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Behavioral ecology of the leaf-cutting ant, Acromyrmex octospinosus (Reich), in Guadeloupe, F.W.I. Therrien, Pierre 1986

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BEHAVIORAL ECOLOGY OF THE LEAF-CUTTING ANT, ACROMYRMEX OCTOSPINOSUS (REICH), IN GUADELOUPE, F.W.I. by PIERRE THERRIEN A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n THE FACULTY OF GRADUATE STUDIES Department of Pla n t Science 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 1986 © P i e r r e T h e r r i e n , 1986 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the of B r i t i s h Columbia, I agree t h a t the 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 study. I f u r t h e r agree 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 copying o f t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the head of my department or by h i s or her r e p r e s e n t a t i v e s . I t i s understood t h a t copying or p u b l i c a t i o n of 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 allowed without my w r i t t e n p e r m i s s i o n . requirements f o r an advanced degree a t the U n i v e r s i t y Department o The U n i v e r s i t y of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 E -6 n/R-n ABSTRACT The f o r a g i n g behavior of the l e a f - c u t t i n g ant, Acromyr-mex oct ospinosus ( R e i c h ) , was s t u d i e d between September 1983 and August 1984 i n 2 areas of Guadeloupe, F.W.I. At Le-mesle, the ants u s u a l l y foraged p r i m a r i l y d u r i n g the day, but o c c a s i o n a l l y were most a c t i v e at n i g h t . At Boi s de Lomard, a d r i e r area, a l l the c o l o n i e s foraged mostly at n i g h t , but c o l o n i e s with part of t h e i r t e r r i t o r y i n the shade sometimes e x h i b i t e d low l e v e l s of a c t i v i t y d u r i n g the day. E f f i c i e n c y of food g a t h e r i n g (% incoming ants laden) v a r i e d d u r i n g the day, but u s u a l l y f o l l o w e d the above pat-t e r n s of a c t i v i t y . Monthly c o r r e l a t i o n s between t r a i l sur-face temperature or vapor d e n s i t y d e f i c i t and t o t a l a c t i v i t y / m i n or e f f i c i e n c y were extremely v a r i a b l e , except fo r c o l o n i e s t h a t d i d not s h i f t the t i m i n g of t h e i r peak a c t i v i t y ; f o r these, 75% of the c o r r e l a t i o n s were s i g n i f i c a n t . Peak a c t i v i t y and e f f i c i e n c y o c c u r r e d d u r i n g the dry season f o r 5 c o l o n i e s , 2 or 3 mo before the n u p t i a l f l i g h t d u r i n g the f i r s t h a l f of May. The average e f f i c i e n c y l e v e l of c o l o n i e s was not c o r r e l a t e d with the average s i z e of t h e i r f o r a g i n g f o r c e . The ants v i s i t e d 83% of the p l a n t s p e c i e s i n t h e i r t e r -r i t o r y , but a c t i v e l y cut only 44% and 31% of them at Bois de Lomard and Lemesle, r e s p e c t i v e l y . A small number of p l a n t s p e c i e s c o n s t i t u t e d most of the d i e t of the a n t s , and the d o m i n a n c e - d i v e r s i t y curves f o r the p r o p o r t i o n of each p l a n t s p e c i e s i n the d i e t r e v e a l e d a l o g normal d i s t r i b u t i o n . The i i colony p r e f e r e n c e f o r a set of p l a n t s p e c i e s was determined by the combined c h o i c e s of a l l f o r a g e r s , although some i n d i -v i d u a l s had a pre f e r e n c e d i f f e r e n t from the o v e r a l l colony c h o i c e . The ants p r e f e r r e d f l o w e r s , f r u i t s , or young l e a v e s . More of the f i r s t 2 p l a n t p a r t s were cut at Lemesle than at Bois de Lomard. The p r o p o r t i o n s of the d i f f e r e n t p l a n t p a r t s c o l l e c t e d v a r i e d each month, as ants responded to the appearance of the p r e f e r r e d p l a n t p a r t s by concen-t r a t i n g t h e i r f o r a g i n g on them or on other p o r t i o n s of the p l a n t s p e c i e s h a r b o r i n g them. The ants c o l l e c t e d an average of 364 kg f r e s h m a t e r i a l / h a / y e a r . The compass d i s t r i b u t i o n of the primary f o r a g i n g t r a i l s around c o l o n i e s was not s i g n i f i c a n t l y d i f f e r e n t from random f o r 3 of 4 c o l o n i e s at Bois de Lomard, and 4 of 5 c o l o n i e s at Lemesle. Temporary t r a i l s were c r e a t e d to t r a n s i t o r y r e -sources, while "permanent" t r a i l s ( l i f e s p a n >120 days) l e d to l e s s t r a n s i e n t r e s o u r c e s . C o l o n i e s near f r u i t bearing t r e e s u s u a l l y had more t r a i l s than c o l o n i e s f a r t h e r away. The s i z e of f o r a g e r s caught at the ends of 0.15, 5.25, and 13 m t r a i l s i n c r e a s e d e x p o n e n t i a l l y with the l e n g t h of the t r a i l s . The pheromone t r a i l e s t a b l i s h e d to p a r t i c u l a r r e -sources p e r s i s t e d about 40 h i n the f i e l d . A t t e n u a t i o n oc-c u r r e d i n stepwise f a s h i o n , with the h i g h e s t r a t e d u r i n g the day. Pheromone t r a i l s exposed to UV l i g h t (253.7 nm) i n the l a b o r a t o r y disappeared twice as f a s t as c o n t r o l t r a i l s . Weather a f f e c t s the t i m i n g of f o r a g i n g a c t i v i t y of Acromyrmex oct ospi nosus . The i n s e c t has developped an opportunistic system of foraging. The amount df biomass cut by colonies is relatively low, and the insect would not be a problem if it did not sometimes concentrate its cutting in subsistance gardens. iv Table of Contents ABSTRACT i i LIST OF TABLES v i i i LIST OF FIGURES x ACKNOWLEDGEMENTS x i i i 1 . INTRODUCTION 1 1 .1 TAXONOMY OF LEAF-CUTTERS 1 1 .2 DISTRIBUTION 1 1.3 NATURAL HISTORY 2 1.3.1 MATING AND COLONY FOUNDING .2 1.3.2 POLYMORPHISM AND DIVISION OF LABOR 4 1.3.3 THE ANT-FUNGUS SYMBIOSIS 5 1.4 GUADELOUPE AND Acromyrmex oct ospi nosus 6 1.4.1 GUADELOUPE 6 1.4.2 Acromyrmex oct ospi nosus 7 2. FORAGING ACTIVITY RHYTHM 9 2.1 INTRODUCTION 9 2.2 MATERIALS AND METHODS 10 2.3 RESULTS 11 2.3.1 GENERAL WEATHER 11 2.3.2 DAILY ACTIVITY CYCLE 16 2.3.2.1 TOTAL ACTIVITY 16 2.3.2.2 PROPORTION OF RETURNING ANTS LADEN 40 2.3.3 SEASONAL ACTIVITY CYCLE 49 2.3.3.1 TOTAL ACTIVITY 49 2.3.3.2 PROPORTION OF RETURNING ANTS LADEN 52 2.4 DISCUSSION 55 v 3. FORAGE COLLECTED AND INDIVIDUAL CHOICE 65 3.1 FORAGE COLLECTED 65 3 .1 .1 INTRODUCTION 65 3 . 1 . 2 MATERIALS AND METHODS 66 3 . 1 . 3 RESULTS 67 3 . 1 . 3 . 1 OVERALL PATTERN 67 3 . 1 . 3 . 2 PLANT PARTS GATHERED 74 3 . 1 . 3 . 3 MONTHLY VARIATIONS 80 3 . 1 . 3 . 4 BIOMASS COLLECTED 87 3 . 1 . 4 DISCUSSION 87 3.2 INDIVIDUAL CHOICE 96 3 .2 .1 INTRODUCTION 96 3 . 2 . 2 MATERIALS AND METHODS 96 3 . 2 . 3 RESULTS 100 3 . 2 . 4 DISCUSSION 106 3.3 LIST OF PLANT SPECIES 111 4. THE FORAGING TRAIL 116 4.1 GENERAL INTRODUCTION 116 4 .1 .1 COMPONENTS OF THE FORAGING TRAILS 116 4 . 1 . 2 ROLES OF PHEROMONES AND TRAILS 117 4.2 FORAGING AND TRAIL DYNAMICS 119 4 .2 .1 INTRODUCTION 119 4 . 2 . 2 MATERIALS AND METHODS 120 4 . 2 . 3 RESULTS 120 4 . 2 . 4 DISCUSSION 126 4.3 PERSISTENCE OF THE PHEROMONE TRAIL 128 4.3 .1 INTRODUCTION 128 v i 4 .3 .2 MATERIALS AND METHODS 128 4 . 3 . 2 . 1 PERSISTENCE IN NATURE 128 4 . 3 . 2 . 2 EFFECT OF ULTRAVIOLET LIGHT 129 4 . 3 . 3 RESULTS . . 130 4 . 3 . 3 . 1 PERSISTENCE IN NATURE 130 4 . 3 . 3 . 2 EFFECTS OF ULTRAVIOLET LIGHT 134 4 . 3 . 4 DISCUSSION 134 4.4 SIZE DISTRIBUTION OF FORAGERS 138 4.4 .1 INTRODUCTION 138 4 . 4 . 2 MATERIALS AND METHODS 139 4 . 4 . 3 RESULTS 139 4 . 4 . 4 DISCUSSION 140 CONCLUSION 148 LITERATURE CITED 152 v i i LIST OF TABLES Table Page I. Average percentages of f o r a g e r s r e t u r n i n g laden to c o l o n i e s 56 I I . Average t o t a l f o r a g i n g a c t i v i t y per min f o r each colony 57 I I I . P r o p o r t i o n s of the t o t a l number of p l a n t s p e c i e s that were v i s i t e d and gathered by f o r a g e r s 68 IV. Concordances between rankings of abundance of p l a n t s p e c i e s i n the t e r r i t o r y , and t h e i r abundance i n the d i e t 72 V. Back-transformed means of the p r o p o r t i o n s of d i f f e r e n t p l a n t p a r t s c o l l e c t e d by f o r a g e r s 75 VI. ANOVA summaries f o r the e f f e c t of area, colony and month on the p r o p o r t i o n of d i f f e r e n t p l a n t p a r t s gathered by f o r a g e r s 76 VIIA. P r o p o r t i o n s of d i f f e r e n t p l a n t p a r t s of the extremely p a l a t a b l e s p e c i e s gathered by f o r a g e r s , Bois de Lomard area 77 VIIB. P r o p o r t i o n s of d i f f e r e n t p l a n t p a r t s of the extremely p a l a t a b l e s p e c i e s gathered by f o r a g e r s , Lemesle area 79 V I I I . Average rank c o r r e l a t i o n s between monthly, rankings of a l l p l a n t s p e c i e s i n the t e r r i t o r y of 6 c o l o n i e s 81 IXA. Monthly v a r i a t i o n s i n p r o p o r t i o n s of the extremely p a l a t a b l e s p e c i e s i n the d i e t of c o l o n i e s , B o i s de Lomard area 82 IXB. Monthly v a r i a t i o n s i n p r o p o r t i o n s of the extremely p a l a t a b l e s p e c i e s i n the d i e t of c o l o n i e s , Lemesle area 84 v i i i Table Page X. Estimated annual dry weight of biomass c o l l e c t e d by c o l o n i e s i n Guadeloupe 88 XI. Estimated annual dry weight of biomass c o l l e c t e d by l e a f - c u t t i n g ant s p e c i e s , as r e p o r t e d i n the l i t e r a t u r e 94 X I I . R e p l i c a t e d G-test r e s u l t s f o r the i n d i v i d u a l c h o i c e s made by f o r a g e r s presented with 4 p l a n t s p e c i e s 103 X I I I . Rankings of c h o i c e s made by 11 i n d i v i d u a l s for 4 p l a n t s p e c i e s 105 XIV. Presence or absence of 11 i n d i v i d u a l s f o r co n s e c u t i v e days of a p l a n t c h o i c e experiment 107 XV. Dynamics of t r a i l s f o r c o l o n i e s of the 2 study areas 121 XVI. Longevity of t r a i l s f o r c o l o n i e s of the 2 study areas 123 XVII. ANOVA summaries f o r the a t t e n u a t i o n of the t r a i l pheromone 133 XVIII. Comparison of the r e g r e s s i o n c o e f f i c i e n t s for the e f f e c t of UV l i g h t on the d u r a t i o n of the t r a i l pheromone 135 XIX. ANOVA summary f o r the r e g r e s s i o n of for a g e r s i z e over the l e n g t h of the t r a i l s they used 143 ix LIST OF FIGURES F i g u r e Page IA. D i s t r i b u t i o n of the l e a f - c u t t i n g ant i n Guadeloupe 12 IB. P a t t e r n s of t o t a l annual p r e c i p i t a t i o n on the 2 main i s l a n d s of Guadeloupe 12 2. Monthly p r e c i p i t a t i o n i n Guadeloupe, September 1983 to August 1984. i A. Bois de Lomard area 14 B. Lemesle area 14 3A. Average d a i l y t r a i l temperature f l u c t u a t i o n i n the 2 study areas 17 3B. Average monthly t r a i l temperature f l u c t u a t i o n in the 2 study areas 17 4A. Average d a i l y vapor d e n s i t y d e f i c i t f l u c t u a t i o n i n the 2 study areas 19 4B. Average monthly vapor d e n s i t y d e f i c i t f l u c t u a t i o n i n the 2 study areas 19 5A. F l u c t u a t i o n s i n d a i l y t o t a l a c t i v i t y of colony 3, Lemesle area 21 5B. F l u c t u a t i o n s i n d a i l y t o t a l a c t i v i t y of colony 5, Lemesle area 21 6A. F l u c t u a t i o n s i n d a i l y t o t a l a c t i v i t y of colony 6, Lemesle area 23 6B. F l u c t u a t i o n s i n d a i l y t o t a l a c t i v i t y of colony 8, Lemesle area 23 7A. F l u c t u a t i o n s i n d a i l y t o t a l a c t i v i t y of colony 2, Lemesle area 25 7B. F l u c t u a t i o n s i n d a i l y t o t a l a c t i v i t y of colony 4, Lemesle area 25 8A. F l u c t u a t i o n s i n d a i l y t o t a l a c t i v i t y of t r a i l "a", co l o n y 12, Lemesle area 27 x Figure Page 8B. Fluctuations in daily total activity of t ra i l "b", colony 12, Lemesle area 27 9. Fluctuations in daily total activity of colony 11, Lemesle area 29 10A. Fluctuations in daily total activity of colony 5, Bois de Lomard area 31 1 OB. Fluctuations in daily total activity of colony 7, Bois de Lomard area 31 IIA. Fluctuations in daily total activity of colony 1, Bois de Lomard area 33 IIB. Fluctuations in daily total activity of colony 3, Bois de Lomard area 33 12A. Fluctuations in daily total activity of colony H , Bois de Lomard area 35 12B. Fluctuations in daily total activity of colony 12, Bois de Lomard area 35 13. Box plot distribution of the monthly corre-lations between total activity/min and the t ra i l temperature for a l l colonies . . . . . .38 14. Box plot distribution of the monthly corre-lations between total activity/min and the t ra i l vapor density deficit for a l l colonies 41 15A. Daily fluctuations in the percentage of ants returning laden to colony 4 at Lemesle 43 15B. Daily fluctuations in the percentage of ants returning laden to colony 11 at Bois de Lomard 43 16. Box plot distribution of the monthly corre-lations between the percentage of laden ants and the t ra i l temperature for a l l colonies 45 17. Box plot distribution of the monthly corre-lations between the percentage of laden ants and the tra i l vapor density deficit for a l l colonies 47 x i F i g u r e Page 18A. Monthly f l u c t u a t i o n s i n f o r a g i n g a c t i v i t y f o r a l l c o l o n i e s of Bois de Lomard 50 18B. Monthly f l u c t u a t i o n s i n f o r a g i n g a c t i v i t y f o r a l l c o l o n i e s of Lemesle 50 19A. Monthly f l u c t u a t i o n s i n e f f i c i e n c y of food g a t h e r i n g f o r a l l c o l o n i e s of Bois de Lomard 53 19B. Monthly f l u c t u a t i o n s i n e f f i c i e n c y of food g a t h e r i n g f o r a l l c o l o n i e s of Lemesle 53 20. D o m i n a n c e - d i v e r s i t y curves f o r s p e c i e s found i n the d i e t of 6 c o l o n i e s 70 21. Percentages of 3 p l a n t c a t e g o r i e s taken by 10 i n d i v i d u a l s , and by the whole colo n y 101 22. Compass d i s t r i b u t i o n of the primary t r a i l s around c o l o n i e s of the 2 study areas 124 23A. P e r s i s t e n c e of the pheromone t r a i l i n nature ...131 23B. E v a p o r a t i o n of the t r a i l pheromone exposed to UV l i g h t i n the l a b o r a t o r y 131 24. T r a i l l e n g t h and the s i z e of f o r a g e r s of the l e a f - c u t t e r ant 141 x i i ACKNOWLEDGEMENTS I wish to thank my t h e s i s d i r e c t o r , Dr. W. G. W e l l i n g t o n . I had the o p p o r t u n i t y to hear him g i v e a s e r i e s of very i n f o r m a t i v e and e n t e r t a i n i n g l e c t u r e s at the U n i v e r -s i t e L a v a l , and I had been impressed enough to want to work under h i s s u p e r v i s i o n . I have not been d i s a p p o i n t e d . H i s experience i n d e a l i n g with a d m i n i s t r a t i v e problems has done much to s i m p l i f y my l i f e when I was working i n the f i e l d , and h i s e f f o r t s are w e l l a p p r e c i a t e d . I am a l s o g r a t e f u l f o r the l o g i s t i c a l support he p r o v i d e d . I extend g r a t i t u d e to the other members of my committee, Dr. T. A. Black, Dr. B. D. F r a z e r , and Dr. V. C. Runeckles, f o r t h e i r i n t e r e s t i n my work. Dr. F r a z e r , e s p e c i a l l y , p r o v i d e d me with l a b o r a -t o r y space and f a c i l i t i e s at the Vancouver Research S t a t i o n , A g r i c u l t u r e Canada, and made h e l p f u l comments on an e a r l y d r a f t of t h i s t h e s i s . I extend g r a t i t u d e to Dr. J . N. McNeil f o r o f f e r i n g me the o p p o r t u n i t y to do r e s e a r c h i n Guadeloupe. Without h i s moral and l o g i s t i c a l support, as w e l l as i n s p i r i n g d i s c u s -s i o n s , t h i s p r o j e c t c o u l d not have been completed. I must thank Drs. A. Kermarrec and G. Febvay f o r the support pro-v i d e d by La S t a t i o n de Zoologie et L u t t e B i o l o g i q u e at the INRA S t a t i o n at P e t i t Bourg, Guadeloupe. T h e i r c o l l a b o r a -t i o n has been i n v a l u a b l e . I thank V a l e r y Ehro, Antoine Jean-Bart and L o u i s M i l l s f o r t h e i r a b l e t e c h n i c a l a s s i s -tance i n the f i e l d and l a b o r a t o r y . I a l s o thank my f r i e n d and c o l l e g u e , Helene Contant, f o r her p a t i e n c e and w i l l i n g x i i i attention during too many interminable discussions. Without her, my stay in British Columbia would not have been so en-joyable . I am honoured and grateful for the generous support provided by Post Graduate Scholarships from NSERC and FCAC. xiv "/ know not what I may appear to the world, but to myself I appear to have been only like a boy playing on the sea-shore, and diverting myself in now and then finding a smoother pebble or a prettier shell than or di nar y, whilst the great ocean of truth lay all undiscovered before me." ISAAC NEWTON xv Chapter 1 INTRODUCTION 1.1 TAXONOMY OF LEAF-CUTTERS Approximately 10,000 s p e c i e s of ants are grouped i n the f a m i l y F o r m i c i d a e . The s p e c i e s of the Subfamily Myrmicinae, T r i b e A t t i n i , grow and eat f u n g i , a h a b i t found o u t s i d e the f a m i l y only i n the macrotermitine t e r m i t e s and some wood-boring b e e t l e s . The T r i b e A t t i n i i n c l u d e s 12 genera, c o n t a i n i n g between 180 and 200 s p e c i e s , and a minimum of 80 subspecies (Weber 1972). The 2 most conspicuous genera, i n terms of the s i z e of t h e i r c o l o n i e s and the many eco n o m i c a l l y damaging s p e c i e s they c o n t a i n , are Atta and Acromyrmex. 1.2 DISTRIBUTION The A t t i n i occur only i n the New World, between 40° North and 44° South l a t i t u d e (Weber 1966). They range from C a l i f o r n i a i n the west and New J e r s e y i n e a s t e r n North America, through C e n t r a l America and most of South America. So f a r , C h i l e has been f r e e of the l e a f - c u t t e r s , presumably because they are unable to invade the d e s e r t coast from the north or to c r o s s the Andes from the east (Weber 1972). The A t t i n i are a l s o found on the major Caribbean i s l a n d s : Cuba, T r i n i d a d , Tobago, Guadeloupe and Curaqao ( C h e r r e t t 1968). The t o t a l absence of the l e a f - c u t t e r s from the O l d World, where the fungus-growing t e r m i t e s (Macrotermitinae) 1 2 r e p l a c e them i s b a f f l i n g . I t c o u l d be a case of c o m p e t i t i v e e x c l u s i o n of one group by the other, or i t may be an a c c i -d e n t a l r e s u l t of the e v o l u t i o n a r y r a r i t y of fungus growing (Wilson 1971). Wilson (1971) f a v o r s the second h y p o t h e s i s , p o i n t i n g out that the l e a f - c u t t i n g ants mostly use fe c e s and f r e s h p l a n t m a t e r i a l as s u b s t r a t e s f o r the fungus, while t e r m i t e s use dead p l a n t m a t e r i a l s . He hyp o t h e s i z e s t h a t i f e i t h e r group were i n t r o d u c e d i n the range of the o t h e r , i t c o u l d c o e x i s t with only minimal i n t e r f e r e n c e . 1.3 NATURAL HISTORY 1.3.1 MATING AND COLONY FOUNDING The l i f e h i s t o r y of a new colony begins with the nup-t i a l f l i g h t of the s e x u a l s . The l i m i t e d data on the mating f l i g h t s of l e a f - c u t t e r s has been reviewed by Weber (1972). There i s c o n s i d e r a b l e r e g u l a r i t y i n f l i g h t time a c c o r d i n g to genus and s p e c i e s . Weber b e l i e v e s that mating oc c u r s when c o n d i t i o n s a re most f a v o r a b l e to s u r v i v a l of the young queens and t h e i r budding c o l o n y . In most t r o p i c a l areas t h i s time would be at the onset of the r a i n y season, when s o i l moistened by the r a i n s w i l l be more e a s i l y p e n e t r a t e d by queens d i g g i n g t h e i r nest c a v i t i e s , and higher humidity w i l l h e l p fungus s u r v i v a l . In the South and North Temperate Zones, mating f l i g h t s occur i n summer and may be syn-c h r o n i z e d by r a i n s . 3 Timing of the f l i g h t d u r i n g the day i s v a r i a b l e between s p e c i e s and ar e a s . F l i g h t s of Acromyrmex versicolor (Per-gande), a de s e r t s p e c i e s , occur at dawn (Wheeler 1917). Acromyrmex octospi nosus (Reich.) and Acromyrmex I undi ( G u e r i n ) , the former a t r o p i c a l s p e c i e s , the l a t t e r a tem-perate one, both f l y i n the e a r l y hours of the morning (Weber 1972). In s u b t r o p i c a l south B r a z i l , At ta s p e c i e s f l y d u r i n g the day, while At ta texana (Buckley) f l y at night i n the U n i t e d S t a t e s (Moser 1967a; Walter et al. 1938), as do Atta sexdens (L.) and Atta cephalotes (L.) i n the t r o p i c s (Weber 1972). Mating takes p l a c e i n f l i g h t . The s y n c h r o n i z a t i o n of f l i g h t by r a i n s promotes i n t e r c o l o n y mating. The males d i e a f t e r the f l i g h t . Females descend a p p a r e n t l y at random over a wide area a f t e r mating, but s u f f e r high m o r t a l i t y . Au-t u o r i (1950, c i t e d by Weber 1972) estimated at 99.95% the m o r t a l i t y r a t e of Atta sexdens rubropilosa ( F o r e l ) females. The remaining 0.05% was a p p a r e n t l y enough to mai n t a i n a con-tinuous a g r i c u l t u r a l hazard. Upon re a c h i n g the ground, the females c a s t o f f t h e i r wings. They then d i g t h e i r way i n t o the s o i l , e x c a v a t i n g a small chamber i n which they w i l l grow t h e i r fungus garden. The d i s c o v e r y of the o r i g i n of the fungus garden was made i n 1898 by von I h e r i n g ( c i t e d by Weber 1972). Before l e a v i n g t h e i r b i r t h p l a c e f o r the mating f l i g h t , f u t u r e qu-eens pack b i t s of mycelia from the p a r e n t a l garden i n t h e i r i n f r a b u c c a l pocket. A f t e r t h e i r new nest has been dug, they 4 s p i t out the wad of mycelia (Autuori 1956). The young gar-den sprouts new hyphae on the f o l l o w i n g day, and the queen begins to l a y eggs. The queen does not seclude h e r s e l f while r e a r i n g her f i r s t brood. She may leave the nest and q u i c k l y forage f o r v e g e t a l s u b s t r a t e f o r the garden. The f i r s t workers appear o u t s i d e the nest i n 2 to 3 mo (Autuori 1942: c i t e d by Weber 1972; A u t u o r i 1956; Moser 1967a). 1.3.2 POLYMORPHISM AND DIVISION OF LABOR Among the A t t i n i , the genera At ta and Acromyrmex show the most polymorphism. For a l l A t t i n i , there i s , a c c o r d i n g to Weber (1972), an i n v e r s e c o r r e l a t i o n between the s i z e of the members of a c a s t e and the d e n s i t y of i n d i v i d u a l s of that c a s t e i n the co l o n y . There i s a l s o a rough correspon-dence between the s i z e of the workers and the task they per-form i n the colo n y . The "minima" workers care f o r the brood and the fungus garden. They prepare the v e g e t a l s u b s t r a t e to be i n c o r -p orated i n t o the fungus garden, n o u r i s h i n g the fungus with f e c a l d r o p l e t s (Martin and M a r t i n 1970a, 1970b). They a l s o care f o r the queen. The "media" workers are the main f o r a -g e r s , excavate nest c a v i t i e s and evacuate o l d vegetable ma-t e r i a l and dead a n t s . The " s o l d i e r s " , the l a r g e s t a n t s , present i n the T r i b e A t t i n i only i n Atta sexdens, guard a g a i n s t p r e d a t o r s or other d i s t u r b a n c e s . They very r a r e l y c a r r y forage to the co l o n y . In other s p e c i e s the r o l e of s o l d i e r s i s f i l l e d by the media workers. A more d e t a i l e d 5 account of the division of labor in Atta sexdens is given by Wilson (1980a, 1980b). 