , however,(Table 22 B) r e l a t i v e s u r v i v a l c l o s e l y p a r a l l e l e d the r e s u l t s of experiments 219 conducted under b r i g h t i l l u m i n a t i o n (Exp. 6.1 and 6.7). The reason(s) f o r the s u p e r i o r s u r v i v a l of P e t i t e Curucaye guppies under dark or dim l i g h t c o n d i t i o n s i s unknown but i t i s tempting to s p e c u l a t e t h a t t h i s p o p u l a t i o n has a more l i g h t - s e n s i t i v e v i s u a l apparatus. I hypothesize t h a t t h i s i s a consequence of s e l e c t i o n by R i v u l u s i n a c o n f i n e d environment dev o i d o f an adequate r e f u g e . A comparative study of the lower l i m i t s of s c o t o p i c v i s i o n u s i n g the technique of Lang (1967) might be informa-t i v e (Lang's domestic guppies had a lower s c o t o p i c l i m i t of — 6 —7 7 x 10 l u x , compared to man's.lower l i m i t o f 7 x 10 l u x ) , together with an anatomical i n v e s t i g a t i o n ( c f . Werner, 1969; King, 1970). The d i s c o v e r y of s u r v i v a l d i f f e r e n c e s among l a b o r a -t o r y stocks i n i t i a t e d a s e r i e s o f d e t a i l e d b e h a v i o r a l experiments aimed a t q u a l i t a t i v e l y and q u a n t i t a t i v e l y e s t a b l i s h i n g the d i f f e r e n c e s among s t o c k s . O v e r a l l t h e r e i s good concordance among measures taken on independent samples from each of the 5 stocks (Table 27). B e h a v i o r a l l y , Lower A r i p o and P a r i a are the 2 most d i s t i n c t stocks ( l e a s t o v e r l a p ) . I have argued t h a t much of the beh a v i o r of Lower A r i p o guppies (and a l s o Guayamare and probably many others) has evolved i n response t o s e l e c -t i v e p r e d a t i o n by c h a r a c i d and c i c h l i d f i s h . The low r e l a t i v e v u l n e r a b i l i t y o f Lower A r i p o f i s h t o pr e d a t o r s (Exp. 6.1, 6.2, 6.7) c l e a r l y i l l u s t r a t e s the s u r v i v a l value 220 TABLE 27. Concordance of b e h a v i o r a l measures taken on 5 stocks of guppies. For each e x p e r i -ment the mean scores are ranked from g r e a t e s t t o l e a s t . The experiment number i s g i v e n i n b r a c k e t s . Stock B e h a v i o r a l measurement LA Guay PCur. UA Par Depth p r e f e r e n c e ( 6 . 3 ) a 1 2 3 4 5 Shore p r e f e r e n c e (6.4)^ 1 4 3 2 5 Reaction d i s t a n c e (6.5) 1 3 5 2 4 S u r v i v a l time (6.7) 1 2 3 4 5 Response t o a e r i a l p r e d a t o r ( 6 . 8 ) c - 2 - - 1 ci S c h o o l i n g behavior (6.9) 1 2 3 4 5 Sum of ranks 5 13 17 16 24 Mean - 15 Ke n d a l l W = 0.76; s =190; •k = 5; p < .01 a b a s e d on Table 2 3A. based on mean number of o b s e r v a t i o n s i n s e c t i o n 4 at T e s t A ( r e f e r t o F i g . 20). °omitted from c a l c u l a t i o n of concordance. d b a s e d on mean of T e s t A and B ( r e f e r t o F i g . 24). 221 of the b e h a v i o r a l d i f f e r e n c e s . For the P a r i a p o p u l a t i o n , the t h r e a t from f i s h p r e d a t o r s i s v i r t u a l l y absent. In a l l t e s t s with r e a l f i s h p r e d a t o r s , t h i s stock was the most v u l n e r a b l e . These r e s u l t s are a t t r i b u t a b l e i n p a r t to a hig h escape t h r e s h o l d , i n a p p r o p r i a t e m i c r o h a b i t a t s e l e c t i o n ( i n or over deep water, away from s h o r e ) , and the absence of s c h o o l i n g b e h a v i o r . Though P a r i a guppies are c l e a r l y unadapted to f i s h p r e d a t i o n , I suggest t h a t some of t h e i r b ehavior does serve an a n t i p r e d a t o r f u n c t i o n . The tendency f o r these f i s h t o l i v e d i s p e r s e d i n deep water may o f f e r some s a f e t y from a e r i a l p r e d a t o r s such as k i n g f i s h e r s and bats which presumably operate most e f f e c t i v e l y i n shallow water. The s u p e r i o r a d a p t a t i o n of P a r i a guppies t o non-aquatic preda-t o r s i s s p e c u l a t i v e ; i t i s e s s e n t i a l t h a t t h i s h y p othesis be t e s t e d with r e a l a e r i a l p r e d a t o r s , p r e f e r a b l y i n s i t u . In the stocks showing i n t e r m e d i a t e p r e d a t o r avoidance, there are s e v e r a l exceptions t o the "predator h y p o t h e s i s " : the long r e a c t i o n d i s t a n c e of Upper A r i p o guppies ,.and the s c h o o l i n g response of P e t i t e Curucaye (at l e a s t i n t e s t A of Exp. 6.9). I have a l r e a d y d i s c u s s e d the q u e s t i o n of r e a c t i o n d i s t a n c e elsewhere and emphasized t h a t a n t i p r e d a t o r behavior must always be viewed w i t h i n the framework of the p h y s i c a l environment. In very s m a l l or t u r b i d streams there i s no avenue f o r s e l e c t i o n t o operate f o r avoidance at g r e a t d i s t a n c e s — a q u a t i c p r e d a t o r s appear suddenly a t a 222 c l o s e range. The b e s t s t r a t e g y f o r escape must be the s e l e c t i o n of a p r e d a t o r - f r e e m i c r o h a b i t a t ( i f a v a i l a b l e ) or the e f f e c t i v e use of escape motor p a t t e r n s (e.g. r a p i d d a r t , s u r f a c e skim and jump). C l e a r l y , i t i s important to r e l y on s e v e r a l measures as an estimate of p o p u l a t i o n d i f f e r e n c e s i n a n t i p r e d a t o r behavior. For example, i f r e a c t i o n d i s t a n c e alone i s used, the a c t u a l v u l n e r a b i l i t y of P e t i t e Curucaye and Upper A r i p o would be over and underestimated, r e s p e c t i v e l y . I suggest t h a t s c h o o l i n g i s a l s o an a n t i p r e d a t o r mechanism i n the guppy. F i e l d and l a b o r a t o r y o b s e r v a t i o n s i n d i c a t e d t h a t t h i s b ehavior operates as a c o u n t e r a d a p t a t i o n to a p r e d a t o r ' s s t r a t e g y of f i x a t i n g a s i n g l e prey f i s h moments bef o r e an a t t a c k . Furthermore, guppies i n a s c h o o l share a g r e a t e r sphere of v i s u a l awareness of p o t e n t i a l danger i n the surrounding environment. Because f i s h preda-t o r s occur along every few metres of stream, i t i s u n l i k e l y t h a t s c h o o l i n g p r o t e c t s guppies simply by r e d u c i n g the p r o b a b i l i t y of. a p r e d a t o r encountering a prey (Brock and R i f f e n b u r g h , 1960; Olson, 1964). At Lower A r i p o , the presence of 1 C r e n i c i c h l a near c o n c e n t r a t i o n s of guppies suggests t h a t both p r e d a t o r and prey may be aware of each ot h e r ' s whereabouts; s e a r c h i n g i s probably not as important as f o r p e l a g i c f i s h p r e d a t o r s , or t e r r e s t r i a l p r e d a t o r s hunting f o r concealed prey (Croze, 1970). My o b s e r v a t i o n s on the adaptive s i g n i f i c a n c e of s c h o o l i n g more c l o s e l y p a r a l l e l the d e s c r i p t i o n s of M a n t e i f e l ' and Radakov (1961), 223 E i b l - E i b e s f e l d t (1962), and Hobson (1968). Though numerous