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The role of vocalizations in spacing out and mate selection in Pacific tree frogs Whitney, Carl Linn 1973

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THE ROLE OF VOCALIZATIONS IN SPACING OUT AND MATE SELECTION IN PACIFIC TREE FROGS by CARL LINN WHITNEY B.S., Iowa State University, 1970 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE In the Department of Zoology We accept t h i s thesis as conforming to the required standard The University of B r i t i s h Columbia September, 1973 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced, degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available f o r reference and study. I further agree that permission for extensive copying of t h i s thesis fo r scholarly purposes may be granted by the Head of my Department or by h i s representatives. It i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of Zoology The University of B r i t i s h Columbia Vancouver 8, Canada Date 1 t September 1973 i i ABSTRACT T h i s t h e s i s i n v e s t i g a t e s the r o l e of male v o c a l i z a t i o n s i n two aspects of P a c i f i c Tree Frog breeding behavior: Spacing out of males on breeding areas and s e l e c t i o n of mates by females. §£acino^out: An a n a l y s i s of nearest neighbor d i s t a n c e s of c a l l i n g f r o g s showed that males space out more than i f d i s t r i b u t e d randomly on the a v a i l a b l e c a l l i n g s i t e s . An a d d i t i o n experiment provided c o r r o b o r a t i v e evidence t h a t males tend not to c a l l too c l o s e t o g e t h e r ; of equal numbers of f r o g s added to an empty (control) e n c l o s u r e and an e n c l o s u r e occupied by c a l l i n g f r o g s , more subsequently c a l l e d i n the c o n t r o l . The s p e c i e s ' "mating" c a l l (D c a l l ) , as w e l l as a t t r a c t i n g females, f u n c t i o n s i n s p a c i n g out; i n an a d d i t i o n experiment, fewer f r o g s c a l l e d i n an e n c l o s u r e occupied only by loudspeakers p l a y i n g back D c a l l s than i n the c o n t r o l . Another v o c a l i z a t i o n (S c a l l ) seems to f u n c t i o n only i n spacing out. I f two c a l l i n g f r o g s come c l o s e together ( l e s s than ca. 50 cm), both u s u a l l y begin making the S c a l l . One f r o g may then move away; i f not, p h y s i c a l combat may f o l l o w . I hypothesized t h a t the S c a l l i s a s t r o n g e r warning to nearby males than the D c a l l . A playback experiment p a r t i a l l y i i i supported t h i s hypothesis. I consider possible functions of spacing out, and suggest that males which maintain spacing may attr a c t more females than they would otherwise, perhaps because females can more ea s i l y locate them. Mate s e l e c t i o n : I attempted to test two hypotheses: F i r s t , females select large males on the basis of th e i r low-pitched vocalizations (there i s an inverse c o r r e l a t i o n between c a l l pitch and body length). A comparison of body lengths and c a l l pitches of males found i n amplexus with a sample of males from the c a l l i n g population did not support t h i s hypothesis. Second, females choose males which i n i t i a t e bouts of c a l l i n g (chorus leaders). Eoth systematic f i e l d observations and a laboratory experiment support t h i s hypothesis. Chorus leaders also end choruses, c a l l at a faster rate during choruses, c a l l more during periods not defined as choruses, and c a l l louder than other frogs. In nature, females may also use these differences as cues in mate sel e c t i o n . The advantages of being a chorus leader should select for frogs c a l l i n g a l l of the time, yet frogs show freguent l u l l s in c a l l i n g . A playback experiment showed that frogs are less responsive (measured by latency to c a l l ) to stimulus c a l l s shortly a f t e r having stopped c a l l i n g than l a t e r in the l u l l period. I hypothesized that fatigue i s responsible for t h i s short-term waning of c a l l i n g tendency. A respirometry i v: experiment suggested (but quite inconclusively) that the fatigue hypothesis i s f e a s i b l e . TABLE OE CONTENTS Page TABLE OF CONTENTS , . i-V ABSTRACT i i LIST OF TABLES . . . v i i LIST OF FIGURES i x ACKNOWLEDGEMENTS X I. INTRODUCTION 1 A. THE PROBLEMS 1 1. Spacing out 2 2. Mate s e l e c t i o n 6 B. THE STUDY ANIMAL 10 1. N a t u r a l h i s t o r y .. 10 2. V o c a l i z a t i o n s 11 C. STUDY AREAS ....15 D. GENERAL METHODS 16 I I . THE ROLE OF VOCALIZATIONS IN SPACING OUT OF CALLING FROGS 17 A. SPACING OUT 17 B. ROLE OF THE D CALL IN SPACING OUT 24 C. ROLE OF THE S CALL IN SPACING OUT 30 D. MOVEMENTS OF MALES ON THE BREEDING GROUNDS 36 E. SUMMARY AND DISCUSSION 38 1. o r g a n i z a t i o n of spa c i n g behavior i n P a c i f i c Free Frogs 38 2. D e f i n i t i o n of t e r r i t o r i a l i t y i n anurans ... 39 3. The function of spacing out 41 I I I . THE ROLE OF VOCALIZATIONS IN HATE SELECTION .' 43 A. CALL PITCH AS A CUE IN HATE SELECTION 43 B. CHORUS-LEADING AS A CUE IN MATE SELECTION ...... 51 1. Chorus leaders 51 2. F i e l d evidence that females choose chorus leaders 54 3. Correlates of chorus-leading .............. 57 4. Use of chorus-leading as a cue i n mate selection 61 5. Short-term changes i n c a l l i n g tendency .... 66 C. SUMMARY AND DISCUSSION 70 LITERATURE CITED 72 APPENDIX I 77 v i i LIST OF/ TABLES Table Page 1. Chronology of the a d d i t i o n experiment ............... 20 2. Numbers of f r o g s c a l l i n g i n the experimental and c o n t r o l e n c l o s u r e s 22 3. Chronology of the a d d i t i o n experiment using loudspeakers 25 4. Numbers of f r o g s c a l l i n g i n the experimental (occupied by f o u r loudspeakers) and c o n t r o l e n c l o s u r e s ..............28 5. Mean d i s t a n c e (cm) to nearest male neighbor of S - c a l l i n g f r o g s and D - c a l l i n g f r o g s ..31 6. Vocal response of D - c a l l i n g males to playback of D c a l l s 40 cm behind them ........................... 32 7. Rates of D - c a l l i n g and S - c a l l i n g and r e l a t i v e i n t e n s i t i e s of the c a l l s ......34 8. A g o n i s t i c responses of males to 1 min playback c f D c a l l s and S c a l l s 35 9. Snout-vent l e n g t h s (mm) of males found i n amplexus compared with c a l l i n g males i n 1972 and 1973 .....46 10. Fundamental frequency and dominant frequency ( c y c l e s / s e c ) of amplectant males and c a l l i n g males ..48 11. Maximum number of choruses s t a r t e d (of a t o t a l cf four) by a member of each of 19 p a i r s of f r o g s ......53 12. Success of chorus l e a d e r s compared with other f r o g s i n a t t r a c t i n g females: F i e l d o b s e r v a t i o n s .....56 13. R e l a t i o n s h i p of c h o r u s - l e a d i n g to chorus-ending .....58 14. Mean c a l l r a t e s ( c a l l s / 3 0 sec) during choruses of chorus l e a d e r s and non-chorus l e a d e r s 60 v i i i 15. Mean number of c a l l s per 15 min during periods not defined as choruses of chorus leaders and non-chorus leaders 60 16. Relationship of fundamental frequency and dominant frequency (both i n cycles /sec) to c a l l i n t e n s i t y ...........62 17. Success of chorus leaders compared with other frogs in attra c t i n g females: Laboratory experiment ,......6<4 18. Mean length of l u l l (sec) i n c a l l i n g when no D c a l l s were played back during the l u l l compared with when D c a l l s were played back (data for the two playback conditions were lumped) .......68 19. Mean latency to c a l l (sec) when stimulus presented in the f i r s t part of a l u l l compared with when presented l a t e r i n a l u l l .................68 i x LIST OF FIGURES F i g u r e Page 1. A. Sound spectrogram of a t y p i c a l d i p h a s i c c a l l (water=14 C; air=8.5 C). B. S e c t i o n through approximately the s i x t h p u l s e of the c a l l ...........12 2. Sound spectrogram of a s t a c c a t o c a l l (water=14 C; air=8.5 C) ..............13 3. Sound spectrogram of a monophasic c a l l (water=14 C; air=8.5 C) . 14 4. Observed (shaded) and expected d i s t r i b u t i o n s of nearest neighbor d i s t a n c e s of c a l l i n g f r o g s .........21 5. The arrangement of loudspeakers i n the experimental e n c l o s u r e f o r the a d d i t i o n experiment. .27 6. Spacing of f r o g s from loudspeaker s t a t i o n s when speakers are p l a y i n g back D c a l l s (shaded) compared with when no speakers are present ..........29 7. R e l a t i o n s h i p of dominant freguency and fundamental frequency (of approximately the s i x t h pulse of the f i r s t phase) of D c a l l s to body len g t h .............44 8. R e l a t i o n s h i p of dominant frequency and fundamental frequency to water temperature ..........47 9. R e l a t i o n s h i p of fundamental frequency (corrected to 12.5 C) to body l e n g t h i n c a l l i n g f r o g s compared with s u c c e s s f u l f r o g s (excluding one f r o g with an aberrant voice) 49 10. R e l a t i o n s h i p of dominant freguency ( c o r r e c t e d to 12.5 C) to body le n g t h i n c a l l i n g f r o g s compared with s u c c e s s f u l f r o g s (excluding one f r o g with an a b e r r a n t voice) ............................50 11. R e l a t i o n s h i p of c a l l i n t e n s i t y to body le n g t h i n chorus l e a d e r s compared with other f r o g s ............59 12. A top view of the arena used i n the l a b o r a t o r y experiment t e s t i n g whether females are a t t r a c t e d to chorus l e a d e r s ..................63 X ACKNOWLEDGEMENTS I am g r a t e f u l t o my s u p e r v i s o r . Dr. J . R. Krebs, f o r a l l o w i n g me to undertake t h i s p r o j e c t , and f o r h i s a s s i s t a n c e and guidance a t a l l stages of the work. I thank D. J . Tapp f o r a i d i n g with the f i e l d work and f o r c r i t i c a l l y r e a d i n g s e v e r a l d r a f t s of t h i s t h e s i s . Dr. C. J . Krebs and Dr. J . D. McPhail a l s o c r i t i c a l l y read the manuscript. R. Corman k i n d l y b u i l t , on s h o r t n o t i c e , the e l e c t r o n i c d e v i c e s necessary f o r s e v e r a l c r u c i a l 'experiments. Thanks are a l s o due D. L a u r i e n t e , S. Borden, and B. Webb f o r a s s i s t a n c e with programming and computing. 1 CHAPTER I INTRODUCTION A. THE PROBLEMS Males of most anuran s p e c i e s make s t e r e o t y p e d v o c a l i z a t i o n s during the breeding season. Observers have f o r many years s p e c u l a t e d about the b i o l o g i c a l s i g n i f i c a n c e of the v o c a l i z a t i o n s , but only with recent experimental i n v e s t i g a t i o n s have they begun to get some answers. The focus of most of these s t u d i e s has been the r o l e of v o c a l i z a t i o n s i n s p e c i e s i d e n t i f i c a t i o n . S e v e r a l s t u d i e s have shown that females are a t t r a c t e d t o c o n s p e c i f i c c a l l s , but not to c a l l s of other sympatric s p e c i e s (e.g. L i t t l e J o h n and L o f t u s - H i l l s 1968); v o c a l i z a t i o n s may thereby serve as pre-mating i s o l a t i n g mechanisms. Males of s e v e r a l s p e c i e s a l s o seem to be a t t r a c t e d to c o n s p e c i f i c c a l l s (e.g. B l a i r 1968 p. 297); presumably t h i s i s a mechanism f o r aggregating males i n t o breeding choruses. T h i s t h e s i s r e p o r t s on i n v e s t i g a t i o n s of two other p o s s i b l e f u n c t i o n s of v o c a l i z a t i o n s i n P a c i f i c Tree Frogs ( H j l a r e a i l l a ) . F i r s t , I wanted to f i n d out i f c a l l s f u n c t i o n i n m a i n t a i n i n g spacing between males on the breeding areas, and second, whether females use d i f f e r e n c e s among males i n c a l l parameters or c a l l i n g behavior as cues i n mate s e l e c t i o n . 