1.3.3 THE ANT-FUNGUS SYMBIOSIS The most fascinating aspect of the leaf-cutters, and certainly their distinguishing feature, is the symbiosis they have developed with their fungus. Inside the nest, the plant material collected by the workers is taken by the minima and prepared for incorpora-tion into the fungus garden. They rasp and chew the materi-al to remove the surface waxes (which are antibiotic) and to create a pulp that is accessible to the hyphae. They then add rectal fluid and incorporate the substrate into the garden. The rectal fluid contains enzymes beneficial to the fungus: proteinase (Martin and Martin 1970b), chitinase, amylase, and cellulase (Boyd and Martin 1975a, 1975b; Martin et al. 1973, 1975). The contribution of the fungus is its ability to ut i -lize cellulose as a source of carbohydrates (Martin and Web-er 1969). More than 50% of the dry weight of the fungus of At t a col ombi ca tonsipes (Santschi) is available as soluble nutrients; 27% of this dry weight being carbohydrates (tre-halose, mannitol, arabinitol, glucose) (Martin et al . 1969a). The other main constituents are free amino acid 4.7%, protein bound amino acid 13%, and lipid 0.2%. Martin et al. (1969a) found no soluble polysaccharides, but De-charme and Issaly (1980), studying the fungi of At ta 6 sexdens, Att a cephalotes, and Acromyrmex octospi nosus l a t e r found s o l u b l e glycogen i n the fu n g i of these s p e c i e s . I t was thought f o r a long time that the fungus was a s u f f i c i e n t source of n u t r i e n t s f o r the l e a f - c u t t e r s and that i t was t h e i r o n l y food source. T h i s i s true f o r the l a r v a e that are fed by the workers on p r e v i o u s l y chewed staphylae (Quinlan and C h e r r e t t 1979). However, the workers d i d not meet t h e i r energy requirements s o l e l y with t h i s d i e t i n the same study of At t a cephalotes. P e r e g r i n e and Mudd (1975) found l e a f c o n s t i t u e n t s i n the pharyngeal glands of the ant s , su g g e s t i n g that s u b s t r a t e components are a s s i m i l a t e d by the workers ( f o r a g e r s when c u t t i n g the p l a n t m a t e r i a l , minima when p r e p a r i n g i t f o r i n c o r p o r a t i o n ) . T h i s a s s i m i l a -t i o n was demonstrated by L i t t l e d y k e and C h e r r e t t (1976) with r a d i o a c t i v e t r a c e r s . I t appears that working ants i n g e s t p l a n t sap, which c o n s t i t u t e s a source of carbohydrate s u f f i -c i e n t to supply t h e i r energy needs f o r about 24 h (Quinlan and C h e r r e t t 1979). 1.4 GUADELOUPE AND ACROMYRMEX OCTOSPINOSUS 1.4.1 GUADELOUPE Guadeloupe i s an a r c h i p e l a g o of i s l a n d s l o c a t e d i n the middle of the Lesser A n t i l l e s b e l t of the West I n d i e s , 61° West l o n g i t u d e and 16° North l a t i t u d e , between the t r o p i c a l A t l a n t i c and the Caribbean Sea. I t comprises 7 d i s t i n c t i s l a n d s , but 2, Grande-Terre and Basse-Terre ( F i g . 1A, page 7 12), c o n s t i t u t e the l a r g e s t p r o p o r t i o n of the t e r r i t o r y . The c l i m a t e of Guadeloupe i s the e q u a t o r i a l i n s u l a r type, s i m i l a r to D j a k a r t a , Colombo and Singapore (Anonyme 1982). Seasons are almost e x c l u s i v e l y marked by changes i n t r o p i c a l e a s t e r l i e s which blow c o n s t a n t l y (Anonyme 1982), and the presence of a volc a n o , La S o u f f r i e r e , r i s i n g 1,467 m on Basse-Terre c r e a t e s a very d i s t i n c t i v e p a t t e r n of p r e c i -p i t a t i o n on the 2 main i s l a n d s of Guadeloupe ( F i g . 1B, page 12) . Because of the p e c u l i a r e f f e c t of the i n t e r a c t i o n bet-ween the t r o p i c a l e a s t e r l i e s and the Basse-Terre massif, Guadeloupe's 2 main i s l a n d s p r o v i d e i d e a l l o c a t i o n s f o r com-p a r a t i v e e c o l o g i c a l s t u d i e s , because p a t t e r n s of weather and v e g e t a t i o n change d r a m a t i c a l l y w i t h i n l e s s than 30 km. 1.4.2 ACROMYRMEX OCTOSPINOSUS Along with Atta sexdens and Atta cephalotes, Acromyrmex octospinosus i s one of the 3 most important s p e c i e s of l e a f - c u t t e r s , because of i t s widespread d i s t r i b u t i o n (Weber 1972) and economic impact (Lewis 1975). In Guadeloupe, Acromyrmex octospinosus was f i r s t found on Grande-Terre i n 1954. Since then, i t has extended i t s d i s t r i b u t i o n to cover most of Grande-Terre and p a r t of Basse-Terre ( F i g . 1A, page 12). C o n s i d e r a b l e e f f o r t i s ex-pended each year to c o n t r o l the i n s e c t (cost US$ 100,000). While e r a d i c a t i o n of Acromyrmex octospinosus may s t i l l be p o s s i b l e on Basse-Terre, on Grande-Terre the ant i s f i r m l y 8 e s t a b l i s h e d . Many s t u d i e s have been p u b l i s h e d on Acromyrmex octo-spinosus i n Guadeloupe. Most of them have been concerned with v a r i o u s a s p e c t s of i t s anatomy, morphology, or p h y s i o -logy (Febvay 1982; Febvay et al . 1984; Febvay and Kermarrec 1981a, 1981b; Kermarrec 1981; Kermarrec and Abud-Antun 1978; Kermarrec and Febvay 1985; Kermarrec et al . 1976; Torre-Grossa et al. 1982). S u r p r i s i n g l y , c o n s i d e r i n g i t s economic importance, and d e s p i t e some papers concerned with c o n t r o l of the animal (Kermarrec 1975; Kermarrec and Mauleon 1975), no e c o l o g i c a l study had been done by 1983, when the present work was s t a r t e d . My major o b j e c t i v e s i n undertaking t h i s study were t o : 1. D e s c r i b e the d a i l y a c t i v i t y c y c l e i n Guade-loupe, with regard to d i f f e r e n c e s between months and between c o l o n i e s under d i f f e r e n t weather regimes; 2. C h a r a c t e r i z e the d i e t of the ant, determine i t s s e asonal v a r i a t i o n i n the 2 study areas, and i n v e s t i g a t e the r o l e of the i n d i v i d u a l f o r a g e r i n shaping the observed p a t t e r n s of p l a n t s e l e c t i o n ; 3. Determine some of the f a c t o r s i n v o l v e d i n food d i s c o v e r y and t r a i l formation i n r e l a -t i o n to food a v a i l a b i l i t y and l o c a t i o n i n the f o r a g i n g a r e a . Chapter 2 FORAGING ACTIVITY RHYTHM 2.1 INTRODUCTION The d a i l y rhythm of f o r a g i n g a c t i v i t y of l e a f - c u t t e r ants of the genus Atta has been r e l a t i v e l y w e l l documented ( C h e r r e t t 1968; Fowler and Robinson 1979a; Lewis et al. 1974a, 1974b; M i n t z e r 1979; P i n t e r a and Z o r r i l l a 1981). Ho-wever, few data are a v a i l a b l e c o n c e r n i n g the f o r a g i n g a c t i -v i t y p a t t e r n of ants of the genus Acromyrmex. Gamboa (1976) found A. versicolor versicolor a c t i v e between temperatures of 7.7 and 44 °C i n A r i z o n a . He a l s o found that the number of ants r e t u r n i n g unladen i n c r e a s e d as the s o i l s u r f a c e tem-pe r a t u r e i n c r e a s e d (Gamboa 1975). The f o r a g i n g a c t i v i t y of A. crassispinus ( F o r e l ) i n Paraguay was c o r r e l a t e d with tem-pe r a t u r e and atmospheric pressure (Fowler 1979). Although f o r a g i n g a c t i v i t y was d i u r n a l i n A p r i l , Fowler (1979) r e -por t e d a swi t c h to n o c t u r n a l a c t i v i t y d u r i n g summer. Acromyrmex oct ospi nosus i s mainly d i u r n a l i n T r i n i d a d (Weber 1972), but my p r e l i m i n a r y o b s e r v a t i o n s i n Guadeloupe r e -ve a l e d a n o c t u r n a l peak of a c t i v i t y f o r some c o l o n i e s of that s p e c i e s . To complicate matters f u r t h e r , C h e r r e t t (1968) observed 2 adjacent c o l o n i e s of A. octospinosus with d i f f e r e n t a c t i v i t y p a t t e r n s , one a c t i v e d u r i n g the day, the other at n i g h t . In order to r e s o l v e these c o n t r a s t i n g o b s e r v a t i o n s , I began a y e a r - l o n g study of the f o r a g i n g rhythm of A. 9 10 oct ospi nosus in Guadeloupe. 2.2 MATERIALS AND METHODS Active colonies were found at Lemesle, near the site where the ant was first observed following its introduction, and at Bois de Lomard (hereafter Lomard), near the limit of the ant's distribution on Grande Terre (Fig. 1A, page 12). Six colonies were init ial ly observed in each area. Every effort was made to obtain a year-long unbroken record for each colony. However observations on some colonies were interrupted. These colonies were immediately replaced by new ones, but as a consequence, only 5 and 3 colonies have complete records at Lomard and Lemesle respectively. The activity of the colonies in both areas was sampled in four 12 h periods (2 from 6:00 to 17:00 h and 2 from 18:00 to 5:00 h, solar time) each month for 12 consecutive months. For 3 min at the beginning of each hour a count was made of the ants leaving the nest and of the ants returning laden and unladen. At night a flashlight used for the count had no apparent effect on the behavior of the ants. The counting sites were usually about 15 cm from the nest entrance. Regardless of the branching of foraging trai ls, these sites ensured that a l l ants were counted. The only exception was colony 2 at Lemesle, where many trails radiated directly from the nest. In this case counts were taken on the most active t ra i l . Thus the picture of the timing of activity for colony 2 is not affected, but the 11 l e v e l s of a c t i v i t y are s l i g h t l y underestimated. Immediately before each 3 min o b s e r v a t i o n p e r i o d , the t r a i l s u r f a c e temperature and r e l a t i v e humidity were rec o r d e d . The temperature was measured with a t h e r m i s t o r thermometer, #K85lO-20 (Cole Palmer Instrument Co., Chicago, I l l i n o i s 60648). The brown wire and sensors were p a i n t e d matte white to a v o i d e x c e s s i v e r a d i a n t h e a t i n g (Fuchs and Tanner 1965). R e l a t i v e humidity was taken with an Hygrocor e l e c t r o n i c psychrometer ( C o r e c i Co., 4 rue Jean Desparmet, B.P. 8237, 69355 Lyon CEDEX 08) c a l i b r a t e d p e r i o d i c a l l y a g a i n s t an Assmann psychrometer. The e l e c t r o n i c psychromet-er was damaged 4 months before the end of the study and hu-m i d i t y data f o r that l a s t p e r i o d are m i s s i n g . The r e l a t i v e h umidity was expressed as vapor d e n s i t y d e f i c i t u s i n g sur-face temperature of the t r a i l as the dry bulb temperature. 2.3 RESULTS 2.3.1 GENERAL WEATHER Lomard has s l i g h t l y l e s s r a i n a n n u a l l y than Lemesle ( F i g . 1B). The seasonal weather p a t t e r n i s c h a r a c t e r i s e d by a four-month dry season from January to A p r i l , and 4 months of heavy r a i n from September to December ( F i g s . 2A and 2B). The 4 months from May to August show a gradual t r a n s i t i o n between the dry and r a i n y seasons. The year of o b s e r v a t i o n l a s t e d from September 1983 to August 1984. In 1983 the r a i n y season was unusual, with amounts of p r e c i p i t a t i o n 12 F i g u r e 1: D i s t r i b u t i o n of the l e a f - c u t t e r ant, Acromyrmex octospi nosus, and p r e c i p i t a t i o n on the 2 main i s l a n d s of Guadeloupe. (A) D i s t r i b u t i o n (shaded area) of the ant i n 1982. The dots i n d i c a t e the l o c a t i o n of the 2 study areas (A: Lemesle; B: Bois de Lomard). (B) T o t a l annual p r e c i p i t a t i o n . C l a s s l i m i t s are (from 1 to 9 ) : < 1.0, 1.0-1.25, 1.25-1.5, 1.5-1.75, 1.75-2.0, 2.0-4.0, 4.0-6.0, 6.0-8.0, > 8.0 m. (Reproduced from L a s s e r r e 1961) 1 3 1 4 F i g u r e 2: Monthly p r e c i p i t a t i o n from September 1983 to Au-gust 1984 i n 2 areas of Guadeloupe compared to the 10 year average f o r each a r e a . (Based on data o b t a i n e d from the Commission meteorologique departementale de l a Guadeloupe) 225-1 (mm) 175- LOMARD • = AVERAGE FOR 10 YEARS ° = OBSERVED [ON PRECIPITAT] 125-75-25-O \ » \ • \ » \ • " \ » ^ p. . . . * » • * • * • » V 1 1 1 1— / \ A /h >N * / ® V ^/ b j i i i i i i SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG MONTH 25-1 1 1 1 1 1 1 1 r 1 1 1 1 SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG MONTH 16 c o n s i s t e n t l y below the 10-year average ( F i g . 2 ) . As a con-sequence, the dry season had worse than u s u a l e f f e c t s on the v e g e t a t i o n s i n c e the dry-season r a i n f a l l , though t y p i c a l f o r the p e r i o d ( F i g . 2), was not s u f f i c i e n t to overcome the ac-cumulated water d e f i c i t i n c u r r e d d u r i n g the r a i n y season. The d a i l y mean temperatures at Lomard were v i r t u a l l y always s l i g h t l y warmer than those at Lemesle ( F i g . 3A). The temperature d i f f e r e n c e was g r e a t e r from l a t e morning to l a t e a f t e r n o o n and decreased at night ( F i g . 3A) . The monthly mean t r a i l temperatures again were s l i g h t l y warmer at Lomard, e s p e c i a l l y d u r i n g the dry season ( F i g . 3B). Very s i m i l a r p a t t e r n s can be seen f o r both the mean d a i l y ( F i g . 4A) and mean monthly vapor d e n s i t y d e f i c i t (VDD) ( F i g . 4B), with t r a i l s at Lomard e x p e r i e n c i n g the g r e a t e s t d e f i c i t s . 2.3.2 DAILY ACTIVITY CYCLE 2.3.2.1 TOTAL ACTIVITY There were 2 g e n e r a l p a t t e r n s of d a i l y f o r a g i n g rhythm apparent at Lemesle. Four c o l o n i e s ( F i g s . 5 and 6) d i s p l a y e d a predominantly d i u r n a l a c t i v i t y f o r the whole p e r i o d of o b s e r v a t i o n . The 4 remaining c o l o n i e s ( F i g s . 7 to 9) a l s o were mainly d i u r n a l f o r a few months, but i n c r e a s e d t h e i r n o c t u r n a l a c t i v i t y i n other months. In most cases the members of these 4 c o l o n i e s c o u l d be found f o r a g i n g at any hour, but p r e f e r r e d 1 7 Figure 3: Micrometeorological data for the trai l temperature of the 2 study areas in Guadeloupe. (A) Average daily t ra i l temperature fluctuation. Al l colonies and months pooled for each area. (B) Average monthly t ra i l temperature fluctuation. Al l colonies and hours pooled for each area. The vertical lines indicate the 9 5 % confidence intervals. 18 u u 18 -f 1 1 1 1 1 1 1 1 1 1 1 S E P O C T N O V D E C J A N F E B M A R A P R M A Y J U N J U L A U G M O N T H 19 F i g u r e 4: M i c r o m e t e o r o l o g i c a l vapor d e n s i t y d e f i c i t data f o r the 2 study areas i n Guadeloupe. (A) Average d a i l y vapor d e n s i t y d e f i c i t f l u c t u a t i o n on t r a i l s . A l l c o l o n i e s and months pooled f o r each a r e a . (B) Average monthly vapor den-s i t y d e f i c i t f l u c t u a t i o n on t r a i l s . A l l c o l o n i e s and hours pooled f o r each area. The v e r t i c a l l i n e s i n d i c a t e the 95% c o n f i d e n c e i n t e r v a l s . 20 21 F i g u r e 5: F l u c t u a t i o n s i n d a i l y t o t a l a c t i v i t y ( a l l ants outgoing and incoming) of c o l o n i e s 3 and 5, Lemesle area, Guadeloupe. Colony 3 was observed f o r 12 mo; colony 5 f o r 5 mo. D e n s i t y expressed i n number of ants/min. Length of day v a r i e d s l i g h t l y with seasons, but s u n r i s e o c c u r r e d at around 5 AM and sundown around 5 PM ( s o l a r t i m e ) . 22 23 Figure 6: Fluctuations in daily total activity of colonies 6 and 8, Lemesle area, Guadeloupe. Both colonies were ob-served for 5 mo. Units of density and length of day as in Fig. 5. 24 25 Figure 7: Fluctuations in daily total activity of colonies 2 and 4, Lemesle area, Guadeloupe. Units of density and length of day as in Fig. 5. 26 27 F i g u r e 8: F l u c t u a t i o n s i n d a i l y t o t a l a c t i v i t y f o r a l l months of o b s e r v a t i o n on 2 t r a i l s of colony 12, Lemesle area, Guadeloupe. Note the g r e a t e r tendency toward noc-t u r n a l a c t i v i t y on t r a i l 12a, p a r t i c u l a r l y i n A p r i l , near the end of the dry season. U n i t s of d e n s i t y and l e n g t h of day as i n F i g . 5. 2 8 29 F i g u r e 9: F l u c t u a t i o n s i n d a i l y t o t a l a c t i v i t y f o r a l l months of o b s e r v a t i o n , colony 11, Lemesle area, Guadeloupe. Note the i n c r e a s e d n o c t u r n a l a c t i v i t y near the end of the dry season (compare F i g . 5A). U n i t s of d e n s i t y and l e n g t h of day as i n F i g . 5. 30 e i t h e r day or n i g h t i n a given month. There was no s t r o n g c o r r e l a t i o n with the m i c r o c l i m a t i c measurements to e x p l a i n the observed s h i f t i n p a t t e r n , but i t i s no-teworthy t h a t , at the end of the dry season (March and/or A p r i l ) , the c o l o n i e s of F i g s . 7 to 9 showed a great amount of n o c t u r n a l a c t i v i t y . T h i s n o c t u r n a l upsurge might have been a response to the e s p e c i a l l y dry environment c r e a t e d by the i n c r e a s i n g water d e f i c i t that •began d u r i n g the p r e c e d i n g , a t y p i c a l l y dry, r a i n y season ( F i g . 2 ). Even when the weather d i d not seem harsh enough to stop f o r a g i n g a l t o g e t h e r d u r i n g the day, a very r e g u l a r m o d i f i c a t i o n of behavior o c c u r r e d i n some c o l o n i e s . During the day, h a l f the c o l o n i e s of Lemesle ( F i g s . 5 and 7) d i s p l a y e d a bimodal a c t i v i t y p a t t e r n . They r e -duced t h e i r f o r aging, d u r i n g the peak i n t r a i l tem-pe r a t u r e and VDD at 12:00-13:00 h ( F i g s . 3A, 4A). C o n t r a r y to the p a t t e r n of Lemesle, where the ants were p r i m a r i l y a c t i v e d u r i n g the day, the c o l o n i e s at Lomard were most o f t e n a c t i v e at night ( F i g s . 10 to 12). C o l o n i e s 5 and 7 ( F i g . 10) were shaded f o r part of the day. Other Lomard c o l o n i e s ( F i g s . 11 and 12), that had t h e i r main f o r a g i n g areas f u l l y exposed to the sun, were f a r more a c t i v e at n i g h t than in the day. There was o c c a s i o n a l daytime a c t i v i t y i n c o l o n i e s 1 and 12 ( F i g s . 11A and 12B), when the ants c o u l d forage i n t e -mporary shade, but even those f o r a g e r s abandoned t h e i r F i g u r e 10: F l u c t u a t i o n s i n d a i l y t o t a l a c t i v i t y f o r a l l months of o b s e r v a t i o n , c o l o n i e s 5 and 7, Bois de Lomard area, Guadeloupe. U n i t s of d e n s i t y and l e n g t h of day as F i g . 5. 32 33 F i g u r e 1 1: F l u c t u a t i o n s i n d a i l y t o t a l a c t i v i t y f o r a l l months of o b s e r v a t i o n , c o l o n i e s 1 and 3, Bois de Lomard area, Guadeloupe. Colony 1 (and a l s o colony 12, F i g . 12B) had daytime shade, but colony 3 (and colony'11, F i g . 12A) were f u l l y exposed to the sun and t h e i r n o c t u r n a l a c t i v i t y was e s p e c i a l l y pronounced. U n i t s of d e n s i t y and l e n g t h of day as i n F i g . 5. 34 35 F i g u r e 12: F l u c t u a t i o n s i n d a i l y t o t a l a c t i v i t y f o r a l l months of o b s e r v a t i o n , c o l o n i e s 11 and 12, Bois de Lomard ar e a , Guadeloupe. Compare with F i g . 11 and c o n t r a s t with F i g . 10. U n i t s of d e n s i t y and l e n g t h of day as i n F i g . 5. 36 37 temporary t r a i l s to go back to the main t r a i l s (not i n shade) as soon as the sun went down. Although c o l o n i e s 3 ( F i g . 11B) and 11 ( F i g . 12A) showed no daytime f o r a g -ing a c t i v i t y , nest-entrance o b s e r v a t i o n s r e v e a l e d that some ants were b u s i l y e v a c u a t i n g r e f u s e from the n e s t . These ants stayed c l o s e to the nest entrance, r a r e l y moving more than 50 cm from i t . The s l i g h t l y h i g h e r t e -mperature and VDD regimes at Lomard a p p a r e n t l y exceeded the upper temperature and moisture l i m i t s of the spe-c i e s , s u p p r e s s i n g d i u r n a l a c t i v i t y except i n p l a c e s where the colony had a l l or p a r t of i t s f o r a g i n g area i n the shade. The d i s t r i b u t i o n p a t t e r n s f o r the c o r r e l a t i o n s bet-ween t o t a l f o r a g i n g a c t i v i t y and t r a i l temperature f o r both areas appear q u i t e s i m i l a r , a l b e i t i n v e r t e d ( F i g . 13). The f i r s t 4 c o l o n i e s of Lemesle (No. 8 to 5) show a s t r o n g p o s i t i v e c o r r e l a t i o n f o r almost a l l the months they were observed. S e v e n t y - f i v e per cent or more of t h e i r c o r r e l a t i o n s are s i g n i f i c a n t . At Lomard, the l a s t 4 c o l o n i e s shown (No. 12 to 3) have s t r o n g l y n e g a t i v e monthly c o r r e l a t i o n s . The 4 c o l o n i e s of each area that showed the s t r o n g e s t responses to t r a i l temperature a l s o had the most c o n s i s t e n t f o r a g i n g p e r i o d s . The remaining c o l o n i e s of each area changed t h e i r f o r a g i n g p e r i o d more f r e q u e n t l y and had more v a r i a b l e p a t t e r n s of c o r r e l a -t i o n . 3 8 F i g u r e 13: Box p l o t d i s t r i b u t i o n of the monthly c o r r e l a t i o n s between t o t a l a c t i v i t y / m i n and the t r a i l temperature f o r a l l c o l o n i e s observed in Guadeloupe. Box p l o t s are a non para-metric r e p r e s e n t a t i o n of a d i s t r i b u t i o n of numeric v a l u e s . There are 15 boxes in t h i s f i g u r e , 1 f o r each c o l o n y except colony 2 at Lomard (the numbers along the b a s e l i n e of the f i g u r e i n d i c a t e the colony numbers). Each box i n c l u d e s 50% of the v a l u e s . The h o r i z o n t a l l i n e i n s i d e the boxes rep r e -sent the l o c a t i o n of the median. The upper and lower l i m i t s of each box i n d i c a t e the l o c a t i o n of the q u a r t i l e s . The v e r t i c a l l i n e s o r i g i n a t i n g from the q u a r t i l e s i n d i c a t e the range of the 25% h i g h e s t and lowest v a l u e s . The s h o r t h o r i -z o n t a l l i n e s at the t i p of the v e r t i c a l l i n e s i n d i c a t e the l o c a t i o n of the h i g h e s t and lowest value of the d i s t r i b u t i o n (the extremes). Asymmetry in a box i n d i c a t e s skewness i n the d i s t r i b u t i o n . The V-shaped hollow (notch) on e i t h e r s i d e of the boxes can be used to t e s t the d i f f e r e n c e i n med-ian l o c a t i o n s between d i s t r i b u t i o n s . Notches t h a t do not o v e r l a p i n d i c a t e a d i f f e r e n c e i n l o c a t i o n (a=0.05) ( M c G i l l et a l . 