workers have documented d i f f e r e n c e s i n the a n t i p r e d a t o r behavior of c l o s e l y r e l a t e d s p e c i e s (Crane, 1952; C u l l e n , 1957; Hoogland e t a l , 1957; Hoar, 1958; Hailman, 1965; Benzie, 1965; Heatwole, 1968; Robinson, 1969; C u r i o , 1969; Baker, 1970), a d e t a i l e d study of geo-gr a p h i c ( i . e . i n t r a s p e c i f i c ) v a r i a t i o n i n a n t i p r e d a t o r behavior has not been c a r r i e d out p r e v i o u s l y (at l e a s t i n f i s h ) . Furthermore, i n some r e p o r t s of geographic d i f f e r -ences (e.g. W i l l i a m s o n , 1952) a g e n e t i c b a s i s f o r the d i f f e r e n c e s has not been e s t a b l i s h e d . Since the tendency to escape may vary w i t h i n the l i f e t i m e of an i n d i v i d u a l as a consequence of maturation, h a b i t u a t i o n , and d i r e c t e x p e r i -ence of a t t a c k s by p r e d a t o r s , i t i s u s u a l l y necessary to conduct some breed i n g experiments and c o n t r o l the p r i o r h i s t o r y o f the o f f s p r i n g . P o p u l a t i o n d i f f e r e n c e s i n a n t i p r e d a t o r a d a p t a t i o n s have been d e s c r i b e d f o r s e v e r a l s p e c i e s . Much of the r e c e n t work on f i s h has cen t e r e d on the s t i c k l e b a c k f a m i l y ( G a s t e r o s t e i d a e ) . McPhail (1969) d i s c o v e r e d an innate d i f f e r e n c e between the escape responses ( i n c l u d i n g r e a c t i o n d i s t a n c e ) of the l a r v a e of 2 forms (taxonbmic s t a t u s unclear) of t h r e e s p i n e s t i c k l e b a c k . He r e l a t e d t h i s d i f f e r e n c e to the presence o r absence of d i f f e r e n t i a l s e l e c t i o n on f r y by a s m a l l c a r n i v o r o u s f i s h , Novumbra hubbsi. 224 Moodie (1972a, 1972b) argued t h a t on the Queen C h a r l o t t e I s l a n d s , s e l e c t i v e p r e d a t i o n (mainly by c u t t h r o a t t r o u t ) has been r e s p o n s i b l e f o r the e v o l u t i o n of a "black" p e l a g i c race o f l a r g e , l o n g - s p i n e d Gasterosteus ac u l e a t u s ( c f . M c P hail, 1969; Semler, 1971). In a l a b o r a t o r y study of s e l e c t i o n by squawfish and c u t t h r o a t t r o u t , Moodie, McPhail, and Hagen (unpubl.).""... found t h a t samples of s t i c k l e b a c k s from one p o p u l a t i o n remained i n a s h e l t e r longer and were consequently l e s s v u l n e r a b l e . However, t h i s d i f f e r e n c e i n behavior may a c t u a l l y be c o r r e l a t e d with the number of l a t e r a l p l a t e s of the s t i c k l e b a c k (see a l s o Hagen and G i l b e r t s o n , 1972). I f t h i s i s confirmed, i t would be an example o f b e h a v i o r a l polymorphism r a t h e r than a p o p u l a t i o n d i f f e r e n c e of the type I have d e s c r i b e d f o r the guppy. L i t t l e work has been done on geographic v a r i a t i o n i n the a n t i p r e d a t o r behavior of o t h e r animals. C u r i o (1961) d e s c r i b e d r a c i a l d i f f e r e n c e s between the mobbing behavior of Spanish and German p o p u l a t i o n s of the p i e d f l y c a t c h e r . The German p o p u l a t i o n mobbed both the redbacked s h r i k e and the owl ( S t r i x aluco) whereas the Spanish p o p u l a t i o n mobbed on l y the l a t t e r . T h i s d i f f e r e n c e c o i n c i d e s with the d i s t r i -b u t i o n of these p r e d a t o r s s i n c e the s h r i k e i s not sympatric with the Spanish p o p u l a t i o n . Johnson (1970) found t h a t 2 g e o g r a p h i c a l l y i s o l a t e d subspecies of the l i z a r d Sceloporus o c c i d e n t a l i s showed 225 d i f f e r e n c e s i n the d i s t a n c e a t which they f l e d at the approach of a human "predator". The subspecies w i t h the s h o r t e s t r e a c t i o n d i s t a n c e was a l s o the most c r y p t i c a l l y c o l o u r e d . A s i m i l a r d i f f e r e n c e was found between 2 s p e c i e s of a n o l i n e l i z a r d s by Heatwole (1968). The c o n c l u s i o n t h a t can be drawn from these s t u d i e s ( i n c l u d i n g the presen t one) i s t h a t p r e d a t i o n can be a d i v e r s i f y i n g f o r c e i n the e v o l u t i o n of behavior. At the i n t r a s p e c i f i c l e v e l t h i s d i v e r s i t y i s u s u a l l y expressed as a q u a n t i t a t i v e or ( l e s s commonly) a q u a l i t a t i v e d i f f e r e n c e i n b e h a v i o r . B e h a v i o r a l d i f f e r e n c e s are o f t e n l i n k e d t o othe r a n t i p r e d a t o r mechanisms such as c r y p t i c or aposematic c o l o r a t i o n , s p i n e s , p r o t e c t i v e armour, e t c . In the guppy, however, m i c r o e v o l u t i o n a r y changes i n behavior are l e s s e a s i l y r e l a t e d t o v i s i b l e m o r p h o l o g i c a l d i f f e r e n c e s , i . e . i t i s very d i f f i c u l t t o p r e d i c t the responsiveness o f a guppy t o a pr e d a t o r simply by i t s c o l o r a t i o n (of course w i t h females i t i s i m p o s s i b l e ) . I suspect t h a t there are many other examples of geographic v a r i a t i o n i n a n t i p r e d a t o r behavior t h a t have remained undiscovered because there are no m o r p h o l o g i c a l c l u e s t o p o p u l a t i o n d i f f e r e n t i a t i o n . For example, had the guppy been a monomorphic s p e c i e s over i t s e n t i r e geographic range, the presen t study might never have been i n i t i a t e d i n the f i r s t p l a c e . P r e d a t i o n may a l s o have e f f e c t s on behavior not d i r e c t l y r e l a t e d t o pr e d a t o r avoidance ( C u l l e n , 1957; W i l z , 226 1971; B a l l i n , 1973). T h i s i s not s u r p r i s i n g because preda-t o r avoidance o f t e n n e c e s s i t a t e s a major change i n the e c o l o g i c a l niche of a p o p u l a t i o n . I t i s tempting to specu-l a t e t h a t i n t h i s manner p r e d a t i o n may r e s u l t i n i n c i p i e n t s p e c i a t i o n . B a l l i n ' s (1973) r e s u l t s are very, important i n t h i s r e g a rd because he has d i s c o v e r e d a tendency f o r v i r g i n females of the P a r i a , Upper A r i p o , and Guayamare stocks to respond s e l e c t i v e l y to t h e i r own males. Because B a l l i n found no support f o r the hypothesis (Haskins e t a l , 1961) t h a t females p r e f e r the humanly more c o n s p i c u o u s l y - c o l o r e d males, he i n f e r r e d a b e h a v i o r a l b a s i s f o r t h i s i n t e r s t o c k d i s c r i m i n a t i o n . In another s e r i e s of t e s t s B a l l i n uncovered q u a n t i -t a t i v e d i f f e r e n c e s i n the frequency and i n t e n s i t y of c o u r t -s h i p and i n t e r m a l e a g g r e s s i v e behavior. Some of these d i f f e r e n c e s are probably an i n d i r e c t r e s u l t of i n t e n s e or r e l a x e d p r e d a t i o n p r e s s u r e . Presumably o n l y i n a r e l a t i v e l y p r e d a t o r - f r e e stream (e.g. P a r i a , Upper Aripo) can male guppies " a f f o r d " to spend a l a r g e p r o p o r t i o n of t h e i r time engaged i n conspicuous c o u r t s h i p and a g g r e s s i v e d i s p l a y s . Summary of Chapter 6 1. N a t u r a l p o p u l a t i o n s of guppies exposed t o preda-t i o n by c h a r a c i d and c i c h l i d f i s h : (a) are more r e s t r i c t e d to shallow water near the stream shore. (b) are more cohesive (show a g r e a t e r tendency to school), (c) a v o i d a p o t e n t i a l p r e d a t o r a t a g r e a t e r r e a c t i o n d i s t a n c e . (d) show escape motor p a t t e r n s a t a lower alarm t h r e s h o l d . 