2 1. Spacing out Spacing out of males on breeding areas has been r e p o r t e d / f o r a number of anuran s p e c i e s : Dendrobates q r a n u l i f e r u s (Goodman 1971; Crump 1972), Dendrobates p u m i l i g (Bunnell 1973), Engystomops pustologus (Brattstrom and Y a r n e l l 1968), Hyla arborea and H. l§£i^ionalis ( P a i l l e t t e 1970), L§£i°^S£iilus melanonotus (Brattstrom and Y a r n e l l 1968), Phyllomedusa h y p o c h o n d r i a l i s (Pyburn and G l i d e w e l l 1971), £seudaghrjne b i b r o n i , P. c o r r o b o r e e , and P. dendi ( P e n g i l l e y 1971), Rana c a t e s b e i a n a (Durham and Bennett 1963; Emleri 1968; Wiewandt 1969), Sana c l a m i t a n s (Martof 1953), Scaphiopus hammondi (Whitford 1967), and Tomodactylus n i t i d u s (Bogert 1960). Observations of " t e r r i t o r i a l c a l l s " and chasing and f i g h t i n g i n males of s e v e r a l o t h e r s p e c i e s suggest t h a t they too space out: Dendrobates a a l i n d o i (Duellman 1966), J v i a a r e n i c o l o r , H. c h r y s o s c e l i s , and H. v e r s i c o l o r ( P i e r c e and R a l i n 1972), Hyla fabor (Lutz 1960), Hjfla r e q i l l a (Snyder and Jameson 1965), Hymenochirus b o e t t g e r i (Rabb and Rabb 1963a), L e p t o d a c t y l u s insularum (Sexton 1962), Pipa pipa (Rabb and Rabb 1963b), P r o s t h e r a p i s panamensis (Duellman 1966), and P c o s t h e r a p i s t r i n i t a t i s (Sexton 1960). In a l l of these r e p o r t s , the authors d e s c r i b e the animals as " t e r r i t o r i a l . " However, most do not p r o v i d e evidence r e g a r d i n g one of the usual p r o v i s o s i n d e f i n i t i o n s of t e r r i t o r i a l i t y t h a t the animals defend " f i x e d areas" (Brown 3 and Orians 1970) or "home ar e a s " (Krebs 1971). Those s t u d i e s which d i d monitor the movements of i n d i v i d u a l s p o i n t out the a r b i t r a r i n e s s of t h i s s t i p u l a t i o n : For example, i n one study (Emleri 1968) male B u l l f r o g s (Rana catesbeiana) defended the same areas n i g h t a f t e r n i g h t , but i n another (Wiewandt 1969) the defended areas were l e s s g e o g r a p h i c a l l y f i x e d . Do the former f r o g s q u a l i f y as t e r r i t o r i a l , but the l a t t e r not? Because of t h i s ambiguity, I w i l l accept, f o r the time being* the a p p l i c a t i o n of the term " t e r r i t o r i a l " to a l l anurans which space out. Although i n most of the above-mentioned s t u d i e s of anurans i t seems c l e a r t h at the animals do space out, none of the authors r i g o r o u s l y t e s t e d f o r i t . T h i s can be done simply by comparing the observed d i s t r i b u t i o n of nearest-neighbor d i s t a n c e s with the d i s t r i b u t i o n expected i f i n d i v i d u a l s s e t t l e d a t random with r e s p e c t t o each other. S e v e r a l workers have e x p e r i m e n t a l l y i n v e s t i g a t e d the r e l a t i o n s h i p of v o c a l i z a t i o n s to anuran t e r r i t o r i a l i t y (Bunnell 1973; Emlen 1968; Jenssen and Preston 1968; P a i l l e t t e 1970; Snyder and Jameson 1965; Wiewandt 1969). The approach i n most of these s t u d i e s was s i m i l a r to that i n the bulk of work on b i r d v o c a l i z a t i o n s and t e r r i t o r y (see e.g. F a l l s 1969): The workers observed whether c o n s p e c i f i c c a l l s , played back w i t h i n the t e s t animal's t e r r i t o r y , e l i c i t e d t e r r i t o r i a l b e havior. The most d e t a i l e d of these s t u d i e s on anurans i s t h a t of B u n n e l l (1973) on Dendrobates p u m i l i o . She found that c a l l s played back two meters or l e s s from males e l i c i t e d i approach b e h a v i o r ; these r e s u l t s concurred with o b s e r v a t i o n s that males space more than two meters apart and t h a t i n t r u d e r s u s u a l l y r e t r e a t upon approach of r e s i d e n t s . Playback a l s o caused changes i n the r a t e and "temporal p a t t e r n i n g " of c a l l i n g , which Bunnell suggested might a l s o warn i n t r u d e r s . Male D. £umilio e v i d e n t l y have only one kind of v o c a l i z a t i o n which serves both as a "mating" c a l l i n a t t r a c t i n g females and as a " t e r r i t o r i a l " c a l l . However, H j l a arborea and H. l§£i^i2Salis ( P a i l l e t t e 1970), H. r e n i l l a (Snyder and Jameson 1965), Rana ca t e s b e i a n a (Eralen 1968; Wiewandt 1969), and R. c l a m i t a n s (Jenssen and Preston 1968) change to v o c a l i z a t i o n s d i s t i n c t from t h e i r r e s p e c t i v e s p e c i e s * mating c a l l s when mating c a l l s are played back w i t h i n t h e i r t e r r i t o r i e s ; these c a l l s may f u n c t i o n o n l y i n t e r r i t o r i a l defense. There has been on l y one d e t a i l e d study, i n any animal, of the r o l e of v o c a l i z a t i o n s i n m a i n t a i n i n g t e r r i t o r y ; t h a t was i n Red-winged B l a c k b i r d s ( J a e l a u i s phoeniceus) (Peeke 1972). Peeke found t h a t muted males are l e s s s u c c e s s f u l than c o n t r o l s i n m a i n t a i n i n g t h e i r t e r r i t o r i e s . V o c a l i z a t i o n s seem to f u n c t i o n only as a " f i r s t l i n e of t e r r i t o r i a l defense" i n t h i s s p e c i e s ; muted males s u f f e r e d an i n c r e a s e d t r e s p a s s i n g r a t e , but they were e q u a l l y as a b l e as c o n t r o l s to e x p e l t r e s p a s s e r s . Peeke c o u l d not, of course, determine from these r e s u l t s which of the nine v o c a l i z a t i o n s t h a t t e r r i t o r i a l males make i s most important i n t e r r i t o r i a l defense, but he argued from i n d i r e c t evidence that i t i s the " a d v e r t i s i n g " song, a 5 vocalization given simultaneously with a wing epaulet display (which Peeke also showed experimentally to function i n t e r r i t o r i a l defense). P a i l l e t t e * s (1970) study of Hyla arborea and H. g e r i d i o n a l i s suggests that vocalizations aid in t e r r i t o r i a l maintainance i n these species. She reported that males moved away from a loudspeaker playing back conspecific mating c a l l s ; and i f confined near the loudspeaker, males would stop c a l l i n g altogether. She also found that the t e r r i t o r i a l c a l l had a greater e f f e c t i n causing frogs to retreat than the mating c a l l . Unfortunately, i t i s d i f f i c u l t to assess the sig n i f i c a n c e of P a i l l e t t e ' s r e s u l t s because she did not report on important aspects of experimental design, such as the use of controls, nor did she present r e s u l t s in a manner suitable for s t a t i s t i c a l analysis. Chapter II describes my studies of spacing and the role of vocalizations i n maintaining spacing among male P a c i f i c Tree Frogs. 6 2. Mate s e l e c t i o n E v o l u t i o n a r y theory p r e d i c t s that animals w i l l show two l e v e l s of d i s c r i m i n a t i o n i n mate s e l e c t i o n : F i r s t , they w i l l choose p a r t n e r s of the same s p e c i e s . Second, they w i l l s e l e c t the " best" a v a i l a b l e c o n s p e c i f i c mates by responding to t r a i t s which are c o r r e l a t e d with d e s i r a b i l i t y as a mate. S t u d i e s of a wide v a r i e t y of s p e c i e s have v e r i f i e d the f i r s t p r e d i c t i o n ; many of these i n c l u d e d e t a i l e d a n a l yses of the cues used. However, the second p r e d i c t i o n has r e c e i v e d very l i t t l e a t t e n t i o n ; indeed, there i s no evidence f o r most s p e c i e s that mate s e l e c t i o n i s other than random. Perhaps the most d e t a i l e d study of the cues used i n i n t r a s p e c i f i c mate s e l e c t i o n i s Ehrman's (e.g. 1972) work on D r o s o g h i l a fiseudoobscura. Using two in b r e d s t r a i n s , she s t u d i e d the mechanisms r e s p o n s i b l e f o r frequency-dependent mating (the p r e f e r e n t i a l s e l e c t i o n of r a r e - t y p e males by f e m a l e s ) . She was a b l e to show t h a t o l f a c t o r y cues alone allow females to s e l e c t r a r e - t y p e males, but her work d i d not exclude the p o s s i b i l i t y t h a t females a l s o use other cues, such as wing v i b r a t i o n r a t e s . B o r i s o v (1970) presented evidence t h a t t h i s system of mate s e l e c t i o n a l s o occurs i n wild p o p u l a t i o n s of D r g s g p h i l a . In v e r t e b r a t e s , the work of K r u i j t and Hogan (1967) on Black Grouse (Lyjrurus t e t r i x ) and Wiley (1970) on Sage Grouse 7 (Centrocercus urgphasianus) i s noteworthy. Males of both s p e c i e s d i s p l a y on l e k s to which females come f o r mating. Males h o l d i n g c e n t r a l t e r r i t o r i e s have by f a r the g r e a t e s t mating success: on a Black Grouse l e k the f o u r most c e n t r a l males performed more than 85% of the c o p u l a t i o n s , while the f i v e or s i x p e r i p h e r a l males performed l e s s than J\5%; c e n t r a l males, t o t a l l i n g l e s s than 10$ of the male p o p u l a t i o n , accomplished more than 75% of the matings on a Sage Grouse l e k . K r u i j t and Hogan suggested t h r e e d i f f e r e n c e s between c e n t r a l and p e r i p h e r a l Black Grouse males t h a t females might use as cues i n mate s e l e c t i o n : (1) C e n t r a l males have s m a l l e r t e r r i t o r i e s (females may be a t t r a c t e d to areas of high male d e n s i t y ) ; (2) C e n t r a l males have a higher l e v e l of g e n e r a l a c t i v i t y ( e s p e c i a l l y f i g h t i n g ) ; (3) In c o u r t i n g females, c e n t r a l males seem to use more e f f e c t i v e " t a c t i c s . " E xperimental evidence i n a number of anuran s p e c i e s shows that females are a t t r a c t e d to c o n s p e c i f i c c a l l s (e.g. Bogert 1960 p. 211; L i t t l e j o h n and L o f t u s - H i l l s 1968; Schmidt 1969). T h i s alone does not e s t a b l i s h t h a t female c h o i c e determines which males w i l l breed. In Rana g r e t i o s a , f o r example, males c a l l i n dense groups, and during the f i r s t few days of the b r i e f breeding season f r e q u e n t l y attempt amplexus with neighboring males ( L i c h t 1969) ; at l e a s t during the e a r l y p a r t of the breeding season, the s u c c e s s f u l males may be those which are f i r s t to rush out and c l a s p females approaching the group. However, i n a number of s p e c i e s , o b s e r v a t i o n s of 8 females approaching males and touching or almost touching them before the males move from t h e i r c a l l i n g s t a t i o n s to c l a s p ( B l a i r 1963 p. 706; Bragg 1959; Brattstrom and Y a r n e l l 1968; Brown and P i e r c e 1965; Emlen 1968; Green 1938; L i v e z e y 1952; Martof and Thompson 1958) i n d i c a t e t h a t , i n these s p e c i e s , female c h o i c e does determine which males w i l l mate. Casual o b s e r v a t i o n s of a breeding chorus of almost any anuran s p e c i e s r e v e a l two kinds of v o c a l v a r i a b i l i t y among males which females may use i n mate s e l e c t i o n . F i r s t , c a l l p i t c h e s vary among i n d i v i d u a l s . Capranica (1965) and Snyder and Jameson (1965) found t h a t c a l l p i t c h i s i n v e r s e l y r e l a t e d to body s i z e i n Rana ca t e s b e i a n a and H_y l a r e ^ i l l a , r e s p e c t i v e l y , and t h i s r e l a t i o n s h i p probably holds f o r other s p e c i e s (Bogert 1960). I t i s i n t u i t i v e l y a p p e a l i n g t o suggest t h a t l a r g e males make b e t t e r mates, and t h a t females choose them on the b a s i s of t h e i r low-pitched v o c a l i z a t i o n s . D i r e c t evidence t h a t females of any s p e c i e s s e l e c t l a r g e males i s l a c k i n g , but A x t e l l (1959) observed s m a l l , n o n - c a l l i n g male Bufo s p e c i o s u s gathered around a l a r g e r c a l l i n g male, and he suggested t h a t the s m a l l e r males may i n t h i s way take advantage of a g r e a t e r a b i l i t y of l a r g e males to a t t r a c t females. The work of Capranica (1965) i s a l s o r e l e v a n t to t h i s d i s c u s s i o n . He found that male Rana ca t e s b e i a n a respond with t e r r i t o r i a l v o c a l i z a t i o n s to playback of c o n s p e c i f i c mating c a l l s of l a r g e males but not to c a l l s of smal l males. The reason, very b r i e f l y , i s t h a t the h i g h - p i t c h e d 9 vocalizations of the small males contain energy in a frequency range which i n h i b i t s the t e r r i t o r i a l response. Although Capranica did not study the responses of females to large and small males* c a l l s , i t seems reasonable to suggest that, i f males do not respond t e r r i t o r i a l l y to small males, small males are not serious competitors for females, perhaps because females are not attracted to t h e i r c a l l s . The second kind of v a r i a b i l i t y i s in c a l l i n g behavior. The sequence of c a l l i n i t i a t i o n by indivi d u a l s in breeding choruses of a large number of species appears to be c l e a r l y non-random (see Bogert 1960 f o r review; Brattstrora and Yarnell 1968; Duellman 1967; Foster 1967; P a i l l e t t e 1970). Workers have speculated that "chorus leaders", the f i r s t i n d i v i d u a l s to c a l l when a chorus begins, are favored by females, but only a few inconclusive f i e l d observations (Brattstrom 1962; Brattstrom and Yarnell 1968) have been made which support this idea. In a sim i l a r system i n Tettigoniidae (Insecta), Busnel (1967) found that s t r i d u l a t i o n leaders are more successful at att r a c t i n g females than other males; however, i t i s not clear what cues the females use i n mate selection because s t r i d u l a t i o n leaders also behave d i f f e r e n t l y i n several other ways from non-leaders. These two hypotheses are not necessarily a l t e r n a t i v e s ; females could r e j e c t small males on the basis of c a l l pitch and select chorus leaders from the remaining males. Chapter 10 I I I d e s c r i b e s my i n v e s t i g a t i o n s of both hypotheses. B. THE STODY ANIMAL 1. Natu r a l h i s t o r y The P a c i f i c Tree Frog i s a member of the n e a r l y cosmopolitan H y l i d a e . I t i s a s m a l l f r o g , h i g h l y v a r i a b l e i n c o l o r , which ranges from southern B r i t i s h Columbia to the t i p of Baja C a l i f o r n i a , and e a s t from the P a c i f i c Ocean to western Montana, Idaho, and Nevada (Stebbins 1966). I t breeds i n shallow, temporary ponds, as w e l l as more permanent water. The breeding season i s r e l a t i v e l y long f o r temperate zone f r o g s ; i n the Lower Mainland of B r i t i s h Columbia, males c a l l f o r a t l e a s t e i g h t weeks. Males c h a r a c t e r i s t i c a l l y c a l l at n i g h t , but d u r i n g the peak of the breeding season they may a l s o c a l l d uring the day. 11 2. V o c a l i z a t i o n s T h i s study i s concerned with two d i s t i n c t v o c a l i z a t i o n s t h a t males make. The s p e c i e s * mating c a l l (D c a l l ) i s a d i p h a s i c v o c a l i z a t i o n ( F i g . 