1978). The notches sometimes extend beyond a quar-t i l e , c r e a t i n g "spurs" on the box. In such a case i t i n d i -c a t e s a low c o n f i d e n c e i n the l o c a t i o n of that q u a r t i l e . Width of each box i s p r o p o r t i o n a l to the square root of the number of months of o b s e r v a t i o n . Monthly c o r r e l a t i o n s f a l -l i n g between the 2 broken l i n e s are not s i g n i f i c a n t l y d i f -f e r e n t from 0 (N=48, <x=0.05). Only 3 c o r r e l a t i o n s were a v a i l a b l e f o r colony 2 at Lomard, and they are shown by s m a l l white squares. 40 When the VDD r e p l a c e s t r a i l temperature, the r e -s u l t s are s t r i k i n g l y s i m i l a r ( F i g . 14), although the range i s reduced f o r most c o l o n i e s . Four months of ob-s e r v a t i o n s are m i s s i n g f o r VDD because of instrument f a i l u r e , however, which may e x p l a i n the s m a l l e r range. 2.3.2.2 PROPORTION OF RETURNING ANTS LADEN The percentage of r e t u r n i n g ants laden on the t r a i l s v a r i e d d u r i n g the day ( F i g . 15), roughly f o l l o w -ing the p a t t e r n of t o t a l a c t i v i t y a l r e a d y d e s c r i b e d f o r the 2 s e l e c t e d c o l o n i e s ( F i g s . 7B and 12A). For colony 4 at Lemesle, e f f i c i e n c y of food g a t h e r i n g (% ants r e -t u r n i n g laden) f o l l o w e d a bimodal p a t t e r n and was r e -duced i n the h o t t e s t hours of the day, f o r months i n which the colony was d i u r n a l ( F i g . 15A, compare with F i g . 7B). At Lomard, where colony 11 was n o c t u r n a l ( F i g . 12A), e f f i c i e n c y was low at sundown, and i n c r e a s e d g r a d u a l l y to a t t a i n a maximum of 70-90% f o r 4 h i n the middle of the n i g h t , and then g r a d u a l l y decreased as the sun rose and f o r a g i n g stopped ( F i g . 15B). The tenden-c i e s f o r other c o l o n i e s i n the 2 areas were s i m i l a r to those i n F i g . 15, although f o r some c o l o n i e s the pat-t e r n s were more v a r i a b l e d u r i n g the a c t i v i t y p e r i o d . When fo o d - g a t h e r i n g e f f i c i e n c y i s examined i n terms of the monthly c o r r e l a t i o n s between the percentages of laden ants and t r a i l temperature ( F i g . 16) and VDD ( F i g . 17), the r e s u l t s are very s i m i l a r to those obtained with mean t o t a l a c t i v i t y ( F i g s . 13 and 14). Most c o l o n i e s 41 F i g u r e 14: Box p l o t d i s t r i b u t i o n of the monthly c o r r e l a t i o n s between t o t a l a c t i v i t y / m i n and the t r a i l vapor d e n s i t y d e f i -c i t f o r a l l c o l o n i e s observed i n Guadeloupe. Other f e a t u r e s are the same as f o r F i g . 13. o I - J K O O o 1.0 0.8 0.6 0.4 0.2 0.0 L E M E S L E • • • • - Q -=9= • L O M A R D -0.2 -0.4 H -0.6 -0.8 ^ -1.0 • • • • 8 3 6 5 2 4 11 12b 12a 9 7 5 12 11 1 3 4 3 Figure 15: Daily fluctuations in the percentage of ants re-turning laden to the colony for a representative colony from each of the 2 study areas in Guadeloupe. Percentage expres-sed in terms of the number of ants returning to the colony, not in terms of the total number of ants on the t ra i l . 44 4 5 F i g u r e 16: Box p l o t d i s t r i b u t i o n of the monthly c o r r e l a t i o n s between the percentage of laden ants and t r a i l temperature f o r a l l c o l o n i e s observed i n Guadeloupe. Other f e a t u r e s are the same as f o r F i g . 13. M O N T H L Y C O R R E L A T I O N S 47 F i g u r e 17: Box p l o t d i s t r i b u t i o n of the monthly c o r r e l a t i o n s between the percentage of laden ants and the vapor d e n s i t y d e f i c i t on the t r a i l f o r a l l c o l o n i e s observed i n Guadeloupe. Other f e a t u r e s are the same as f o r F i g . 13. M O N T H L Y C O R R E L A T I O N S i o o U i o co cn _l i L_ I o I o ro i o o o to p I o as o bo i CO CO a> cn ro 4^ ro cr ro • • • •ae 3 • • CD • • w CO t - 1 M ro cn ro co • • • • • • • t-1 O > tt 49 d i s p l a y e d a range of monthly c o r r e l a t i o n s e q u i v a l e n t to those i n F i g s . 13 and 14. Colony 1 of Lomard, however, no longer d i s p l a y e d the h i g h negative c o r r e l a t i o n s i t e x h i b i t e d with mean t o t a l a c t i v i t y . I t s e f f i c i e n c y of food g a t h e r i n g was very p o o r l y c o r r e l a t e d with t r a i l t e -mperature and VDD. 2.3.3 SEASONAL ACTIVITY CYCLE 2.3.3.1 TOTAL ACTIVITY There was a h i g h l e v e l of v a r i a t i o n i n mean monthly t o t a l s of f o r a g i n g a c t i v i t y per min (outgoing and incom-ing workers) both between months f o r a p a r t i c u l a r co-lony, and between c o l o n i e s f o r 1 area ( F i g . 18). Some common p a t t e r n can be seen, however. From September to December, a c t i v i t y was q u i t e v a r i a b l e . T h i s p e r i o d u s u a l l y r e c e i v e s the h i g h e s t f r e -quencies of r a i n but, as a l r e a d y e x p l a i n e d , the amount of p r e c i p i t a t i o n was uncommonly low i n 1983. For most c o l o n i e s at Lomard a c t i v i t y was lower at the beginning of the dry season (January and/or Fe-bruary) than at other times of the year ( F i g . 18). Co-lony 11 was an e x c e p t i o n , i t s f o r a g i n g a c t i v i t y i n c r e a s -ing d u r i n g that time. At Lemesle, the p a t t e r n was l e s s c l e a r ( F i g . 18). Colony 3 f o l l o w e d the decrease of the b e g i n n i n g of the dry season seen at Lomard. C o l o n i e s 2 and 4 had an o p p o s i t e change, t h e i r f o r a g i n g i n c r e a s i n g d u r i n g that time. 50 F i g u r e 18: Monthly f l u c t u a t i o n s i n f o r a g i n g a c t i v i t y f o r co-l o n i e s i n the 2 study areas i n Guadeloupe. N u p t i a l f l i g h t o c c u r r e d i n the f i r s t h a l f of May. Numbers i n legend's i n -d i c a t e colony numbers. S E P O C T N O V D E C J A N F E B M A R A P R M A Y J U N J U L A U G M O N T H 30 2; ~ 25 ~ 20 15 10 < W 5 H B L E M E S L E D = 2 = 3 S E P O C T N O V D E C J A N F E B M A R A P R M A Y J U N J U L A U G M O N T H 52 In the second h a l f of the dry season (March and/or A p r i l ) , c o l o n i e s 1 and 3 at Lomard, and c o l o n i e s 2, 3 and 12 at Lemesle i n c r e a s e d t h e i r a c t i v i t y l e v e l . O b s e r v a t i o n s of sexuals at the nest entrance and t h e i r c a p ture i n emergence t r a p s p l a c e d on top of the nests suggested that the n u p t i a l f l i g h t of Acromyrmex octo-spinosus would occur i n the f i r s t h a l f of May i n Guadeloupe. Such t i m i n g would u s u a l l y synchronize the f l i g h t with the f i r s t r a i n s a f t e r the dry season ( F i g . 2). Thus i t seems that these 5 c o l o n i e s i n c r e a s e d t h e i r f o r a g i n g i n the few months bef o r e the n u p t i a l f l i g h t , i n a p e r i o d of maximum p r o d u c t i o n of s e x u a l s . F o l l o w i n g a g e n e r a l low a c t i v i t y l e v e l i n May, about h a l f the c o l o n i e s of both areas have a gradual i n -crease i n a c t i v i t y between June and September. T h i s i s the t r a n s i t i o n p e r i o d between the dry and the r a i n y sea-son, and r a i n u s u a l l y occurs more o f t e n d u r i n g t h i s per-i o d ( F i g . 2). C o l o n i e s 5 and 7 at Lomard, and colony 11 at Lemesle were 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 . 2.3.3.2 PROPORTION OF RETURNING ANTS LADEN As with a c t i v i t y , there was a l s o great v a r i a b i l i t y i n e f f i c i e n c y of food g a t h e r i n g ( F i g . 19). E f f i c i e n c y was r a r e l y more than 80%, and o f t e n l e s s than 50%. In both a r e a s , the peak e f f i c i e n c y o c c u r r e d d u r i n g the dry season, i n the few months p r e c e d i n g the n u p t i a l f l i g h t . D e c l i n e i n e f f i c i e n c y s t a r t e d i n March f o r c o l o n i e s 7 and 12 at Lomard ( F i g . 19A), but f o r other c o l o n i e s of 53 F i g u r e 19: Monthly f l u c t u a t i o n s i n e f f i c i e n c y of food gath-e r i n g f o r c o l o n i e s i n the 2 study areas i n Guadeloupe. Le-gend i s the same as i n F i g . 18. 54 90 20 -j 1 1 1 1 1 T 1 1 — i 1 1 1 — S E P O C T N O V D E C J A N F E B M A R A P R M A Y J U N J U L A U G M O N T H 90 20 H 1 1—: 1 1 - i 1 1 1 1 1 1 I S E P O C T N O V D E C J A N F E B M A R A P R M A Y J U N J U L A U G M O N T H 55 both areas, i t began a f t e r the n u p t i a l f l i g h t , i n June. Values c o n t i n u e d to drop i n J u l y and August at Lomard, while at Lemesle they g r a d u a l l y i n c r e a s e d d u r i n g the same p e r i o d f o r 2 of the 5 c o l o n i e s . From September to December, e f f i c i e n c y i n both areas was most v a r i a b l e between c o l o n i e s , but was g e n e r a l l y lower than d u r i n g the dry season. O v e r a l l e f f i c i e n c y values (Table I) were 60.8% at •Lemesle and 55.7% at Lomard; they were not s i g n i f i c a n t l y d i f f e r e n t (t-test=1 . 831, 14 DF). Thus the e f f i c i e n c y of each colony d i d not appear to be r e l a t e d to i t s mean t o t a l f o r a g i n g a c t i v i t y per min (Table II) (r=0.14, 14 DF, n . s ) , i n d i c a t i n g that the observed e f f i c i e n c y ap-p a r e n t l y was not dependent on the s i z e of the f o r a g i n g f o r c e each co l o n y committed. 2.4 DISCUSSION Much a t t e n t i o n has been given to the e f f e c t s of a b i o t i c f a c t o r s on the d a i l y f o r a g i n g a c t i v i t y of l e a f - c u t t e r s , as i t i s the p r e r e q u i s i t e to f u r t h e r e c o l o g i c a l s t u d i e s . Atta cephalotes has been d e s c r i b e d as d i u r n a l i n Panama, with l i g h t i n t e n s i t y i n i t i a t i n g the a c t i v i t y i n the morning (Hod-gson 1955). In Guyana, however, the same s p e c i e s was mostly n o c t u r n a l d u r i n g a 58-day study ( C h e r r e t t 1968). In T r i n i -dad At t a cephalotes made s h i f t s from day to n i g h t a c t i v i t y and v i c e - v e r s a (Lewis et al. 1974a) that were not c o r r e l a t e d with l i g h t i n t e n s i t y , humidity, a i r temperature or 56 TABLE I : Average percentages of f o r a g e r s r e t u r n i n g laden i n c o l o n i e s of the l e a f - c u t t e r a n t , Acromyrmex oct ospi nosus , i n Guade loupe . AREA NEST N 1 % EFFICIENCY (X ± SEM) LEMESLE r1 2a 249 (7) 73. 1 + 1 .5 M 2b 168 (7) 67. 1 + 2.3 3 336 (12) 66.5 + 1 .6 2 390 (12) 64.0 + 1 .6 1 1 134 (7) 63.7 + 2.7 6 108 (5) 60.3 + 3.5 5 93 (5) 56.7 + 3.6 4 458 (12) 55.5 + 1 .4 8 103 (5) 49.5 + 3.1 MEAN (± SEM) 60.8 + 2.4 LOMARD 1 1 282 (12) 65.2 + 1 .6 5 329 (12) 60.7 + 1 .9 1 528 (12) 57.0 + 1 .3 7 239 (12) 54.7 + 2.3 3 344 (12) 53.5 + 1 .6 2 62 (3) 51 .0 + 4.8 1 2 241 (8) 48. 1 + 2.1 MEAN (± SEM) 55.7 + 2.2 OVERALL MEAN ( ± SEM) 59.2 + 1.8 1. Numbers i n parenthese s r e p r e s e n t the number of months of o b s e r v a t i o n . 57 TABLE I I : Average t o t a l f o r a g i n g a c t i v i t y per min f o r each colony of Acromyrmex octospinosus of 2 areas i n Guadeloupe. AREA NEST N 1 TOTAL ACTIVITY/MIN (x ± SEM) LEMESLE 4 548 (12) 16.7 + 0.8 r1 2a 304 (7) 10.1 + 0.8 M 2b 308 (7) 2.8 + 0.3 2 548 (12) 7.1 + 0.5 3 548 (12) 2.1 + 0.1 8 240 (5) 1 .7 + 0.2 6 240 (5) 1 .2 + 0.1 1 1 308 (7) 1 .2 + 0.1 5 240 (5) 1 .0 + 0.1 LOMARD 1 574 (12) 13.1 + 0.5 3 574 (12) 11.0 + 0.7 1 1 574 (12) 8.2 + 0.5 1 2 384 (8) 2.9 + 0.2 5 575 (12) 1 .6 + 0.1 7 575 (12) 1 .0 + 0.1 2 1 42 (3) 0.9 + 0.1 1. Numbers i n parentheses represent the number of months of observat i o n . 58 atmospheric pressure (Lewis et al. 1974b). That s w i t c h i n g p a t t e r n was a l s o present i n Co s t a - R i c a , where i t was se a s o n - r e l a t e d , with n o c t u r n a l f o r a g i n g d u r i n g the dry sea-son and d i u r n a l a c t i v i t y d u r i n g the r a i n y season f o r both Atta cephalotes and At ta col ombi ca (Guerin) (Rockwood 1975). Lugo et al. (1973) d i d not observe t h i s p a t t e r n i n Costa R i c a with Att a col ombi ca, but t h e i r study l a s t e d only a month (August). S e a s o n - r e l a t e d s w i t c h i n g i n the times of a c t i v i t y was a l s o observed f o r At ta sexdens (Fowler and Ro-binson 1979a) and At t a vo l I enweideri ( F o r e l ) (Robinson and Fowler 1982) i n Paraguay, and f o r Atta mexicana (F. Smith) i n the Mexican Sonoran d e s e r t (Mintzer 1979). In the sou-thern U.S.A., f o r a g i n g of Atta texana was temperature-dependent and o c c u r r e d at t r a i l temperatures between 11 and 29 °C (Moser 1967b). Atta insular is (Guerin) was mostly n o c t u r n a l i n Cuba d u r i n g a yea r - l o n g study ( P i n -t e r a and Z o r r i l l a 1981). In the genus Acromyrmex, fewer data are a v a i l a b l e , but a s e a s o n - r e l a t e d t i m i n g of f o r a g i n g a c t i v i t y has been r e -p o r t e d f o r Acromyrmex versicolor versicolor i n A r i z o n a (Gam-boa 1976); and f o r Acromyrmex crassispinus (Fowler 1979) and Acromyrmex landolti fracticornis ( F o r e l ) i n Paraguay (Robin-son and Fowler 1982). Although my o b s e r v a t i o n s of the d a i l y f o r a g i n g of A. oct ospi nosus show a switch i n the ti m i n g of a c t i v i t y analo-gous to those observed i n many At t a spp, i t i s a p p a r e n t l y random r a t h e r than s e a s o n - r e l a t e d . Lewis et al. (1974b), 59 unable to c o r r e l a t e the onset of a c t i v i t y of Atta cephalotes w i t h e i t h e r long-" or short-term p h y s i c a l c y c l e s , proposed t h a t the changing p e r i o d i c i t i e s were dependent on a complex i n t e r a c t i o n of f a c t o r s i n v o l v i n g the r e p r o d u c t i v e c y c l e wi-t h i n a colony and the n u t r i t i o n a l needs of the brood and f o r a g e r s . They noted that p l a n t s e x h i b i t a d i e l c y c l e of primary and secondary m e t a b o l i t e accumulation, thus o f f e r i n g a v a r y i n g amount and mixture of n u t r i e n t s to a n t s , depending on the time of f o r a g i n g . However, as Lewis et al. (1974b) noted, t h i s h y p o t h e s i s , though s e d u c t i v e , i s s t i l l based onl y on c i r c u m s t a n t i a l evidence. N e v e r t h e l e s s , what may h o l d f o r At ta cephalotes i n T r i n i d a d , may a l s o serve f o r A. octospinosus i n Guadeloupe. Except f o r s p e c i e s that have a d i s t i n c t s e a s o n - r e l a t e d switch i n t i m i n g , one simply cannot be sure at t h i s p o i n t why such s h i f t s occur. If Acromyrmex oct ospi nosus can a p p a r e n t l y vary i t s time of f o r a g i n g i n more f o r g i v i n g c l i m a t e s , i t c l e a r l y cannot i n harsher environments. The c l i m a t i c d i f f e r e n c e s between Le-mesle and Lomard may seem s m a l l , but they are s u f f i c i e n t to l i m i t the times of f o r a g i n g at Lomard. Since my study l a s t e d only a year, I do not know i f these d i f f e r e n c e s i n ti m i n g between the 2 areas can be observed r e g u l a r l y . The p o s s i b i l i t y cannot r e j e c t e d that the s t r i k i n g d i f f e r e n c e s observed i n 1983-84 were induced by the e x c e p t i o n a l l y dry c o n d i t i o n s that p r e v a i l e d d u r i n g the f i r s t 8 months of study. Even so, i t should not be s u r p r i s i n g that the seem-i n g l y small d i f f e r e n c e s i n c l i m a t e between the 2 study areas 60 produced such d i f f e r e n t f o r a g i n g p a t t e r n s . C o l o n i e s at Lomard were very c l o s e to the southeastern l i m i t of the pre -sent d i s t r i b u t i o n of the s p e c i e s i n Guadeloupe, and Andre-wartha and B i r c h (1954) have shown that e f f e c t s of c l i m a t e are most l i k e l y to be seen at the edge of a p o p u l a t i o n ' s d i s t r i b u t i o n . The work of Andrewartha and B i r c h a l s o sug-gests that some c l i m a t e i n f l u e n c e on f o r a g i n g might never-t h e l e s s be d e t e c t e d at Lomard i n more normal c o n d i t i o n s . • C o r r e l a t i o n s between parameters of f o r a g i n g ( t o t a l number of ants on t r a i l , % incoming workers laden) and en-vironmental f a c t o r s (temperature, RH, barometric p r e s s u r e ) have been r e p o r t e d f o r Acromyrmex landolti fract i corni s (Fowler and Robinson 1979b; Robinson and Fowler 1982), Acromyrmex crassispinus (Fowler 1979), and Atta cephalotes (Lewis et al . 1974b). Two of the short-term s t u d i e s (Fowler 1979; Robinson and Fowler 1982) had s i g n i f i c a n t c o r r e l a -t i o n s , while c o r r e l a t i o n s i n the t h i r d (Fowler and Robinson 1979b), and i n the ye a r - l o n g study (Lewis et al. 1974b) were not s i g n i f i c a n t . My data g e n e r a l l y agree with p u b l i s h e d r e s u l t s . Four c o l o n i e s i n each area had a c o n s i s t e n t f o r a g -ing p e r i o d ( e i t h e r night or day), and they had many s i g n i f i -cant c o r r e l a t i o n s with the 2 environmental f a c t o r s measured. But the c o r r e l a t i o n s o b t a i n e d f o r the other co-l o n i e s d i f f e r e d widely from month to month, and were o f t e n not s i g n i f i c a n t . P u b l i s h e d c o r r e l a t i o n s have been t e n t a t i v e l y used to p r e d i c t f o r a g i n g a c t i v i t y of ants, with l i t t l e success 61 (Fowler and Robinson 1979b; Lewis et al. 1974b). The high v a r i a t i o n between months i n my study suggests that c o n c l u -s i o n s c o n c e r n i n g the p r e d i c t i v e power of environmental f a c -t o r s , based only on a few months of sampling, should be avoided. The rhythm of f o r a g i n g a c t i v i t y i s undoubtedly i n -f l u e n c e d by other f a c t o r s than weather, and u n t i l more of these other f a c t o r s have been i d e n t i f i e d and q u a n t i f i e d , p r e d i c t i o n s of colony a c t i v i t y o u t s i d e the nest are not l i k e l y to improve. Seasonal changes i n the amounts of f o r a g i n g a c t i v i t y occur f o r many s p e c i e s . In Costa R i c a , At ta col ombi ca and At ta cephalotes both reduced t h e i r f o r a g i n g at the end of the r a i n y and the dry seasons (Rockwood 1975), but i n c r e a s e d t h e i r f o r a g i n g at the beginning of the r a i n y season. In Paraguay, Fowler and Robinson (1979a) found that At ta sex-dens had a lower f o r a g i n g r a t e d u r i n g the c o o l e r months. At t a insular is i n Cuba forages l e a s t i n November and Decem-ber, a f t e r which i t i n c r e a s e s from January to June, j u s t be-f o r e the swarming p e r i o d of l a t e May through e a r l y June ( P i n t e r a and Z o r r i l l a 1981). My data show that f o r a g i n g e f f o r t v a r i e s with season f o r Acromyrmex oct ospi nosus c o l o n i e s . There i s l e s s a c t i v i -ty i n c o o l e r months and a l s o an i n c r e a s e of a c t i v i t y f o r 5 c o l o n i e s i n the few months pre c e d i n g swarming. There i s a l s o a g r a d u a l i n c r e a s e of f o r a g i n g a c t i v i t y i n the i n c r e a s -i n g l y r a i n y t r a n s i t i o n a l months between the dry and r a i n y seasons, f o l l o w i n g a g e n e r a l low a c t i v i t y l e v e l i n May. 62 T h i s seasonal response i s not l i m i t e d to t o t a l f o r a g i n g a c t i v i t y , but i s a l s o apparent i n the i n c r e a s i n g p r o p o r t i o n of s u c c e s s f u l f o r a g e r s r e t u r n i n g to the colo n y i n the few months p r e c e d i n g swarming. T h i s i s a p p a r e n t l y the f i r s t time that such a d i s t i n c t i o n has been made f o r a l e a f - c u t t e r ant. The o v e r a l l mean e f f i c i e n c y of f o r a g i n g i s u s u a l l y low in l e a f - c u t t e r s . A c c o r d i n g to Fowler (1979), Acromyrmex crassispinus i s 50% e f f i c i e n t ; Atta col ombi ca between 50% ( C h e r r e t t p e r s . com. to Lugo et al . 1973) and 70% (Lugo et al. 1973); and At ta cephalotes between 42% i n Guyana (Cher-r e t t 1972b) and 87% i n T r i n i d a d (Lewis et al. 1974a). The l a t t e r s p e c i e s was only 27% e f f i c i e n t i n a l a b o r a t o r y study ( L i t t l e d y k e and C h e r r e t t 1976), while i n the same study Acromyrmex octospinosus was 46% e f f i c i e n t . My f i e l d data f o r A. octospinosus are c o n s i s t e n t with the p r e v i o u s r e p o r t s , the o v e r a l l mean being 59%. However i t i s c l e a r that an o v e r a l l v alue i s somewhat m i s l e a d i n g s i n c e e f f i c i e n c y , l i k e t o t a l a c t i v i t y , tends to vary with c o l o n i e s , seasons and time of day. The problem of f o r a g e r s r e t u r n i n g to the nest unladen has not yet been completely r e s o l v e d with the l e a f - c u t t e r s . L i t t l e d y k e and C h e r r e t t (1976) and S t r a d l i n g (1978) showed that f o r a g e r s r e t u r n i n g a p p a r e n t l y unladen to Atta cepha-lotes and Acromyrmex octospinosus c o l o n i e s had i n g e s t e d some p l a n t sap. Quinlan and C h e r r e t t (1979) demonstrated that the fungus garden does not p r o v i d e workers with a s u f f i c i e n t 63 n u t r i t i o n a l source to meet t h e i r needs, and that ants supplement t h e i r d i e t with p l a n t sap. I t was thus proposed by L i t t l e d y k e .and C h e r r e t t (1976) that ants r e t u r n i n g u n l a -den might have gone out on the t r a i l to feed on p l a n t j u i c e s . Lugo et al . (1973) proposed that unladen workers were the t r a i l maintenance f o r c e i n At t a colombica. A t e s t of t h i s h y p o t h e s i s c a r r i e d out on many A t t i n e s p e c i e s found 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 the t o t a l number of ants r e t u r n i n g unladen and a t r a i l development index combin-ing l e n g t h and width of the t r a i l s (Fowler 1978). However, the " p r o p o r t i o n " of ants r e t u r n i n g unladen was only weakly c o r r e l a t e d with the t r a i l index. Fowler (1978) concluded that i t was u n l i k e l y that a l l r e t u r n i n g unladen f o r a g e r s comprised the t r a i l maintenance f o r c e ; he proposed that some of these workers might be new f o r a g e r s l e a r n i n g t h e i r way along the c o l o n y ' s f o r a g i n g t e r r i t o r y . P r o d u c t i o n of sexuals i s an enormous d r a i n on the c o l o n y ' s economy, and i t i s not s u r p r i s i n g that an i n c r e a s e d energy input i s r e q u i r e d d u r i n g t h i s p e r i o d . Many avenues are a v a i l a b l e to c o l o n i e s to s a t i s f y t h e i r i n c r e a s e d needs. The o u t s i d e f o r a g e r s may comprise a s m a l l p r o p o r t i o n of the p o p u l a t i o n of a colony [from 10 to 20% i n Acromyrmex I ando-I t i j r a c t i corni s , a c c o r d i n g to Fowler and Robinson (1979b)]. C o l o n i e s c o u l d i n c r e a s e t h i s p r o p o r t i o n by com-m i t t i n g p a r t or a l l of the u s u a l l y i d l e workers found i n the nest (Oster and Wilson 1978) to f o r a g i n g . Another, more 64 c o s t l y way, would be to i n c r e a s e the a b s o l u t e number of f o r -agers i n the c o l o n y . Lewis (1975) observed p r e c i s e l y t h i s i n A. octospinosus i n T r i n i d a d . S i m i l a r o b s e r v a t i o n s have a l s o been made f o r Acromyrmex coronat us (Fabr.) by P e r e i r a - d a - S i l v a et al. (1981). F i n a l l y , an obvious s o l u -t i o n would be to i n c r e a s e the p r o p o r t i o n of ants r e t u r n i n g laden. T h i s t a c t i c i s l e s s c o s t l y than new worker produc-t i o n , and i t i s s u r p r i s i n g that i t has not been observed b e f o r e . The l i m i t to t h i s s o l u t i o n would be 100% f o r a g e r s r e t u r n i n g laden, but i n A. octospinosus, and no doubt i n other l e a f - c u t t e r s , t h i s i d e a l s i t u a t i o n i s not l i k e l y ever to be a t t a i n e d due to other c o n s t r a i n t s on f o r a g i n g . For example, I saw that f o r colony 4 of Lemesle ( F i g . 