2 . To elude a p o t e n t i a l f i s h p r e d a t o r , an i n d i v i d u a l guppy may use one or s e v e r a l of 6 main escape motor p a t t e r n s . These p a t t e r n s appear to be arranged on a s c a l e of i n c r e a s i n g r e a c t i v i t y ranging from " p r e c a u t i o n a r y " to "emergency" behavior. No q u a l i t a t i v e d i f f e r e n c e s were observed i n these motor p a t t e r n s i n f i s h taken from d i f f e r e n t p o p u l a t i o n s . 3. The primary sensory m o d a l i t y used i n p r e d a t o r avoidance i s v i s i o n . 4. Experiments with n a t u r a l p r e d a t o r s demonstrated t h a t guppies taken from a p o p u l a t i o n exposed to c h a r a c i d and c i c h l i d p r e d a t o r s were l e s s v u l n e r a b l e than f i s h taken from p o p u l a t i o n s exposed to R i v u l u s . 5. Experiments with C r e n i c i c h l a and p r e d a t o r - n a i v e guppies (second to approximately f i f t h g e n e r a t i o n l a b o r a -t o r y stock) d e r i v e d from 5 n a t u r a l p o p u l a t i o n s r e v e a l e d d i f f e r e n c e s i n r e l a t i v e v u l n e r a b i l i t y p a r a l l e l t o those found among wild-caught specimens. With one e x c e p t i o n ( P e t i t e Curucaye), stock d i f f e r e n c e s were o n l y e v i d e n t when the s c o t o p i c v i s i o n of guppies was not g r e a t l y impaired by 228 very low l i g h t l e v e l s . I t i s hypothesized t h a t P e t i t e Curucaye guppies have evolved a more l i g h t - s e n s i t i v e v i s u a l system to contend with n o c t u r n a l p r e d a t i o n by R i v u l u s . 6. P a r i a stock p r e f e r r e d to remain near the bottom of a deep aquarium s i g n i f i c a n t l y more than Lower A r i p o , Upper A r i p o , Guayamare, and P e t i t e Curucaye. There was a tendency f o r Guayamare and Lower A r i p o stocks t o spend r e l a t i v e l y more time near the s u r f a c e . 7. Lower A r i p o and P a r i a stocks s e l e c t e d d i f f e r e n t microenvironments when p l a c e d i n a simulated shore g r a d i e n t . The former tended to remain c l o s e r t o shore and the l a t t e r away from shore. 8. The mean r e a c t i o n d i s t a n c e of the 5 stocks of guppies to a m o t i o n l e s s , dead C r e n i c i c h l a was ( i n rank o r d e r ) , Lower Aripo>Upper Aripo>Guayamare>Paria>Petite Curucaye. When the "predator" was animated, the r e a c t i o n d i s t a n c e doubled f o r a l l 5 s t o c k s , however, the same rank o r d e r p e r s i s t e d . 9. When Lower A r i p o , Guayamare, and Upper A r i p o stocks were exposed to a m o t i o n l e s s , dead C r e n i c i c h l a , the modal escape motor p a t t e r n was avoidance d r i f t ; f o r P e t i t e Curucaye and P a r i a stocks the modal p a t t e r n was weak avoidance d r i f t . The m o t i o n l e s s p r e d a t o r e l i c i t e d no response from 20% of P e t i t e Curucaye and 12% of P a r i a f i s h . The a d d i t i o n of motion to the p r e d a t o r evoked a more r e a c t i v e escape motor p a t t e r n from the m a j o r i t y of Lower 229 A r i p o , Guayamare, and Upper A r i p o f i s h but had a l e s s e r e f f e c t on P a r i a and P e t i t e Curucaye stock. 10. The s u r v i v a l time of i n d i v i d u a l Lower A r i p o males p l a c e d with a hungry R i v u l u s was s i g n i f i c a n t l y g r e a t e r than t h a t of P e t i t e Curucaye, Upper A r i p o , and P a r i a males. Of the 5 stocks t e s t e d , P a r i a was caught i n the s h o r t e s t time. S u r v i v a l time was p o s i t i v e l y c o r r e l a t e d with the frequency of r a p i d d a r t s and s u r f a c e jumps. 11. The P a r i a stock was s i g n i f i c a n t l y more respon-s i v e to a simulated a e r i a l p r e d a t o r than Guayamare. 12. Groups of Lower A r i p o , Guayamare, and P e t i t e Curucaye guppies swam about a strange environment i n a more cohesive s c h o o l than Upper A r i p o or P a r i a f i s h . A l l stocks were l e s s c o h e s i v e a f t e r a 5-hour e x p l o r a t i o n p e r i o d but t h i s d i s p e r s i o n was l e s s pronounced i n Lower A r i p o and Guayamare f i s h . 13. F i e l d and l a b o r a t o r y o b s e r v a t i o n s suggested t h a t s c h o o l i n g behavior f u n c t i o n s to i n c r e a s e the p e r c e p t u a l awareness of a group of guppies and a l s o serves t o thwart the decision-making mechanisms of the p r e d a t o r ("confusion e f f e c t " ) . I t i s d i f f i c u l t t o demonstrate the s u r v i v a l v alue of p o p u l a t i o n d i f f e r e n c e s i n s c h o o l i n g behavior because the tendency to s c h o o l i s l i n k e d to other b e h a v i o r a l t r a i t s which a l s o serve as a n t i p r e d a t o r mechanisms. 14. I conclude, t h a t much of the geographic v a r i a t i o n i n a n t i p r e d a t o r behavior i s caused by g e n e t i c d i f f e r e n c e s 230 t h a t have evolved i n response to d i f f e r e n t i a l p r e d a t i o n p r e s s u r e s . I t appears t h a t i n some cases (e.g. Lower A r i p o and Guayamare vs. P e t i t e Curucaye) these m i c r o e v o l u t i o n a r y b e h a v i o r a l d i f f e r e n c e s are maintained without a major b a r r i e r to gene flow. CHAPTER 7 GENERAL DISCUSSION AND CONCLUSIONS Organic d i v e r s i t y i s i m p r e s s i v e , wonderful, f a s c i n a t i n g , or e x a s p e r a t i n g , a c c o r d i n g t o one's t a s t e s and tempera-^ ment. Does i t have some b i o l o g i c a l f u n c t i o n and meaning? (Dobzhansky, 1970: 24) The purpose of t h i s chapter i s to review the major f i n d i n g s of t h i s t h e s i s . I s h a l l a l s o i n d i c a t e the areas where a d d i t i o n a l r e s e a r c h might be most p r o d u c t i v e . Because t h i s study i s the f i r s t of a s e r i e s ( L i l e y and Seghers, unpubl.; B a l l i n , 1973; L i l e y , unpubl.) concerned w i t h geographic v a r i a t i o n i n the behavior and morphology of T r i n i d a d guppies, i t was necessary t o p r o v i d e a s u f f i c i e n t l y broad background f o r f u t u r e work. Consequently only a few aspects have been i n v e s t i g a t e d i n t e n s i v e l y and many q u e s t i o n s l e f t unanswered. The Environment In Chapter 3 I d e s c r i b e d the b i o t i c