1a). Both phases are pulsed, the f i r s t phase averaging about 12 p u l s e s ; the second, 4 p u l s e s . As many as seven freguency bands occur (only the lowest f i v e can be seen i n F i g . 1). The "dominant" freguency i s i n the second harmonic ( F i g . 1b); i t ranged from 1790 to 2330 c y c l e s / s e c i n c a l l s of 60 f r o g s from Marion Lake, B r i t i s h Columbia. C a l l s show a s m a l l e r peak of energy i n the f i r s t harmonic; t h i s "fundamental" frequency ranged from 850 to 1250 c y c l e s / s e c i n Marion Lake f r o g s . The higher harmonics c o n t a i n even l e s s energy. The second v o c a l i z a t i o n s t u d i e d i s a " s t a c c a t o " c a l l (S c a l l ) which males make i n response to o t h e r s c a l l i n g nearby ( l e s s than c a. 50 cm). T h i s c a l l i s not d i v i d e d , i s much longer than the D c a l l , and has a' much slower pulse r a t e ( F i g . 2 ) . A t h i r d v o c a l i z a t i o n (M c a l l ) i s mentioned i n the t h e s i s , but i t s s i g n i f i c a n c e was not s t u d i e d . The M c a l l i s s i m i l a r i n pulse r a t e to the D c a l l , but monophasic ( F i g . 3). Males seem t o give t h i s c a l l i n response to movements of nearby n o n - c a l l i n g i n d i v i d u a l s (males or females). These c a l l c h a r a c t e r i s t i c s agree l a r g e l y with those 12 Figure 1. a. Sound spectrogram of a t y p i c a l diphasic c a l l (Water=14 C Air=8.5 C). b. Section through approximately the s i x t h pulse of the c a l l . 13 Figure 2. Sound spectrogram of a staccato c a l l (Water=14 C; Air=8.5 C). C y c l e s Per S e c o n d —^ GO (Jl O O O o o o o o o _ l I - I 1 1_ 14 Figure 3. Sound spectrogram of a monophasic c a l l (Water=14 C; Air=8.5 C). S e c o n d s 1 5 r e p o r t e d f o r P a c i f i c Tree Frogs by Snyder and Jameson (1965), from whom I borrowed the d e s c r i p t i v e names f o r the c a l l s . C. STUDY AREAS Most p a r t s of the study were done at Marion Lake, a s m a l l (800 m by 200 m) lake l y i n g at 300 m e l e v a t i o n i n the Coast Mountains 50 km east o f Vancouver, B r i t i s h Columbia. E f f o r d (1967) d e s c r i b e d the lake i n d e t a i l . P a c i f i c Tree Frogs c a l l from emergent v e g e t a t i o n around the pe r i p h e r y of the lake d u r i n g A p r i l , May, and June. Some work was done i n s m a l l ponds on the U n i v e r s i t y of B r i t i s h Columbia Endowment Lands near Vancouver. These ponds are u s u a l l y temporary d r y i n g up i n l a t e summer. Here f r o g s c a l l d uring March, A p r i l , and May. Vancouver f r o g s average s l i g h t l y s m a l l e r than Marion Lake f r o g s , but otherwise no morphological or b e h a v i o r a l d i f f e r e n c e s were noted. 16 D. GENERAL METHODS Most f i e l d work was done at n i g h t . I observed and captured f r o g s with the a i d of a s i x - v o l t l a n t e r n . S h i n i n g the l i g h t on f r o g s u s u a l l y caused them to stop c a l l i n g , but i f not f u r t h e r d i s t u r b e d , most would resume c a l l i n g i n a few minutes. Frogs were captured by hand, measured from snout to vent to the nearest m i l l i m e t e r , and t o e - c l i p p e d f o r i n d i v i d u a l i d e n t i f i c a t i o n . C a l l s were recorded with Uher 4000 Report-L tape r e c o r d e r s equipped with Dher M514 microphones. Recording speed was always 9.5 cm/sec. A f t e r each r e c o r d i n g , a i r temperature 1 cm above the water s u r f a c e and water temperature 1 cm below the s u r f a c e were taken to the nearest 0.5 C. C a l l s were analyzed f o r fundamental and dominant frequency using a Kay model 675 sound spectrograph. A s m a l l s o u n d - l e v e l meter ( R e a l i s t i c 33-1028) was used to measure c a l l i n t e n s i t i e s . 17 CHAPTER I I THE ROLE OF VOCALIZATIONS IN SPACING OUT OF CALLING FROGS A. SPACING OUT Snyder and Jameson (1965) reported evidence that male P a c i f i c Tree Frogs space out on the breeding grounds: I f a c a l l i n g male i s approached by another male, i t changes from the D c a l l to the S c a l l . Both f r o g s make the S c a l l u n t i l " e i t h e r the i n v a d i n g male has r e t r e a t e d or the r e s i d e n t male accommodates to the i n v a d e r ' s presence." As a f i r s t step i n my study, I attempted to t e s t the hypothesis that males space out. A d i r e c t t e s t of the hypothesis would answer the q u e s t i o n : "Do c a l l i n g f r o g s space out more than i f d i s t r i b u t e d at random on the a v a i l a b l e c a l l i n g s i t e s ? " To answer t h i s q u e s t i o n , I made a "nearest-neighbor" a n a l y s i s of the d i s p e r s i o n of c a l l i n g f r o g s , comparing the observed d i s t r i b u t i o n of f r o g s to a random d i s t r i b u t i o n . I a l s o t e s t e d the hypothesis i n d i r e c t l y by means of an a d d i t i o n experiment, i n which equal numbers of f r o g s were added to an occupied (by c a l l i n g frogs) area and an unoccupied area. I p r e d i c t e d that i f the presence of c a l l i n g f r o g s i n an area prevents o t h e r s from c a l l i n g t h e r e , more of the added f r o g s would c a l l i n the p r e v i o u s l y unoccupied a r e a . 18 Methods Both the nearest-neighbor analysis and the addition experiment were done at Marion Lake i n f i e l d enclosures. The enclosures measured approximately 3.2 m by 3.2 m with walls constructed of polyethylene i n a manner such that frogs could not escape. leasestzliexahbor_ana 1 j s i s : I introduced 12 male frogs to an enclosure, allowed them approximately 24 hr to s e t t l e , then determined, with the aid of a gri d placed over the enclosure, the position of each c a l l i n g frog to the nearest 10 cm for both the X and Y coordinates. The positions of c a l l i n g frogs for f i v e separate introductions were determined, the number c a l l i n g being, respectively, 5, 10, 8, 6, and 6. The distance to nearest c a l l i n g neighbor was calculated for each frog, yieldin g a t o t a l of 35 nearest neighbor distances. This observed d i s t r i b u t i o n of nearest-neighbor distances was compared with a simulated expected d i s t r i b u t i o n obtained by allowing "frogs" to s e t t l e at random on the 56 d i f f e r e n t c a l l i n g s i t e s used during the period of observations. These 56 s i t e s include those which were used during subsequent observations of several of the f i v e groups of frogs (these observations were not included i n the spacing analysis because of possible non-independence from the f i r s t observation for each group). The only other r e s t r i c t i o n placed on where frogs could s e t t l e was that they could not s e t t l e on any s i t e 19 already occupied by another frog. The simulation was done 10 times, using a random numbers table, for each of the f i v e observed group s i z e s . The sum of the average d i s t r i b u t i o n s of nearest neighbor distances for the f i v e groups was compared with the observed d i s t r i b u t i o n . j^Mfeig" exE§Iiinen-t: Two f i e l d enclosures were used. A group of c a l l i n g frogs was established i n the experimental enclosure, then equal numbers of frogs were added to both enclosures. Table 1 describes i n d e t a i l the chronology of the experimental procedures. For purposes of analysis, the number of frogs already c a l l i n g in the experimental enclosure before addition of frogs to both enclosures was defined as the maximum number observed c a l l i n g on nights two and three; the number of frogs c a l l i n g a f t e r the addition i n each enclosure was the maximum number c a l l i n g on nights four and f i v e . The experiment was run three times, the roles of the two enclosures alternating each time. The main reason for doing the experiment i n enclosures was to ensure high densities of c a l l i n g frogs. Because of the frequent and substantial movements of frogs, the densities in unenclosed areas vary widely i n time, making i t d i f f i c u l t to obtain s u f f i c i e n t and consistent experimental r e s u l t s . For a l l three t r i a l s , the absolute density in the experimental enclosure (10, 8, 6/ca. 10 m2, respectively) was equal to or greater than the maximum observed outside. However, I did not 20 Table 1, Chronology of the addition experiment. Ma n ipu 1 a t i o n s a nd 0bserya t ign§ Night Experimental Enclosure Contro1_Enclosure 1 Add 12 frogs 2 Count number of c a l l i n g frogs 3 Count number of c a l l i n g frogs Add 8 frogs Add 8 frogs 4 Count number Count number of c a l l i n g frogs of c a l l i n g frogs 5 Count number count number of c a l l i n g frogs of c a l l i n g frogs consider that t h i s would in v a l i d a t e the r e s u l t s ; I reasoned that i f c a l l i n g frogs i n an area i n h i b i t others from c a l l i n g there, t h i s e f f e c t w i l l simply be greater, not q u a l i t a t i v e l y d i f f e r e n t , when the densities are higher. Hesults_and^Discussign Nearest neighbor analysis: Figure 4 shows that c a l l i n g males space out more than, i f distributed randomly; no frogs c a l l e d closer together than 40 cm, and a spacing effect i s noticeable up to 50 cm. Addition experiment: Fewer frogs c a l l e d after being added to the enclosure already occupied by c a l l i n g frogs than c a l l e d 21 Figure 4. Observed (shaded) and expected distributions of nearest neighbor distances of calling frogs. The difference i s significant, p<0.01 (Kolmogorov-Smirnov test). 