15), the p r o p o r t i o n of ants r e t u r n i n g laden tended to f l u c t u a t e with microweather c o n d i t i o n s , being lowest d u r i n g the h o t t e s t p e r i o d of the day. At the i n d i v i d u a l l e v e l , the i n c e n t i v e necessary to mo-t i v a t e workers who u s u a l l y go out j u s t to feed on sap with-out 'bringing back s u b s t r a t e f o r the fungus (an e s s e n t i a l l y u s e l e s s t r i p f o r the colony) c o u l d come from a r e a c t i o n to the d e p l e t e d s t a t e of the fungus which i s o f t e n seen p r i o r to the p r o d u c t i o n of sexuals (Dias, Kermarrec p e r s . com.). More s t u d i e s on t h i s aspect of the behavior of f o r a g e r s would i n c r e a s e the understanding of the i n d i v i d u a l ' s r e s -ponses to v a r y i n g demands of the s o c i e t y . Chapter 3 FORAGE COLLECTED AND INDIVIDUAL CHOICE 3.1 FORAGE COLLECTED 3.1.1 INTRODUCTION The economic consequences of the p e c u l i a r f o r a g i n g be-h a v i o r of l e a f - c u t t e r ants have been widely r e c o g n i z e d . The l a r g e amounts of v e g e t a t i o n cut as s u b s t r a t e f o r fungus gar-dens can cause great damage to economically important p l a n t s p e c i e s (Echols 1966; Lewis 1975). L e a f - c u t t e r s have a com-plex f o r a g i n g p a t t e r n that i s not easy to understand (Blan-ton and Ewel 1985; C h e r r e t t 1968, 1972b; Fowler and Robinson 1977; Fowler and S t i l e s 1982; P i n t e r a 1983; Rockwood 1976, 1977). Ants o f t e n harvest from i n d i v i d u a l specimens of a p l a n t s p e c i e s f a r from the nest when other specimens of the same s p e c i e s are present much c l o s e r to i t ; abundant s p e c i e s may be ignored while l e s s abundant ones are r e a d i l y c u t . Some of t h i s v a r i a t i o n has been t r a c e d to chemical com-ponents ( B a r r e r and C h e r r e t t 1972; B e r i s h 1986; Chen et al . 1983, 1984; C h e r r e t t and S e a f o r t h 1970; Febvay et al. 1985; H u b b e l l et al. 1983, 1984; L i t t l e d y k e and C h e r r e t t 1975, 1978a; Mudd et al. 1978; Mullenax 1979; S t r a d l i n g 1978; Wiemer and A l e s 1981), and p h y s i c a l ones ( C h e r r e t t 1972a, 1972b; W a l l e r 1982a, 1982b), but more f i e l d s t u d i e s are needed. 65 66 The results of an 11-month sampling of type and amount of vegetation cut by Acromyrmex octospinosus in 2 areas of Guadeloupe are reported herein. 3.1.2 MATERIALS AND METHODS Sampling of the plant species gathered by Acromyrmex octospinosus was conducted from November 1983 to August 1984 for 3 colonies in each of the 2 study areas, at the same time that the activity rhythm was being studied (Chapter 2). Every other hour, after measurements of the amount of activity were finished, the material gathered by every third returning forager was taken until 30 plant pieces had been obtained. No collection was made when activity on the t ra i l was less than 15 ants/3 min, as picking up the pieces then took so long that the normal flow of ants was s t i l l somewhat disturbed when observations on the amount of activity had to be resumed. The plant material collected was brought to the labora-tory and frozen until i t could be identified through herbar-ium collections and a guide to the flora of the French Antilles (Fournet 1978). Between 2 and 9% of the plant ma-terial was too small or too damaged to permit identification. Such material was excluded from the analysis. After identification, the plant pieces were oven dried at 105 °C for 24 h and weighed. An overall fresh/dry weight ratio was also estimated. To obtain this ratio, 15 plant species from each area were 67 chosen because they were r e g u l a r l y c o l l e c t e d by the a n t s , and 1 specimen of each was brought to the l a b o r a t o r y . For each s p e c i e s , 5 p i e c e s of the p l a n t p a r t most o f t e n cut by the ants were weighed, d r i e d , and weighed a g a i n . Thus, the o v e r a l l r a t i o i s based on 150 p l a n t fragments cut from 30 p l a n t s p e c i e s . To f a c i l i t a t e i d e n t i f i c a t i o n of the p l a n t m a t e r i a l , I made a thorough census of the p l a n t s p e c i e s found i n the t e r r i t o r y of each co l o n y sampled. An estimate of the abun-dance of each s p e c i e s was obtained by measuring the area co-vered by every p l a n t patch of each s p e c i e s . To do t h i s , each patch was a s s i g n e d to the c l o s e s t geometric f i g u r e ( u s u a l l y a c i r c l e or r e c t a n g l e ) . Any t r a c e of a t t a c k by Acromyrmex octospinosus on any p l a n t s p e c i e s was then duly noted. 3.1.3 RESULTS 3.1.3.1 OVERALL PATTERN The number of p l a n t s p e c i e s on the t e r r i t o r y of a colony v a r i e d from 38 to 94 at Lomard, and from 40 to 95 at Lemesle (Table I I I ) . On average, 83% of the s p e c i e s showed some evidence of ant v i s i t s . The 2 area means were not d i f f e r e n t (approximated t - t e s t , t*=0.268< t**=2.776, n . s ) . The highest percentage of v i s i t s was 89.4%, f o r c o l o n y 1 at Lomard and c o l o n y 2 at Lemesle. The lowest v a l u e was 62.5% f o r colony 3 at Lemesle. 68 TABLE I I I : P r o p o r t i o n s of the t o t a l number of p l a n t s p e c i e s on the t e r r i t o r y of c o l o n i e s of the l e a f - c u t t e r ant, Acromyrmex octospinosus, that were v i s i t e d and gathered by f o r a g e r s . AREA COLONY TOTAL # % SPECIES % SPECIES SPECIES VISITED FOUND IN DIET 1 LOMARD LEMESLE 1 94 89.4 39.4 1 2 38 86.8 — 1 1 85 85.9 47.6 3 72 84.7 44.4 5 43 79. 1 — MEAN2 [95% CI] 85.3 [80.4)r 89 .6] [33 43.8 .7; 54 2 84 89.4 38. 1 1 2 79 88.6 25.3 4 95 80.0 29.5 1 1 63 77.8 — 3 40 62.5 — MEAN2 [95% CI] 80.5 [66.0;91 .6] [16 30.8 .2;47 OVERALL MEAN2 83.0 [95% CI] [77.2;88.0] 1. Expected i n a sample of 400 p l a n t p i e c e s . Computed us-ing the formula given by H u r l b e r t (1971). 2. A r c s i n back-transformed means. 69 Not a l l s p e c i e s v i s i t e d by the ants were found i n the d i e t . D i r e c t comparisons of the s p e c i e s r i c h n e s s i n the d i e t showed that ants gathered the f o l l o w i n g percen-tages of s p e c i e s i n t h e i r t e r r i t o r y : at Lomard, 53% f o r col o n y 1; 57% f o r colony 3; and 59% f o r colony 11. At Lemesle, 25% for colony 12; 44% f o r colony 2; and 45% f o r c o l o n y 4. Although percentages were a p p a r e n t l y h i g h e r at Lomard, d i r e c t comparisons can be m i s l e a d i n g . The t o t a l sample s i z e v a r i e d between c o l o n i e s from 434 to 1,681 p l a n t fragments. I t i s w e l l known that s p e c i e s r i c h n e s s v a r i e s with sample s i z e , so that the l a r g e s t samples tend to have the g r e a t e s t r i c h n e s s (Peet 1974). T h e r e f o r e i t i s not a p p r o p r i a t e to compare d i r e c t l y co-l o n i e s with v a r y i n g sample s i z e s . H u r l b e r t (1971) g i v e s a formula to f i n d the expected number of s p e c i e s from each sample i f a l l samples were reduced to a common s i z e . Table III pr e s e n t s the c a l c u l a t e d v a l u e s f o r a common sample s i z e of 400 fragments. An average of 31% of the s p e c i e s on the t e r r i t o r y would be i n c l u d e d i n the d i e t of c o l o n i e s at Lemesle i f only 400 fragments were sampled from the t r a i l s . At Lomard, the average would be 44%. Thus c o l o n i e s at Lomard a p p a r e n t l y i n c l u d e d more s p e c i e s i n t h e i r d i e t than c o l o n i e s at Lemesle ( t - t e s t , t=2.84, 4 DF, P<0.05). F i g u r e 20 shows the d o m i n a n c e - d i v e r s i t y curves f o r the p l a n t s p e c i e s found i n the d i e t of each c o l o n y . F o l l o w i n g a procedure of c l a s s i f i c a t i o n begun by 7 0 100 0.01J  R A N K F i g u r e 20: D o m i n a n c e - d i v e r s i t y c u r v e s for s p e c i e s found i n the d i e t of 6 c o l o n i e s of the l e a f - c u t t e r a n t , Acromyrmex oct ospi nosus, i n Guade loupe . B l a c k dots a t the upper end of each c u r v e i n d i c a t e ex treme ly p a l a t a b l e s p e c i e s (%>x+SD). B l a c k dot s at the lower end of each c u r v e i n d i c a t e m a r g i n a l -l y p a l a t a b l e s p e c i e s (%<x-SD). P a l a t a b l e s p e c i e s are i n w h i t e . Separate c o l o n i e s are i d e n t i f i e d by t h e i r code numb-e r s a t the tops of the c u r v e s . 71 Rockwood (1976), the species can be placed into 3 pala-tability categories. Extremely palatable species were found in the diet in proportions greater than 1 standard deviation (SD) above the mean proportion for a l l species in the diet of the colony; palatable species were taken in proportions not exceeding ±1 SD of the mean; and marginally palatable species were collected in smaller proportions than 1 SD below the mean. A complete l is t of the species as well as their rank in the diet is g i -ven in section 3.3. The form of the dominance-diversity curves was close to that expected from a log normal distribution for a l l but 1 colony; colony 12, which had the smallest number of species in its diet, had a geometric series distribution. For a l l colonies, a small number of spe-cies constituted the greatest proportion of the diet. Extremely palatable species, for instance, represent between 9 to 25% of the species found in the diet, but constituted between 62 to 82% of the material collected by the ants. Thus, colonies concentrate on harvesting a few highly palatable species present, but also include apparently less acceptable species, although in much smaller proportions. Correlations between the rank-abundance of the spe-cies on the territory, as measured by the percentage cover, and their rank-abundance in the diet were small and non significant (Table IV). If the plant species 72 TABLE IV: Concordances between rankings of abundance of plant species in the territory and their abundance in the diet for 6 colonies of the leaf-cutter ant, Acromyrmex octo-spinosus, in 2 areas of Guadeloupe. AREA COLONY DF SPEARMAN'S r KENDALL'S r LEMESLE LOMARD 4 1 2 2 1 1 1 3 94 78 83 84 93 71 0.27 n.s 0.19 n.s 0.11 n.s 0.17 n.s 0.12 n.s 0.01 n.s 0.22 n.s 0.14 n.s 0.08 n.s 0.14 n.s 0.09 n.s 0.02 n.s 73 had been taken i n p r o p o r t i o n to t h e i r abundance i n the f i e l d , 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 s would be expected. The small c o r r e l a t i o n s i n d i c a t e that the ants a c t i v e l y s e l e c t among the s p e c i e s on t h e i r t e r r i t o r y . Of the t o t a l of 15 extremely p a l a t a b l e s p e c i e s at Lomard ( l i s t e d f o r each colony i n Tables VIIA and IXA), 11 (73%) were found i n the t e r r i t o r y of a l l 3 c o l o n i e s of the ar e a . However only 3 of these 11 s p e c i e s (27%) were found to be i n the extremely p a l a t a b l e c a t e g o r y f o r every colony, however (Euphorbia hirta, Senecoi des cinera, and Si da rhombifolia rhombifol i a). At Lemesle, 6 of the 12 s p e c i e s of the area that were t r e a t e d as ex-tremely p a l a t a b l e by at l e a s t one colony were common to the 3 c o l o n i e s of the a r e a . But none of these 6 s p e c i e s was t r e a t e d as extremely p a l a t a b l e food by a l l 3 c o l o n i e s . T h i s d i s p a r i t y seems to i n d i c a t e t h a t pre-ference f o r a given s p e c i e s of p l a n t v a r i e s c o n s i d e r a b l y between c o l o n i e s . S e c t i o n 3.3 q u i c k l y confirms t h i s v a r i a t i o n i n pre-f e r e n c e . Euphorbia hirta, one of the 3 extremely p a l a t -a b l e s p e c i e s common to the 3 c o l o n i e s of Lomard was ranked f i r s t by c o l o n i e s 1 and 3, but s i x t h by colony 11. D i s c r e p a n c i e s were even g r e a t e r f o r other s p e c i e s . El echum brownei was ranked i n n i n t h p l a c e by colo n y 1, but was at rank 28 f o r colony 11 and t i e d at rank 32 for colony 3. Comparable d i f f e r e n c e s were seen among the pl a n t s p e c i e s of Lemesle. 74 3.1.3.2 PLANT PARTS GATHERED The p r o p o r t i o n s of d i f f e r e n t p l a n t p a r t s found i n the d i e t d i f f e r r e d (P<0.001) f o r the 2 areas (Tables V and V I ) , but f o r c o l o n i e s w i t h i n an area they were r e -markably c o n s i s t e n t (P>0.05) (Tables V and V I ) . Propor-t i o n s a l s o v a r i e d each month (not i l l u s t r a t e d ) (P<0.001) (Table V I ) . At Lemesle, c o l o n i e s c o l l e c t e d 2 times few-er leaves (approximated t - t e s t , t=5.32, 39.2 DF, P<0.0001) and stems (approximated t - t e s t , t=3.03, 39.3 DF, P<0.004) compared to Lomard, but c o l l e c t e d 6 times more flower p a r t s (approximated t - t e s t , t=4.59, 36.7 DF, P<0.0001) and 5 times more f r u i t p i e c e s (approximated t - t e s t , t=2.31, 40.1 DF, P<0.026) (Table V ) . R e s u l t s f o r the extremely p a l a t a b l e s p e c i e s i n the d i e t are c o n s i s t e n t with the colony and area r e s u l t s de-s c r i b e d i n the pr e v i o u s paragraph (Table VIIA and VI I B ) . With the exc e p t i o n of a few s p e c i e s f o r each co-lony, the hi g h e s t p r o p o r t i o n of p l a n t p i e c e s c o l l e c t e d at Lomard c o n s i s t e d of l e a f fragments. The exceptions were Mangifera indica, mango, that was c o l l e c t e d i n l a r g e r p r o p o r t i o n f o r i t s f r u i t s , Haemat oxyl on campe-c hi a num. that was v i s i t e d as much f o r i t s seeds as f o r i t s l e a v e s , and Caj anus cajan and Bidens pilosa pilosa that were both c o l l e c t e d f o r t h e i r f l o w e r s . Ants of c o l o n i e s at Lemesle, i n marked c o n t r a s t , gathered mostly flowers and f r u i t s from t h e i r extremely p a l a t a b l e s p e c i e s (Table V I I B ) . S p e c i e s l i k e Euphorbia TABLE V: Back transformed means of the proportions of different plant parts collected by foragers of the leaf-cutter ant, Acromyrmex octospinosus, for 3 colonies in areas of Guadeloupe. PLANT PART AREA COLONY SAMPLE SIZE LEAF FLOWER FRUIT SEED STEM OTHER' LEMESLE 8 1 1 24 . 5 46.2 24.4" 16.7 19.0 14.3 2 . 1 2.6 3. 1 6.6 0.01 0.20 12 30.5 49 . 1 2.2 0.5 7.0 0.00 MEAN • [95% CI] LOMARD 25 35.2 26.2 11.8 [23.1;48.4] [15.2;38.9] [3.6;24.0] 1.8 5.4 0.10 [0.2;4.6] [2.8;8.8] [0.00002;0.2] 1 1 75.0 2.9 2.7 1 .4 8.2 0.02 79.5 2.4 0.3 0.7 13.4 0.03 1 1 10 67.3 6.4 3.7 1.0 14.2 0.04 MEAN [95% CI] 30 73.9 3.7 2.0 [66.9:80.4] [1.5:6.9] [0.2;5.5] 1.0 11.6 0.03 [0.4;2.0] [9.3:14.2] [0.0004;0.1] 1. Includes bread, insect cuticle, wood. 76 TABLE VI: ANOVA summaries f o r the e f f e c t of area, colony and month on the p r o p o r t i o n of d i f f e r e n t p l a n t p a r t s gathered by f o r a g e r s of the l e a f - c u t t e r ant, Acromyrmex octospi nosus, i n Guadeloupe. SOURCE OF VARIATION DF SS MS F 1 MODEL 95 34.232 0 .360 9. 47 **** AREA 1 0.013 0 .013 — COLONY(AREA) 4 0.053 0 .013 — MONTH 10 0.062 0 .006 — PLANT PART 5 23.325 4 .665 5. 13 * AREA*PLPART 5 4.552 0 .910 12. g4 **** COLONY*PLPART 20 1 .444 0 .072 0. 75 n. s MONTH*PLPART 50 4.783 0 .096 2. 53 **** ERROR 234 8.967 0 .038 TOTAL 329 43.199 1. * P<0.05, ** P<0.01, *** P<0.005, **** P<0.001. TABLE VIIA: Proportions of different plant parts of the extremely palatable species gathered by foragers of the leaf-cutter ant, Acromyrmex octospinosus, In the Bois de Lomard area of Guadeloupe. PLANT PART SPECIES N LEAF FLOWER FRUIT SEED STEM a) Colony 1 Euphorbi a hirta 235 78. 1 7.0 0.0 0.0 14.9 Ipomoea tiltacea 130 100.0 0.0 0.0 0.0 0.0 Senecioides cinera 190 86. 1 1.6 0.0 0.0 12.3 Mangffera indica 99 8.1 3.0 85.9 0.0 3.0 Sesbania sesban 96 84.9 0.0 0.0 0.0 15. 1 Haematox 11 on campechianum 46 41.3 0.0 o.o 50.0 8.7 Si da rhombifolia rhomblfolia 75 89.3 4.0 0.0 0.0 6.7 Bidens pllosa pllosa 39 92.3 2.6 0.0 0.0 5. 1 Blechum brownei 29 93. 1 0.0 0.0 0.0 6.9 Cajanus cajan 35 '. 4.3 77.1 5-7 5.8 7 . 1 b) Colony 3 Euphorbi a hirta 342 71.2 7.9 0.9 0.0 20.0 Senecioides cinera 191 84.8 2.4 0.0 0.0 12.8 St da rhombifolia rhombifolia 172 91.0 1 .2 0.0 O.O 7.8 Acacia sp. 38 84.2 0.0 0.0 . 0.0 15.8 Corchorus siliquosus 48 86.5 0.0 0.0 0.0 13.5 Cracca or Teramnus' 84 75.0 8 . 3 0.0 9.5 7.2 Haematoxylon campechianum 42 42.9 0.0 2.4 52.3 2.4 Continues on next page TABLE VIIA: Continued from previous page SPECIES LEAF FLOWER PLANT PART FRUIT SEED STEM c) Colony 11 Acacia sundra Sida rhombtfolia rhombifolia Manglfera indica Senecioides drier a Bidens pilosa pilosa Euphorbia hirta Cassia obtusifolia Sesbani a sesban 171 56 .6 12 .9 0 .0 1 .8 28 .7 154 83 . 1 0 .0 0 .0 0. •0 16 .9 126 22 .2 0, .8 76. ,2 " 0, .0 O .8 94. 89 • 4 1 . . 1 0. .0 0. 0 9. .5 89 14 .6 83. 2 0. .0 0. .0 2 .2 57 69 .9 6. .7 0. o 1. .8 21 . .6 74 95, .9 0. .0 0. 0 . 0. .0 1 . .4 51 91. .2 0. 0 0. .0 0. .0 8 .8 1. Cracca caribea Benth. and Teramnus labialis Spreng. In the samples could only be distinguished by the color of their flowers. Since flowers were often absent from the samples, these 2 species were grouped under the same heading. TABLE VIIB: Proportions of different plant parts of the extremely palatable species gathered by foragers of the leaf-cutter ant, Acromyrmex octospinosus, 1n the Lemesle area of Guadeloupe. PLANT PART SPECIES N LEAF FLOWER FRUIT SEED STEM a) Colony 2 Artocarpus a l t i l i s 378 13.0 23.3 61.0 0.3 2.4 Tabebuia pallida 58 12.0 46.6 o.o 41.4 0.0 Cordia collococca 65 0.0 100.0 0.0 0.0 0.0 Cent rosema pubescens 25 4.0 96.0 0.0 O.O 0.0 Cassia fistula 23 34.8 0.0 0.0 65.2 O.O Senec ioides cinera 33 93.9 0.0 0.0 1.5 4.6 b) Colony 4 Cracca or Teramnus' 690 21 .7 59.4 0.0 14.0 4.9 Mangifera indica 259 10.8 19.3 64.0 0.0 5.9 Senecioides cinera 273 69.0 15.2 0.2 0.0 15.6 Euphorbia hirta 101 82.2 0.0 9.9 . 0.0 7.9 Si da rhombifoil a rhombifolia 53 28.3 55.7 0.0 11.3 4.7 c) Colony 12 TamarIndus Indica 146 25.3 71.2 0.0 0.0 3.5 Artocarpus a l t i l i s 75 0.0 62.7 37.3 0.0 0.0 Desmanthus depressus 24 0.0 100.0 0.0 0.0 0.0 Mangifera indica 80 19.4 48. 1 28 .8 0.0 3.7 1 . See note 1 Table VIIA. 80 hirta, Senecioides cinera and Mangifera i n d i c a , ( i n the extremely p a l a t a b l e category f o r c o l o n i e s of both areas) were foraged c o n s i s t e n t l y i n both areas. The f i r s t 2 s p e c i e s were e x p l o i t e d f o r t h e i r l e a v e s , and the t h i r d f o r f r u i t s and fl o w e r s . The other 2 s p e c i e s of t h i s c a -tegory common to c o l o n i e s i n both areas, Cracca or Tera-mnus and Sida rhombifolia rhombifolia, were not v i s i t e d c o n s i s t e n t l y f o r the same p l a n t p a r t s . At Lomard, •leaves were mostly gathered, whereas mostly flowers were cut at Lemesle. 3.1.3.3 MONTHLY VARIATIONS The average rank c o r r e l a t i o n between monthly pre-f e r e n c e s shown by c o l o n i e s of the l e a f - c u t t e r f o r a l l s p e c i e s found i n t h e i r t e r r i t o r i e s was s i g n i f i c a n t f o r a l l c o l o n i e s of both areas (Table V I I I ) , but was not very h i g h . Thus even i f there was a gene r a l c o n s i s t e n c y between months f o r some s p e c i e s (most l i k e l y the margin-a l l y p a l a t a b l e s p e c i e s and s p e c i e s that were never c o l -l e c t e d and so always ranked l a s t ) , p r e f e r e n c e f o r more a t t r a c t i v e s p e c i e s a p p a r e n t l y v a r i e d from month to month. The monthly v a r i a t i o n s f o r the extremely p a l a t a b l e s p e c i e s (Tables IXA and IXB) c o n f i r m t h i s impression. A given s p e c i e s d i d not represent the same p r o p o r t i o n of the d i e t i n each month. For example, i n October, No-vember, January and February, Euphorbia h i r t a and Sene-cioides cinera were the 2 s p e c i e s most f r e q u e n t l y 81 TABLE V I I I : Average rank c o r r e l a t i o n s between monthly rank-ings of a l l p l a n t s p e c i e s i n the t e r r i t o r y of 6 c o l o n i e s of Acromyrmex octospinosus i n Guadeloupe. AREA COLONY DF FRIEDMAN'S KENDALL'S F 1 W1 LEMESLE 4 94 0.44 * 0.50 * 12 78 0.22 * 0.35 * 2 83 0.25 * 0.34 * LOMARD 1 93 0.41 * 0.46 * 11 84 0.40 * 0.46 * 3 71 0.37 * 0.44 * 1. * P<0.001 . TABLE IXA: Monthly variations 1n proportions of the extremely palatable species 1n the diet of the leaf-cutter ant, Acromyrmex octosplnosus, in the Bois de Lomard area of Guadeloupe. MONTH' SPECIES OCT NOV DEC JAN FEB . MAR APR MAI JUN JUL AUG a) Colony 1 (240) (138) (107) (232) (79) ( 176) (90) (46) (69) (51) (46) Euphorbi a hirta 19 • 6t 27 . 5» 21 .5 38 8t 19 .0 5 .7 o .0 0. .0 10 . 1t 9 .8 0.0 Ipomoea ttliacea 0 •P 2 .2 3 .7 3 .4 8 .9 37 .5 . 0. .0 0. 0 0 .0 7 .8 82.6 Senecioides cinera 26 .7* 23 . 2» 7 .5 20 .7 22 • 8t 6 .3 0. .0 ' 19. 6. 0 .0 0 .0 0.0 Mangifera Indica 0 .4 0 .0 3 .7 2 .2 0 .0 1 . 1 91 . . 1* 0. 0 2 .9 2 .0 4.3 Sesbania sesban 11 .3 7 .2 6 .5 0 .9 6 .3 4 .0 0. ,0 15. 2 37 .7 7 .8 2.2 Haematoxylon campechlanum 1 .7 0, .7 0 .0 0 •0 0 .0 10 .8" 2. 2» 32. 6» 5 .8 0 .0 2.2 Sida rhombifolia rhombifolia 10 8« 9, .4* 1 1 2t 7 .3 1 .3 1 . 1 0. 0 2. 2 1 , .4 3 .9 0.0 Bidens pilosa pilosa 4. .2 1 . 4 0 .0 0, 9 10 . 1 1 .7 0. 0 6. 5 4, .3 9 .8 6.5 Blechum brownel 0. .0 1 . 4 2 .8 1 . .3 1 .3 0 .0 0. 0 0. 0 4. .3 33 .3* O.O Cajanus cajan 0. .0 0. o 31 8t 0. 4 0. .0 0 .0 0. 0 0. 0 0. •°, 0 .0 0.0 ALL OTHERS 25. . 3 27. .0 11 .3 24. , 1 30. .3 31 . .8 6. 7 23. 9 33. ,5 25 .6 2.2 b) Colony 3 (233) ( 128) ( 109) ( 179) ( 155) (225) (--) (-•) (77) (57) (72) Euphorbia hirta 38. 6t 30. 5t 50. 5t 39. 1t 14 , 8 7 6 2. 6 59. 6» 16.7 Senecioides cinera 0. 0 10. 2t 18. 3t 14 . ot 45. 2t 26. 2t 0. .0 0. .0 5.6 Sida rhombifolia rhombifoil a 25. 8t 21 . 1 1 1 . 9t 25. 7 5. ,2 8. .0 0. ,0 0. .0 0.0 Acacia sp. 0. 0 0. 8 0. 9 1 . 1 1 . 9 0. 0 40. 3* o. .0 0.0 Corchorus si 1tquosus 0. 0 1 . 6 1 . 8 0. 0 1 . 9 9. .3 0. 0 5 . ,3 23.6 Cracca or Teramnus' 8. 6 1 1 . 7t 0. 0 2. 8 2. 6 17 . 8- 0. 0 0. .0 0.0 Haematoxi1 on campechlanum 0. 0 0. 0 0. 9 0. 0 0. 6 9. 3- 19. 5» 5. ,3 1 .4 ALL OTHERS 27. 0 24. 1 15. 7 17. 3 27 . 8 21 . 8 37. 6 29. 8 52.7 Continues on next page TABLE IXA: Continued from previous page MONTH1 SPECIES OCT NOV DEC OAN FEB MAR APR MAI JUN JUL AUG c) Colony 11 (117) (99) (110) (217) (152) (125) (-) (66) (156) (248) (33) Acacia sundra 12 .8* 0. .0 21 8t 2 8t 13 .2 . 8 .8 - 47 .0- 3 .2 14 .9 66 .7 SI da rhombifol1 a rhombifolia 15 .4* 0; .0 8 .2 33 .2* 0, • 9 0 .0 O .0 34 ,6« 0 . 4 0 .0 Mangifera indica 1 .7 7. 1 0. .0 12, 4* 0. ,7 24. .8* -• 1 . 5* 17 3* 1 1 . 3* 6 . 