and a b i o t i c environment o f the guppy with s p e c i a l r e f e r e n c e to the n a t u r a l h i s t o r y , d i v e r s i t y , and d i s c o n t i n u o u s d i s t r i b u t i o n of p o t e n t i a l p r e d a t o r s , and the major and minor b a r r i e r s to gene m i g r a t i o n . I concluded t h a t H o p l i a s malabaricus, C r e n i c i c h l a a l t a , Astyanax bimaculatus, and R i v u l u s h a r t i i 231 232 are the most important present-day p r e d a t o r s of the guppy. Because the d i s t r i b u t i o n of R i v u l u s normally does not over^ l a p with the d i s t r i b u t i o n of the o t h e r p r e d a t o r s , i t p r o v i d e d a unique o p p o r t u n i t y to search f o r p o t e n t i a l geographic v a r i a t i o n i n a n t i p r e d a t o r a d a p t a t i o n s . F i e l d o b s e r v a t i o n s and c o l l e c t i o n s q u i c k l y r e v e a l e d a wealth of i n t e r p o p u l a t i o n d i v e r s i t y i n the sex r a t i o , c o l o r a t i o n , body s i z e , and behavior of guppies. The o b j e c t of t h i s t h e s i s was to assess to what extent ( i f any) t h i s v a r i a t i o n was a t t r i b u t a b l e t o d i f f e r e n t i a l s e l e c t i o n by p r e d a t o r s . The most c o n c l u s i v e evidence was o b t a i n e d f o r b e h a v i o r a l v a r i a t i o n . Although t h i s may be r e l a t e d i n p a r t to the g r e a t e r e f f o r t expended on t h i s aspect/, I f e e l t h a t behavior i s l i k e l y the most s e n s i t i v e i n d i c a t o r of a micros e v o l u t i o n a r y response to p r e d a t i o n , C o l o r a t i o n and the sex r a t i o The primary concern of Chapter 4 was to r e - e v a l u a t e the "Haskins h y p o t h e s i s " , v i z . t h a t v a r i a t i o n i n c o l o r a t i o n i s governed by the c o n f l i c t i n g p r e s s u r e s imposed by preda-t i o n and sexual s e l e c t i o n . I d i s c o v e r e d t h a t t h i s hypo-t h e s i s was d i f f i c u l t to t e s t s a t i s f a c t o r i l y . I attempted to understand the s i g n i f i c a n c e of d i f f e r e n c e s i n c o l o r a t i o n by q u a n t i f y i n g v a r i a t i o n i n the c o l o u r p a t t e r n s of p o p u l a t i o n s l i v i n g i n a d i v e r s e range of h a b i t a t s ( L i l e y and Seghers, unpubl.) and by s t u d y i n g 233 the mechanisms r e s p o n s i b l e f o r sex r a t i o d i f f e r e n c e s , Extreme departures from a t h e o r e t i c a l 1:1 Mendelian sex r a t i o were c o r r e l a t e d with the presence of dense p o p u l a t i o n s of R i v u l u s . Laboratory experiments r e v e a l e d t h a t unbalanced r a t i o s ( i n favour of females) were not caused by g e n e t i c d i f f e r e n c e s i n the sex de t e r m i n a t i o n system. In a d d i t i o n , sex r a t i o s were not c o r r e l a t e d with sexual dimorphism i n c o l o u r . P r e d a t i o n experiments wi t h R i v u l u s demonstrated t h a t male guppies were not s e l e c t i v e l y a t t a c k e d but were l e s s adept than females a t a v o i d i n g c a p t u r e . S i z e - s e l e c t i v e p r e d a t i o n by R i v u l u s probably p l a c e s males at a s e l e c t i v e disadvantage as w e l l . Though I was unable t o t e s t the Haskins hypothesis adequately, the c i r c u m s t a n t i a l evidence suggests t h a t the c o l o u r of male guppies i s not a g r e a t l i a b i l i t y . Because B a l l i n (1973) c o u l d f i n d no evidence of a female p r e f e r e n c e f o r c o n s p i c u o u s l y - c o l o r e d males, the q u e s t i o n of the s i g n i f i c a n c e of geographic v a r i a t i o n and polymorphism i n c o l o r a t i o n i s s t i l l open. Although I concluded t h a t sex r a t i o s have l i t t l e b e a r i n g on the problem of the adapt i v e s i g n i f i c a n c e ( i f any) of c o l o u r p a t t e r n v a r i a t i o n , the work i n Chapter 4 does r a i s e some i n t e r e s t i n g q u e s t i o n s r e g a r d i n g sex r a t i o s per se. There i s a voluminous l i t e r a t u r e on t h i s t o p i c , however, most i n v e s t i g a t o r s have j u s t t a b u l a t e d sex r a t i o s without comment, some have o f f e r e d p l a u s i b l e hypotheses f o r 234 sex r a t i o d i f f e r e n c e s , but very few have ever attempted to t e s t these i d e a s . S i m i l a r l y , my study has s i n g l e d out p r e d a t i o n as an important environmental v a r i a b l e determining the sex r a t i o s of guppies, but t h i s was c o n s i d e r e d o n l y w i t h i n a narrow frame of r e f e r e n c e (colour v a r i a t i o n ) . An i n t e r e s t i n g q u e s t i o n i s what e f f e c t an extremely unbalanced sex r a t i o (e.g. Tompire T r i b u t a r y , P e t i t e Curucaye) has on the p o p u l a t i o n dynamics and mating system of the guppy (and perhaps even the r a t e of e v o l u t i o n a r y change, c f . G i e s e l , 1972). In a d d i t i o n , the q u e s t i o n s r a i s e d i n Chapter 4 con-c e r n i n g geographic v a r i a t i o n i n the sex d i f f e r e n c e s i n a n t i p r e d a t o r b e h a v i o r , and the p o s s i b i l i t y of mimicry i n the guppy-Rivulus system, are worthy of f u r t h e r e x p l o r a t i o n . For example, how does R i v u l u s discrim.ina.te between i t s own f r y and guppy f r y ? Body s i z e The s t r i k i n g geographic v a r i a t i o n i n the body s i z e of a d u l t guppies was c o n s i d e r e d i n Chapter 5. T h i s t o p i c was t r e a t e d o n l y c u r s o r i l y because c o n t r o l l e d experiments ( L i l e y , unpubl.) were s t i l l underway to determine i f there was a g e n e t i c b a s i s f o r p o p u l a t i o n d i f f e r e n c e s i n mean a d u l t body s i z e . N e v e r t h e l e s s , p r e l i m i n a r y f i e l d obserya^ t i o n s suggested t h a t g e n e t i c d i f f e r e n c e s were very probable ( L i l e y ' s experiments subsequently confirmed t h i s f o r s e v e r a l p o p u l a t i o n s ) even though a s u b s t a n t i a l p o r t i o n of the s i z e 235 d i f f e r e n c e s was undoubtedly a d i r e c t phenotypic response to environmental d i f f e r e n c e s , p r i m a r i l y temperature (I found a s i g n i f i c a n t n e g a t i v e c o r r e l a t i o n between body s i z e and stream temperature). Of a m u l t i t u d e of p o t e n t i a l s e l e c t i v e f a c t o r s t h a t might be r e s p o n s i b l e f o r g e n e t i c d i f f e r e n c e s i n body s i z e , I i n v e s t i g a t e d o n l y one, s i z e - s e l e c t i v e p r e d a t i o n . F i e l d and l a b o r a t o r y evidence supported the hypothesis t h a t l a r g e guppies enjoy an advantage with r e s p e c t to R i v u l u s p r e d a t i o n but are more v u l n e r a b l e to l a r g e p r e d a t o r s such as H o p l i a s and C r e n i c i c h l a . In the l a b o r a t o r y , one aspect of s i z e -s e l e c t i v e p r e d a t i o n , the h a n d l i