21a 12-T 1 l l V I I I I I I I 1 1 1 1 I r i 1 i 1 i I * i 2 0 4 0 6 0 8 0 100 120 140 160 180 2 0 0 2 2 0 240 2 6 0 D i s t a n c e t o N e a r e s t N e i g h b o r ( c m ) 22 aft e r being added to the empty enclosure (Table 2). Most of Table 2. Numbers of frogs c a l l i n g i n the experimental and control enclosures. Number C a l l i n g Experimental,,Enclosure Contrgl_ Enclosure Before after ( A f t e r - 1 Before After ( A f t e r - 1 T r i a l Addition Addition Before) Addition Addition Before) 1 10 13 3 0 7 7 2 8 11 3 0 6 6 2 6 7 1 0 5 5 Total 24 31 7 0 18 18 ^ s i g n i f i c a n t l y d i f f e r e n t ^ p<0.001 (t=18.7) the frogs which did not c a l l were observed on or near the walls of the enclosures, evidently attempting to escape. These r e s u l t s are not simply a consequence of having more frogs than c a l l i n g s i t e s i n the experimental enclosure; at any one time, the number of possible c a l l i n g s i t e s (as judged by the number of d i f f e r e n t s i t e s used during the entire experimental period) far exceeded the number of c a l l i n g frogs. The r e s u l t s of both the nearest-neighbor analysis and the addition experiment show that frogs tend not to c a l l close together. The re s u l t s of the addition experiment are 23 p a r t i c u l a r l y i n t e r e s t i n g i n showing that the presence of c a l l i n g f r o g s i n an area can i n h i b i t c a l l i n g i n other f r o g s c o n f i n e d to the area. In nature, f r o g s presumably avoid areas having high d e n s i t i e s of c a l l i n g i n d i v i d u a l s , and s e t t l e elsewhere u n l e s s a l l p o s s i b l e c a l l i n g areas are " f i l l e d up." There i s no evidence t h a t t h i s i s the case a t Marion Lake. As mentioned above, the d e n s i t i e s i n s m a l l areas f l u c t u a t e d g r e a t l y i n time, many times f o r no apparent reason; t h i s should not have o c c u r r e d i f spacing behavior was l i m i t i n g the p o p u l a t i o n . A l s o , many areas which appeared to have s u i t a b l e c a l l i n g s t a t i o n s never had f r o g s . T h e r e f o r e , spacing behavior probably does not l i m i t the p o p u l a t i o n of c a l l i n g males i n the lake as a whole, although i t q u i t e l i k e l y l i m i t s d e n s i t i e s i n s m a l l areas. The r o l e of v o c a l i z a t i o n s i n spacing out can not be deduced from these r e s u l t s ; that s u b j e c t w i l l be taken up i n the f o l l o w i n g s e c t i o n s . Observations of f r o g s both i n the en c l o s u r e s and at l a r g e i n the lake did r e v e a l another means of m aintaining s p a c i n g , however. On s e v e r a l o c c a s i o n s I saw a n o n - c a l l i n g male swim near a D - c a l l i n g male, stop, then begin D - c a l l i n g . The r e s i d e n t f r o g would u s u a l l y begin S - c a l l i n g , f o l l o w e d immediately by the i n t r u d e r . I f one of the f r o g s d i d not soon r e t r e a t , one or both would begin a d i s t i n c t i v e "bouncing" behavior which a t f i r s t might not be o r i e n t e d toward the other f r o g . The f r o g s would soon begin to bounce toward each o t h e r , making p h y s i c a l c o n t a c t with bouncing blows 24 of v o c a l sac a g a i n s t v o c a l sac. T h i s behavior might be f o l l o w e d by the f r o g s t r y i n g to c l a s p each other, a l l the while S - c a l l i n g and k i c k i n g s t r o n g l y . U s u a l l y the f i g h t would end a f t e r one f r o g c l a s p e d the o t h e r ( u s u a l l y a n t e r i o r to the f r o n t l e g s and around the v o c a l s a c ) , d e f l a t i n g i t . The d e f l a t e d , defeated f r o g would then swim away. B, ROLE OF THE D CALL IN SPACING OUT In a d d i t i o n to i t s known f u n c t i o n i n a t t r a c t i n g females (Snyder and Jameson 1965), the D c a l l may f u n c t i o n i n m a i n t a i n i n g s p a c i n g between males. A s i m i l a r dual r o l e has long been p o s t u l a t e d f o r b i r d song, and P a i l l e t t e ' s (1970) r e s u l t s suggest that the mating c a l l s of two other H y l i d s serve both f u n c t i o n s . In t h i s s e c t i o n , I d e s c r i b e an experiment designed to t e s t whether the D c a l l p l a y s a r o l e i n spacing of males. One method f o r s t u d y i n g the r o l e of v o c a l i z a t i o n s i n s p a c i n g out i s t o mute the experimental animals, as Peeke (1970) d i d i n h i s study of Red-winged B l a c k b i r d s . However, i f the study animal has more than one kind of v o c a l i z a t i o n , using t h i s method, one can only i n f e r the r o l e of s p e c i f i c v o c a l i z a t i o n s from i n d i r e c t evidence. A more d i r e c t approach i s to observe whether a p a r t i c u l a r v o c a l i z a t i o n , played back through a loudspeaker, i n h i b i t s animals from c a l l i n g near the speaker. T h i s approach was taken to examine the r o l e of the D 25 c a l l i n spacing out. An a d d i t i o n experiment, s i m i l a r i n r a t i o n a l e to the one d e s c r i b e d above, was designed to answer two q u e s t i o n s : "Do f r o g s space out from loudspeakers p l a y i n g back D c a l l s ? " and "Does the presence of D c a l l s i n an area i n h i b i t f r o g s c o n f i n e d to the area from c a l l i n g ? " Experimental procedures (Table 3) were b a s i c a l l y the same as f o r the f i r s t a d d i t i o n experiment, except that t h i s time the experimental e n c l o s u r e was occupied only by f o u r loudspeakers p l a y i n g back D c a l l s . Methods Table 3. loudspeakers. Chronology of a d d i t i o n experiment using M a nipulations and Observations Niqht Experimental E n c l o s u r e C o n t r o l E n c l o s u r e 1 Turn on loudspeakers Add 12-13 f r o g s Add 12-13 f r o g s 2 Count number of c a l l i n g f r o g s Count number of c a l l i n g f r o g s A minor d i f f e r e n c e i n procedure was t h a t I counted the number of c a l l i n g f r o g s only on the f i r s t n i g h t a f t e r the \ 26 addition, because I had found no difference i n the number c a l l i n g on the f i r s t and second nights aft e r addition i n the f i r s t addition experiment. The arrangement of loudspeakers (Altec 405A) in the enclosure i s shown in Figure 5. Each speaker played back D c a l l s at approximately the rate at which they were o r i g i n a l l y recorded (30 calls/min; water=15 C; air=10 C). This was accomplished with a loop tape containing nine i d e n t i c a l D c a l l s , spaced to play back at 120 calls/min. The loop tape was played on a single tape recorder, connected to a stepping relay which turned each speaker on f o r only every fourth c a l l . Thus, the speakers played back c a l l s seguentially, corresponding to the antiphonal c a l l i n g of r e a l frogs (although in a chorus of four frogs, frequent overlapping of c a l l s would occur). The i n t e n s i t y of the c a l l s (92 db 50 cm i n front of the speaker) was within the range of natural c a l l i n g i n t e n s i t i e s . Since frogs t y p i c a l l y c a l l in bouts several minutes long, then are s i l e n t for several minutes before c a l l i n g again, a variable on-off timer was used to set the amount of c a l l i n g approximately equal to that of the frogs i n the lake. The speakers were placed about 4 cm above the surface of the water on wooden stands. The stands remained i n both enclosures for a l l t r i a l s of the experiment, but I alternated the speakers between the enclosures. For the spacing analysis, I computed the distance of each c a l l i n g frog from the nearest speaker stand and compared the d i s t r i b u t i o n obtained f o r frogs i n the experimental (speakers 27 Figure 5. The arrangement of loudspeakers i n the experimental enclosure for the addition experiment. Each speaker faced a corner of the 2 enclosure. The 1 m area i n the middle of the enclosure was fenced to keep frogs out because, unlike the calls of real fro.gs, the sound from the loudspeakers was unidirectional. w loudspeaker ~— enclosure wall 28 playing) enclosure with that of frogs in the control enclosure. Results and Discussion Frogs spaced out more from the speaker stations when the loudspeakers were playing back D c a l l s , but the difference was not s i g n i f i c a n t (Fig. 6 ) . However, i n a l l six t r i a l s , more frogs c a l l e d i n the control enclosure than in the experimental (Table 4) ; t h i s difference was s i g n i f i c a n t . Table 4, Number of frogs c a l l i n g in the experimental (occupied by four loudspeakers) and control enclosures. Number Ca l l i n g T r i a l 1?P§£imental_ Enclosure* Control^Enclosure * 1 8 9 2 6 7 3 3 5 4 7 8 5 8 10 6 9 11 Total 41 5 0 ^ S i g n i f i c a n t l y d i f f e r e n t , p<07001 (t=12.5J I 29 Figure 6. Spacing of frogs from loudspeakers when speakers are playing back D calls (shaded) compared with when no speakers are present. The distributions are not significantly different, 0.20>*p>0.10 (Kolmogorov-Smimov test) . 29a 20 -20 40 60 80 100 120 140 160 Distance to Nearest Speaker Station (cm) 30 T h e s e l a t t e r r e s u l t s show t h a t D c a l l s do f u n c t i o n i n m a i n t a i n i n g s p a c i n g ; t h e p r e s e n c e o f D c a l l s i n an a r e a , w i t h o u t any o t h e r b e h a v i o r a l m e c h a n i s m s , s u c h a s f i g h t i n g , t h a t f r o g s p o s s e s s f o r m a i n t a i n i n g s p a c i n g , c a n i n h i b i t f r o g s f r o m c a l l i n g t h e r e . T h u s , a s w e l l a s a t t r a c t i n g f e m a l e s , D c a l l s p l a y a r o l e i n m a l e - m a l e c o m m u n i c a t i o n . C. BOLE OF THE S CALL IN SPACING OUT S n y d e r and Jameson (1965) s u g g e s t e d t h a t t h e S c a l l i s a t e r r i t o r i a l v o c a l i z a t i o n ; my own f i e l d o b s e r v a t i o n s a g r e e w i t h t h i s i n t e r p r e t a t i o n . Many t i m e s I o b s e r v e d D - c a l l i n g m a l e s s h i f t t o S - c a l l i n g when a n o t h e r male began c a l l i n g n e a r b y ( l e s s t h a n c a . 50 cm). The u p s h o t was u s u a l l y t h a t one o f t h e m a l e s w o u l d l e a v e t h e a r e a o r t h a t t h e m a l e s w o u l d f i g h t , a f t e r w h i c h one w o u l d l e a v e . I n t h i s s e c t i o n , I e x a m i n e t h e r o l e o f t h e S c a l l i n s p a c i n g o f m a l e s . M j e a r e s t - n e i g h b o r distances_gf__S-,.,,and -D2calling_ m a l e s : To t e s t t h e h y p o t h e s i s t h a t t h e S c a l l i s g i v e n i n r e s p o n s e t o t h e a p p r o a c h o f o t h e r m a l e s , I d i d a s e r i e s o f s y s t e m a t i c f i e l d o b s e r v a t i o n s . The m ethods were t o f i r s t m e a s ure t h e d i s t a n c e t o t h e n e a r e s t male n e i g h b o r o f any S - c a l l i n g male e n c o u n t e r e d , t h e n move 5 m i n a random d i r e c t i o n , s e l e c t t h e n e a r e s t D - c a l l i n g m a l e , and m e asure t h e d i s t a n c e t o i t s n e a r e s t m a l e n e i g h b o r . The r e s u l t s s u p p o r t t h e h y p o t h e s i s ( T a b l e 5 ) . H owever, t h e r e s u l t s by no means e x c l u d e t h e 31 Table 5. Mean distance (cm) to nearest male neighbor of S- c a l l i n g frogs and D-calling frogs. Table includes data for 10 pairs of frogs. S-Ca11in^_Frogs D-Calling_Frogs 36 125 S i g n i f i c a n t l y d i f f e r e n t , p<0.001 (Hilcoxon test) p o s s i b i l i t y that males also S c a l l in response to nearby females; i n the type of observations made, one would not be l i k e l y to observe this since, at any one time on the breeding grounds, males greatly outnumber females. Voca 1_reSJDOnse_to_mgdels: To examine the p o s s i b i l i t y that males also s h i f t to S-cal l i n g in response to nearby females, I made two plast e r - o f - p a r i s models of frogs, painted to resemble r e a l frogs. One model had an i n f l a t e d vocal sac (to resemble a male); the other resembled either a female or a "deflatedV male (females d i f f e r very l i t t l e from deflated males, the major difference being the l i g h t e r color of females under the lower jaw). Each model, hung from a wire fastened to the end of a pole, was placed on the water surface 30 cm in front of a D-calling frog, then moved slowly toward the frog,. No frogs responded with S c a l l s to presentation of either model (N=10 for each model); instead, a l l frogs responded with the M c a l l . .These r e s u l t s agree with f i e l d observations of frogs giving 32 the M c a l l when either deflated, non-calling males or females approached them. Thus, v i s u a l detection of nearby frogs, even i n f l a t e d males, does not appear to e l i c i t S - c a l l i n g , although the p o s s i b i l i t y remains that the i n f l a t e d model was not r e a l i s t i c enough to e l i c i t the response. y°g,aJ:^£g§E9I]§g-to Elaybgg]^,, of D,calls: Snyder and Jameson (1965) found that D-calling males respond with S c a l l s to playback of D c a l l s near them. I repeated t h i s experiment by playing back D c a l l s 40 cm behind (to avoid v i s u a l disturbance) D-calling males at a volume within the range of natural c a l l i n t e n s i t i e s . For controls, the loudspeakers was s i l e n t . A response was defined as one or more D c a l l s made during the 1 min presentation period. My r e s u l t s (Table 6) are s i m i l a r to Snyder and Jameson's, Table 6. Vocal response of D-calling males to playback of D c a l l s 40 cm behind them. Playback Change tp_S-Calling Hot Change to S-Calling D c a l l s 9 1 None 0 10 S i g n i f i c a n t l y d i f f e r e n t , p<0,005 (Fisher's test) 33 These r e s u l t s i n d i c a t e that the S c a l l i s given only i n response to the c a l l i n g of nearby males. I t seems reasonable to argue that i t f u n c t i o n s i n spacing out, and that i t s f u n c t i o n i s not i d e n t i c a l with t h a t of the D c a l l . O b s e r v ations t h a t S - c a l l i n g "bouts" between two males u s u a l l y l e a d to r e t r e a t by one of the males