1 Seneci oi des cinera 10. .3* 8. 1» 1 1 . 8» 15. • 7t 9. 2 6. .4 - 4 , .5 0. 6 0. .4 o. O Bidens p l l o s a p l l o s a 0 .0 2,. 0 0. .0 0 .5 56. 6t 0. .0 - 0 . .0 0. .0 0. .0 0. .0 Euphorbia h i r t a 7, .7* 18 . 2t 5. .5 4. . 1» 0 .0 5. 6t 3. .0 1 . .3 3. .0 5 .0 Cassia obtuslfolla 7 . 7» 8 . 1« 4 . 5 0. .0 1 . .3 3. .2 ' - 0 .0 2 .6 16 . 9» 0 o Sesbania sesbans 0. .9 7. 1 1 1 . ,8 8. .3 0. ,7 2. .4 - 0. .0 0. 6 2. 8 0. .0 ALL OTHERS 43. 5 49. 4 36. ,4 23. .0 18. .3 48. ,8 - 44 , 0 39. .8 50. ,3 22. ,2 1. Number in parentheses represent the sample size for the month. The special character to the right of a percentage Indicates that material of the following transient resources were found 1n samples containing that species: •= young leaves; tr flowers; t= fruit pieces; •= seeds. 2. See note 1 Table VIIA. TABLE IXB: Monthly variations In proportions of the extremely palatable species 1n the diet of the leaf-cutter ant, Acromyrmex octosplnosus, In the Lemesle area of Guadeloupe. MONTH' SPECIES OCT NOV DEC JAN F.EB MAR APR MAI . JUN JUL AUG a) Colony 2 • (161) (109) (-) (150) (116) (103) (72) (60) (-) (34) (-) Artocarpus a l t i l i s 2.5+ 64. 2t — 100.0* 50. 6* 19.5+ 62.5+ 40.0+ ' — 17 .6+ — Tabebula pal 11 da 0.0 5.5 0.0 10.3 + 1 .9 23.6 + 35.0" - 0.0 -Cordla collococca 0.0 0.0 - 0.0 0.0 63. 1 + 0.0 0.0 - O.O -Cent rosema pubescens 0.6 0.9 - 0.0 1.7+ 0.0 5.6+ 0.0 - 50. Ot -Cassia fistula 3. 1 0.0 • • - 0.0 2.6 0.0 0.0 25.0" - 0.0 -Senecioides cinera 7.5. 3.7 - 0.0 3.4 12.6 0.0 0.0 - 0.0 -ALL OTHERS 86.3 25.7 - 0.0 31.1 2.9 8.3 0.0 — 32.4 — b) Colony 4 (172) (242) (392) (264) (153) ( 107) (28) (80) (54) (26) (86) Cracca or Teramnus' 50.Ot 34.7- 41 .6» 67.8« 46.4- 58.9 + 39.3+ 31 .3 + 5.6 0.0 5.8 Manglfera indica 1 .7 0.9 6.9+ 22.0+ 15.0+ 28.0+ 42.9+ 63.8 + 64 .8* 0.0 20.9 Senecioides cinera 21 .6t 20.7 + 34.7 + 3.0 9.8 4.7 3.6 1.3 11.1 ;38.5+ 4.7 Euphorbia hirta • 11 .0 14.0+ 3.1 3.8 0.7 0.9 0.0 0.0 0.0 3.8 26.7 + Sida rhombifolia rhombifolia 0.6 11.6 + 0.3 0.8 11.8+ 0.9 3.6 0.0 O.O O.O 1 .2 ALL OTHERS 15.2 18. 1 13.4 2.6 16.3 6.6 10.6 3.6 18.6 57.7 40.7 c) Colony 12 (-) (-) (-) (-) (167) (38) (58) (-) (4) (32)* (125) Tamari ndus indica — — — — 0.6 0.0 0.0 — 50.0+ 90.6 + 91 .2 + Artocarpus a l t i l i s - - - - 21.6 + 7.9+ 56.9+ 0.0 0.0 2.4 + Desmanthus depressus - • - - - o.o 63.2 + 0.0 - • 0.0 0.0 0.0 Manglfera indica - - - - 45.5 + 7.9 + 1 .7 ' - . O.O O.O O.O ALL OTHERS - , - - - 32.3 21.0 41.4 — 50.0 9.4 6.4 1. 2 . See note 1 Table IXA. See note 1 Table VIIA. 00 85 gathered by colony 1 at Lomard (Table IXA). In A p r i l , however, they were not seen in the d i e t , but were r e -p l a c e d i n s t e a d by Mangifera indica. In December, Ca-janus cajan was the most gathered s p e c i e s , but was ab-sent from the d i e t i n the other months. S i m i l a r changes o c c u r r e d f o r other s p e c i e s of p l a n t s and f o r the other 5 ant c o l o n i e s . The p r o p o r t i o n s of each type of p l a n t p a r t c o l -l e c t e d from each p l a n t s p e c i e s d u r i n g each month provide f u r t h e r i n s i g h t on the response p a t t e r n s of f o r a g e r s to changes i n food q u a l i t y . At Lemesle, where 3 c o l o n i e s were l o c a t e d c l o s e to f r u i t - b e a r i n g t r e e s , and where p l a n t p a r t s such as flowers or f r u i t s were most abundant i n the d i e t , i t appears t h a t changes in p r e f e r e n c e rank-ings of a p a r t i c u l a r s p e c i e s were o f t e n a s s o c i a t e d with e x p l o i t a t i o n of a t r a n s i e n t r e s o u r c e . In Tables IXA and IXB, a s p e c i a l c h a r a c t e r appearing to the r i g h t of a percentage i n d i c a t e s that a t r a n s i e n t resource was found i n the foraged m a t e r i a l cut from that s p e c i e s . For example, p l a n t m a t e r i a l cut by colony 2 from Artocarpus alt i I i s , the bread f r u i t t r e e , i n October and December (Table IXB, a) c o n s i s t e d of 70 to 75% of flower p a r t s . In January, o n l y f r u i t p i e c e s were cut from that s p e c i e s . Increases i n c u t t i n g of Cordia collococca i n March, and of Centrosema pubescens i n J u l y , and to a l e s s e r degree i n February and A p r i l , c o i n c i d e d with the p r o d u c t i o n of f l o w e r s , which became the dominant 86 material foraged from these species. Even if a transient resource was not collected in large quantities, its presence may induce cutting of other parts on a plant. Appearance of fruit on Euphor-bia hirta at Lemesle induced an increase in cutting of this species in November and August, even though fruit comprised only 15-20% of the cut material, with leaves constituting the remainder. For the same species at Lomard (Table IXA), presence of flowers favored in-creased cutting, but they comprised only 2-20% of the material taken from that species. With other species at Lomard, the ants often responded to the production of young leaves, and they collected them from species such as Senecioides cinera and Euphorbia hirta. Consequently, attempts to relate the monthly varia-tion in proportions of the extremely palatable species collected, with the variations in the proportions of the different plant parts cut, disclose that a change in ac-ceptability of a species is often associated with an in-crease in the proportions of the preferred plant parts collected from that species. This relationship suggests that ants react to the appearance of an acceptable tran-sitory resource, and concentrate their cutting on the new forage. However, the increased cutting is not always concentrated on the actual transient resource, but may include other parts of the same plant. 87 3 . 1 . 3 . 4 BIOMASS COLLECTED Combining the average numbers of laden f o r a g e r s r e -t u r n i n g to a c o l o n y tha t were o b t a i n e d from the o b s e r v a -t i o n s of the a c t i v i t y rhythm w i t h the average dry weight of a l l fragments c o l l e c t e d , a l l o w e d me to e s t imate the d r y weight of biomass c u t by the c o l o n i e s of Acromyrmex octospinosus. Amounts cut v a r i e d from 0.4 to 21.9 k g / y e a r / c o l o n y a t Lemes le ; and from 0.7 to 11.1 k g / y e a r / c o l o n y at Lomard (Tab le X ) . There was no d i f -f e r e n c e between the means f o r the 2 areas ( t - t e s t , t=0.324, 13 DF, n . s ) , and the o v e r a l l mean was e s t i m a t e d at 5.3 kg dry m a t t e r / y e a r / c o l o n y . The o v e r a l l f r e s h / d r y weight r a t i o was 4.1 ± 0.2 (x ± SEM). Thus an average c o l o n y would cu t 21.7 kg of f r e s h m a t t e r / y e a r . On G r a n d e - T e r r e , between 1976 and 1980, the d e n s i t y of A. octospinosus has been e s t i m a t e d at 16.8 ± 1.8 (x ± SEM) c o l o n i e s / h a (data p r o v i d e d by M. B e r t a u x , S e r v i c e de l a P r o t e c t i o n des Vegetaux, G u a d e l o u p e ) . Based on t h a t e s t i m a t e , t h e r e f o r e , the ant s would c o l l e c t an average of 364 kg f r e s h m a t e r i a l / h a / y e a r . 3 . 1 . 4 DISCUSSION The f i e l d r e s u l t s f or f o r a g i n g of Acromyrmex octospino-sus a re c o n s i s t e n t w i t h r e s u l t s o b t a i n e d f o r o ther l e a f - c u t t e r s p e c i e s . Atta colombica, At t a cephalotes ( B l a n -ton and Ewel 1985; C h e r r e t t 1968; Rockwood 1976, 1977), and 88 TABLE X: Estimated annual dry weight of biomass c o l l e c t e d by the c o l o n i e s of Acromyrmex octospinosus s t u d i e d i n Guade-loupe . AREA COLONY # MONTHS OBSERVED DRY WEIGHT (kg/year) LEMESLE LOMARD r12a 1-1 2b 4 2 3 1 1 6 5 8 1 1 1 3 1 2 5 2 7. 7 7 1 2 1 2 1 2 7 5 5 5 MEAN (± SEM) 1 2 1 2 1 2 8 1 2 3 12 MEAN (± SEM) OVERALL MEAN (± SEM) 18.2 3.7 12.0 8.6 1 .4 0.9 0.8 0.7 0.4 5.8 ± 2.8 11.1 9.6 7.9 1 .7 1 .3 0.7 0.7 4.7 ± 1.7 5.3 ± 1.6 89 Atta insularis ( P i n t e r a 1983), a l l e x h i b i t d e f i n i t e pre-f e r e n c e s f o r some s p e c i e s of p l a n t s in t h e i r f o r a g i n g area, do not c o l l e c t s p e c i e s i n p r o p o r t i o n to t h e i r a v a i l a b i l i t y i n the t e r r i t o r y , and vary t h e i r p r e f e r e n c e over time. V a r i a b i l i t y i n p r e f e r e n c e between the c o l o n i e s of the l e a f - c u t t e r s they s t u d i e d has a l s o been r e c o g n i z e d by the p r e v i o u s a u t h o r s , and by Fowler (1982) f o r Acromyrmex I an-dolti fracti corni s . Fowler observed that p r e f e r e n c e s f o r d i f f e r e n t grasses were e n v i r o n m e n t a l l y induced i n t h i s spe-c i e s , the ants being c o n d i t i o n e d to p r e f e r a p a r t i c u l a r spe-c i e s by e a r l y experience and l e a r n i n g . He h y p o t h e s i z e d that comparable l e a r n i n g i s a l s o i n v o l v e d i n such s p e c i e s as At ta col ombi ca and At ta cephalotes, which may h e l p to e x p l a i n v a r i a t i o n s i n p r e f e r e n c e between c o l o n i e s . D e f i n i t e p r e f e r e n c e s f o r s p e c i f i c p l a n t p a r t s have a l s o been noted f o r the above 3 At ta s p e c i e s , wherein new l e a v e s , f l o w e r s , and f r u i t s were the most f r e q u e n t l y gathered p l a n t p a r t s . P r e f e r e n t i a l f o r a g i n g by ants on p l a n t s p e c i e s har-b o r i n g t r a n s i e n t r e s o u r c e s has a l s o been noted by Rockwood (1976) and P i n t e r a (1983). C h e r r e t t (1972a) observed that l a b o r a t o r y c o l o n i e s of At ta cephalotes were a t t r a c t e d to no-v e l food sources. Rockwood suggested that ants i n the f i e l d were r e a c t i n g to the n o v e l t y of flowers and f r u i t s of a p l a n t s p e c i e s i n t h e i r f o r a g i n g area, and p r e f e r e n t i a l l y ex-p l o i t e d those patches c o n t a i n i n g these novel r e s o u r c e s . T h i s e x p l a n a t i o n probably c o u l d be a p p l i e d to Acromyrmex oc-tospinosus as w e l l , but a genuine p r e f e r e n c e f o r the 90 transitory plant parts, not simply a positive reaction to their novelty, also seems to be involved in this species. Flowers or fruits of some species were s t i l l collected after being present for many months, so that the novelty effect is unlikely to be a factor then (e.g. Artocarpus altilis, Crac-ca or Teramnus, and Mangifera indica; colonies 2 or 4 at Le-mesle, Table IXB). The results of the study of visits to plants, as well as the plant collection results, suggest that even though ants sample a high proportion of the plants on their terr i -tory, they usually collect the most palatable of these to carry back to the nest. Similar conclusions were drawn by Rockwood (1976) who found that At ta col ombi ca and At ta ce-phalotes visited between 49 to 77% of a l l species in their territory, but collected only 31.4 and 22% of a l l species respectively; and by Blanton and Ewel (1985) for At t a cepha-lotes. The differential cutting of plant parts between the 2 areas is difficult to explain. Differences in the availabi-l i ty of transient resources explain some of the variation, but not entirely. For example, although ants prefer flowers or fruits for species like Artocarpus altilis and Tabebui a pallida, and these species were present at Lomard, they were immature and did not produce flowers or fruits. Therefore such plant products could not be collected there. In con-trast Mangifera indica, mature and producing flowers and fruits in both areas, was collected consistently for flowers 91 and f r u i t s from one area to the o ther ( T a b l e s V I I A and V I I B ) . But t h i s e x p l a n a t i o n does not h o l d f o r a l l the o b s e r v e d d i f f e r e n c e s . Si da rhombifolia rhombifolia and Cracca or Teramnus were p r e s e n t i n both a r e a s . A l t h o u g h the same p l a n t p a r t s were c o l l e c t e d in both a r e a s , at Lomard l eaves were c o l l e c t e d more than f l o w e r s , f r u i t s , or seeds . The t i m i n g of the f o r a g i n g a c t i v i t y i n the r e s p e c t i v e areas might h e l p to e x p l a i n t h i s d i f f e r e n c e . Ants at Lomard were n o c t u r n a l , w h i l e a t Lemesle they were mos t ly d i u r n a l . F l o w -e r s , f r u i t s or seeds may be more a t t r a c t i v e a n d / o r e a s i e r to f i n d d u r i n g the d a y . Working at n i g h t , f o r a g e r s at Lomard then might f i n d them l e s s a t t r a c t i v e or l e s s obv ious and t h e r e f o r e would not cu t them as o f t e n . The s m a l l e r p r o p o r t i o n of the t o t a l a r r a y of p l a n t s w i -t h i n a c o l o n y ' s t e r r i t o r y that was found i n the d i e t of c o -l o n i e s at Lemesle compared to c o l o n i e s a t Lomard might be r e l a t e d to the d i f f e r e n c e in a v a i l a b i l i t y of the p r e f e r r e d p l a n t p a r t s between a r e a s . Lemesle was r i c h i n s p e c i e s p r o -d u c i n g the f l o w e r s , f r u i t s or seeds p r e f e r r e d by the a n t s . A l a r g e p r o p o r t i o n of t h e i r d i e t a r y requ irements thus was met by these s p e c i e s ; c o n s e q u e n t l y the dominance of a few s p e c i e s at Lemesle can be observed i n F i g . 20. These spe-c i e s a p p a r e n t l y a t t r a c t e d the l a r g e s t p a r t of the f o r a g i n g f o r c e , thus l e a v i n g few other f o r a g e r s to e x p l o i t o ther r e s o u r c e s . At Lomard, on the o t h e r hand , t h e r e was l e s s emphasis on a few h i g h l y p r e f e r r e d s p e c i e s . S i n c e there was 92 l e s s pronounced channeling of the f o r a g i n g f o r c e , the ants of Lomard e x p l o i t e d a wider range of s p e c i e s . These c o n c l u -s i o n s are supported by Shepherd (1985) who a l s o observed, between 2 c o l o n i e s of Atta cephalotes, a c o n t r a c t i o n i n d i e t with g r e a t e r a v a i l a b i l i t y of h i g h ranking food s p e c i e s . The 2 adjacent c o l o n i e s were f o r a g i n g i n 2 d i f f e r e n t a r e a s : 1 i n an o l d secondary f o r e s t , the other i n a younger one. The younger f o r e s t was r i c h e r i n h i g h q u a l i t y s p e c i e s , and the colony e x p l o i t i n g i t foraged on a s m a l l e r p r o p o r t i o n of the a v a i l a b l e s p e c i e s , and s p e c i a l i z e d on a s m a l l number of the high q u a l i t y p l a n t s ; the other colony foraged on more spe-c i e s of lower q u a l i t y . The d i f f e r e n c e i n d i e t breadth observed i s c o n s i s t e n t with p r e d i c t i o n s of optimal f o r a g i n g theory with r e f e r e n c e to the o p t i m a l ' d i e t of p r e d a t o r s (Krebs 1979, Pyke et al . 1977). Krebs (1979) wrote "... the models p r e d i c t t h at pre-d a t o r s should be more s e l e c t i v e when food a v a i l a b i l i t y (more p r e c i s e l y a v a i l a b i l i t y of high q u a l i t y prey) i s h i g h , than when food i s s c a r c e . Rather more s u r p r i s i n g i s the p r e d i c -t i o n t h a t whether or not low ranking prey should be i n c l u d e d i n the d i e t depends only on the a v a i l a b i l i t y of h i g h ranking types, and not on the encounter r a t e with low ranking prey themselves. In other words, i f good q u a l i t y prey are common enough, the predator should never 'take time out' to eat un-p r o f i t a b l e prey, even i f they are abundant". E s s e n t i a l l y what i s p r e d i c t e d by the theory i s that a c h a n n e l i n g of the e f f o r t of the f o r a g i n g f o r c e should occur when h i g h ranking 93 food types are p r e s e n t . T h i s i s e x a c t l y what was observed f o r Acromyrmex octospinosus. S e v e r a l i n v e s t i g a t o r s have devoted c o n s i d e r a b l e a t t e n -t i o n to the amount of biomass c o l l e c t e d by l e a f - c u t t e r s (Table X I ) . ' Comparisons of estimates between d i f f e r e n t s t u -d i e s are d i f f i c u l t to make as the methods and the s i z e s of c o l o n i e s used vary widely (Lugo et al. 1973). N e v e r t h e l e s s , i t i s c l e a r from Table XI that the amount of v e g e t a t i o n cut by Acromyrmex s p e c i e s i s not of the same order of magnitude as t h a t of At t a s p e c i e s . The o v e r a l l mean dry biomass c o l -l e c t e d f o r a l l s t u d i e s presented i n Table XI i s 2.5 kg/year/colony f o r Acromyrmex s p e c i e s , and 482.7 kg/year/colony f o r s p e c i e s of Atta, n e a r l y 200 times as much. If o n l y the amount of biomass c o l l e c t e d i s c o n s i d e r e d , the pest s t a t u s of A. octospinosus might be doubted. In order to compare l e a f - c u t t e r s with a more f a m i l i a r animal, Fowler and Robinson (1975, c i t e d by Jonkman 1980) used as an example a 200 kg cow consuming grass at the r a t e of 4.9 kg dry matter/day. An average ant colony c o l l e c t s 5.1 kg dry matter/year, about the same as the cow i n 1 day. The c a r r y -ing c a p a c i t y of p a s t u r e s , even under the most i n t e n s i v e c a t t l e r a i s i n g methods, does not exceed 2 cows/ha without a d d i t i o n a l f e e d i n g (Jonkman 1980). These 2 cows would con-sume 3,576 kg dry matter/ha/year. At d e n s i t i e s of 16.8 n e s t s / h a , A. oct ospi nosus c o l l e c t s 85.7 kg dry matter/ha/year on average, about 40 times l e s s than the 2 TABLE XI: Estimated annual dry weight of biomass collected by leaf-cutter ant colonies as reported 1n the literature. SPECIES LOCATION NEST SIZE DRY WEIGHT (kg/yr/nest) REFERENCE a) Atta cap I guara Paraguay 149, . 1 Robinson and Fowler (1982) cephalotes Costa Rica 37.8 ni' 560. 6 Markham (1966)' Costa Rica • 4 trails 2540. .4 Gara (1970)' Costa R1ca — 150. ,7 Parsons (1968)' Guyana 56 m' 95. .5 Cherret (1968)' Guyana 56 m' 1 14 . .8 Cherret (pers. com.)1 Panama 2 trails 746 . 4 Hodgson (1955) co1ombi ca Costa Rica 3 trails 366. 2 Emmel (1967)5 Costa Rica 44.8 m' 416. 1 Lugo et al. ( 1973) Panama 2.5-35.3 m! 130. 5 Haines (1978) sexdens Brazl1 — • 417. 9 Autuori (1940)' vo 1 1 enive ideri Costa R1ca "Average" 39-85 Jonkman (1980) Paraguay — 201.3-217.8 Robinson and Fowler (1982) sp. Costa R1ca Medium-Large 700. 8 Harris (1969)' Costa Rica — 1033. 6 Lloyd (1967)' Costa Rica — 367. 9 Ferrand (1966)' Costa Rica 144 . 5 Wood ( 1966)1 b) Acromyrmex landoltl Guyana — 2. 2 Cherret et al. (1974)' 1. fracticornis Paraguay 0. 9 Robinson and Fowler (1982) Paraguay — 1 . 5 Fowler and Robinson (1975) octospinosus Trinidad Smal1-Medium 2.8 [0.7. 5.8] Lewis (1975) Guadeloupe Sma11-Large 5.3 [0.4 I. 21.9] This study 1. Cited by Harris (1969). 2. Cited by Jonkman (1980). 3. Cited by Lugo et al. (1973). 4. Cited by Robinson and Fowler (1982), 5. Cited by Weber (1966). 95 cows. Since the ants rarely cut grasses, they cannot be compared directly with cattle. However the above calcula-tions show that the economic impact of A. octospinosus would be very small if only the amounts of biomass cut were consi-dered. But much depends on the location of the colonies, when flowers, fruits, or young leaves are preferred by the ants. In Trinidad, Lewis (1975) estimated that A. octospinosus de-foliated and destroyed 6-17% of newly planted cacao, whereas in older plantations, about 3,000 flowers/ha were cut each day. In citrus groves, 20-25 three-year old trees/ha would be largely defoliated. The density of colonies used by Le-wis for his calculations was 36 nests/ha, about twice the Grande-Terre density. Nevertheless, these figures emphasize the potential for damage of A. octospinosus when it is lo-cated among cultivated crops. Further economic damage not incorporated in this analy-sis includes interference with cattle feeding in colony territories. In both study areas of Guadeloupe, cattle were seen grazing on the ant's territories, and Robinson and Fowler (1982) and Fowler and Saes (1986) have shown that, in such circumstances, the cows spend more time in motion to avoid the ants than in grazing. 96 3.2 INDIVIDUAL CHOICE 3.2.1 INTRODUCTION C o l o n i e s of Acromyrmex oct ospi nosus e x h i b i t a c l e a r p r e f e r e n c e f o r some p l a n t s p e c i e s i n t h e i r environment, while i g n o r i n g other s p e c i e s , even abundant ones ( S e c t i o n 3.1). In s o c i a l i n s e c t s , more than any other type, the r e -sponses and v i a b i l i t y of the colony r e f l e c t how w e l l the i n d i v i d u a l ' s behavior i s c o o r d i n a t e d with that of the other members (Blum 1977; S c h n e i r l a 1952). Any understanding of a colony's breadth of d i e t w i l l u l t i m a t e l y depend on a pro-found knowledge of the range of c h o i c e s an i n d i v i d u a l can make. T h i s problem has not yet been adequately s t u d i e d i n the l e a f - c u t t i n g ants, so i t i s not known how w e l l i n d i v i d u -a l c h o i c e s agree with observed colony s e l e c t i o n s . T h e r e f o r e I i n v e s t i g a t e d the i n d i v i d u a l c h o i c e s of f o r a g e r s p r e s e n t e d with 4 p l a n t s p e c i e s . 3.2.2 MATERIALS AND METHODS Two days p r i o r to the t e s t i n g of i n d i v i d u a l p r e f e r e n c e , an a c t i v e l a b o r a t o r y nest, f ed with flowers of Hibiscus rosa-si nensis (L.) f o r at l e a s t a year, was chosen and some 60 f o r a g i n g workers were caught. They were i n d i v i d u a l l y marked by g l u e i n g a 2 mm c o l o r e d and numbered p l a s t i c tag to the d o r s a l p a r t of the abdomen and then r e l e a s e d back i n t o the nest. 97 T h e o b s e r v a t i o n a r e n a w a s a 1 0 x 1 0 cm g l a s s s q u a r e d i -v i d e d i n t o 4 r o w s a n d c o l u m n s o f s m a l l e r s q u a r e s ( 2 . 