n g e f f i c i e n c y of the predator/ was shown to be an important f a c t o r determining the r e l a t i v e v u l n e r a b i l i t y of guppies of d i f f e r e n t s i z e s . Under n a t u r a l c o n d i t i o n s , however, s e v e r a l a d d i t i o n a l s i z e ' - s p e c i f i c mechanisms presumably i n t e r a c t to determine r e l a t i v e v u l n e r a b i l i t y (e.g. the conspicuousness, a c c e s s i b i l i t y , and escape behavior of the prey; the p r i o r experience and hunger of the p r e d a t o r ) . Although s i z e - s e l e c t i v e p r e d a t i o n i s a p l a u s i b l e e x p l a n a t i o n f o r the e v o l u t i o n of d i f f e r e n c e s i n body s i z e , i t does not adequately e x p l a i n a l l the s i z e t r e n d s . D e t a i l e d work w i l l be r e q u i r e d on o t h e r s e l e c t i v e f a c t o r s such as sexual s e l e c t i o n , water v e l o c i t y , and temperature. A n t i p r e d a t o r behavior In Chapter 6 I attempted to assess the f u n c t i o n a l 236 and e v o l u t i o n a r y s i g n i f i c a n c e of geographic v a r i a t i o n i n a n t i p r e d a t o r behavior. F i e l d o b s e r v a t i o n s r e v e a l e d t h a t where c h a r a c i d and c i c h l i d p r e d a t o r s were pr e s e n t , guppies were more r e s t r i c t e d t o the stream shore, showed a g r e a t e r tendency to s c h o o l , avoided a p o t e n t i a l p r e d a t o r a t a g r e a t e r d i s t a n c e , and had a lower "alarm t h r e s h o l d " . I formulated the hy p o t h e s i s t h a t these b e h a v i o r a l t r a i t s served as a n t i p r e d a t o r mechanisms and t h a t the geographic v a r i a t i o n r e f l e c t e d m i c r o e v o l u t i o n a r y ( i . e . g e n e t i c ) d i f f e r e n c e s . To t e s t t h i s I conducted p r e d a t i o n experiments wi t h samples of wild-caught and p r e d a t o r - n a i v e ( l a b o r a t o r y -bred) guppies t h a t o r i g i n a t e d from 5 n a t u r a l p o p u l a t i o n s . These p o p u l a t i o n s encompassed the range of p r e d a t i o n p r e s s u r e (by f i s h ) , and the degree of geographic i s o l a t i o n found i n the Northern Range. O v e r a l l , the r e s u l t s of these experiments supported my h y p o t h e s i s , i . e . f i s h e i t h e r taken, or descended from, p o p u l a t i o n s exposed t o c h a r a c i d s and c i c h l i d s were r e l a t i v e l y l e s s v u l n e r a b l e than those exposed t o R i v u l u s . To determine why some of the p r e d a t o r - n a i v e stocks were taken non-randomly, I compared t h e i r h a b i t a t p r e f e r e n c e s , s c h o o l i n g behavior, r e a c t i o n d i s t a n c e , and escape motor p a t t e r n s . S i g n i f i c a n t i n t e r s t o c k d i f f e r e n c e s were found f o r s e v e r a l of the measures; g e n e r a l l y , these were c o n s i s t e n t w i t h the f i e l d o b s e r v a t i o n s and a l s o the performance of each stock i n " c o m p e t i t i v e " p r e d a t i o n experiments. For example, Lower 237 A r i p o , one of the l e a s t v u l n e r a b l e s t o c k s , had a long r e a c -t i o n d i s t a n c e t o a standard "predator", was a cohesive s c h o o l e r , and responded (on f i r s t exposure) t o a r e a l preda-t o r with v i g o r o u s escape motor p a t t e r n s , P a r i a guppies, on the o t h e r hand, tended to be l e s s c o h e s i v e , had a s h o r t e r r e a c t i o n d i s t a n c e , and responded weakly t o a p r e d a t o r . Consequently they were more v u l n e r a b l e . I t i s important to s t r e s s t h a t these g e n e r a l i z a t i o n s apply o n l y t o a " t y p i c a l " i n d i v i d u a l o f any giv e n stock (behavior scores f a l l i n g near the mean); i n a l l behavior t e s t s there was some o v e r l a p between even the most d i v e r g e n t stocks ( P a r i a vs. Lower A r i p o ) , Presumably some of t h i s i n t r a s t o c k v a r i a b i l i t y a l s o has a g e n e t i c b a s i s and forms the raw m a t e r i a l upon which s t a b i l i z i n g and d i r e c t i o n a l s e l e c t i o n can operate. Although the main purpose of Chapter 6 was to t e s t the hypothesis t h a t b e h a v i o r a l differences,among n a t u r a l p o p u l a t i o n s of guppies are h e r i t a b l e and serve as a n t i -p r e d a t o r mechanisms, l i t t l e a t t e n t i o n was giv e n to the r o l e of l e a r n i n g i n pr e d a t o r avoidance. I t i s well-known, however, t h a t the tendency to show a n t i p r e d a t o r behavior i s dependent to some degree on e x p e r i e n t i a l f a c t o r s (Nice and Ter Pelkwyk, 1941; Hinde, 1954; S c h l e i d t , 1961; Melzack, 1961; George, 1960; Veselov, 1964; Popov, 1953, c i t e d i n M a n t e i f e l ' and Radakov, 1961; Benzie, 1965; C u r i o , 1969; P i l l , 1972). Though l e a r n i n g undoubtedly o c c u r r e d d u r i n g the 238 course of my long-term experiments, I i m p l i c i t l y assumed t h a t the r a t e s of change i n behavior were the same between st o c k s . T h i s may not be c o r r e c t . In Exp. 6.9 schools of P e t i t e Curucaye f i s h tended to d i s s o c i a t e more r a p i d l y than e i t h e r Lower A r i p o or Guayamare. A l s o i n Exp. 6.2, i t appeared t h a t a f t e r a few hours of exposure to a C r e n i c i c h l a , Lower A r i p o and Guayamare guppies became r e l a t i v e l y more r e a c t i v e ( i n c r e a s e i n r e a c t i o n d i s t a n c e and decrease i n the t h r e s h o l d f o r s u r f a c e jumps, etc.) than P a r i a guppies. These o b s e r v a t i o n s might r e f l e c t d i f f e r e n t stock ( i . e . genotype) x environment i n t e r a c t i o n s . Whether such d i f f e r e n c e s occur between p o p u l a t i o n s of f i s h i s unknown at p r e s e n t , but i n view of s t u d i e s on, (a) s t r a i n d i f f e r -ences i n avoidance c o n d i t i o n i n g i n mice and r a t s ( C o l l i n s , 1964; Bovet e t a l , 1969; Wahlsten, 1972), (b) responses of domestic r a t p o p u l a t i o n s to a r t i f i c i a l s e l e c t i o n f o r high and low r a t e s of avoidance c o n d i t i o n i n g (Bignami, 1965) and (c) d i f f e r e n c e s between s p e c i e s of f i s h i n the e f f e c t of experience w i t h a p r e d a t o r (Benzie, 1965) or other s t i m u l i (Wodinsky e t a l , 1962) i t seems a t l e a s t p o s s i b l e t h a t guppy p o p u l a t i o n s may d i f f e r i n t h e i r r a t e s of avoidance c o n d i -t i o n i n g and h a b i t u a t i o n to p r e d a t o r s . P r e l i m i n a r y i n v e s t i -g a t i o n s of h a b i t u a t i o n ( R u s s e l l , 1967a) and avoidance c o n d i t i o n i n g (Werboff and L l o y d , 1963) have been c a r r i e d out u s i n g domestic guppies but not w i t h i n the framework of a r e a l i s t i c p r e d a t o r - p r e y system. Perhaps these techniques 239 c o u l d be m o d i f i e d f o r a comparative study of l e a r n