or to f i g h t i n g suggest t h a t the S c a l l i s a " s t r o n g e r warning" to nearby males than the D c a l l . O p e r a t i o n a l l y , I d e f i n e " s t r o n g e r warning" as a s i g n a l which communicates more imminent h i g h e r - l e v e l a g g r e s s i o n than a weaker warning. In t h i s case, the h i g h e r - l e v e l a g g r e s s i o n would be e i t h e r to r e t r e a t or to a t t a c k , depending upon the i n t e r n a l s t a t e of the r e c e i v e r . Aggnis -ti ic_resppnses t g m p l a y b a c k of ,,S_calls_and - O x g a l l s : To t e s t the hypothesis that the S c a l l i s a s t r o n g e r warning than the D c a l l , I d i d a playback experiment. The g e n e r a l procedure was to playback e i t h e r S c a l l s or D c a l l s 40 cm behind D - c a l l i n g males, and r e c o r d the l a t e n c y of each f r o g to " a t t a c k " or " r e t r e a t " d u r i n g the 1 min p r e s e n t a t i o n p e r i o d . Attack was d e f i n e d as the d i s t i n c t i v e "bouncing" behavior that f r o g s show before f i g h t s ; r e t r e a t , as e i t h e r moving 30 cm or more away from the loudspeaker or c e a s i n g c a l l i n g f o r the r e s t of the p r e s e n t a t i o n p e r i o d . The experiment was done with p a i r e d playbacks of D c a l l s and S c a l l s , each to d i f f e r e n t f r o g . The order of p r e s e n t a t i o n was a l t e r n a t e d each time to avo i d confounding by the p o s s i b l e e f f e c t s of such v a r i a b l e s as time of n i g h t and temperature, and by the e f f e c t of playback 34 on nearby f r o g s (sometimes experimental animals were c l o s e t o g e t h e r ) . To ensure that the c a l l s were played back at r a t e s and i n t e n s i t i e s s i m i l a r t o ' those of r e a l f r o g s , I d i d a p r e l i m i n a r y playback experiment i n which f i r s t the D c a l l s of a f r o g were recorded f o r approximately 15 sec, then S c a l l s , e l i c i t e d by p l a y i n g back on another tape r e c o r d e r a 1 min r e c o r d i n g of the f r o g D - c a l l i n g , were recorded f o r 1 min. R e l a t i v e c a l l i n t e n s i t i e s of D c a l l s and S c a l l s were measured by playback from a tape r e c o r d e r with the average i n t e n s i t y of D c a l l s f o r each f r o g s et as c l o s e l y as p o s s i b l e t o 90 db 50 cm away from the loudspeaker (about average f o r f r o g s ) . The average c a l l r a t e s and r e l a t i v e i n t e n s i t i e s are shown i n Table 7. Table 7. Rates of D - c a l l i n g and S - c a l l i n g and r e l a t i v e i n t e n s i t i e s of the c a l l s . D C a l l s S C a l l s Rate (n=8) 41/60 sec 19/60 sec I n t e n s i t y (n=6) 90.3 db 88.1 db An S c a l l and a D c a l l from one i n d i v i d u a l were recorded 35 onto loop tapes and played back at these r e s p e c t i v e r a t e s and i n t e n s i t i e s . Most f r o g s f i r s t responded to playback of e i t h e r S c a l l s or D c a l l s by changing to S - c a l l i n g , with more responding i n t h i s way to S c a l l s (92%) than to C c a l l s {62%). F u r t h e r , more f r o g s a t t a c k e d or r e t r e a t e d i n response to S c a l l s (Table 8). T able 8. A g o n i s t i c responses of males to 1 min playback of D c a l l s and S c a l l s . Number A t t a c k i n g Number not A t t a c k i n g C a l l , P l a y e d Back or R e t r e a t i n g or R e t r e a t i n g D c a l l 0 13 S c a l l . 7 1 6 S i g n i f i c a n t l y d i f f e r e n t , p<0.005 ( F i s h e r ' s t e s t ) ^Includes four r e t r e a t s and three a t t a c k s Looking at a t t a c k s and r e t r e a t s s e p a r a t e l y , s i g n i f i c a n t l y more f r o g s r e t r e a t e d i n response to playback of S c a l l s (p<0.05), but the d i f f e r e n c e i n number of a t t a c k s was not s i g n i f i c a n t (0.20>p>0.10). These r e s u l t s c l e a r l y show t h a t f r o g s respond d i f f e r e n t l y to S c a l l s and D c a l l s . They suggest that the S c a l l i s a 3 6 s t r o n g e r warning to nearby males, but t h i s i n t e r p r e t a t i o n i s not c o n c l u s i v e s i n c e f r o g s d i d not show s i g n i f i c a n t l y more of both a t t a c k i n g and r e t r e a t i n g i n response to S c a l l s . An a c c e p t a b l e a l t e r n a t i v e hupothesis i s t h a t the S c a l l e l i c i t s more "submissive" behavior than the D c a l l , as P a i l l e t t e (1970) r e p o r t e d f o r the e q u i v a l e n t c a l l s of Hjrla arboreal and H. m e r i d i o n a l i s . However, the f a c t t h a t s e v e r a l f r o g s d i d a t t a c k i n response to S c a l l s and none d i d i n response to D c a l l s suggests t h a t i f more data were c o l l e c t e d my o r i g i n a l h y p o thesis would be upheld. D. MOVEMENTS OF MALES ON THE BREEDING GROUNDS In many s p e c i e s , most notabl y b i r d s , i n d i v i d u a l s defend f i x e d areas f o r long p e r i o d s of time (e.g. f o r an e n t i r e breeding season) . I t i s not s u r p r i s i n g t h a t birds': t e r r i t o r i e s are g e o g r a p h i c a l l y f i x e d s i n c e they are o f t e n a s s o c i a t e d with nest s i t e s . Some f r o g s defend nests (see e.g. Lutz 1960), but i n other s p e c i e s , i n c l u d i n g ( P a c i f i c Tree Frogs, there i s no obvious advantage to defending f i x e d areas. N e v e r t h e l e s s , Jameson (1957) found i n a 16-day mark-recapture study d u r i n g the P a c i f i c Tree Frog breeding season t h a t males moved very l i t t l e , most i n d i v i d u a l s only a few i n c h e s . To e v a l u a t e the g e n e r a l i t y of Jameson's r e s u l t s , I d i d a mark-recapture study on a s m a l l pond i n Vancouver. I 37 Methods Males were captured on 15 d i f f e r e n t nights from 17 March to 26 A p r i l from a roughly triangular area (with sides approximately 25 m and 20 m) i n the pond. Only males which were c a l l i n g or which had their vocal sacs i n f l a t e d were captured. The position of each capture s i t e was determined to the nearest 10 cm for both the X and Y coordinates. Frogs were replaced at t h e i r capture s i t e s and any movements occurring during the following 60 sec were recorded. Besults_anfl Discussion The frogs made substantial movements: for 19 individuals captured a t o t a l of 68 times, the mean distance between successive capture s i t e s was 4.6 m (range=0-19.4 m; mean time between successive captures=5.7 days). Data for 8 additional frogs captured a t o t a l of 13 times indicate that the movements can not be attributed to my disturbance. I observed these frogs resume c a l l i n g at t h e i r capture s i t e s after replacement, yet when next captured, they had moved an average of 4.6 m (range=0-11.6 m; mean time between successive captures=5.5 days). Nor did any obvious change in the use of any part of the c a l l i n g area occur which" could account for the movements by i n d i v i d u a l s . On the basis of a small amount of mark-recapture data, the movement behavior of Marion Lake frogs seems to be similar 38 to t h a t of Vancouver f r o g s . E. SUMMARY AND DISCUSSION Male P a c i f i c Tree Frogs space out on the breeding grounds, using v o c a l i z a t i o n s and p h y s i c a l combat to maintain s p a c i n g . The s p e c i e s ' mating c a l l (D c a l l ) , as w e l l as a t t r a c t i n g females, f u n c t i o n s i n maintaining s p a c i n g ; the presence of D c a l l s alone i n an area can prevent males from c a l l i n g t h e r e . Another v o c a l i z a t i o n , the S c a l l , which i s given only when another male begins c a l l i n g nearby, may act as a s t r o n g e r warning than the D c a l l ; males are more l i k e l y to r e t r e a t and perhaps a l s o to a t t a c k when the S c a l l i s played back near them. The defended areas of i n d i v i d u a l f r o g s are not g e o g r a p h i c a l l y f i x e d ; a mark-recapture study showed that f r o g s make e x t e n s i v e movements during the breeding season. 1. O r g a n i z a t i o n of spacing behavior i n P a c i f i c Tree Frogs The spacing behavior of P a c i f i c Tree Frogs seems, i n g e n e r a l , to f i t a " t h r e e - l e v e l " system of t e r r i t o r i a l defense proposed by Peeke (1972) f o r Red-winged B l a c k b i r d s . Peeke's three l e v e l s are 1) " a d v e r t i s i n g song which f u n c t i o n s p r i m a r i l y as a long-range warning system to r e p e l p o t e n t i a l 39 t r e s p a s s e r s a t a d i s t a n c e " ; 2) v i s u a l d i s p l a y s "which f u n c t i o n at i n t e r m e d i a t e ranges to discourage a c t u a l t r e s p a s s e r s or neighboring males at t e r r i t o r i a l boundaries"; and 3) "chase and a t t a c k . " The D c a l l f u n c t i o n s i n a manner analagous to the Red-wing's a d v e r t i s i n g c a l l i n a d v e r t i s i n g the s p e c i e s , sex, and l o c a t i o n of the c a l l e r . Indeed, i t should be e s p e c i a l l y adapted f o r these f u n c t i o n s s i n c e females use i t i n l o c a t i n g mates (Snyder and Jameson 1965). A f r o g ' s second l e v e l of defense i s the S c a l l (an a c o u s t i c s i g n a l r a t h e r than v i s u a l as i n Red-wings) which i s given o n l y when another i n d i v i d u a l begins c a l l i n g nearby. T h i s c a l l seems to be a " s t r o n g e r warning" than the D c a l l , warning the other f r o g of imminent " t h i r d l e v e l " defense p h y s i c a l a t t a c k . 2. D e f i n i t i o n of t e r r i t o r i a l i t y i n anurans According t o many d e f i n i t i o n s , to be considered t e r r i t o r i a l , animals must show s i t e attachment; t h a t i s , they must remain i n "home a r e a s . " The meaning attached to "home area" (or i t s e q u i v a l e n t s ) by v a r i o u s authors, although never s t a t e d e x p l i c i t l y , seems to be: "A f i x e d geographic area occupied f o r a r e l a t i v e l y long p e r i o d of time." In b i r d s , the group i n which t e r r i t o r i a l i t y has been most s t u d i e d , t h i s d e f i n i t i o n presents few problems s i n c e , i n most cases, the nest p r o v i d e s an obvious s t a t i o n a r y f o c a l p o i n t f o r the t e r r i t o r y . However, when reasons f o r defending f i x e d s i t e s no are obscure, as i n many anurans, two d i f f i c u l t i e s a r i s e : F i r s t , as mentioned i n the i n t r o d u c t i o n , t h e time requirement of the d e f i n i t i o n makes i t s a p p l i c a t i o n a r b i t r a r y . How long i s long enough? Are P a c i f i c Tree Frogs i n some p l a c e s (e.g. Jameson's (1957) study area) t e r r i t o r i a l , but i n o t h e r s (e.g. my study areas) not? Second, the f a c t that an animal remains a t a p a r t i c u l a r s i t e does not n e c e s s a r i l y imply that i t has a p o s i t i v e "attachment" f o r the s i t e ; i n s t e a d , remaining at the s i t e may simply be a b e t t e r s t r a t e g y than expending the energy to move. For anurans, i t might be o p e r a t i o n a l l y more u s e f u l to d e f i n e " s i t e attachment" as: "The tendency of e x p e r i m e n t a l l y d i s p l a c e d (to a d i s t a n c e s e v e r a l times g r e a t e r than the d i s t a n c e t h a t i n d i v i d u a l s space out) animals to r e t u r n to t h e i r o r i g i n a l capture s i t e s " ; and t h e r e f o r e , "home are a " as: "The area to which d i s p l a c e d animals r e t u r n . " With "home ar e a " d e f i n e d i n t h i s way, I accept Krebs' (1971 p. 4) d e f i n i t i o n of t e r r i t o r i a l i t y as " a l l cases of spacing out of i n d i v i d u a l ' s home ar e a s . " The r e s u l t s of the mark-recapture study i n d i r e c t l y suggest t h a t Vancouver and Marion Lake P a c i f i c Tree Frogs do not show s i t e attachment a c c o r d i n g to my d e f i n i t i o n . I a l s o have a n e c d o t a l evidence t h a t f r o g s r e l e a s e d s e v e r a l meters away from t h e i r capture s i t e s d i d not r e t u r n any more o f t e n than expected on a random b a s i s . T h e r e f o r e , I would 41 tent a t i v e l y argue that P a c i f i c Tree Frogs are not t e r r i t o r i a l . Their spacing behavior can perhaps be best described simply as "i n d i v i d u a l distance" (Condor 1949), with frogs using the c a l l i n t e n s i t y of neighbors as a cue i n determining distance. 