5 cm o n a s i d e ) w i t h a w a t e r - b a s e d i n k m a r k e r . F o u r d i f f e r e n t m a r k s , e a c h f o u n d 4 t i m e s , w e r e d r a w n i n s i d e t h e 16 s m a l l e r s q u a r e s t o f o r m a L a t i n s q u a r e a r r a n g e m e n t o n t h e f o r a g i n g a r e n a . D u r i n g a n e x p e r i m e n t , t h i s a r e n a w a s p l a c e d 50 cm f r o m t h e e x p e r i m e n t a l c o l o n y , a n d i t w a s v i s i t e d b y m a r k e d a n d u n m a r k e d f o r a g e r s . E a c h d a y d u r i n g t h e e x p e r i m e n t , 50 c i r c u l a r l e a f p i e c e s ( 6 mm i n d i a m e t e r ) w e r e c u t w i t h a h o l e p u n c h f r o m f r e s h l y h a r v e s t e d l e a v e s o f Mani hot esculent a ( C r a n t z . ) v a r . A ( c a s -s a v a A ) , Mani hot esculent a v a r . B ( c a s s a v a B ) , Ipomoea bata-tas ( L . ) ( s w e e t p o t a t o ) , a n d Di os cor ea cayennensis cayennen-s i s ( L a m . ) ( y e l l o w y a m ) . T h e 2 c a s s a v a c l o n e s w e r e k n o w n t o b e p r e f e r r e d b y t h e a n t s ( B e l l o t t i a n d v a n S c h o o n h o v e n 1 9 7 8 ; B l a n t o n a n d E w e l 1 9 8 5 ; W e b e r 1 9 7 2 ) . T h e y w e r e c h o s e n t o t e s t w h e t h e r t h e a n t s c o u l d d i s c r i m i n a t e b e t w e e n c l o s e l y r e -l a t e d p l a n t v a r i e t i e s . T h e y e l l o w y a m i s a v o i d e d b y c o -l o n i e s o f A. octospinosus ( F e b v a y et al. 1 9 8 5 ) . T o a v o i d p o s s i b l e c h e m i c a l c o n t a m i n a t i o n w h e n c u t t i n g t h e d i f f e r e n t p l a n t s p e c i e s , t h e h o l e p u n c h w a s c a r e f u l l y w a s h e d w i t h 95% m e t h y l a l c o h o l b e f o r e t h e n e x t s p e c i e s w a s c u t . U n t i l t h e y w e r e n e e d e d t h e l e a f p i e c e s w e r e k e p t o n m o i s t f i l t e r p a p e r i n a p e t r i d i s h , t o e n s u r e t h a t n o d i f f e r e n t i a l w a t e r l o s s b e t w e e n s p e c i e s c o u l d t a k e p l a c e , s i n c e l e a f w a t e r - c o n t e n t c a n a f f e c t p l a n t s e l e c t i o n b y l e a f - c u t t e r a n t s ( B a r r e r a n d C h e r r e t t 1 9 7 2 ; B o w e r s a n d P o r t e r 1 9 8 1 ; C h e r r e t t a n d S e a f o r t h 98 1970; Fowler and Robinson 1979b; Haines 1975). Before an experiment, each p l a n t s p e c i e s was randomly p a i r e d with one of the 4 d i s t i n c t marks i n the o b s e r v a t i o n arena, and an a p p r o p r i a t e l e a f d i s c was p l a c e d on each of the 16 sma l l squares. Thus, the ants were presented with 4 l e a f d i s c s of 4 p l a n t s p e c i e s i n a L a t i n square, to minimize the e f f e c t s of the d i r e c t i o n from which they a r r i v e d at the arena. The random p a i r i n g of a s p e c i e s with a mark ensured that the r e l a t i v e p o s i t i o n of the p l a n t s p e c i e s was changed each day, so that no l e a r n i n g of the p o s i t i o n of a p a r t i c u -l a r s p e c i e s c o u l d occur. Furthermore, the arena was r o t a t e d c l o c k w i s e s e v e r a l times i n the course of one experiment to ensure that the p o s s i b l e i n f l u e n c e of a pheromone t r a i l l e a d i n g to a p a r t i c u l a r l o c a t i o n i n i t was n e u t r a l i s e d . When a l e a f d i s c was taken by a f o r a g e r , i t was immedi-a t e l y r e p l a c e d by another d i s c of the same s p e c i e s . The p i e c e s were handled with s p e c i e s - s p e c i f i c f o r c e p s . A given r e p l i c a t e of the experiment ended when a l l 50 l e a f d i s c s of 1 s p e c i e s had been taken. Each r e p l i c a t e l a s t e d no more than 30 min, and 5 r e p l i c a t e s were made i n as many days. Choices were noted i n d i v i d u a l l y f o r each marked f o r a g e r . Unmarked i n d i v i d u a l s c o l l e c t i n g p l a n t p i e c e s were noted under the c o l l e c t i v e heading of "colony". The data were an a l y z e d with a G - t e s t , with i n d i v i d u a l s being the u n i t of experiment. In a l l t e s t s a 5% type I e r r o r was used. The G-test was chosen f o r i t s ANOVA-like p r o p e r t i e s (Sokal and Rohlf 1981). Because of the low p i c k - u p f r e q u e n c i e s f o r 99 each individual in each replicate, the individual collec-tions for the 5 days were pooled before analysis. Any indi-vidual that took fewer than 13 pieces during the 5 days was discarded from the G-test, and its frequencies were assigned to the "colony" category. As a result, only 11 of the 60 individuals remained eligible for analysis. In addition, since it was almost never chosen, the yellow yam category had to be combined with the sweet potato category in testing 1 hypothesis. Nevertheless, some individuals had expected frequencies between 1.5 and 4 for the sweet potato-yellow yam category. A Monte Carlo study conducted by Larntz (1978) showed that the G-test rejects the null hypothesis much too often in such conditions. The same study also showed that, under those conditions, the conventional x 2 statistic gives values very close to the nominal values. However, because the x 2 statistic is not purely additive, it cannot provide the ANOVA-like table of the G-test (Sokal and Rohlf 1981). Despite its problems, therefore, the G-test was preferable for this analysis. To avoid any inflated type I error, the x 2 statistic was also calculated for each individual, and the 2 outcomes compared. Any individual contribution to the overall G-test was rejected if there was a discrepancy between the G- and x 2 test results. Only 1 contribution had to be rejected. 100 3.2.3 RESULTS The colony as a whole chose the 4 p l a n t s i n the f o l l o w -ing p r o p o r t i o n s : 42.5% cassava A, 34.5% cassava B, 16% sweet p o t a t o , and 7% yellow yam ( F i g . 21). The e x t r i n s i c n u l l hy-p o t h e s i s of random pick-up was r e j e c t e d by a s i n g l e c l a s s i -f i c a t i o n G-test (Table X I I ) . I n d i v i d u a l f o r a g e r s d i f f e r e d i n t h e i r s e l e c t i o n of l e a f d i s c s ( F i g . 21). The r e s u l t s of the G-test under 2 e x t r i n s -i c n*ull hypotheses (random pick-up, and i n d i v i d u a l p i c k - u p same as colony pick-up) i n Table XII c o n f i r m t h i s o b s e r v a t i o n . Under both hypotheses there was a s i g n i f i c a n t h e t e r o g e n e i t y between i n d i v i d u a l s , suggesting d i f f e r e n c e s i n t h e i r responses to the 4 p l a n t s p e c i e s o f f e r e d . A G - t e s t of independence based on the marginal t o t a l s a l s o supports the f i n d i n g of h e t e r o g e n e i t y between i n d i v i d u a l s (G=55.912, 20 DF, P<0.001). The t e s t of the random p i c k - u p n u l l hypothesis (Table XII) shows that only 3 of the 11 i n d i v i d u a l s d i d not d i s c r i -minate between the p l a n t s . The pooled r e s u l t s f o r the 11 i n d i v i d u a l s show a group c o l l e c t i o n as s i g n i f i c a n t l y d i f -f e r e n t from random as that which was shown by the whole c o l o n y . In f a c t the G v a l u e s f o r the pooled i n d i v i d u a l s and f o r the colony are very s i m i l a r , which i s not s u r p r i s i n g c o n s i d e r i n g the r e s u l t s of the G - t e s t of the second n u l l hy-p o t h e s i s (Table X I I ) . The pooled i n d i v i d u a l s i n the a n a l y -s i s d i d not appear to choose d i f f e r e n t l y from the colony, but 5 i n d i v i d u a l s made c h o i c e s d i f f e r e n t from the colony 101 F i g u r e 21: Percentages of l e a f d i s c s i n each p l a n t c a t e g o r y taken by 10 i n d i v i d u a l s and by a colony of the l e a f - c u t t e r ant, Acromyrmex octospinosus, i n the l a b o r a t o r y . The colony c h o i c e i s represented by the white square. White dots r e -present i n d i v i d u a l s whose c h o i c e s do not d i f f e r from the co-lony p r e f e r e n c e . Black dots represent i n d i v i d u a l s whose c h o i c e s are s i g n i f i c a n t l y d i f f e r e n t from the colony c h o i c e s . 103 TABLE X I I : R e p l i c a t e d G - t e s t r e s u l t s f or the i n d i v i d u a l c h o i c e s of f o r a g e r s of the l e a f - c u t t e r a n t , Acromyrmex octo-spinosus, f or 4 p l a n t s p e c i e s under 2 h y p o t h e s e s . INDIVIDUAL 1 DF H 0:RANDOM DF H 0 : INDIVIDUAL PICK-UP OF PICK-UP=NEST PLANT P I E C E S 2 P I C K - U P 2 ' 3 1 (16) 3 11.60 ** 2 1 .64 n . s 2 (29) 3 33.46 * * * * 2 10.59 ** 3 (24) 3 25.25 * * * * 2 8.50 * 4 (14) 3 1 .47 n . s 2 1.15 n . s 5 (25) 3 0.45 n . s 2 7.88 * 6 (19) 3 14.18 * * * 2 0.62 n . s 7 (32) 3 20.81 * * * * 2 3.27 n . s 8 (27) 3 6.76 n . s 2 0.68 n . s 9 (18) 3 11.79 ** 2 6.69 * 10 (13) 3 24.88 * * * * 2 11.92 * * * 1 1 (15) 3 21.40 * * * * TOTAL 33 172.05 **** 20 52.94 **** POOLED 3 100.75 * * * * 2 2.79 n . s HETEROGENEITY 30 71.30 * * * * 18 50.15 * * * * COLONY 4 (287) 3 100.98 * * * * 1. Numbers i n p a r e n t h e s e s r e p r e s e n t the sample s i z e . 2. * P<0.05, * * P<0.01, * * * P<0.005, * * * * P<0.001. 3. NEST: CA=42.51%, CB=34.49%, and SP+YY=23% (16%+7%). 4. S i n g l e c l a s s i f i c a t i o n G - t e s t . 1 04 c h o i c e s ( F i g . 21). There i s l i t t l e doubt that d i f f e r e n c e s e x i s t between the i n d i v i d u a l s i n the percentages of each p l a n t they took. But are these d i f f e r e n c e s s u f f i c i e n t to modify the pre-ference rankings of the 4 p l a n t s between the i n d i v i d u a l s ? Table XIII p r e s e n t s the p e r t i n e n t d a t a . The d i s p l a y e d ranks were c a l c u l a t e d f i r s t by ranking each p l a n t f o r each i n d i v i -d ual on each day, then summing the ranks, and f i n a l l y rank-ing the sums. T h i s procedure g i v e s a b e t t e r estimate than ranking the o v e r a l l f r e q u e n c i e s f o r each p l a n t f o r each i n -d i v i d u a l (Gibbons 1971). There i s some agreement between the rankings of each i n d i v i d u a l (W=0.74, x 2=24.308, 3 DF, P<0.001), but i t i s c l e a r t h at i t comes from the almost unanimous r e j e c t i o n of yellow yam and sweet potato. N e v e r t h e l e s s 1 i n d i v i d u a l (No. 5) p r e f e r r e d the sweet potato, and another (No. 7) p l a c e d i t i n second p l a c e . The d i s c r e p a n c i e s i n rank between cassava A and cassava B are much g r e a t e r . I f K e n d a l l ' s W i s c a l c u -l a t e d f o r the 2 cassava o n l y , W=0.00 (x 2=0.00, 1 DF, n . s ) . There i s no c o r r e l a t i o n between the 11 rankings of these 2 v a r i e t i e s . Thus, there i s a p p a r e n t l y more v a r i a t i o n i n pre-ference between i n d i v i d u a l s f o r s p e c i e s that are a c c e p t a b l e than f o r s p e c i e s that are not. In other words, f o r a g e r s agree on which s p e c i e s are not a c c e p t a b l e , but do not have i d e n t i c a l p r e f e r e n c e s f o r s p e c i e s that are a c c e p t a b l e . There was much v a r i a t i o n between days (or r e p l i c a t e s ) i n t h i s experiment. Both the colony (G=18.153, 8 DF, TABLE X I I I : Rankings of c h o i c e s made by 11 i n d i v i d u a l s of the l e a f - c u t t e r ant, Acromyrmex octospinosus, f o r 4 p l a n t spec i e s . PLANT INDIVIDUAL CASSAVA CASSAVA SWEET YELLOW A B POTATO YAM 1 1 2 3 4 2 2 1 3 4 3 2 1 3 4 4 2 1 3 4 5 3 2 1 4 6 . 1 2 3 4 7 1 3 2 4 8 1 .5 1.5' 3 4 9 2 1 3 4 1 0 1 2 3.5 3.5 1 1 1 2 3.5 3.5 NOTE: K e n d a l l ' s W=0.74, Spearman's r=0.71 (x 2=24.308, 3 DF, P<0.001). Using only the 2 cassava v a r i e t i e s , K e n d a l l ' s W=0.00, Spearman's r=-0.!0 (x 2=0.00, 1 DF, n . s ) . 106 P<0.025) and the pooled 11 i n d i v i d u a l s (G=30.466, 8 DF, P<0.001) were heterogeneous i n t h e i r c h o i c e s from day to day. Due to the low p i c k - u p f r e q u e n c i e s on each day, i t was not p o s s i b l e to t e s t the homogeneity of response between days f o r each i n d i v i d u a l independently. Not a l l marked f o r a g e r s were c o n s i s t e n t l y seen on each day of the experiment (Table XIV). Only 2 i n d i v i d u a l s (2 and 7) were seen on each day. On each of the f i r s t 3 days of experiment, 2 i n d i v i d u a l s were m i s s i n g . On the l a s t 2 days, the number of marked f o r a g e r s absent i n c r e a s e d to 6 or 7, perhaps due to m o r t a l i t y . 3.2.4 DISCUSSION The problem of host p l a n t d i s c r i m i n a t i o n i n phytopha-gous i n s e c t s has given r i s e to a voluminous l i t e r a t u r e (Brues 1920; D e t h i e r 1954; Kennedy 1965; Rausher 1983; M i l -l e r and S t r i c k l e r 1984). I t i s now widely accepted that a l a r g e v a r i e t y of v i s u a l , o l f a c t o r y , chemotactic, and gusta-t o r y s i g n a l s are important i n t h i s process (Thornsteinson 1960), and that t h e i r i n f l u e n c e depends on the r e p r o d u c t i v e or n u t r i t i o n a l s t a t u s of the i n s e c t (Dethier 1982). For the A t t i n i , the d e c i s i v e c o n t r i b u t i o n of chemical a r r e s t a n t s or r e p e l l e n t s ( C h e r r e t t 1972a; C h e r r e t t and Sea-f o r t h 1970; Febvay et al. 1985; Hubbell et al. 1983, 1984; L i t t l e d y k e and C h e r r e t t 1975, 1978a; Mudd et al. 1978; Ro-ckwood and Glander 1979; S t r a d l i n g 1978) and of p h y s i c a l c h a r a c t e r i s t i c s of the p l a n t ( C h e r r e t t 1972b; Wall e r 1982a, TABLE XIV: Presence or absence of 11 i n d i v i d u a l s of the l e a f - c u t t e r ant, Acromyrmex octospinosus, f o r a p l a n t c h o i c e experiment repeated f o r 5 c o n s e c u t i v e days. DAY OF EXPERIMENT INDIVIDUAL 1 2 3 4 5 1 + + _ _ _ 2 + + + + + 3 - .+ + + + 4 + _ + 5 + + + + — 6 + + + - — 7 + + + + + 8 + — + — + 9 + + + _ _ 10 - + + + -11 + + - - -+: Present, —: Absent. 108 1982b) to host acceptance has been well recognized. Since the size of the discs offered was small enough to make them easily handled by the ants, any physical property affecting the cutting of the plant by the ants can be ruled-out in ex-plaining the differential pick-up of the 4 species, which leaves differences in the chemical components as probable cause. For example, the avoidance of yellow yam by A. octo-spinosus has been traced to the presence in the plant of sa-ponin antifeedants (Febvay et al. 1985). But if chemical differences between plant species can explain the hierarchy of preference for one individual for-ager, they cannot, in my controlled conditions, explain the observed variations in preference between foragers. Dif-ferences in the perception of the plant chemical components undoubtedly were involved but they have many possible origins. Prior conditioning to one of the 4 plants, as seen for many insects (Phillips 1977, Cassidy 1978) and some leaf-cutters (Littledyke and Cherrett 1975), was probably not a factor, since the nest used had been in the laboratory for at least a year, during which only flowers of Hibiscus rosa-si nensi s had been given as food. In addition, after each experiment, the leaf discs were not left in the forag-ing arena, but were replaced by Hibiscus flowers, so the ants could not develop any conditioning to the new food sources during the course of the experiment. This assump-tion is further supported by an independent experiment per-formed on colony conditioning. After 8 days in which 109 c o l o n i e s were given only sweet potato l e a v e s , no p r e f e r e n c e i n d i c a t i n g s u c c e s s f u l c o n d i t i o n i n g c o u l d be d e t e c t e d . Two b e t t e r a l t e r n a t i v e s than c o n d i t i o n i n g are d i f f e r e n c e s i n age and i n d i v i d u a l g e n e t i c make-up. Age has been shown to a f f e c t the sensory c a p a b i l i t i e s of the b l o w f l y , Phormia regina (Meigen), in which 30-50% of the l a b e l l a r chemoreceptors were i n o p e r a t i v e i n a 25-day o l d f l y ( S t o f f o l a n o 1973). A s i m i l a r age e f f e c t i n A. octo-spinosus c o u l d e x p l a i n c h o i c e s of l e s s a c c e p t a b l e p l a n t s by some i n d i v i d u a l s . D i f f e r e n c e s i n genotype c o u l d a l s o e x p l a i n the d i f -f e r e n c e s i n chemosensory p e r c e p t i o n between i n d i v i d u a l s . M u l t i p l e mating i n A t t i n i has been demonstrated f o r Atta sexdens and Acromyrmex I andol t i (F.) (Weber 1972). T h i s i s evidence that s e g r e g a t i o n and simultaneous use of spermato-zoa from d i f f e r e n t males occurs i n the honeybee (Taber 1955; Page and M e t c a l f 1982). If something s i m i l a r occurs i n A. octospinosus, g e n e t i c v a r i a t i o n between i n d i v i d u a l s might be s u f f i c i e n t to e x p l a i n t h e i r v a r i a b l e behavior p a t t e r n s . Whatever i t s r o o t s , the v a r i a b i l i t y i n behavior i s h i g h l y b e n e f i c i a l f o r the c o l o n y . I t ensures t h a t a wide v a r i e t y of p o t e n t i a l host p l a n t s w i l l be sampled i n the f i e l d . With a wide range of p l a n t s a v a i l a b l e , the colony would not be s u s c e p t i b l e to the a c c i d e n t a l e l i m i n a t i o n of a p r e f e r r e d h o s t . Furthermore, the colony would be b e t t e r a b l e to t r a c k changing c o n d i t i o n s i n the p l a n t resources i f the i n d i v i d u a l s e x h i b i t v a r i a b l e p r e f e r e n c e s than i f they 1 10 were a l l c o l l e c t i n g from the same s p e c i e s . And i f t h i s var-i a b i l i t y has a g e n e t i c o r i g i n , the p l a s t i c i t y of the s p e c i e s can be f u r t h e r improved on a long term b a s i s by e v o l u t i o n a r y p r o c e s s e s . The f a c t that not a l l marked f o r a g e r s were seen on every day of the experiment has i n t e r e s t i n g i m p l i c a t i o n s . C h e r r e t t and S e a f o r t h (1970) and C h e r r e t t (1972b), in w e l l c o n t r o l l e d and s t a n d a r d i z e d experiments, found a l a r g e unex-p l a i n e d v a r i a b i l i t y i n r e p l i c a t e s of p r e f e r e n c e experiments. There was a s i m i l a r v a r i a b i l i t y i n my e x p e r i -ment, both f o r the colony and f o r the 11 i n d i v i d u a l s combined. Since i n d i v i d u a l s o f t e n e x h i b i t a p a r t i c u l a r pre-ference d i f f e r e n t from that of other i n d i v i d u a l s , and s i n c e the colony p r e f e r e n c e appears to be determined by the combi-ned p r e f e r e n c e of the i n d i v i d u a l s , i t i s p o s s i b l e that the observed v a r i a t i o n s i n colony p r e f e r e n c e might be caused by a d i f f e r e n t mix of f o r a g e r s f o r each r e p l i c a t e of an e x p e r i -ment . 111 3.3 LIST OF PLANT SPECIES The t a b l e on the next and f o l l o w i n g pages prese n t s a complete l i s t of a l l p l a n t s p e c i e s found on the t e r r i t o r i e s of the c o l o n i e s s t u d i e d . For each colony, a number f o r one sp e c i e s i n d i c a t e s that i t was a l s o found i n the d i e t and the number i s the rank p r e f e r e n c e f o r that s p e c i e s as d i s p l a y e d by that c o l o n y . Species found i n the t e r r i t o r y but not i n the d i e t are i n d i c a t e d by a V ( f o r v i s i t e d ) i f t r a c e s of c u t t i n g were seen; and by a T i f none was seen. Species that are not on the t e r r i t o r y of a p a r t i c u l a r colony are shown by an X. The f o l l o w i n g c o n v e r s i o n makes t h i s l i s t compatible with the p a l a t a b i l i t y c a t e g o r i e s used by Rockwood (1976) and P i n t e r a (1983): CATEGORY COLONY EXTREMELY PALATABLE PALATABLE MARGINALLY PALATABLE 1 3 1 1 RANK<10 RANK< 7 RANK< 8 11<RANK<42 8<RANK<31 9<RANK<42 RANK>43 RANK>32 RANK>43 2 4 1 2 RANK< 6 RANK< 5 RANK< 4 7<RANK<29 6<RANK<36 5<RANK<14 RANK>30 RANK>37 RANK>15 Odd numbered c o l o n i e s were at Lomard. Even numbered ones at Lemesle. SPECIES 1 3 11 2 4 12 AbiIdgaardia monostachya V a h l T T T T X T Acacia gl auca Moench. X X V X X X Acacia guadal upensis DC. X X X X T X Acacia macracantha H . & B. X T X X X X Acacia muricata W i l l d . X 1 7 V X X X Acacia sundra Roxb. 28 X 1 X X X Acacia tortuosa W i l l d . X X 34.5 X X X Acaci a sp 1 2 4 18 X X X Achyranthes aspera L . X X 27 10 20 X Achyranthes indica M i l l . X X X 29 X X Aechmea serrata Mez. X X X T X X Aeschynomene americana L . 47 27 X X X X Ageratum conyzoides L . V V V X V V Amaranthus dubi us M a r t . 29 V 25 X X X Amyris elemifera L . X X X V X V Ana cardi um occidenta Ie L. V X X X X X Ananas comosus M e r r . T X X X X X Annona muricata L . X X V X V V Arachis hypogea L . 50 X 40 X 33.5 X Artocarpus altilis F o s b e r g 46 X X 1 X 2 Axonopus compressus Beauv. V X V V V V Bel onyx molendra L . X 28.5 X X 25 X Beurreria succulenta J a c q . X X X V X X Bidens pilosa pilosa S t e h l e 8 1 1 5 1 5 1 2 X Blechum brownei J u s s . 9 32.5 28 19 10 V Borreria laevis G r i s e b . 1 6 18 33 20 6 V Brachiaria er ucaef ormi s G r i s e b . X V X X X X Brachiaria pur pur as cens Hems. X X X X T X Brachiaria reptans G a r d . & Hubb. X X T T T V Br as si ca ra pa L . X X X X T X Bursera simaruba S a r g . T V X X X V Caj anus cajan Mi l i s p . 10 38 X X V X Caladium bi col or V e n t . X X X V X X Calophyllum calabra L . X X X X X V Cal opogoni um mucunoides Desv . 44 X X 31 .5 V 1 6 Caperoni a palustris S t . H i l . X X 44 X X X Capparis indica L . X X X X T T Capraria bi fl or a L . X X 42 X X X Capsicum frutescens L . V X X V V V Capsicum sinense Jacques V T X X X V Car di osper mum microcarpum Hbk. X X X X T V Cassia bi capsular is L . X X X X 23 X Cassia fistula L . X 40.5 1 4 5 14.5 X Cassia obtusi folia L . 15 1 5 7 1 2 8 1 2 Cassia occidental is L . 42 9 X X 39.5 X Cassia sop her a L . X X 23 X X X Cent r os ema pubescens B e n t h . 24 37 1 7 4 36 10 Ce nt r os ema virginianum B e n t h . 34.5 X X X 28 X 1 13 SPECIES 1 3 COLONY 1 1 2 4 1 2 Chaptalia nut ens P o l a k . X X X X X T Cissus sicyoides L . T T X V X V Citharexylum spinosum L . V 28.5 V X X V Citrus aur ant if olia Swing le V X T 17 X X Citrus aur ant ium L . X X X V X X Citrus sinensis Osbek 45 19 X V X 16 Co cos nuci f er a L . T X X T 35 X Coffea ar abi ca L . X X X V X X Coffea liberica B u l l . X X X V X X Cor chorus si I i quosus L . 19 5.5 11 X 32 V Cordia col I ococca L . V X V 3 X V Cor di a globosa Hbk . X X 46 X X X Cordia sulcata DC. 21 24 1 3 23 13 5 Commel i na diffusa Burm. X X X V X V Cracca or Teramnus } 1 3 5.5 1 9 1 1 1 8 Cr ocus sat i vus L . V X X X X X Cr ot on flavens L . 26 X T 13 X X Cr ot on I obat us L . 18 1 3 48.5 X 18 19.5 Cueumi s sat i vus L . X X V X X X Cucurbit a moschata Duch. X 40.5 V X T X Cupani a americana L . X T X V X X Cynodon dactylon P e r s . V T X X V X Desmanthus depressus H . & B . X 21 X X V 3 Desmanthus virgatus W i l l d . X X X 36.5 X X Desmodium canum S c h i n z 40 V V V 1 4.5 V Desmodium tortuosum DC. X X X X 28 1 2 Di chant ium annul at um R o b e r t y X 31 X X X X Digit aria decumbe ns S t e n t . V V X V X V Digit aria horizontal is W i l l d . X X X X T X Digit aria insular is Mez. X X X X T X Dioscorea al at a L . V X X X X X Di oscore a cayennensis Lam. V X X X X X Di phol i s s a I i ci f ol i a DC. X X X V V T El eusine indica G a e r t n . X X X X V X Emi I i a s one hi jol i a DC. 23 32.5 24 X 17 X Eriochloa polystachya Hbk. X X X X T X Erythroxylon ovatum C a v . X X V V 22 V Eugenia monticola I at i folia KR. X X X V V V Euphorbia graminea J a c q . V X X X V X Euphor bi a hirta L . 1 1 6 1 6 4 V Euphorbia hyper i ci f ol i a L . 27 16 21 9 9 V Fi cus citrifolia M i l l . X X X T X X Gal act i a dubia typica S t e h l e 31.5 T X 22 X X Gi basis geniculatum Rohweder X X X 25 38 X Grami nae sp 22 1 2 1 6 27.5 16 X Guettarda scabra Lam. X X T X X X Haemat oxylo n campechlanum L . 6 7 29 X 42 X 1 1 4 SPECIES 1 3 COLONY 1 1 2 4 1 2 Hemi di odi a ocymi folia Schum. 34.5 X X X X X Heteropogon cont or t us Beauv. V V V V V V Het er opt er ys purpurea Hbk. T T 34.5 27. 5 V 7 Hibiscus esculent us L . X X X X V X Hyptis vert ici I I at a J a c q . X X 38 T X T Ichnanthus pal I ens Munro X V X 31.5 V V Indigofera t i net ori a L . 43 35 10 X X X Inga ingoides L . X X X T X V Ipomoea batatas L . V X X X V X Ipomoea tiliacea C h o i s y 2 V X 1 4 V X Just i ci a pectoral is J a c q . X X X X X T Lantana camara L . 37.5 X 1 5 X 37 X Lantana involucrata L . V 20 37 X X V Laportea aesluans Chew. 30 V 48.5 V V T Leonotis nepetaefolia B r . X X X X V X Leucaena I eucocephal a De Wit X X T X X X Macfadyena ungui s-cat i G e n t r y X X X V T X Malachra fasciata J a c q . X X X X T X Mai vast rum coromendel i anum Garcke V 1 4 22 30 42 X Mammea americana L . X X X V X X Mangifera indica L . 