i n g by u s i n g more n a t u r a l "predator" s t i m u l i and q u a n t i f y i n g a change i n behavior known to be a d a p t i v e . One of the major weaknesses of t h i s work i s t h a t I have s t u d i e d a n t i p r e d a t o r behavior p r i m a r i l y i n a " s i m p l i -f i e d " l a b o r a t o r y environment devoid of much of the s p a t i a l and temporal h e t e r o g e n e i t y known to occur i n nature (Appendix, Tables 1 and 2). Some of these f a c t o r s , n o t a b l y stream morphometry, water v e l o c i t y , t u r b i d i t y , s u b s t r a t e composition, l i g h t i n t e n s i t y , and the presence of other f i s h s p e c i e s (non-predators), may p l a y an important r o l e i n the way a n t i p r e d a t o r behavior c o n t r i b u t e s to s u r v i v a l . More s p e c i f i c a l l y , they may e x p l a i n some of the anomalous r e s u l t s I o b t a i n e d i n c e r t a i n t e s t s . For example-, the mean r e a c t i o n d i s t a n c e of Guayamare and P e t i t e Curucaye stock was r e l a -t i v e l y s h o r t e r than I had expected on the b a s i s of p r e d a t i o n p r e s s u r e alone. However, these r e s u l t s are l e s s s u r p r i s i n g when i t i s known t h a t these streams are , r e s p e c t i v e l y , t u r b i d and very s m a l l . Suggestions f o r f u t u r e r e s e a r c h With the completion of t h i s t h e s i s , the a n a l y s i s of the c a u s a t i o n of i n t r a s p e c i f i c v a r i a t i o n i n guppies has o n l y begun. I hope the f o l l o w i n g suggestions may s t i m u l a t e f u r t h e r work on t h i s t o p i c . 1. For each of the p r i n c i p a l p o p u l a t i o n s of guppies, 240 " p r e d a t i o n p r e s s u r e " ( i . e . p r e d a t o r biomass x frequency of p r e d a t i o n on guppies) should be estimated q u a n t i t a t i v e l y . T h i s w i l l r e q u i r e an i n t e n s i v e sampling program ( i n c l u d i n g suspected a e r i a l p r e dators) t o be c a r r i e d out i n a l l seasons. 2. The s u r v i v a l value o f behavior should be demon-s t r a t e d by t r a n s p l a n t experiments i n the f i e l d . T h i s might answer some of the ques t i o n s concerning the r o l e of l i g h t i n p r e d a t o r avoidance and the e x i s t e n c e of p r e d a t o r - s p e c i f i c a n t i p r e d a t o r a d a p t a t i o n s . 3. As an expansion of suggestion 2, s e v e r a l l a r g e -s c a l e (and long-term!) t r a n s p l a n t experiments c o u l d be conducted. The streams f l o w i n g o f f the no r t h e r n face of the Northern Range (e.g. P a r i a , Marianne, Yarra) are devoid of c h a r a c i d s and c i c h l i d s (see F i g u r e 3), Neve r t h e l e s s the e c o l o g i c a l c o n d i t i o n s f o r such s p e c i e s as C r e n i c i c h l a and Astyanax appear t o be i d e a l . A r e v e a l i n g experiment would be t o stock the P a r i a R. with C r e n i c i c h l a and monitor the immediate and long-term changes ( i f any) i n (a) the c o l o r a -t i o n o f males, (b) the sex r a t i o , (c) body s i z e , (d) a n t i -p r e d a t o r b e h a v i o r — e s p e c i a l l y h a b i t a t s e l e c t i o n , r e a c t i o n d i s t a n c e , and s c h o o l i n g b e h a v i o r , (e) c o u r t s h i p and aggres-s i v e behavior ( c f . B a l l i n , 1973), and (f) p o p u l a t i o n para-meters such as age s t r u c t u r e and f e c u n d i t y , A p a r a l l e l - f l o w i n g stream (Yarra) a l s o devoid of C r e n i c i c h l a c o u l d serve as a c o n t r o l . (The c o l o u r p a t t e r n s 241 [ L i l e y and Seghers, unpubl.] and a n t i p r e d a t o r behavior [ p e r s o n a l o b s e r v a t i o n s ] of guppies i n t h i s stream are very s i m i l a r t o Pa r i a . ) I f , a f t e r the i n t r o d u c t i o n of C r e n i c i c h l a , P a r i a guppies tended t o converge towards a Lower A r i p o pheno-type, t h i s would be c o n v i n c i n g proof o f the e f f i c a c y o f a p i s c i v o r e i n n a t u r a l s e l e c t i o n . Another r e v e a l i n g experiment would be to stock a small stream devoid o f f i s h (there are s e v e r a l along the no r t h c o a s t of T r i n i d a d ) with a founder p o p u l a t i o n o f Lower A r i p o guppies and monitor the same mor p h o l o g i c a l and b e h a v i o r a l parameters as above. With a n a t u r a l g e n e r a t i o n time of 3-4 months, I suspect t h a t over s e v e r a l years there would be ample o p p o r t u n i t y (through s e g r e g a t i o n and recombination) f o r concealed v a r i a t i o n to be r e l e a s e d and exposed t o new s e l e c t i o n p r e s s u r e s , v i z . r e l a x e d or i n t e n s i f i e d p r e d a t i o n . Mather (1970) has s t a t e d t h a t i t i s not uncommon i n a r t i -f i c i a l s e l e c t i o n experiments f o r the mean of a q u a n t i t a t i v e t r a i t t o be pushed beyond the range of the o r i g i n a l popula-t i o n i n about 12 g e n e r a t i o n s . Whether s e l e c t i o n can proceed t h i s r a p i d l y i n a w e l l - b u f f e r e d n a t u r a l p o p u l a t i o n remains to be demonstrated. 4. The work i n Chapter 6 should be extended i n t o a more sy s t e m a t i c study o f the g e n e t i c s of a n t i p r e d a t o r behavior. There i s a c o n s i d e r a b l e p s y c h o l o g i c a l l i t e r a t u r e on the beh a v i o r g e n e t i c s of " f e a r " and "boldness' 1 i n mammals 242 (Dawson, 1932; F o s t e r , 1959; Whitney, 1970; P l u t c h i k , 1971) but i n f o r m a t i o n f o r f i s h (and lower v e r t e b r a t e s i n general) i s v i r t u a l l y n o n - e x i s t e n t . As Thiessen. (1972? 116) r i g h t l y p o i n t s out, " B e h a v i o r a l e v o l u t i o n has been l e f t p r i m a r i l y to the e t h o l o g i s t s , who, i n s p i t e of t h e i r i m pressive accomplishments, l a c k g e n e t i c s o p h i s t i c a t i o n , . , ." Never-t h e s l e s s , both B r u e l l (1967) and T h i e s s e n (1972) s t r e s s t h a t the kinds of i n f o r m a t i o n which are now a v a i l a b l e f o r the guppy (adaptive s i g n i f i c a n c e of behavior d i f f e r e n c e s ) are a necessary p r e r e q u i s i t e f o r f u t u r e progress i n the s c i e n c e of behavior g e n e t i c s . i propose, t h a t a d e t a i l e d study be c a r r i e d out on the behavior of the h y b r i d s (F^, F 2 , and baekcrosses) of some of the more d i v e r g e n t s t o c k s (e.g. Lower A r i p o x P a r i a ) . Another approach would be to d e l i b e r a t e l y s e l e c t (or l e t a p r e d a t o r s e l e c t ) f o r animals showing s u p e r i o r escape behavior. I t would be of c o n s i d e r a b l e t h e o r e t i c a l i n t e r e s t , f o r example, t o determine i f a r t i f i c i a l s e l e c t i o n f o r i n c r e a s e d r e a c t i o n d i s t a n c e has any e f f e c t on o t h e r , p o s s i b l y t i g h t l y - l i n k e d , or "coadapted" b e h a v i o r a l t r a i t s such as s c h o o l i n g b e h a v i o r , h a b i t a t p r e f e r e n c e , c o u r t s h i p d i s p l a y , e t c . 