3. The function of spacing out If the function of spacing out in P a c i f i c Tree Frogs i s to protect certain resources, such as food or spawning s i t e s , one would expect i n d i v i d u a l s to show s i t e attachment. As well as the above-mentioned evidence that they do not, other l i n e s of evidence argue against these functions. Males space out only during the generally nocturnal periods of c a l l i n g , yet they evidently feed mostly during the day (I only once observed a male attempt to capture a prey item at night). With regard to defense of spawning s i t e s , observations of amplectant pairs suggest that females do not spawn i n the areas defended by their mates; instead, they lay the i r eggs i n a number of small, well-dispersed masses. Spacing out might be accounted for by natural selection i f i t reduced an in d i v i d u a l ' s chance of being taken by a predator of the type which reacts to the discovery of a prey item by inte n s i v e l y searching the area f o r other prey (Tinbergen et a l . 1967). Potential nocturnal predators of frogs on.my study areas include Raccoons (Procvon l o t o r ) , Striped Skunks (Mephitis mephitis), and owls (especially Otus 42 agio). Raccoons and skunks, e s p e c i a l l y , may practice "area r e s t r i c t e d searching", but i t seems unlikely that t h i s would select for spacing out i n frogs. The disturbance of capturing a frog would cause nearby frogs to dive to the bottom of the pond, where they would probably be either inaccessible to or overlooked by the predator. Thus, spacing out may not be necessary as an anti-predator mechanism in something as mobile as frogs. Spacing out i s more e a s i l y explained by sexual selection. One obvious way that i t could increase a male's chances of mating i s by reducing the interference to approaching females by neighboring males. Yet t h i s does not seem to account for the entire area defended. Observations indicate that females must touch or nearly touch males before the males leave t h e i r c a l l i n g stations to clasp. If t h i s i s so, why i s i t necessary to space out 50 cm or more? The answer may be that females can more e a s i l y locate males which are spaced-out. In my own attempts to locate i n d i v i d u a l males, I found i t easiest when densities were low; Pierce and Ralin (1972) had the same experience with several other species of HyJLa and suggested a si m i l a r hypothesis for the function of spacing out. This hypothesis i s e a s i l y testable only to the extent of determining whether females select males which are more spaced-out over those which are less spaced-out; to elucidate the causation of the response would be much more d i f f i c u l t . 43 CHAPTER III THE ROLE OF VOCALIZATIONS IN KATE SELECTION This chapter reports on investigations of the method of mate selection i n P a c i f i c Tree Frogs. Two hypotheses were examined: F i r s t , females select large males on the basis of their' low-pitched c a l l s , and second, females choose chorus leaders, frogs which i n i t i a t e bouts of c a l l i n g . A. CALL PITCH AS A CUE IN HATE SELECTION For females to select large males by cueing in to low-pitched vocalizations requires that c a l l pitch be inversely related to body s i z e . Snyder and Jameson (1965) found t h i s to be the case with P a c i f i c Tree Frogs, and my own data are in accordance with t h e i r r e s u l t s : I recorded the D c a l l s of 40 Marion Lake males and determined the fundamental frequency and dominant freguency of one c a l l of each frog. Figure 7 shows that both frequencies decrease with increasing snout-vent length (r=-0.61, p<0.001; r=-0.60, p<0.001, respectively). If the c a l l pitch hypothesis i s v a l i d , mated males should 1) be larger than average and 2) have low-pitched voices. F i e l d data were collected to test these two predictions. 44 Figure 7. Relationship of dominant frequency and fundamental frequency (of approximately the sixth pulse of the f i r s t phase) of D calls to body length. 44a C o o CD 2 4 0 0 -2 2 0 0 -2 0 0 0 -1 8 0 0 H D O M I N A N T F R E Q U E N C Y Y = - 5 2 . 1 1 X + 3 9 1 3 N = 4 0 CD 1 6 0 0 -Q. CO CD O * 1 4 0 0 -F U N D A M E N T A L F R E Q U E N C Y 1 2 0 0 i Y = - 3 2 . 4 8 X + 2 1 1 2 N = 4 0 1 0 0 0 -8 0 0 -"~r 1 1 1 1 1 1 1 1 1 1— 3 1 3 3 3 5 3 7 3 9 41 4 3 S n o u t - V e n t L e n g t h ( m m ) 45 Methods Sizes o f j i a t e d males: The d i s t r i b u t i o n s of snout-vent lengths of a l l males found in amplexus at Marion Lake i n 1972 and 1973 were compared with the respective length d i s t r i b u t i o n s of males captured from the c a l l i n g population i n the two years. The samples of c a l l i n g males represent a l l of the frogs captured (indiscriminately with respect to size),and marked on the study area i n each year. C a l l pitches of mated males: The c a l l pitches of males found i n amplexus at Marion Lake i n 1972 were compared with those of a sample of males from the c a l l i n g population. Amplectant males were separated from females, placed in f i e l d enclosures, and recorded i f they subsequently c a l l e d . The sample of c a l l s from males in the c a l l i n g population was collected by recording c a l l i n g frogs before capturing them f o r various experiments. Since, for a l l frogs recorded at Marion Lake i n 1972, both fundamental frequency and dominant frequency are p o s i t i v e l y correlated with water temperature (r=0.14, 0.4>p>0.2; r=0.31, p<0.01, resp e c t i v e l y ) , a l l frequencies were corrected to standard water temperature according to the relationships shown in Figure 8 (fundamental and dominant frequency are also correlated with a i r temperature (r=0.08, p>0.5; r=0.21, 0.1>p>0.05, respe c t i v e l y ) , but since these correlations are weaker, and since a i r temperature i s correlated with water temperature (r=0.62. 46 p<0.001), I c o r r e c t e d only f o r water temperature). l g s u l t s _ a n d _ D i s c u s s i o n Size§^of_mated_malgs: Amplectant males were s l i g h t l y l a r g e r than c a l l i n g males i n 1972, and s l i g h t l y s m a l l e r i n 1973 (Table 9) ; i n n e i t h e r case was the d i f f e r e n c e s i g n i f i c a n t . Table 9. Snout-vent l e n g t h s (mm) of males found i n amplexus compared with c a l l i n g males i n 1972 and 1973. C a l l i n g Males Am£lectant Males Year N Mean Length N Mgan_Length 1972 185 37.281 22 37.641 1973 132 38.442 9 37.91 2 *Not s i g n i f i c a n t l y d i f f e r e n t , 0.50>p>0.40 (t=0.70) 2Not s i g n i f i c a n t l y d i f f e r e n t , 0.50>p>0.40 (t=0.73) C a l l p i t c h e s of mated males: C a l l p i t c h e s of amplectant males were not lower (Table 10); i n f a c t , even though the average s i z e of the amplectant males whose c a l l s were recorded (38.17 mm) was s l i g h t l y g r e a t e r than t h a t of the c a l l i n g males (37.15 mm), fundamental frequency of amplectant males was s i g n i f i c a n t l y higher (p<0.01), and dominant frequency was higher but not s i g n i f i c a n t l y (0.4>p>0.2). T h i s was p a r t l y the 47 Figure 8. Relationship of dominant frequency and fundamental frequency to water temperature. These relationships were obtained by regressing 2 Y on X and on X + X and choosing the equation which minimized the residual sum of squares. 47a 2 4 0 0 A D O M I N A N T F R E Q U E N C Y Y = 7 7 . 1 4 X - 2 . 4 3 X 2 + 1 4 1 5 N = 6 4 2 2 0 0 H © © •o c o o GO CD a co 2 0 0 0 1 1 8 0 0 1 6 0 0 H £ 1 4 0 0 O F U N D A M E N T A L F R E Q U E N C Y Y = 4 . 4 2 X + 9 0 4 N = 6 4 1 2 0 0 -1000H 8 0 0 -g © & 0 I » * © © © © 0 © © © ® @ O "I i 1 1 1 1 1 1 1 I I 7 9 11 1 3 1 5 1 7 W a t e r T e m p e r a t u r e ( C ) 48 Table 10, Fundamental frequency and dominant frequency (cycles/sec) of amplectant males and c a l l i n g males. 1mplectant_Males Calling,Males Iz .11 N= 111 ;40 Frequency Mean S.E. Mean Mean Fundamental 1019 28. 8 996 18.9 940 9. 1 Dominant 2035 40.7 2007 32.6 1994 13. 9 *Data for frog with aberrant voice (fundamental frequency=1272; dominant frequency=2339) not included. e f f e c t of including an amplectant male with an aberrantly high voice i n the analysis. But even without t h i s frog, both fundamental and dominant freguency of amplectant males are higher than for c a l l i n g males (Table 10). Since amplectant males are larger, on the average, than c a l l i n g males, i t appears that, for a given body s i z e , amplectant males had higher-pitched c a l l s . An analysis of covariance confirms t h i s for fundamental frequency (Fig. 9), but not for dominant frequency (Fig. 10). In section B of thi s chapter I w i l l present an explanation f o r the higher-pitched c a l l s , in r e l a t i o n to body length, of males which have attracted females. In summary, neither analysis supports the hypothesis that females choose large males on the basis of the i r low-pitched c a l l s . 49 Figure 9. Relationship of fundamental frequency (corrected to 12.5 C) to body length in calling frogs compared with successful frogs (excluding one frog with an aberrant voice). Lines are significantly different, p<0.01 (F=14.88). 49a o 1 2 0 0 H "O c o o CD CO CD Q . % 1000-o > N o S U C C E S S F U L M A L E S ( • ) Y = - 7 . 6 6 X + 1 2 8 7 N = 11 o Y = - 1 2 . 3 7 X + 1 3 . 9 9 N = 4 0 8 0 0 41 -i r~ 4 3 3 1 3 3 3 5 3 7 3 9 S n o u t - V e n t L e n g t h ( m m ) 50 Figure 10. Relationship of dominant frequency (corrected to 12.5 to body length in calling frogs compared with successful frogs (excluding one frog with an aberrant voice). Lines are not significantly different, p:>0.50 (F=1.26). 50a 2 4 0 0 H -D 2 2 0 0 c o o CD CO CD a CD o >. O 2000H S U C C E S S F U L M A L E S ( a ) Y = - 2 1 . 1 8 X + 2 1 8 1 n N = 11 C A L L I N G P O P U L A T I O N ( ° ) Y = - 1 8 . 3 8 X + 2 6 7 7 N = 4 0 1800H ~i 1 1 \ 1 1 1 ' — 3 7 3 9 4 1 4 3 —i 1 1 ~i 1— 3 1 3 3 3 5 S n o u t - V e n t L e n g t h ( m m ) 51 B. CHORUS-LEADING AS A CUE IN MATE SELECTION 1. Chorus l e a d e r s A p r e r e q u i s i t e f o r t h i s hypothesis of the method of mate s e l e c t i o n i s t h a t c e r t a i n males s t a r t more choruses than expected by chance. F o s t e r (1967) r e p o r t e d a n e c d o t a l evidence f o r t h i s i n P a c i f i c Tree Frogs, but as a f i r s t step i n my study, I attempted to gather some more c o n c l u s i v e evidence. In s m a l l ponds, such as those i n Vancouver, where at most only about 50 males c a l l at one time, the p e r i o d s of c a l l i n g are r e l a t i v e l y synchronous f o r a l l i n d i v i d u a l s ; t y p i c a l l y , a l l f r o g s c a l l f o r s e v e r a l minutes, then a l l are s i l e n t f o r s e v e r a l minutes before c a l l i n g a gain. Here, the f i r s t f r o g to c a l l a f t e r a p e r i o d of s i l e n c e would be chorus leader f o r a l l f r o g s i n the pond. The s i t u a t i o n i s more complex a t Marion Lake, where the work r e p o r t e d i n t h i s s e c t i o n was done. The c a l l i n g of a l l f r o g s i n the lake i s not u s u a l l y i n phase. The s i z e of groups f o r which c a l l i n g p e r i o d s are synchronous v a r i e s from o n l y a few i n d i v i d u a l s i n areas where d e n s i t i e s are low, to perhaps s e v e r a l hundred i n prime c a l l i n g areas. At Marion Lake, t h e r e f o r e , an i n d i v i d u a l can i n i t i a t e a bout of c a l l i n g f o r anywhere from one to s e v e r a l hundred other f r o g s , depending upon where he i s l o c a t e d i n the l a k e . To f a c i l i t a t e data c o l l e c t i o n , I d i d not attempt to keep t r a c k of which f r o g s i n i t i a t e d choruses f o r an e n t i r e group; I 52 observed only two c a l l i n g males at a time, always a pair of nearest neighbors, reasoning that i f one of these started more choruses for the pair than expected by chance, even though i t was not necessarily chorus leader for an entire group, th i s would strongly suggest that chorus i n i t i a t i o n s are non-randomly divided among in d i v i d u a l s in the group as a whole. Methods Using a stop watch, I determined whether each frog c a l l e d during 15 sec periods. Choruses were a r b i t r a r i l y defined as occurring when both frogs c a l l e d i n at lea s t two consecutive 15 sec periods. A chorus i n i t i a t i o n could occur only a f t e r one or both of the frogs had been s i l e n t for at least one 15 sec period ( i f one of the frogs c a l l e d during t h i s period, i t was the leader for the next chorus). I list e n e d to 19 pairs of frogs, each for four consecutive chorus i n i t i a t i o n s . The d i s t r i b u t i o n of four chorus i n i t i a t i o n s between two frogs could be that both s t a r t two choruses; one s t a r t s three and the other, one; or, one s t a r t s a l l four. According to the binomial d i s t r i b u t i o n , the pro b a b i l i t y of each of these three events occurring i s respectively, 0.545, 0.364, and 0.091. The expected number of each occurring i n 19 observations equals the respective probability times 19. The observed d i s t r i b u t i o n was compared with t h i s expected d i s t r i b u t i o n . 