4 X 3 X 2 4 Mani hot esculent a C r a n t z . V X 20 X X X Mel i coccus bijugatus J a c q . X X 48.5 V V V Miconia laevigata DC. 31.5 X X X X T Mimosa pudi ca L . 36 35 X T 24 X Moghania strobilifera S t . H i l l . X X X X X V Morinda citri folia L . X V V X X X Musa acuminata*bal bi si ana T X X X X X Ocimum basilicum L . 48.5 X X 8 33.5 V Oxalis bar r el i e r i L . X X X X V X Pani cum fasciculatum Sw. V X 36 V T V Pani cum maximum J a c q . T T T X T X Paspalidium gemi nat um S t a p f V X X X X X Paspalum conjugatum B e r g . V X X T T T Pas pal um fimbriatum Hbk. V X X X X X Paspalum paniculatum L . V V T X T X Pas si flora foetida L . X X X 18 V X Phaseolus adenanthus Meyer X 26 X X X X PhylI ant hus debilis K l e i n . 1 7 8 9 X 21 X Picramnia pentandra Sw. X X X 21 V V Pi lea microphylla microphylla L . X X X 24 X T Pimenta r acemos a Moore X X V V X V Piper amal ago L . X X X V V V Piriqueta ci s t oi des Meyer X X X X T X Pisonia fragrans Dumont-Cours X X V V X V Poinsettia heterophylla K l . X X X X X V Pol ypodi um aureum L . X X X T X X 1 1 5 COLONY SPECIES 1 3 1 1 2 4 1 2 Priva lappulacea P e r s . 1 4 23 32 36.5 39. 5 18 Ps eudelephant opus spi cat us G leason X X X 34 1 1 V Psidium guajava L . V X V V X V Psychotria nervosa Sw. 33 X 31 X V V Ri ci nus communis L . V V X X X X Ruellia tuberosa L . 39 25 48.5 V 26 V Sans evi eri a metallica Gerome X X X V X X Schaefferi a frutescens J a c q . X X V X X X Schi zachyri um condensatum Roberty X X X X X T Scleria pterota P r e s l . T T X V T V Senecioides cinera K t z e . 3 2 4 6 3 6 Sesbania sesban M e r r i l l . 5 10 8 X 19 X 5/ da rhombifol i a rhombifol i a L . 7 3 2 V 5 16 Si da s pi nos a L. V X X X V X SIoana berteriana C h o i s y X X X X X 1 2 Solanum racemosum ignaeum S c h u l z V V X V X V Solanum racemosum inerme S t e h l e X X T V X X Solanum seafort hi anum S t e h l e X X X V X V Solanum torvum genuinum S e n d t n . V X T V V 19.5 Sorgum halepense Roberty V X X X X X Spermacoce tenuior L . X X X X X V Spondias mombin L . X X X X X V Sporobol us i ndi cus R. B r . T T T X V X St achyt arphet a jamaicensis V a h l 1 1 30 1 2 X 30 V Stictocardia til i ae folia H a l l . f . X X X X V X Tabebuia pallida M i e r s . 37.5 X 30 2 X 9 Tabernaemont ana citrifolia L . X V V 34 X X Tamarindus indica L . 48.5 35 26 7 31 1 Teramnus uncinatus Sw. X X 44 X X X Tour nef or t i a hirsutissima L . X X X X T X T our ne f or t i a v ol ubiI i s L . X X V X X X Tragi a volubilis L . 25 22 V 26 28 X Tragus bert eroni anus S c h u l t . 41 X X X X X Triphasia trifolia W i l s o n X X X V X X Triumfetta I appul a L . X V T X X X Vangueria madagascarensi s Gmel . V X 44 X X X Verbe si na al at a L . X X X X 7 X Waltheria indica L . X V X X X X Wedelia calycina R i c h . V V V X T X Wedelia trilobata H i t c h . 20 39 X 34 42 1 4 Zant hoxylum monophylIum W i l s . V V V V V V Zizyphus mauritiana Lam. V V V X V X SP 1 X X 39 X X X SP 2 X X 41 X X X 1. See note 1 T a b l e V I I A . Chapter 4 THE FORAGING TRAIL 4.1 GENERAL INTRODUCTION 4.1.1 COMPONENTS OF THE FORAGING TRAILS F o r a g i n g t r a i l s of the l e a f - c u t t i n g ants are charac-t e r i z e d by chemical and p h y s i c a l components. The chemical component, a p p a r e n t l y common to a l l A t t i n i (Robinson et al. 1974) and to many other ant s p e c i e s (Wilson 1971; C a r r o l l and Janzen 1973), i n v o l v e s marking the s u b s t r a t e with a c h a r a c t e r i s t i c pheromone produced by the poison gland (Blum et al. 1964; Cross et al. 1979, 1982; Moser and Blum 1963; R i l e y et al. 1974; Tumlinson et al. 1971, 1972). T h i s com-ponent i s present alone i n newly formed t r a i l s . The second component i s found i n a l l s p e c i e s of At ta and many s p e c i e s of Acromyrmex (Fowler 1978; Weber 1972). The t r a i l s are w e l l d e f i n e d and c l e a r e d of a l l d e b r i s , grass or l e a v e s . Well e s t a b l i s h e d t r a i l s are o f t e n sunken s e v e r a l m i l l i m e t e r s i n t o the ground (Fowler 1976; Weber 1972), and when extendi n g i n t o grassy areas v e g e t a t i o n o f t e n c o v e r s and obscures them. The t r a i l s may even be underground f o r some of t h e i r l e n g t h i n some s p e c i e s (Fowler 1985; Mintzer 1979) or covered by l e a v e s , forming a tunnel (Fowler 1976, 1985). Well maintained t r a i l s favor i n c r e a s e d t r a v e l speeds. Since c l e a r i n g i s c o s t l y i n energy (Lugo et al. 1973), t r a i l s o f t e n f o l l o w logs or r o o t s , even i f they are not i n the 1 1 6 1 17 d i r e c t i o n of the t r a i l . F o r aging t r a i l s i n the f i e l d have c h a r a c t e r i s t i c pat-t e r n s s i m i l a r to the m u l t i p l e branching of a r i v e r . The segment of t r a i l c l o s e to the nest i s wider than other p a r t s of the t r a i l and i s o f t e n c a l l e d a "trunk t r a i l " (e.g. Fowl-er 1978). In t h i s chapter i t w i l l be c a l l e d a "primary" t r a i l . The smal l e r t r a i l s branching from i t w i l l be c a l l e d "secondary" t r a i l s . 4.1.2 ROLES OF PHEROMONES AND TRAILS The pheromone t r a i l i s of primary importance f o r mass rec r u i t m e n t and f o r fo r a g e r o r i e n t a t i o n to food sources (Bradshaw and Howse 1984; J a f f e and Howse 1979; Weber 1972; Wilson 1971). I t helps the f o r a g e r s to go back to a s p e c i f -i c area without redundant searches ( C a r r o l l and Janzen 1973) and, thus, i s an o r g a n i z e r of f o r a g i n g (Shepherd 1982). The c o n c e n t r a t i o n of the pheromone on the t r a i l can p r o v i d e a feedback mechanism ab l e to r e g u l a t e the d e n s i t y of f o r a g e r s e x p l o i t i n g a given food source (Hangartner 1969; J a f f e and Howse 1979; J a f f e and V i l l e g a s 1985; J a f f e et a l . 1985; Wi-l s o n 1962), or t r a n s p o r t of the l e a f m a t e r i a l (Bradshaw et a l . 1986). The pheromone has at l e a s t 2 d i f f e r e n t chemical compounds. The recruitment chemical u s u a l l y has a much s h o r t e r l i f e t i m e than the o r i e n t a t i o n c h e m i c a l . In o l d t r a i l s , only the o r i e n t a t i o n chemical remains, and onl y a l -ready "motivated" f o r a g e r s use the t r a i l . A "nest e x i t pheromone" has r e c e n t l y been r e p o r t e d i n A t t a c e p h a l o t e s , 118 (Holldobler and Wilson 1986). It is apparently used in or-ientation to the nest by returning foragers, and increases the rate of t rai l laying and food gathering. A ful l appreciation of the odor t ra i l as an orientation mechanism can be gained by reading the classic paper of Wil-son (1962) on the pheromone t ra i l of the fire ant, Soleno-psis saevissima (F. Smith). Using a calculation method based on Shannon's information theory and first proposed by Haldane and Spurway (1954), Wilson was able to show that the pheromone tra i l conveyed as much information on both the direction and distance of a food source as the waggle dance of honeybees. No wonder that he characterized it as "The most elaborate of a l l the known forms of chemical communication". Being so effective in orientation, the pheromone trails can increase the time available for forag-ing by allowing foraging on completely overcast days or at night (Holldobler 1976). In addition to this prominent role in resource exploi-tation, the t ra i l may also be important in reducing or avoiding intra- or interspecific competition, and in parti-tioning resources in situations of high colony density. Mass recruitment ensures a protective force adjusted to the size of the resource and its needed rate of removal when there is much temporal or spatial heterogeneity in the dis-tribution of requisites (Carroll and Janzeh 1973; Fowler and Stiles 1980). Although no study is available on this aspect of t ra i l function for the Attini, it has been shown to occur 1 19 f o r 4 s p e c i e s of Pogonomyrmex ( H a r r i s o n and Gentry 1981; H o l l d o b l e r 1976; Jorgensen and P o r t e r 1982). 4.2 FORAGING AND TRAIL DYNAMICS 4.2.1 INTRODUCTION Given the importance of the t r a i l i n the f o r a g i n g be-h a v i o r and resource e x p l o i t a t i o n of the A t t i n i , more i n f o r -mation on the dynamics of the f o r a g i n g t r a i l s and t h e i r r e -l a t i o n to food sources would be v a l u a b l e . C a r r o l l and Jan-zen (1973), and Fowler and S t i l e s (1980) b e l i e v e that phys-i c a l t r a i l s would only develop to permanent r e s o u r c e s , wher-eas t r a i l s l e a d i n g to t r a n s i t o r y ( s h o r t l i v e d ) r e s o u r c e s would have only a pheromone component. C h e r r e t t (1968) s t u -dying Atta sexdens f o r 58 days suggested, even though he co-u l d observe f o r a g i n g i n only 1 p a r t of the t e r r i t o r y of the colony, that abandonment of t r a i l s and c r e a t i o n of new ones would u l t i m a t e l y r e s u l t i n f a i r l y g e n e r a l coverage of the f o r a g i n g a r e a . C o l o n i e s of Acromyrmex octospinosus c o l l e c t many spe-c i e s of p l a n t s , and have a p r e f e r e n c e f o r v a r i o u s temporary resources such as f l o w e r s , f r u i t s and seeds (Chapter 3 ) . Thus c o l o n i e s of A. octospinosus represent an i d e a l s i t u a -t i o n i n which to t e s t the p r e d i c t i o n s formulated by the e a r -l i e r w r i t e r s . 1 20 4.2.2 MATERIALS AND METHODS Field sampling of the compass direction and rates of appearance and disappearance of trails was done in both ar-eas each time activity rhythms were observed (Chapter 2). On each occasion, the presence or absence of a t r a i l , its compass direction and, when possible, the food source ex-ploited by foragers on that t ra i l were noted separately for each colony. 4.2.3 RESULTS The variances of the mean monthly number of trails per colony were significantly different (Bartlett, x 2 =37.68, 13 DF, P<0.005) and increased with increase of the means, so the values were log-transformed to stabilize the variance. The highest value of mean monthly trails was at Lemesle for colony 2 (Table XV).. Colonies 3 and 4 at Lemesle, and colony 3 at Lomard, each had more than 2 trails on average. While foraging, colonies often temporarily desert a t ra i l to create another one at a different location, only to come back to the abandoned tra i l at a later time. These te-mporarily deserted trails were recorded more than once in the "total # trails" variable (Table XV), as a t ra i l was counted each time it was used anew. The large differences between the "# different trails" column and the "total # trails" column for colonies 2 and 3 at Lemesle (Table XV), indicates that they temporarily deserted already existing trails to create new ones more often than other colonies. 121 TABLE XV: Dynamics of t r a i l s of c o l o n i e s of the l e a f - c u t t e r ant, Acromyrmex octospinosus, i n 2 study areas of Guade-loupe, from September 5, 1983 to August 16, 1984. AREA COLONY 1 MEAN MONTHLY # TRAILS 2 TOTAL # TRAILS # DIFFERENT TRAILS RATIO # FORMED/ ABANDONED LEMESLE 2 (12) 4.3 a 44 25 1 .03 4 (12) 2.8 ab 13 9 1 .00 3 (12) 2.6 a be 21 8 1 .00 1 2 (7) 2.6 abed 1 1 10 1.17 8 (5) 2.0 bede 2 2 1 .00 5 (5) 1 .9 bede 4 4 1 .00 6 (5) 1 .9 bede 4 3 1 .00 1 (11) 1 .3 e 10 6 0.90 1 1 (5) 1 .2 cde 4 3 1 .00 LOMARD 3 (12) 2.6 abc 8 7 0.67 1 2 (8) 1 .9 bede 5 3 1 .00 5 (12) 1 .5 cde 8 5 1 .20 1 (12) 1 .4 de 6 4 1 .25 1 1 (12) 1 .2 e 2 2 1/0 7 (7) 1 . 1 e 3 2 1 .00 1. Numbers i n parentheses represent the number of months of observat i o n . 2. Back-transformed means. Numbers f o l l o w e d by the same l e t t e r are not s i g n i f i c a n t l y d i f f e r e n t (Tukey-Kramer, a(experiment)=0.05). 1 22 The sequence of t ra i l creation and abandonment did not result in gain or loss of trails for most colonies. The ra-tio of trails formed/abandoned was 1.00 or near this value for most colonies (Table XV). Colonies usually have a relatively low number of long-lived "permanent" trails (> 120 days), and most co-lonies, especially at Lemesle, had a relatively high number of short-lived trails (^  30 days), the majority of which led to flowers or fruits (Table XVI). The other short-lived trails either did not lead to such resources or the food source could not be recorded. Except for colony 5 at Lomard and colony 2 at Lemesle, none of the directional distribution of the primary trails was significantly different from random (Fig. 22). Thus, over the study period no particular compass direction was favored by most colonies for the creation of their primary trai ls. Trails could be found a l l around the nest, even for colony 2 of Lemesle, which nevertheless showed a significant polarity. Colonies 1 and 5 of Lomard are obvious exceptions to this generalization, however (Fig. 22). Both showed a strong polarity, although the mean vector length was not quite significant for colony 1. The position of these 2 co-lonies relative to each other was such that their mean angle pointed toward the same mango tree, some 3 m from each of them. New primary trails apparently were formed in response to the presence of this preferred food source. With the ex-ception of one t ra i l from each nest that led to mango 1 23 TABLE XVI: Longe v i t y of t r a i l s of c o l o n i e s of the l e a f - c u t t e r ant, Acromyrmex octospinosus, i n 2 d i f f e r e n t study areas of Guadeloupe, from June 15, 1983 to August 16, 1984. TRAIL DURATION (DAYS) 1 AREA COLONY 15< 30< 60< 90< X X X X X X <15 <30 <60 <90 <120 >120 LEMESLE LOMARD 1 1 (1) 3(3) 3 3 0 0 2 13(10) 6 1 5 2 2 2 3 2 (2) 4(2) 10 1 0 2 4 0 1 2 2 1 3 1 2 3 (3) 1 0 0 0 2 1 0 2(2) 2 0 0 1 3 1 3(2) 1 0 0 3 5 0 K D 2 2 0 1 1 2 0 0 1 0 0 2 1. Numbers i n parentheses i n d i c a t e t r a i l s l e a d i n g to f l o w e r s , f r u i t s or seeds. 1 24 Figure 22: Compass direction of the primary trails of colonies of the leaf-cutter ant, Acromyrmex oct ospi nosus, in 2 study areas of Guadeloupe, June 15, 1983 to August 16, 1984. In the diagrams, the number at upper left is the colony number. Black octagons indicate permanent trai ls, white octagons indicate a l l other trai ls . r is the length of the mean vector; $ is the mean angle; S is the mean angular deviation and P indicates the probability that the observed angular distribution is even (or random) ac-cording to the Rayleigh test (Batschelet 1981). L O M A R D 1 26 le a v e s , a l l new primary t r a i l s appeared d u r i n g the f r u i t i n g season and l e d to the mango f r u i t s . 4.2.4 DISCUSSION The r e s u l t s of the l i f e s p a n and compass d i s t r i b u t i o n of the t r a i l s of Acromyrmex o c t o s p i n o s u s are f u r t h e r proof that t h i s s p e c i e s i s h i g h l y o p p o r t u n i s t i c i n i t s search f o r and e x p l o i t a t i o n of food, e s p e c i a l l y at Lemesle, where c o l o n i e s cut l a r g e q u a n t i t i e s of t r a n s i e n t resources (Chapter 3). C o l o n i e s 2, 3, 4, and 12 were a l l l o c a t e d near a mango or bread f r u i t t r e e , and had the most t r a i l s . They a l s o had many s h o r t - l i v e d t r a i l s , most of which were l e a d i n g to t r a n -s i e n t r e s o u r c e s . At Lomard, the presence of the mango t r e e l e d to some p o l a r i t y i n the c r e a t i o n of new primary t r a i l s . Most of these t r a i l s were formed a f t e r d i s c o v e r y of a mango f r u i t on the ground. Thus, p r o x i m i t y of p r e f e r r e d t r a n s i -t o r y resources seems to be c o r r e l a t e d with h i g h f r e q u e n c i e s of s h o r t - l i v e d t r a i l s i n Acromyrmex o c t o s p i n o s u s . The t r a i l s a s s o c i a t e d with the t r a n s i e n t resources d i d not e x h i b i t a p h y s i c a l component. They were most probably pheromone t r a i l s , l a i d on a d r i e d l e a f s u r f a c e at Lemesle and on bare ground at Lomard. Only the permanent t r a i l s and t r a i l s of more than a month's d u r a t i o n , developed i n t o phys-i c a l t r a i l s . And, even i f flowers o c c a s i o n a l l y were c a r r i e d back to the nest, l e a v e s were the dominant p l a n t p a r t c o l -l e c t e d by f o r a g e r s u s i n g these p h y s i c a l t r a i l s . Since main-tenance of p h y s i c a l t r a i l s i s a continuous energy consuming 1 27 process (Lugo et al. 1973), i t i s only u s e f u l when the ants use i t f o r p e r s i s t e n t or permanent resources ( C a r r o l l and Janzen 1973; Shepherd 1982). O b s e r v a t i o n s of primary t r a i l d i r e c t i o n s leave no doubt that a l l the t e r r i t o r y would be covered i n time as suggested by C h e r r e t t (1968) i n h i s 58-day study of At t a cephalotes. However, the d i s t r i b u t i o n of the primary t r a i l s around the nest i s not the same as the d i s t r i b u t i o n of the f o r a g i n g e f f o r t . For each colony s t u d i e d , the m a j o r i t y of f o r a g i n g and c r e a t i o n of secondary t r a i l s was c o n c e n t r a t e d on per-manent t r a i l s and, t h e r e f o r e , the g r e a t e s t p r o p o r t i o n of forage came from 1 or 2 d i r e c t i o n s c o r r e s p o n d i n g to the l o -c a t i o n of the more permanent r e s o u r c e s . Thus, while f o r a g -ing t r a i l s can be c r e a t e d i n any d i r e c t i o n i n response to food a v a i l a b i l i t y , a l l o c a t i o n of e f f o r t i s a f u n c t i o n of the abundance and d u r a b i l i t y of the re s o u r c e . T h i s r e s u l t i s i n c o n t r a s t to C h e r r e t t ' s (1968) h y p o t h e s i s implying that the p a t t e r n of f o r a g i n g e f f o r t would f o l l o w the t r a i l formation p a t t e r n and be uniform around the nest . Fowler and Robinson (1979a) r e p o r t e d that At ta sexdens i n Paraguay c o n s t r u c t a s e r i e s of short t r a i l s , evenly d i s -t r i b u t e d around the nest, when the d e n s i t y of v e g e t a t i o n i s low. In these c o n d i t i o n s , the ants searched a l l the t e r r i -t o r y , c o l l e c t i n g food at random as i t was encountered, w i t -hout concern f o r q u a l i t y (Fowler and S t i l e s 1980). T h i s type of t r a i l d i s t r i b u t i o n around an Atta nest may appear analogous to the one observed f o r Acromyrmex octospinosus, 1 28 but it is not so. In my study, Acromyrmex responded to the presence of preferred, but quantitatively small, food patches, not to a generally low density of food sources as was the case for At ta sexdens. Acromyrmex evenly covered directions successively in time, whereas At ta covered them with many trails simultaneously. Moreover, Acromyrmex octo-spinosus actively selected its food and did not collect at random (Chapter 3) like A. sexdens did. In both cases, however, there are similar ramifications. Repeated t ra i l creation and abandonment enabled the ants to track the changing conditions in food availability. 4.3 PERSISTENCE OF THE PHEROMONE TRAIL 4.3.1 INTRODUCTION Given the transitory nature of most pheromone trai ls, the persistence of the pheromone laid down by the foragers of Acromyrmex octospinosus was examined. 4.3.2 MATERIALS AND METHODS 4.3.2.1 PERSISTENCE IN NATURE The duration of the t ra i l pheromone was determined by the technique described by Jander and Jander (1979). Three sticks were placed end to end on an active t ra i l so that foragers using the t ra i l traveled along them when leaving or entering the nest. After 24 h period, the sticks were removed and replaced by 3 fresh unmarked 1 29 sticks until the end of the experiment. Care was taken not to touch the marked surfaces of the sticks when manipulating them. Between the test periods, the sticks were kept in a clearing out of reach of the ants. To determine the pheromone potency on the sticks, the middle stick of a set s t i l l on the t ra i l was removed and replaced by a new pair: one that had already been marked with pheromone at a given time before the test, and an unmarked (control) stick. The choices made by 25 foragers were recorded, then the positions of the 2 sticks were reversed, to avoid any possible location bias, and the choices of a second group of 25 foragers were recorded. A G-test of independence (Sokal and Rohlf 1981) was used to compare the 2 groups and, as the difference bet-ween them was never significant, they were pooled. The t ra i l pheromone was assumed to have completely disap-peared when, after a series of tests at 12 h intervals, as many foragers chose the unmarked sticks as did the marked ones. The experiment was carried out 3 times with 3 sticks each time. 4.3.2.2 EFFECT OF ULTRAVIOLET LIGHT During the previous experiment, some casual obser-vations led me to suspect an effect of UV light on the potency of the t ra i l pheromone. Thus an experiment to test the possible influence of UV on the persistence of the t ra i l was carried out. 1 30 Four s t i c k s were marked by ants as above and brought to the l a b o r a t o r y . There, 2 marked s t i c k s were kept as c o n t r o l s and the other 2 were p l a c e d 1 m from a Mazda TG 15 germicide lamp (Mazda Co.29 rue de Lisbonne, 75008 P a r i s ) f o r 30 min/12 h. The lamp emits more than 90% of i t s r a d i a n t f l u x at 253.7 nm and has an i r r a -diance of 37 microwatts/cm 2 at 1 m. The i r r a d i a t e d and c o n t r o l s t i c k s were t e s t e d i n •the f i e l d as d e s c r i b e d e a r l i e r . T h i s experiment was r e -peated twice. .3 RESULTS 4.3.3.1 PERSISTENCE IN NATURE Disappearance of the pheromone i n nature was not constant over time ( F i g . 23A and Table XVIIa). Ins t e a d , a t t e n u a t i o n of the pheromone appeared to occur i n a s t e -pwise f a s h i o n . I t was slow at n i g h t , but f a s t e r i n d a y l i g h t . The v a r i a n c e s d i f f e r e d among the t e s t hours (B a r t l e t t = 9 . 6 9 , 3 DF, P<0.05), and i t i s c l e a r that as the pheromone l o s t potency the c h o i c e of the ants became more and more v a r i a b l e ( F i g . 23A). Even when the pheromone was f r e s h , some f o r a g e r s d i d not choose the marked s t i c k ( F i g . 23A). A f t e r 36 h the marked s t i c k s were chosen s i g n i f i c a n t l y more o f t e n (X 2=13.69, df=1, P<0.001) i n d i c a t i n g that the pheromone was s t i l l a c t i v e . However, c o n s i d e r i n g the r a p i d a t t e n -u a t i o n o c c u r r i n g i n d a y l i g h t , i t i s d o u b t f u l that 131 F i g u r e 23: P e r s i s t e n c e of the t r a i l pheromone of the l e a f - c u t t e r ant, Acromyrmex octospinosus i n Guadeloupe. (A) Evaporation i n nature. P a t t e r n of a l t e r n a t i n g black and white s t r i p s i n d i c a t e s the p h o t o p e r i o d i c i t y . (B) Evapora-t i o n i n the l a b o r a t o r y . C o n t r o l ( s o l i d l i n e ) , and under UV e x p o s i t i o n of 30 min/12 h (broken l i n e ) . V e r t i c a l l i n e s i n d i c a t e the 95% con f i d e n c e i n t e r v a l f o r the mean. 1 32 o E-GO Q W « < o CO o o CJ CO H <: 100 18 24 30 36 T I M E F R O M L A S T M A R K I N G ( H O U R ) 0 10 20 30 40 T I M E F R O M L A S T M A R K I N G ( H O U R ) 1 33 TABLE X V I I : ANOVA summaries f o r the a t t e n u a t i o n of the pheromone t r a i l of Acromyrmex octospinosus i n Guade loupe . SOURCE OF DF SS MS F 1 VARIATION a) NATURE BETWEEN HOURS 3 8991 . 