5. The f i n a l suggestion i s t h a t work should be done on the proximate f a c t o r s r e s p o n s i b l e f o r the b e h a v i o r a l d i f f e r e n c e s . Why, on i t s f i r s t meeting wi t h a hungry R i v u l u s , does a Lower A r i p o guppy respond w i t h a r a p i d d a r t 243 f o l l o w e d by a s e r i e s of s u r f a c e jumps, when under the same circumstances a P a r i a guppy h a r d l y responds a t a l l ? Why d i d 20% of P e t i t e Curucaye and 12% of Paria, guppies not "re c o g n i z e " the mot i o n l e s s C r e n i c i c h l a i n Exp, 6.6? These q u e s t i o n s have a d i r e c t b e a r i n g on the search f o r the e l u s i v e "innate r e l e a s i n g mechanism'1 (IRM) under-l y i n g a n t i p r e d a t o r b e h a v i o r . Though i n t e r e s t i n g hypotheses can be generated from e t h o l o g i c a l analyses (e.g. C u r i o , 1969), i t w i l l e v e n t u a l l y be necessary t o t r a n s l a t e o v e r t b e h a v i o r a l d i f f e r e n c e s i n t o g e n e t i c e f f e c t s on the s t r u c t u r e or f u n c t i o n of the nervous system (Manning, 1967) * T h i s c o u l d pose q u i t e a c h a l l e n g e because the n e u r a l c o r r e l a t e s of avoidance behavior have been s t u d i e d i n only a few animals (e.g. Ewert, 1970); furthermore very l i t t l e i s known about the g e n e t i c aspects of i n t r a s p e c i f i c v a r i a t i o n i n sensory mechanisms (Marler and Hamilton, 1966). In the guppy, some of the geographic d i f f e r e n c e s i n behavior may be caused simply by d i f f e r e n c e s i n the s t r u c t u r e or s e n s i t i v i t y of the eye. 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APPENDIX Contents TABLE Page 1 P h y s i c a l and chemical c h a r a c t e r i s t i c s of streams i n the Northern Range r e g i o n 262 2 The f a m i l i e s and s p e c i e s of f i s h c o l l e c t e d a t 23 s i t e s i n the Northern Range r e g i o n . . . 268 3 Summary of the d i e t s of the main p o t e n t i a l p r e d a t o r s of the guppy . . . . . . 269 4 S i z e s t a t i s t i c s of guppies used i n Exp. 6.2. . . 270 FIGURE 1 Map of the experimental s e c t i o n of the P e t i t e Curucaye R , . » , , . , 271 2 S i z e d i s t r i b u t i o n of the e n t i r e guppy p o p u l a t i o n of the experimental s e c t i o n of P e t i t e Curucaye R i v e r 272 3 S i z e d i s t r i b u t i o n of the e n t i r e R i v u l u s p o p u l a t i o n of the experimental s e c t i o n of P e t i t e Curucaye R i v e r . . . . . . . . . . . 273 261 262 TABLE 1. P h y s i c a l and chemical c h a r a c t e r i s t i c s of streams i n the Northern Range r e g i o n . The measurements were made i n 1969. For methods r e f e r t o Chapter 2. For stream l o c a t i o n s and a b b r e v i a t i o n s r e f e r to Chapter 2, F i g u r e 1, Key to symbols Weather: S - f u l l sun; no l a r g e clouds SP - sunny p e r i o d s ; no r a i n but i n t e r m i t t e n t c l o u d cover SPR - sunny p e r i o d s w i t h r a i n (L) - l i g h t r a i n (H) - heavy r a i n C - cloudy with no sunny p e r i o d s CR - cloudy w i t h r a i n (L) or (H) HR - heavy continuous r a i n C h a r a c t e r i s t i c s of stream: W - mean width of stream i n metres D - mean depth of stream i n metres a - a constant used i n c a l c u l a t i n g volume of flow 0.9 - smooth sand, s i l t , mud 0.8 - rough r o c k s , l a r g e pebbles R - volume of flow (discharge) i n m^/sec V - water v e l o c i t y of main p o r t i o n of stream i n m/sec T - water temperature i n °C DH - t o t a l degrees of water hardness i n ppm C - degree of overhead cover (shade) 0 - no shade 1 - s m a l l amount of shade r e s t r i c t e d mainly t o streambank 2 .-- medium: shade' (50% cover) 3 - medium to dense (75% cover w i t h few exposed p a r t s ) 4 - very dense cover with v i r t u a l l y complete shading Turb - water t u r b i d i t y 0 - always c l e a r 1 - t u r b i d o n l y a f t e r heavy r a i n s 2 - t u r b i d throughout year TABLE 1 (Continued) Stream Date Time Weather W D a R V T p H D H C Turb 20 Mar 1100 S 0 . 6 0 0 . 0 5 0 . 9 0 . 0 0 7 2 0 . 2 6 6 2 4 . 7 7 . 5 300 4 28 1100 SP 0 . 4 0 0 . 0 4 0 . 0 0 5 2 0 . 3 6 3 2 4 . 5 7 . 3 300 14 Apr 1400 SPR(H) 0 . 6 0 0 . 0 5 0 . 0 0 5 4 0 . 2 0 0 2 5 . 0 7 . 2 280 27 1530 — 0 . 6 0 0 . 0 5 0 . 0 0 6 4 0 . 2 3 5 2 5 . 0 7 . 2 — 16 May 1645 SP — — — -- 2 5 . 1 7 . 2 260 17 1400 s 0 . 7 0 0 . 0 4 0 . 0 0 4 8 0 . 1 9 0 2 5 . 1 7 . 2 250 19 1630 s — — • — 2 5 . 0 7 . 2 250 20 1030 SP 0 . 7 5 0 . 0 6 0 . 0 2 0 2 0 . 5 0 0 2 5 . 1 7 . 2 240 20 1630 HR — — — 2 4 . 8 7 . 2 220 22 0915 S — — — — 2 4 . 8 7 . 2 220 22 1445 S - — — — 2 5 . 5 7 . 2 220 27 1545 SPR(H) — — — 2 5 . 1 7 . 2 200 7 Jun 1400 C 0 . 9 0 0 . 0 7 0 . 0 1 4 2 0 . 2 5 0 2 5 . 0 7 . 2 210 13 1100 SPR — — — ••— 2 5 . 3 7 . 2 230 18 1315 S 0 . 5 0 0 . 0 5 0 . 0 0 6 4 0 . 2 8 6 2 5 . 2 7 . 2 180 1 J u l 1500 SP 0 . 6 0 0 . 0 4 0 . 0 0 6 6 0 . 3 0 8 2 5 . 1 7 . 2 150 5 1730 S — -- — 2 5 . 0 7 , 2 170 Means 0 . 6 3 0 . 0 5 0 . 0 0 8 5 0 . 2 8 9 2 5 . 0 7 . 2 230 GCur 22 Apr 1100 SP 1 . 0 0 0 . 1 5 0 . 8 0 . 0 1 2 8 0 . 1 0 7 2 5 , 1 7 . 0 — 3 20 May 0920 C -- -- — — 2 5 . 1 7 . 2 200 Means 1 . 0 0 0 . 1 5 0 . 0 1 2 8 0 . 1 0 7 2 5 . 1 7 , 1 200 UCur 13 Mar 1330 S 0 . 8 — 2 4 . 8 7 . 4 305 3 0 TT 25 Mar 1400 SP 0 . 9 0 0 . 1 1 0 . 9 0 0 2 6 . 2 7 . 2 — 4 0 TABLE 1 (Continued) Stream Date Time Weather W D a . R V T pH DH C Turb UA(N) 29 Mar 1030 SP . . _ _ 0.9 _ _ 24.5 7.5 210 3 0 13 Apr 1120 S — -- -- — 24.8 7.6 190 3 J u l 1100 SP — — — 24.5 7.4 150 Means 24.6 7.5 183,3 UA(X) 29 Mar 0930 SP 0.8 — — 24.0 7.6 220 3 0 13 May 1330 SP 3.00 0.08 0.0480 0.250 28.1 7.6 190 3 Jun 1500 SPR(L) — 28.8 7.6 130 15 1300 CR(H) 4.00 0.22 0.7040 1.000 24.1 7.0 110 21 1200 CR(H) — 26.3 7.2 90 3 J u l 1200 SP 4.00 0.08 0,1024 0.400 26,4 7,6 150 Means 3.67 0,13. . 0,2848 . 0.550 2.6,3 7.4 148,3 UTac 15 Apr 1630 SP 1.30 0.15 0,8 0.0780 0.500 26.9 7.0 100 2 0 24 May 1520 S 1.10 0,09 0.0297 0.375 27.8 7.0 90 Means 1.20 0.12 0.05 39 0.438 2.7.4 7.0 .9.5. UArouc 2 May 1500 S 2,00 0,06 0,9 0.0348 0.323 26 .8 7.0 2 0 UGuan 2 May 1200 S 5.00 0.10 0.8 0.2667 0.667 26.5 7.0 2 0 TABLE 1 (Continued) Stream Date Time Weather w D a R V T pH DH C Turb BB 6 May 1220 SP 0 . 8 0 0 . 0 8 0 . 8 0 . 0 2 7 9 0 . 5 4 5 2 4 . 9 7 . 6 3 0 29 1300 SPR(L) — -- 0 . 5 0 0 2 4 . 9 7 . 4 140 24 Jun 1400 SPR(L) — -- ••— -- 2 5 . 2 7 . 6 150 Means 0 . 8 0 0 . 0 8 0 . 0 2 7 9 0 . 5 2 3 2 5 . 0 7 . 5 145 Mar 19 Mar 1 5 3 0 SP 4 . 5 0 0 . 2 0 0 . 8 0 . 3 6 0 0 . 5 0 0 2 3 . 5 7 . 1 ,140 2 0 29 Jun 1200 SP 5 . 0 0 0 . 1 1 0 . 2 5 9 0 . 5 8 8 2 5 . 1 7 . 2 100 Means 4 . 7 5 0 . 1 6 0 . 3 0 9 0 . 5 4 4 2 4 . 3 7 . 2 120 Oro 8 Mar 1500 S 0 . 8 — „ _ 2 6 . 0 7 . 8 250 2 0 - 1 25 Apr 1030 SP 6 . 0 0 0 . 2 0 1 . 1 2 9 1 . 1 7 6 2 6 . 8 7 . 2 r-T-10 May 1200 S — — 2 7 . 2 7 . 4 190 Means 6 . 0 0 0 . 