53 Results_and_Discussion Table 11 shows that, far more times than expected, one of the frogs started a l l four choruses. Thus, for pairs of Table 11. Maximum number of choruses started (of a t o t a l of four) by a member of each of 19 pairs of males. Number of Choruses Two Three Four Expected/19 10.36 6.91 1.73 Observed/19 1 5 13 Difference i s s i g n i f i c a n t , p<0.01 (Kolmogorov-Smirnov test) nearest-neighbor males over short periods of time (mean time from s t a r t of f i r s t chorus to s t a r t of fourth chorus for the 19 pairs equalled 11.0 min), one of the frogs i s usually a chorus leader. This, as well as abundant anecdotal evidence, suggests that, i n larger groups of frogs, certain individuals s t a r t most of the choruses. 54 2. F i e l d e v i d e n c e t h a t f e m a l e s c h o o s e c h o r u s l e a d e r s Two s e p a r a t e i n v e s t i g a t i o n s were made a t M a r i o n L a k e t o e v a l u a t e t h e h y p o t h e s i s t h a t f e m a l e s s e l e c t i v e l y mate w i t h c h o r u s l e a d e r s . S y s t e m a t i c f i g l d _ o b s e r v a t i o n s : The h y p o t h e s i s c a n be t e s t e d d i r e c t l y by o b s e r v i n g w h e t h e r t h e c h o r u s l e a d e r f o r a g r o u p o f f r o g s i s t h e most s u c c e s s f u l a t a t t r a c t i n g f e m a l e s . F i e l d _ e x p e r i m e n t : I f f e m a l e s c h o o s e c h o r u s l e a d e r s , one w o u l d e x p e c t t h a t m a l e s f o u n d i n a m p l e x u s , i f s e p a r a t e d f r o m t h e f e m a l e s a nd r e p l a c e d on t h e b r e e d i n g g r o u n d s , w o u l d become c h o r u s l e a d e r s . M e t h o d s ? Y 5 t e m a t i c _ f i g l d , _ o b s e r v a t i o n s : A f r o g w h i c h h a s a t t r a c t e d a f e m a l e c a n be r e c o g n i z e d by c h a n g e s i n i t s c a l l i n g b e h a v i o r : I t c h a n g e s f r o m D - c a l l i n g t o f a s t M - c a l l i n g when i t d e t e c t s t h e f e m a l e ' s a p p r o a c h , t h e n a b r u p t l y s t o p s c a l l i n g when i t c l a s p s t h e f e m a l e . The f i e l d p r o c e d u r e was t o l i s t e n t o a g r o u p o f f i v e o r more c a l l i n g f r o g s and d e t e r m i n e w h i c h f r o g f i r s t a t t r a c t e d a f e m a l e , a l w a y s c o n f i r m i n g t h e a u d i t o r y e v i d e n c e by v i s u a l i n s p e c t i o n . The s u c c e s s f u l f r o g c o u l d be e i t h e r t h e c h o r u s l e a d e r f o r t h e p r e v i o u s c h o r u s (a c h o r u s was d e f i n e d a s two o r more f r o g s c a l l i n g f o r a t l e a s t 30 s e c ; a c h o r u s e n d e d when a l l o r a l l b u t one o f t h e f r o g s s t o p p e d 55 c a l l i n g f o r at least 15 sec) or another frog. Ii51d_experifent: Amplectant pairs were collected when encountered on the breeding grounds, and separated. A control male for each successful male was obtained by capturing the f i r s t c a l l i n g male encountered i n a random di r e c t i o n from the capture s i t e of the amplectant pair. Both males were placed i n a f i e l d enclosure. On the following night, I determined which frog f i r s t started three consecutive choruses for the pair (chorus defined as on p. 52), and defined i t as the chorus leader. Results and Discussion Systematic f i e l d observations: The results obtained were very scanty owing to the slow rate at which females come onto the breeding grounds; however, a l l observations supported the hypothesis that females select chorus leaders (Table 12). Field_gxEgriment: The experiment yielded only three usable t r i a l s (others f a i l e d because I could not locate one or both of the frogs i n the enclosure). A l l three t r i a l s supported the hypothesis: In two t r i a l s , the previously amplectant male c a l l e d and the control did not; i n the other t r i a l , the successful male was the chorus leader. The r e s u l t s , however, are not unequivocal; one could argue that the previously amplectant males called more a c t i v e l y because of some ef f e c t of having been i n amplexus or of having been 56 Table 1 2 , Success of chorus leaders compared with other frogs i n a t t r a c t i n g females: F i e l d observations, « Successful Frog Chorus leader 1 2 20 Chorus leader 1 3 5 Chorus leader 1 p=0.002 (Binomial test) 1Was"chorus -leader f o r " at l e a s t - two choruses immediately preceding amplexus, separated from their mates. The r e s u l t s of both investigations suggest that chorus leaders are more l i k e l y to mate; however, they do not establish that females use chorus-leading jper se as a cue in mate s e l e c t i o n , for chorus leaders also show other differences in c a l l i n g behavior. Casual observations suggested that chorus leaders are more active vocally than other frogs in several ways: They end choruses, c a l l at a fas t e r rate during choruses, are more l i k e l y to c a l l during general l u l l s in c a l l i n g , and c a l l louder. Minimum Number of Frogs Observation i n Chorus 1 57 3. Correlates of chorus-leading Systematic f i e l d observations were made to rigorously determine whether other differences in c a l l i n g behavior are correlated with chorus-leading. Methods To determine whether chorus leaders end choruses, c a l l at a faster rate during choruses, and are more l i k e l y to c a l l during general l u l l s i n c a l l i n g than other frogs, I made f i e l d observations s i m i l a r to those made to determine i f certain frogs are chorus leaders. For these analyses, I defined chorus leaders as frogs which started three or more consecutive choruses. The investigation of whether chorus leaders c a l l louder than other frogs was done afte r I had already determined from the above observations that chorus leaders c a l l at a faster rate than other frogs. Therefore, I adopted a less time-consuming procedure: I simply measured the c a l l rates of two nearest-neighbor males, either simultaneously or sequentially, each for two 30 sec periods. If one frogs c a l l e d at a faster rate both times, i t was defined as the chorus leader. Then the c a l l i n t e n s i t y of each frog was measured by placing the sound l e v e l meter 50 cm behind the frog. The maximum reading in decibels for a 15 sec recording 58 of each frog was used i n the analysis. Since c a l l i n t e n s i t y for a l l frogs recorded was correlated with body length (r=0.56, p<0.01), to avoid confounding by t h i s variable, an analysis of covariance of c a l l i n t e n s i t y on body length was done for chorus leaders versus non-chorus leaders. Table 13. Relationship of chorus-leading to chorus-ending. Table includes data for the f i r s t chorus recorded for 26 pairs of frogs. Frog Ending Chorus Chorus_Leader Non-chorus Leader Expected/26 13 13 Observed/26 21 5 S i g n i f i c a n t l y d i f f e r e n t , p<0.01 (Chi Square=9.98) The r e s u l t s show that chorus leaders do tend to end choruses (Table 13), c a l l at a faster rate during choruses (Table 14), c a l l more during periods not defined as choruses (Table 15), and c a l l louder than other frogs (Fig. 11). Again, the frogs defined as chorus leaders for these analyses were not necessarily (or l i k e l y ) the chorus leaders for the 59 Figure 11. Relationship of c a l l intensity to body length i n chorus leaders compared with other frogs. Lines are significantly different, p"<0.025 (F=6.87). 59a S n o u t - V e n t L e n g t h (mm) 60 Table 14. Mean c a l l rates (calls/30 sec) during choruses of chorus leaders and non-chorus leaders. Table includes data for 23 pairs of frogs (for three other pairs of frogs observed, no c a l l rate data were co l l e c t e d ) . C ho rus w Leaders ff°l££hg.¥Sf .,iig a3ers 22.96 21.05 S i g n i f i c a n t l y d i f f e r e n t p<0.01 (Hilcoxon test) Table 15. Mean number of c a l l s per 15 min during periods not defined as choruses of chorus leaders and non-chorus leaders. Table includes data for 26 pairs of frogs. £horus_Leaders Non-Chorus Leaders 1.56 0.36 S i g n i f i c a n t l y d i f f e r e n t , p<0.01 (Wilcoxon test) groups i n which they were c a l l i n g , but I would argue that " r e a l " chorus leaders, compared to the other frogs in their groups^ show s i m i l a r differences i n c a l l i n g behavior. An explanation f o r the higher-pitched c a l l s , i n r e l a t i o n to body s i z e , of males found i n amplexus compared to other males (see p. 46) can now be attempted: Results thus f a r presented show that 1) successful males are chorus leaders, 2) chorus leaders c a l l louder than other frogs, and 3) males captured i n amplexus tend to be chorus leaders when replaced 6 1 on the breeding grounds. Given these relationships, the c a l l s of males captured i n amplexus w i l l be high-pitched i n r e l a t i o n to body size i f c a l l pitch i s p o s i t i v e l y related to c a l l i n t e n s i t y , as suggested by f i e l d observations (the f i r s t few c a l l s an i n d i v i d u a l makes afte r a l u l l i n c a l l i n g are sometimes of r e l a t i v e l y low i n t e n s i t y ; these c a l l s seem lower-pitched than louder c a l l s ) . This hypothesis was tested by comparing the fundamental frequencies and dominant frequencies of two c a l l s , d i f f e r i n g i n loudness by at least 2 db, recorded from each of f i v e males. In a l l cases, both fundamental and dominant frequency were higher for the louder c a l l (Table 16) , thus upholding the hypothesis. 4. Use of chorus-leading as a cue in mate selection The vocal behavior of chorus leaders d i f f e r s i n several ways from that of other frogs; any of these differences might be used by females i n mate select i o n . However, because chorus-leading was the variable measured in f i e l d observations of mate se l e c t i o n , a laboratory experiment was done to rigorously examine the use of t h i s cue. 62 Table 16. Relationship of fundamental frequency and dominant frequency (both in cycles/sec) to c a l l i n t e n s i t y . Fundamental.Frequency DominantFrequency Frog Quieter C a l l Louder_Ca11 0.uieter_Call Louder_Call 1 910 1000 1930 1990 2 , 1020 1030 2080 2180 3 880 910 1910 1940 4 900 930 1930 1990 5 870 890 1910 1940 Methods The experiment was done in a c i r c u l a r arena (Fig. 12) having four loudspeakers on the circumference playing back D c a l l s via a system si m i l a r to that used in the second addition experiment (see p. 26). A randomly chosen speaker was the chorus leader for each t r i a l ; for each chorus i t c a l l e d three times before the other speakers were turned on (not an unusual number of times for a chorus leader to c a l l before other frogs j o i n ) . Choruses were two minutes long, with one minute l u l l s between. The arena was f i l l e d to,a depth of 10 cm with 15 C water (the same temperature as the D c a l l was recorded a t ) . A female c o l l e c t e d on breeding areas either in Vancouver or at Marion Lake was held i n a c i r c u l a r cage i n the center of the arena u n t i l about 15 sec aft e r the s t a r t of the fourth chorus. Then, the hidden observer removed the cage top by r a i s i n g i t 63 Figure 12. A top view of the arena used i n the laboratory experiment testing whether females are attracted to chorus leaders. Females were placed in the circular cage (with i t s floor slightly submerged) in the center of the arena. The wooden platforms i n front of each speaker were slightly submerged; females frequently rested on their edges during experiments. 