773 2997. 258 35.37 * * * REGRESSION 1 7808. 041 7808. 041 13.19 n . s ERROR(REGRESSION) 2 1 183. 732 591 . 866 6.98 ** WITHIN HOURS 31 2626. 913 84. 739 TOTAL 34 11618. 686 b) CONTROL BETWEEN HOURS 3 4500. 698 1 500. 233 182.31 *** REGRESSION 1 4486. 1 26 4486. 1 26 615.72 * * * ERROR(REGRESS I ON) 2 14. 572 7. 286 0.89 n . s WITHIN HOURS 1 2 98. 750 8. 229 TOTAL 1 5 4599. 448 c) TEST(UV) BETWEEN HOURS 2 51 00. 677 2550. 339 170.02 * * * REGRESSION 1 51 00. 500 51 00. 500 30002.94 * * * ERROR(REGRESSION) 1 0. 1 77 0. 1 77 0.01 n . s WITHIN HOURS 9 1 35. 000 15. 000 TOTAL 1 1 5235. 677 1. * * P<0.005, * * * P<0.001. 1 34 pheromone 36 h old could have lasted for another 12 h. Using the estimated rate of pheromone disappearance in daylight (2.4%/h) to predict a time of extinction, another 4 h would have been needed. Therefore the tra i l pheromone would last an estimated 40 h under my experi-mental conditions. Considering the complex molecular structure of the tra i l pheromone of Acromyrmex octospinosus (Cross et al. 1982) and the high susceptibility of such molecules to UV light, we hypothesized that the stepwise fade-out pattern observed could be the result of an UV effect on the sticks. The next section gives the results of the experiment conducted to test this hypothesis. 4.3.3.2 EFFECTS OF ULTRAVIOLET LIGHT As might be expected, the stepwise attenuation dis-appeared when the sticks were kept in the laboratory under UV light (Fig. 23B). For both the test and con-trol groups,' the pheromone attenuation can be succes-sfully modeled by a regression (Table XVIIb,c). A com-parison of slopes between the 2 groups shows that the pheromone was inactivated twice as fast on the UV-treated sticks as on the control sticks (Table XVIII, Fig. 23B). 4.3.4 DISCUSSION The UV experiment does not prove conclusively that the natural range of ultraviolet is a major factor in the 1 35 TABLE XVIII: Comparison of the r e g r e s s i o n c o e f f i c i e n t s f o r the e f f e c t of UV l i g h t on the d u r a t i o n of the t r a i l pheromone of the l e a f - c u t t e r ant, Acromyrmex octospinosus, i n Guadeloupe. SOURCE OF VARIATION DF SS MS F 1 BETWEEN SLOPES 1 1262.644 1262.644 257.02 * WITHIN REGRESSIONS 3 14.743 4.914 1. * P<0.001. 1 36 degradation of the pheromone in the field. The only UV source available emitted at a wavelength not found in nature (Silberglied 1979). Thus even though high energy UV at 253.7 nm was very effective, I do not know how effective UV of longer wavelength and less energy would be. Furthermore, persistence of the pheromone naturally depends on many fa-ctors, such as temperature, humidity, the blend of chemicals involved, and the nature of the substrate (Bradshaw and Howse 1984). The last 2 factors were similar in a l l the tests. However temperature and humidity conditions vary, as does UV, between night and day and could have been investigated. But in Guadeloupe there was no environmental chamber to control these 2 factors and test their relative importance. Nevertheless the UV experiment shows that short wavelengths can accelerate the disappearance of the t ra i l pheromone when the source is intense. The relative impor-tance of UV compared with temperature and humidity remains to be investigated with better experimental faci l i t ies. Whatever the complex of factors responsible for the ra-pid fading of the t ra i l pheromone, the process is very im-portant to the ants. Moser and Silverstein (1967) showed that Atta texana workers can follow a pheromone t ra i l wi-thout being directly in contact with i t . Workers of Iri-diomyrmex humi I i s (Mayr) can orient to a pheromone t ra i l from a distance when wind is present (van Vorhis Key and Baker 1982). These observations indicate that the t ra i l pheromone contains a volatile component. In addition, 1 37 Littledyke and Cherrett (1978b) demonstrated that a pheromone t ra i l disturbed by moving air causes foragers to follow it in a loop downwind of the air stream. Thus, if the pheromone of A. octospinosus was long lived, wind close to the nest could potentially create a network of inter-mingled signals coming from current and abandoned trails " that would reduce the efficiency of the foragers, wasting their time and energy by diverting them to no longer used trai ls . It is noteworthy that not a l l foragers follow the pheromone assiduously even when it is s t i l l fresh. This lack of precision could be accidental due to a low sensiti-vity threshold or a low competence, or it could be part of an ant's tasks. In either case, the individual leaving the established t ra i l explores new terrain, an act that can cer-tainly be advantageous for the colony where there are undis-covered food sources (Pasteels et al. 1982). Few data are available in the literature on the durabi-l i ty of the foraging trails of ants. Talbot (1967) showed that trails of Polyergus lucidus (Mayr), a slave-making ant, last for only 2 h. This longevity is apparently sufficient as their resources are short lived; a nest that has been raided is no longer a useful source of slaves. The primi-tive leaf-cutting ant, Trachymyrmex urichi (Forel), has a tra i l pheromone that lasts 1 h (Jaffe and Villegas 1985). A longer duration is apparently not needed as the pheromone is used only in orientation toward the food source, and not in 1 38 o r i e n t a t i o n back to the nest. The ant OecophylI a smaragdina (Fabr.) has a t r a i l d u r a b i l i t y of 3 days (Jander and Jander 1979). T h i s s p e c i e s maintains t e r r i t o r i e s , so a r e l a t i v e l y p e r s i s t e n t t r a i l pheromone i s advantageous. The 40 h l o n g e v i t y of the t r a i l pheromone of A. octo-spinosus i n sunny c o n d i t i o n s would allow the i n s e c t to main-t a i n a t e r r i t o r y l i k e 0. smaragdina, but a c t u a l f i g h t i n g or t e r r i t o r i a l defence r a r e l y occurs (Jutsum 1979; Jutsum et al. 1979). The food resources of A. octospinosus are l e s s t r a n s i t o r y than the s l a v e s resources of P. lucidus, and con-t r a r y to T. urichi, A. octospinosus c o n s t a n t l y uses i t s pheromone t r a i l as an o r i e n t a t i o n cue. Thus the f o r a g i n g p a t t e r n of A. oct ospi nosus f a l l s somewhat between the pat-t e r n s of the 3 other s p e c i e s , and i t s pheromone l a s t s an i n -termediate time. 4.4 SIZE DISTRIBUTION OF FORAGERS 4.4.1 INTRODUCTION B e r n s t e i n (1979), s t u d i e d the r e l a t i o n s h i p between s i z e of f o r a g e r and n i c h e breadth i n 13 communities of d e s e r t a n t s . She found t h a t c o l o n i e s with l a r g e workers a l s o had l a r g e t e r r i t o r i e s . She hypothesized that l a r g e workers i n a colony t r a v e l f a r t h e r from the nest than smaller ones. None of the s p e c i e s she s t u d i e d was a l e a f - c u t t e r , b u t f o r a g e r s of many d i f f e r e n t s i z e s can be seen on t r a i l s of A. octospino-sus and other l e a f - c u t t i n g ants (Fowler and Robinson 1979a; 1 39 Wilson 1980a). I t e s t e d B e r n s t e i n ' s hypothesis with A. oc-tospinosus. 4.4.2 MATERIALS AND METHODS Sampling of the f o r a g e r s was done at Lemesle on 3 t r a i l s of c o l o n y 12. T r a i l l e n g t h s were 0.15, 5.25, and 13 m. The 2 s h o r t e r t r a i l s l e d to r i p e bread f r u i t , and the t h i r d was a permanent t r a i l and l e d to a Tamarindus indica t r e e that was n e i t h e r f l o w e r i n g nor f r u i t i n g at the time. On 2 c o n s e c u t i v e days, at the end of each t r a i l , every t h i r d f o r a g e r coming back to the nest was captured u n t i l 20 f o r a g e r s were gathered f o r each day. During sampling, 1 of these f o r a g e r s was l o s t from 1 of the t r a i l samples. To maintain equal sample s i z e s , one f o r a g e r from each of the 2 other t r a i l s was randomly removed, g i v i n g a t o t a l of 39 ants f o r each t r a i l . Forager s i z e , as expressed by headwidth a c r o s s the eyes, was measured i n the l a b o r a t o r y , with a micrometer und-er a 50 mm o b j e c t i v e . In Acromyrmex the measure i s c l o s e , but not i d e n t i c a l , to the standard i n s e c t head-width used by taxonomists. 4.4.3 RESULTS There was no s i g n i f i c a n t d i f f e r e n c e between the means of f o r ager s i z e f o r each day on a given t r a i l , so the 39 i n -d i v i d u a l s f o r each t r a i l were pooled. The v a r i a n c e s i n f o r -ager s i z e f o r the 3 t r a i l s were unequal ( B a t t l e t t , x 2=45.5, 1 40 2 DF, P<0.001) and i n v e r s e l y p r o p o r t i o n a l to the means. Consequently a weighted r e g r e s s i o n a n a l y s i s was used (Box and H i l l 1974). As the le n g t h of the t r a i l s i n c r e a s e d , the s i z e of the f o r a g e r s i n c r e a s e d e x p o n e n t i a l l y ( F i g . 24). The r e g r e s s i o n model gave a s i g n i f i c a n t r e g r e s s i o n with no s i g n i f i c a n t de-v i a t i o n from the r e g r e s s i o n (Table XIX). Although a s i g n i -f i c a n t l i n e a r model was f i t t e d to the data, the d e v i a t i o n from r e g r e s s i o n was a l s o s i g n i f i c a n t , so the l i n e a r form was r e j e c t e d . P a r a l l e l i n g t h i s i n c r e a s e i n forager s i z e , as a l -ready noted, the v a r i a n c e decreased. T h i s r e l a t i o n s h i p sug-ge s t s t h a t l a r g e f o r a g e r s can be found at any d i s t a n c e from the nest, but that s m a l l e r ones forage only c l o s e to home. As the f o r a g e r s were c o l l e c t e d on the 0.15 m long t r a i l , many ants of the minor c a s t e were seen g a t h e r i n g bread f r u i t . Thus i t appears t h a t , given a s u f f i c i e n t l y h i g h ranking r e s o u r c e , some minors w i l l abandon t h e i r fungus- and bro o d - c a r i n g tasks and j o i n those f o r a g e r s c o l l e c t i n g food. 4.4.4 DISCUSSION Acromyrmex octospi nosus i s a very o p p o r t u n i s t i c s p e c i e s t a k i n g advantage of chance appearance of temporary r e s o u r c e s . In such circumstances i t i s presumably adap t i v e to be a b l e t o commit to f o r a g i n g an otherwise i d l e p a r t of the c o l o n y ' s work f o r c e (Herbers 1981; Oster and Wilson 1978; Wilson 1971), or to have p a r t of the most numerous workers, the minors (Weber 1972; Wilson 1980a, 1980b), 141 F i g u r e 24: T r a i l l e n g t h and the s i z e of f o r a g e r s of the l e a f - c u t t e r ant, Acromyrmex octospinosus. The sample s i z e i s 39 f o r each mean shown. The v e r t i c a l bars i n d i c a t e the 95% c o n f i d e n c e i n t e r v a l f o r the means. 2.25 TRAIL L E N G T H (m) 143 T A B L E X I X : ANOVA s u m m a r y f o r t h e r e g r e s s i o n o f s i z e o f f o r -a g e r s o f t h e l e a f - c u t t e r a n t , Acromyrmex octospinosus, w i t h t h e l e n g t h o f t h e f o r a g i n g t r a i l t h e y u s e d . SOURCE OF V A R I A T I O N DF S S MS F 1 BETWEEN L E N G T H S 2 2 4 . . 4 8 3 1 2 . , 2 3 0 12. ,23 *** R E G R E S S I O N 1 2 4 . . 3 8 8 2 4 . . 3 8 8 256. .72 *** E R R O R ( R E G R E S S I O N ) 1 0 . . 0 9 5 0 . . 0 9 5 0 . . 1 0 n . s W I T H I N L E N G T H S 1 1 4 1 1 4 . . 0 7 0 1 , . 0 0 1 TOTAL 1 1 6 1 3 8 . . 5 5 3 1 . * * * P < 0 . 0 0 0 1 . 1 44 t e m p o r a r i l y n e g l e c t t h e i r u s u a l t a s k s and h e l p i n food gath-e r i n g . In some other A t t i n i , minima workers can be found any-where on the t r a i l . For example, i n Atta cephalotes, minima workers o f t e n r i d e back to the nest on l e a f fragments c a r -r i e d by l a r g e r f o r a g e r s . These minima p r o t e c t the l a r g e r workers from p a r a s i t i c p h o r i d f l i e s ( E i b l - E i b e s f e l d t and E i b l - E i b e s f e l d t , c i t e d by Weber 1972; Wilson 1971, 1980a), and l i c k the l e a f fragments to c l e a n them of d e b r i s and a l i e n organisms ( S t a h e l 1943; S t a h e l and G e i j s k e s 1939, both c i t e d by Weber 1972). Fowler and Robinson (1979a) found that about 25% of the f o r a g e r s c a p t u r e d i n t h e i r 330 p i t f a l l t r a p s set up i n the t e r r i t o r y of an Atta sexdens c o l o n y were minima. Minima were found at 13m from the main f o r a g i n g t r a i l s and a l s o , although not s p e c i f i c a l l y mentioned, along t h e i r t o t a l l e n g t h . Fowler and Robinson d i d not d i s c u s s the r o l e of these workers i n the f o r a g i n g p r o c e s s . The mechanism by which A. octospinosus a c h i e v e s a size-dependent f o r a g i n g d i s t a n c e i s not known. Larger f o r -agers might have b e t t e r n a v i g a t i o n a l a b i l i t i e s a s s o c i a t e d with t h e i r l a r g e r s i z e ( B e r n s t e i n and B e r n s t e i n 1969). I f so, the poorer n a v i g a t i o n a l a b i l i t i e s of minor workers would prevent them from ranging f a r from the nest, e s p e c i a l l y i f they l a c k f o r a g i n g e x p e r i e n c e . A l t e r n a t i v e l y , the ants might be l i m i t e d by t h e i r hygrothermal requirements. Other f a c t o r s being e q u a l , the s i z e of an ectotherm d i r e c t l y i n f l u e n c e s i t s heat balance 145 ( S t e v e n s o n 1985; W i l l m e r 1982). L a r g e i n s e c t s a t t a i n t h e h i g h e s t t e m p e r a t u r e s , b u t t a k e l o n g e r t o r e a c h them (Casey 1981; D i g b y 1955; P a r r y 1951; P o r t e r a nd G a t e s 1969; W i l l m e r and Unwin 1981) t h a n s m a l l e r o n e s . C o n s e q u e n t l y , as l o n g a s t h e y c a n a v o i d i n t e n s e i n s o l a t i o n , w h i c h h a s t h e p o t e n t i a l t o c a u s e o v e r h e a t i n g , l a r g e i n s e c t s c a n t o l e r a t e more v a r i -a b l e t h e r m a l c o n d i t i o n s t h a n s m a l l e r i n s e c t s . F u r t h e r m o r e , l a r g e i n s e c t s a r e a l s o a t an a d v a n t a g e a t n i g h t , s i n c e t h e i r c o o l i n g r a t e i s s l o w e r t h a n t h a t o f s m a l l i n s e c t s ( B a r t h o -lomew 1981; W i l l m e r 1982; W i l l m e r and Unwin 1981). Body w e i g h t o f Acromyrmex octospinosus v a r i e s f r o m 1.4 t o 20 mg. The l a r g e r a n t s a r e p r e s u m a b l y b e t t e r a d a p t e d t o work o u t s i d e t h e n e s t i n n a t u r a l a s w e l l a s a r t i f i c i a l s e t -t i n g s , a n d c o u l d r a n g e f a r t h e r away f r o m i t t o f i n d f o o d . T h e r e i s p r e s u m a b l y l i t t l e d a n g e r o f o v e r h e a t i n g where t h e r e i s good p l a n t g r o w t h f o r , a s n o t e d i n t h e i n t r o d u c t i o n t o t h i s c h a p t e r , t h e f o r a g i n g t r a i l s a r e u s u a l l y p a r t l y h i d d e n by c o v e r i n g v e g e t a t i o n . Water b a l a n c e i s a l s o v e r y i m p o r t a n t f o r most i n s e c t s ( B a r t o n - B r o w n e 1964; S t o b b a r t a nd Shaw 1974). T h e i r haemo-lymph has an e q u i v a l e n t o f 99.5 t o 99.8 %RH. As a c o n s e -q u e n c e , u n l e s s t h e y a r e i n a s a t u r a t e d a t m o s p h e r e , t h e g r a -d i e n t o f w a t e r exchange w i l l be d i r e c t e d o u t w a r d . As p o i n t e d o u t by W i l l m e r ( 1 9 8 2 ) , "whereas w i t h t e m p e r a t u r e i n -s e c t s may cha n g e t h e i r b e h a v i o u r a nd p h y s i o l o g y t o p e r m i t e i t h e r h e a t i n g o r c o o l i n g , w i t h w a t e r b a l a n c e t h e p r o b l e m i s g e n e r a l l y t h a t o f r e t a i n i n g w a t e r a nd r e p l a c i n g t h e 146 inevitable losses...". Once again, when insects differ only in size, the larger ones wil l have the advantage. In terms of percentage water loss, which is a function of the surface-volume ratio, they wil l lose less water and their body fluids wil l concentrate less rapidly than those of smaller insects. Once again, larger individuals of A. octo-spinosus would be better suited for foraging farther from the nest. Some circumstantial evidence that water balance or thermoregulatory requirements might be cr i t ical factors for leaf-cutters is scattered through the literature, but it has not previously attracted attention. One obvious way to re-plenish lost water is to drink, and insects wil l readily do so if offered liquid (Barton-Browne 1964; Cloudsley-Thompson 1975). In leaf-cutters, Littledyke and Cherrett (1976), and Stradling (1978) have separately shown that foragers drink sap from the plant material as they cut i t . This sap gives them energy, but it also provides water. The most decisive evidence for a water problem, however, has been reported by Bowers and Porter (1981). They have shown that, as Atta co-lombica foragers travel farther from the nest, they tend to cut leaves with an increasing water content. These authors also state that the ants they observed might have been water-stressed, since their sampling took place at the end of the dry season. The point is, however, that those forag-ers farthest, and therefore longest, from home had more time to lose water. The fact that they then cut leaves with a 1 47 hi g h water content suggests that t h e i r changing water ba-lanc e t r i g g e r e d a form of compensatory behavior. In At t a cephalotes, the minima that r i d e on the l e a f fragments t h e i r l a r g e r s i b l i n g s c a r r y are r e l a t i v e l y f a r from the ground, and thus l e s s exposed to he a t i n g by the ground-emitted long-wave r a d i a t i o n . Furthermore, t h e i r l i c k i n g behavior, o s t e n s i b l y to remove f o r e i g n m a t e r i a l , c o u l d a l s o p r o v i d e them with water i f they t r e a t the r e c e n t l y - c u t edges of the fragments as w e l l as the s u r f a c e . These a s p e c t s of t h e i r behavior might be worth f u r t h e r i n v e -s t i g a t i o n i n subsequent s t u d i e s . CONCLUSION "Science consists in grouping facts so that general laws or conclusions may be drawn from them." CHARLES DARWIN "The joy of research must be found in doing, since every other har v e s t is uncertain." THEOBALD SMITH In Guadeloupe, the timing of the foraging activity of Acromyrmex octospinosus is influenced by environmental factors. During the unusually dry seasons of 1983-84 when this study took place, colonies at Lemesle foraged mostly during the day. Most colonies reduced their activity during the hottest hours of the day. Some colonies at Lemesle also shifted their activity to the night towards the end of the annual dry season. In contrast, colonies at Lomard, a more xeric area, were active only during the night, unless part of their territory was shaded, in which case very low levels of activity were seen occasionally during the day. The high variances between monthly correlations of activity and foraging efficiency with the t ra i l temperatures and vapor density deficits suggest that other factors than weather also influence the foraging activity of the ants. More information will be needed on these other factors be-fore one can predict adequately the foraging activity levels 148 1 49 of c o l o n i e s . On a seasonal b a s i s , some c o l o n i e s a p p a r e n t l y respond to the i n c r e a s e d energy demands of producing sexuals by i n -c r e a s i n g t h e i r average a c t i v i t y l e v e l s and the p r o p o r t i o n of r e t u r n i n g f o r a g e r s c a r r y i n g food. In the months f o l l o w i n g the n u p t i a l f l i g h t , a c t i v i t y l e v e l s and f o r a g i n g e f f i c i e n c y decrease f o r most c o l o n i e s , and i n t e r c o l o n y v a r i a n c e i n c r e a s e s . I t i s p o s s i b l e that the observed environmental e f f e c t s on f o r a g i n g were e s p e c i a l l y e vident due to the ex-c e p t i o n a l l y dry weather c o n d i t i o n s d u r i n g my study. Since c o l o n i e s at Lomard are c l o s e to the present l i m i t of the d i s t r i b u t i o n of the s p e c i e s , i t i s probable that any t r e n d toward more normal c o n d i t i o n s would have more impact there than at Lemesle. C o l o n i e s do not c o l l e c t t h e i r food p l a n t s i n p r o p o r t i o n to t h e i r abundance i n the t e r r i t o r i e s . They e x h i b i t a c l e a r p r e f e r e n c e f o r some s p e c i e s . Thus, although they v i s i t 83% of the p l a n t s p e c i e s i n t h e i r t e r r i t o r i e s , they cut only a small subset of the s p e c i e s a v a i l a b l e . Foragers respond q u i c k l y to the appearance of t r a n s i t o r y r e s o u r c e s such as f l o w e r s , f r u i t s , or young l e a v e s . At Lemesle, c o l o n i e s were l o c a t e d near f r u i t b e a r i n g t r e e s , and they c o n c e n t r a t e d t h e i r f o r a g i n g e f f o r t on flowers and f r u i t s . T h i s concen-t r a t i o n caused an apparent r e d u c t i o n i n the p r o p o r t i o n of the t o t a l p l a n t s p e c i e s a v a i l a b l e that were cu t by the a n t s , compared with the s i t u a t i o n at Lomard. Such a r e d u c t i o n i n d i e t breadth i n the presence of h i g h - r a n k i n g food sources i s 150 predicted by optimal foraging theory. The preferences of individual foragers for plant species may d i f f e r , and at least half the foraging force may d i f f e r from the whole colony's preference. Individual ants apparently agree on which species are not acceptable, but individual variations in preference increase as the plant species offered are more acceptable. This response probably ensures that more than one plant resource w i l l be exploited, even when the foraging e f f o r t i s channeled to a p a r t i c u l a r high-ranking resource. In their e x p l o i t a t i o n of short-lived resources, the ants have developed an opportunistic system of foraging. They react quickly to the appearance of t r a n s i t o r y resources by concentrating t h e i r cutting on them, and the manner in which they e s t a b l i s h and abandon their temporary foraging t r a i l s r e f l e c t s t h e i r opportunism. Their pheromone t r a i l s l a s t only for some 40 h i f they are not a c t i v e l y maintained, and thus rapidly fade when they are rendered obsolete by the disappearance of the food resources to which they led. The ants cut 85.7 kg dry matter/ha/year on Grande-Terre, about 40 times less than c a t t l e on the same substrate, and the insect would not be a pest in Guadeloupe i f i t did not sometimes concentrate on s p e c i f i c plants such as cassava or yam. The fact that they tend to concentrate th e i r cutting on flowers or f r u i t s further increases their p o t e n t i a l for economically important damage. Since there was v i r t u a l l y no information on the behav-i o r a l ecology of Acromyrmex octospinosus in Guadeloupe 151 before this study began, the results of even this compara-tively brief investigation can be used to improve the cur-rent attempts to control the insect. Many important ques-tions could not be answered, however, in the 12 mo that were available for field work. 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