2 0 1 . 1 2 9 1 . 1 7 6 2 6 . 7 7 . 5 220 Guay 16 Mar 1000 SP — — _ — 0 . 9 — — 2 8 . 0 7 . 1 110 1 2 7 Apr 1115 SP -- -- — 2 9 . 2 7 . 0 120 9 1600 S — - — _ 3 0 . 9 7 . 0 130 12 1100 C — — 2 8 . 6 7 . 0 120 17 1500 S -- — — 3 0 . 9 7 . 0 30 0630 S -- — -- 2 8 . 5 7 . 0 12 May 1400 S — -- -- 3 1 . 0 7 . 0 130 10 Jun 1500 S — — -T-. • 2 7 . 9 6 . 8 70 30 1330 SPR(H) 2 . 0 0 1 .50 1 . 0 8 * 0 . 4 0 0 2 7 , 0 7 . 0 80 Means 2 . 0 0 1 . 5 0 1 . 0 8 0 . 4 0 0 2 9 . 1 7 . 0 1 0 8 . 6 *rough a p p r o x i m a t i o n — t h e volume of flow of lowland r i v e r s i s very v a r i a b l e depending on season and weather. TABLE 1 (Continued Stream Date Time Weather W D a R V T PH DH C Turb Cap 24 Apr 1100 S 0.9 28.0 7.0 0 2 17 Jun 1600 CR(H) - r - ~- 26.2 6.8 110 30 1200 CR(H) 5.00 1.50 2.25* 0.333 26.5 7.0 110 Means 5.00 1.50 2.25 0.333 26.9 6.9 110 Car 7 Apr 1100 SP _ 0.9 30.2 7.0 170 0 2 30 Jun 1400 SPR(H) 25.0 3.00 22.50* 0.333 27.2 7.0 90 Means 25.0 3.00 22.50 0.333 28.7 7.0 130 Yar 19 Mar 1700 SP 0.75 0.07 0.8 0.0280 0.667 24.0 7.1 140 2 0 29 Jun 1400 CR(L) 0.80 0.13 0.0289 0.348 26.0 7.0 80 Means 0.78 0.10 0.0285 0.508 25.0 7.1 110 Par 18 Apr 1215 SPR 3.00 0.20 0.8 0,0828 0,172 25.0 7.1 2 0 11 Jun 1400 SP — -- •— 26.5 7.0 100 22 1300 SPR(L) — -- — -i- 25.0 7.0 100 29 1600 CR(L) 4.00 0.14 0.8 0.0729 0.163 25.1 7.0 90 Means 3.50 0.17 0,0779 0.168 25.4 7.0 96.7 to CTi TABLE 1 (Continued) Stream Date Time Weather w D a R V T pH DH C Turb LA 2 Apr 1115 SP 5.00 0.12 0.8 0.3200 0.667 26.5 7.5 230 2 0-1 3 Jun 1600 CR — — 25.5 7.2 90 21 1600 SPR(L) — — — 25.6 7.0 90 3 J u l 1300 SP 7.00 0.13 0 . 4 C 4 0 0.555 26.4 7.2 100 Means 6.00 0.13 0.3620 0.611 26.0 7.2 127.5 LTac 5 Apr 1000 CR 3.00 0.15 0.8 0 0 28.4 8.0 + 190 2 0-1 10 1330 — — — — 33.5 8.0+ 190 3 J u l 1530 SP 3.00 0.15 0.150 0.417 30.0 7.6 100 Means 3.00 0.15 0.075 . 0.209 .30.6 -*- 160 13 Mar 1500 S — — 0.8 — 29.9 • . — - r -28 1330 SP — 28.4 7.2 300 14 Apr 1530 SPR(H) 8.00 0.20 0.768 0.600 27.4 7.2 280 16 May 1715 SP -- — — 27.5 7.0 250 5 J u l 1700 S — — - T- — 27,9 7.2 160 Means 8.00 0,20 0.768 0.600 28,2 7,2 247,5 268a TABLE 2. The f a m i l i e s and s p e c i e s o f f i s h c o l l e c t e d a t 23 s i t e s i n the Northern Range r e g i o n . For a l i s t of stream a b b r e v i a t i o n s , r e f e r t o F i g . 1. FAMILY AND SPECIES UCur PCur SLR MV TT GCur UA B3 Par Yar UArouc UGuan UTac LPar Mar LCur SC Oro LTac LA Cap Car Guay F. Poeciliidae Poecilia reticulata + + + + + + + + + + + + + + + + + + + + t + + F. Cyprinodontidae Rivulus h a r t i i + + + + + + + + + + + F. Cichlidae Aeguidens pulcher + + + + + + + + Cichlasoma biroaculatum + + Crenicichla alta + + + + + + + b b + Tilapia mossambica + F. Characidae Astyanax bimaculatus + + + + + + + Hemibrycon sp. + + + + Curiroata argentea + + + + + + Roboides dayi  Kopliai~malabc  Corynopoina r i j Hemigrammus~unilineatus P r i s t e l l a r i d d l e i or Aphyocharax axelrodi s abaricus a a + + + + a + + nopom i s e i + + + + + + + F. Loricariidae Hypostomus ro b i n i i + a + + + + + + + + + t Ancistrus cirrhosus + + + F. Pimelodidae Rharr.dia sp. + + + + + + + F. Synbranchidae Synbranchus marmoratus + + F. Callichthyidae Corydoras aeneus + + F. Mugilidae Agonostornus F.onticola (?) + F. Gobiidae Sicydium sp. (?) + + + + Unidentified goby + M • . O a - species not seen or collected in this study but presence determined through personal communication with local °* residents. b - species present in river according to Boeseman (1960). 269a TABLE 3. Summary of the d i e t s of the main p o t e n t i a l p r e d a t o r s of the guppy. The data are expressed as the per cent frequency of occurrence of each food item. RIVULUS CRENICICHLA AEQUIDENS HEMIBRYCON ASTYANAX HOPLIAS L. Aripo L. Aripo L. Tacarigua L. Tacarigua Maracas Caroni Guayamare Number examined 259 14 42 64 28 7 Number empty 5 0 5 3 1 6 Size range (mm) 22-105 60-195 80-148 62-90 45-116 90-392 B. Basin L. Aripo L. Aripo L. Aripo Source Tomp. T r i b . Maracas L. Tacarigua U. Arouca P. Curucaye Maracas Maracas Contents P. r e t i c u l a t a . 10.2 14.3 1.6 18.5 Other f i s h 42. 9 a 3.7a Oligochaeta 3.7 Crustacea 3.7 Mollusca 14.3 29.7 Ephemeroptera 7.1 4.9 29.6 Tr i c h o p t e r a 5.5 7.1 2.7 1.6 3.7 Diptera 3.9 3.3 11.1 Coleoptera 10.6 8.2 3.7 Plecoptera 0.8 Komcptera 0.8 Hemiptera 7.1 3.3 Arachnida 3.9 3.7 Hymenoptera c 42.1 2.7 22.9 25.9 Odonata 0.8 1.6 7.4 Orthoptera 1.2 Chilopoda 0.8 Diplopoda 2.8 Thysanoptera 0.4 Others^ 3.5 64.3 10.8 27.9 18.5 Algae 3.1 2.7 27.9 40.7 Vascular plants 5.9 2.7 3.3 25.9 F i s h vertebrae, s c a l e s . C r e n i c i c h l a a l t a . CA11 Formicidae. U n i d e n t i f i e d l a r v a l and adult i n s e c t p a r t s . TABLE 4. S i z e s t a t i s t i c s of guppies used i n Exp. 6.2. A l l s i z e s are i n mm t o t a l length, n = 50 f o r each mean. A. 12 l i g h t — 1 2 dark B. 12 l i g h t — 1 2 dim l i g h t x SD SE Range x SD SE Range Females 1. Lower A r i p o 20. 34 1. 61 0. 23 17. 0 -- 24 .0 24.40 3. 32 0 .47 18 .5 - 30 .0 Guayamare 21. 08 1. 56 0. 22 17. 0 -- 25 .0 25.30 4. 31 0 .61 17 .0 - 33 .0 2. Lower A r i p o 21. 54 2. 32 0. 33 18. 0 -- 25 .5 20.76 1. 84 0 .26 16 .0 - 25 .0 P e t i t e Curucaye 22. 07 2. 16 0. 31 18. 0 -- 25 .5 20.59 2. 19 0 .31 17 .0 - 26 .5 3. Lower A r i p o 22. 32 2. 48 0. 35 17. 5 -- 26 .5 20.92 5. 01 0 .71 16 .0 - 34 .0 P a r i a 21. 43 3. 67 0. 52 17. 0 -- 28 .5 21.95 4. 13 0 .58 16 .0 - 31 .5 4. Lower A r i p o 22. 40 2. 99 0. 42 18. 0 -- 29 .0 21.66 2. 26 0 .32 18 .5 - 27 .0 Upper A r i p o 22. 21 2. 43 0. 34 18. 0 r - 28 .0 20.68 1. 97 0 .28 17 .0 - 25 .5 Males 1. Lower A r i p o 20. 86 1. 84 0. 26 17. 0 -- 27 .5 19.31 1. 92 0 .27 16 .0 - 23 .5 Guayamare 20. 64 1. 70 0. 24 18. 0 -- 25 .0 18.54 2. 02 0 .29 15 ,0 - 23 .0 2. Lower A r i p o 21. 10 1. 48 0. 21 19. 0 -- 25 .0 20.39 1. 48 0 .21 18 .0 - 23 .5 P e t i t e Curucaye 21. 45 1. 67 0. 24 18. 5 -- 25 .0 20.42 1. 67 0 .24 17 .5 - 24 .0 3. Lower A r i p o 21. 00 1. 43 0. 20 18. 0 -- 24 .0 20.74 1. 42 0 .20 18 .0 r- 23 .5 P a r i a 20. 39 1. 38 0. 19 17. 5 -- 23 .0 20.64 1. 31 0 .19 17 .5 r- 24 • 0 4. Lower A r i p o 21. 19 1. 18 0. 17 18. 5 -- 24 .0 18.79 2. 62 0 .37 16 .0 - 25 .0 Upper A r i p o 21. 50 1. 82 0. 26 17. 5 -- 24 .5 20.18 3. 09 0 .44 16 .0 r- 26 .0 271a FIGURE 1. Map of the experimental s e c t i o n of the P e t i t e Curucaye R. 271b P E T I T E CURUCAYE R I V E R EXPERIMENTAL SECTION (July 1,1969 ) 5 10 METRES \ \ \ \ X \ \ \ \ \ SCREEN 1 POOL 1 (max. depth 17cm) POOL 2 (max. depth 28cm) POOL 3 (max. depth 16 cm) 4 (max. depth 8 cm) N SCREEN 3 iPOOL 5 v|"OUT (to SANTA CRUZ R.-1.6 km) l T 272a FIGURE 2. S i z e d i s t r i b u t i o n of the e n t i r e guppy p o p u l a t i o n of the experimental s e c t i o n of P e t i t e Curucaye R i v e r . FREQUENCY (%) 273a FIGURE 3. S i z e d i s t r i b u t i o n of the e n t i r e R i v u l u s p o p u l a t i o n of the experimental s e c t i o n of P e t i t e Curucaye R i v e r . 273b 0 10 20 30 40 50 60 70 80 90 100 T O T A L L E N G T H (mm) 

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