1 m e t e r  ^ l o u d s p e a k e r w o o d e n p l a t f o r m c a g e for f e m a l e a r e n a w a l l 64 with an attached s t r i n g . A "choice" occurred when a female came within 2 cm horizontal distance of a speaker. If a female did not make a choice within 60 min after release, the experiment was terminated; likewise, i t was terminated i f a female began to climb the screen fence around the arena. Non-responding females (most frequently ones which were not captured i n amplexus) were given 1000 I.U. of human chorionic gonadotrophin i n t r a p e r i t o n e a l l y to induce ovulation (Schmidt 1969) and retested the next day. Results and _ Discussion The r e s u l t s confirm that females are attracted to chorus leaders (Table 17). The mean time for females to make a Table 17. Success of chorus leaders compared with other frogs i n a t t r a c t i n g females: laboratory experiment. / Number_of_Females_Attracted Chorus Leader Others Expected/11 2.75 8.25 Observed/11 9 2 S i g n i f i c a n t l y d i f f e r e n t , p<0.001 (Binomial test) choice aft e r release from the central cage was 18 min (range: 65 1-r43 min) , but even a t the moment of r e l e a s e ( a f t e r hearing only f o u r chorus i n i t i a t i o n s ) , females appeared to be more " i n t e r e s t e d " i n the chorus l e a d e r ; 8 of 11 were f a c i n g w i t h i n 45 degrees of the chorus l e a d e r speaker (p<0.01). These r e s u l t s do not exclude the p o s s i b i l i t y t h a t i n nature females a l s o cue i n to the c o r r e l a t e s of c h o r u s - l e a d i n g chorus-ending, c a l l i n g a t a f a s t e r r a t e d u r i n g choruses, c a l l i n g more duri n g g e n e r a l l u l l s i n c a l l i n g , and c a l l i n g louder than other f r o g s . Gerhardt (1968) ( c i t e d by P i e r c e and R a l i n 1972) r e p o r t e d evidence t h a t female Hyla c i n e r e a may p r e f e r males with loud v o i c e s ; when he i n c r e a s e d the playback i n t e n s i t y of a mating c a l l from a h y b r i d H. c i n e r e a X H. g r a t i g s a male to 5 db g r e a t e r than t h a t of a H. c i n e r e a male i n a l a b o r a t o r y c h o i c e experiment, female H. c i n e r e a r e v e r s e d t h e i r n a t u r a l p r e f e rence f o r the c o n s p e c i f i c c a l l . However, i n the d i n of a chorus i n v o l v i n g more than a very few f r o g s , i t would seem d i f f i c u l t f o r females to d i s t i n g u i s h i n d i v i d u a l d i f f e r e n c e s i n loudness or i n c a l l r a t e , except at very s h o r t d i s t a n c e s . On the other hand* c a l l s i n i t i a t i n g or ending choruses and c a l l s during l u l l p e r i o d s are given a t times of minimal a c o u s t i c a l i n t e r f e r e n c e , and, at l e a s t f o r the human observer, are e f f e c t i v e a i d s i n l o c a t i n g males. 66 5. Short-term changes in c a l l i n g tendency Since females are attracted to males which s t a r t choruses, and also perhaps to males which end choruses and c a l l more than other frogs during l u l l s , the question a r i s e s , "Why do males not c a l l continuously during the nocturnal periods of breeding a c t i v i t y as the resu l t of each male attempting to c a l l during these times when no or few other frogs are c a l l i n g ? " The l u l l s i n c a l l i n g by ind i v i d u a l frogs seem to be the re s u l t of a short-term waning of responsiveness to the stimulus of other frogs c a l l i n g . Anecdotal evidence for th i s comes from attempts I made to stimulate frogs to c a l l (for the purpose of recording th e i r vocalizations) by playing back D c a l l s to them. I soon learned that playback shortly after the beginning of a l u l l period was less e f f e c t i v e than playback l a t e r i n the l u l l . A controlled experiment was designed to more rigorously test t h i s idea. / 67 Methods Defining a l u l l as a period 15 sec or longer between two 30 sec or longer bouts of c a l l i n g , I measured the length of a l u l l (length=X sec) for an i n d i v i d u a l frog, then measured the lengths of i t s following three l u l l s under three d i f f e r e n t stimulus conditions: no stimulus, stimulus presented 20% of X after the beginning of the l u l l , and stimulus presented after 60% of X. For example, i f the length of the f i r s t l u l l was 200 sec, under the second condition the stimulus would be presented 40 sec after the beginning of the l u l l ; under the - t h i r d condition, 120 sec. The order of the three experimental conditions was randomized for each t r i a l . The stimulus was a D c a l l played back at normal rate and volume approximately 4 m away from the frog. Results and_Discussion a preliminary analysis of the res u l t s confirmed that D c a l l s stimulate frogs to c a l l (Table 18); and, as expected, the latency of response (defined as the length of time from the beginning of playback to the beginning of a 30 sec or longer bout of c a l l i n g by the frog) was longer when the stimulus was presented e a r l i e r in the l u l l period (Table 19). These r e s u l t s are interpreted as showing an increasing c a l l i n g tendency during l u l l periods; a compensating waning of c a l l i n g tendency preseumably occurs during choruses. according to 68 Table 18. Mean l e n g t h of l u l l (sec) i n c a l l i n g when no D c a l l s were played back during the l u l l compared with when D c a l l s were played back (data f o r the two playback c o n d i t i o n s were lumped):. Table i n c l u d e s data f o r 12 f r o g s . No D C a l l s Played Back D_Callg„Played,..Back 202 107 S i g n i f i c a n t l y d i f f e r e n t , p<0.01 (Wilcoxon t e s t ) T a b l e 19. Mean l a t e n c y t o c a l l (sec) when s t i m u l u s presented i n the f i r s t p art of a l u l l compared with when presented l a t e r i n a l u l l . Table i n c l u d e s data f o r 12 f r o g s . D C a l l s Played Back - D C a l l s Played Back E a r l y _ i n _ a _ l u l l L a t e r i n a _ l u l l 60 22 S i g n i f i c a n t l y d i f f e r e n t , p<0.005 (Wilcoxon t e s t ) t h i s model, chorus l e a d e r s are the f r o g s which f i r s t reach a th r e s h h o l d f o r c a l l i n g "spontaneously" during the in c r e m e n t a l phase; they are u s u a l l y the l a s t to stop c a l l i n g at the end of a chorus because they r e g u i r e l e s s s t i m u l a t i o n than other f r o g s to keep c a l l i n g . I t i s a p p e a l i n g to suggest t h a t muscular f a t i g u e i s r e s p o n s i b l e f o r the r e c u r r e n t waning of c a l l i n g tendency. 69 Other mechanisms such as habituation or sensory adaptation, which are mechanisms for adaptive reduction of response, seem unlikely to apply i n t h i s case, since i t should instead be adaptive for frogs to continue c a l l i n g u n t i l energy requirements force a halt. The fatigue hypothesis would be d i f f i c u l t to test d i r e c t l y , but i t s c r e d i b i l i t y would be enhanced by finding that c a l l i n g takes a s i g n i f i c a n t amount of energy. A respirometry experiment was attempted to determine i f this i s the case. The procedure was to compare the rate of oxygen uptake of c a l l i n g frogs with that of non-calling frogs in a d i f f e r e n t i a l respirometer, using 5 ml of 25% potassium hydroxide as the carbon dioxide absorbant. D c a l l s were played back through a loudspeaker near the respirometer to stimulate c a l l i n g . Unfortunately, however, only one frog c a l l e d in the respirometer. This frog c a l l e d for 150 sec, using 0.101 ml of oxygen; in the control period, i t used only 0.040 ml. These r e s u l t s suggest that c a l l i n g requires large amounts of energy, and thus provide tentative support for the fatigue hypothesis; firmer conclusions w i l l require much more data. 70 C. SUMMARY AND DISCUSSION The r e s u l t s presented i n t h i s chapter i n d i c a t e that females use i n d i v i d u a l d i f f e r e n c e s among males i n c a l l i n g behavior, but not i n c a l l p i t c h , as cues i n mate s e l e c t i o n . Females choose males which are chorus l e a d e r s , using c h o r u s - l e a d i n g and perhaps the c o r r e l a t e s of c h o r u s - l e a d i n g chorus-ending, c a l l i n g a t a f a s t e r r a t e d u r i n g choruses, c a l l i n g more during g e n e r a l l u l l s , and c a l l i n g louder than other f r o g s as cues. Another t r a i t which may enhance mating success i n males i s s p a c i n g out; as d i s c u s s e d i n Chapter I I (p. 42), spacing out may f a c i l i t a t e the l o c a t i o n of i n d i v i d u a l males by females. I f so, one would expect chorus l e a d e r s to hold an advantage over other males i n t h i s r e s p e c t a l s o : S i n c e chorus l e a d e r s have louder v o i c e s , f r o g s should space out f a r t h e r from them.. T h i s p r e d i c t i o n remains to be t e s t e d . I t i s t h e r e f o r e advantageous f o r an i n d i v i d u a l male to c a l l more and a l s o perhaps to c a l l l o uder than other f r o g s . I f the mechanism r e s p o n s i b l e f o r the r e c u r r e n t waning of c a l l i n g tendency i n i n d i v i d u a l s i s , as I have argued, muscular f a t i g u e , the proposed system of mate s e l e c t i o n would have females choosing the most " r o b u s t " males, the males which f a t i g u e l e a s t e a s i l y . I n t u i t i v e l y , as w e l l as according to the theory of n a t u r a l s e l e c t i o n , one would p r e d i c t t h a t these m a l e s make t h e b e s t m a t e s . 72 LITERATURE CITED A x t e l l ; R. W. 1959. 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Comparative breeding behavior of the Bed-legged Frog (Rana aurora aurora) and the Western Spotted Frog (Rana p r e t i o s a _pretiosa) i n southwestern B r i t i s h Columbia? Can. J . Z o o l . 47:"l287-1299. L i t t l e j o h n , M. J . and J . J . L o f t u s - H i l l s . 1968. An experimental e v a l u a t i o n of premating i s o l a t i o n i n the Hj^la gwinga complex (Anura: H y l i d a e ) . E v o l u t i o n 227659-663. L i v e z e y , R. L. 1952. Some o b s e r v a t i o n s on P s e u d a c r i s n j g r i t a t r i s e r i a t a (Wied) i n Texas. Amer. M i d i . Natur. 47:572-587. L u t z , B. 1960. F i g h t i n g and an i n c i p i e n t n o t i o n of t e r r i t o r y i n male t r e e f r o g s . Copeia 1960:61-63. Martof, B. S. 1953. T e r r i t o r i a l i t y i n the Green Frog, Rana c l a m i t a n s . Ecology 34:165-176. Martof, B. S. and E. F. Thompson, J r . 1958. Reproductive behavior of the Chorus Frog, P s e u d a c r i s n i g r i t a . Behavior 13:243-258. P a i l l e t t e , M. 1970. La n o t i o n de t e r r i t o i r e chez l e s amphibiens anoures, et p l u s p a r t i c u l i e r e m e n t l a v a l e u r de emissions sonores dans l e comportement t e r r i t o r i a l des h y l i d e s : Hyla arborea e t Hyla m e r i d i o n a 1 i s , p. 35-48. In G. Richard (ed.) T e r r i t o i r e et Domaine V i t a l . 75 Peeke, F. W. 1972. An experimental study of the t e r r i t o r i a l f u n c t i o n of v o c a l and v i s u a l d i s p l a y i n the male Red-winged B l a c k b i r d (Agelaius phoeniceus). Anim. Behav. 20:112-118. P e n g i l l e y , R. K. 1971. C a l l i n g and a s s o c i a t e d behaviour of some s p e c i e s of Pseudophrjrne (Anura: L e p t o d a c t y l i d a e ) . J . Z o o l . Lond. 163:73-927 P i e r c e , J . R. and D. B. R a l i n . 1972. V o c a l i z a t i o n s and behavior of the males of three s p e c i e s i n the Hjrla v e r s i c o l o r complex. H e r p e t o l o g i c a 28:329-337. Pyburn, W. F. and J . R. G l i d e w e l l . 1971. Nests and breeding behavior of Phyllomedusa h i j a o c h o n d r i a l i s i n Columbia. J . Herpet. 5:49-52. Rabb, G. B. and M. S. Rabb. 1963a. A d d i t i o n a l o b s e r v a t i o n s on breeding behavior of the Surinam Toad, P i p a p i p a . Copeia 1963:636-642. Rabb, G. B. and M. S. Rabb. 1963b. On the behavior and breeding b i o l o g y of the A f r i c a n p i p i d f r o g Hymenochirus b o e t t g e r i . , Z . T i e r p s y c h o l . 20:215-241. Schmidt, R. S. 1969. P r e o p t i c a c t i v a t i o n of mating c a l l o r i e n t a t i o n i n female anurans. Behaviour 35:114-127. Sexton, o . J . 1960. Some as p e c t s of the behavior and of the t e r r i t o r y o f a dendrobatid f r o g . P r o s t h e r a p i s t r i n i t a t u s . Ecology 41:107-115. Sexton, 0. J . 1962. Apparent t e r r i t o r i a l i s m i n l e p t o d a c t y l u s insularum Barbour. H e r p e t o l o g i c a 18:212-214. Snyder, W. F. and D. L. Jameson. 1965. M u l t i v a r i a t e geographic v a r i a t i o n of mating c a l l i n p o p u l a t i o n s of the P a c i f i c Tree Frog (H£la r e g i l l a ) . Copeia 1965:129-142. Stebb i n s , R. C. 1966. A f i e l d guide to western r e p t i l e s and amphibians. Houghton M i f f l i n Co., Boston. Tinbergen, N., M. Impekoven, and D. Franck. 1967. An experiment on s p a c i n g out as a defense a g a i n s t p r e d a t i o n . Behaviour 28:307-321. Whitford, W. G. 1967. Observations on t e r r i t o r i a l i t y and a g g r e s s i v e behavior i n the Western Spadefcct Toad, Seaphiopus hammondii. H e r p e t o l o g i c a 23:318. 76 Wiewandt, T. A. 1969. V o c a l i z a t i o n s , a g g r e s s i v e behavior, and t e r r i t o r i a l i t y i n the B u l l f r o g , Rana c a t e s b e i a n a . Copeia 1969:276-285. Wiley, R. H. 1970. T e r r i t o r i a l i t y and non-random breeding i n the Sage Grouse (Centrpcercus urophasianus). Unpublished Ph. D. T h e s i s , The R o c k e f e l l e r U n i v e r s i t y . 77 APPENDIX I DIFFERENTIAL SELECTION FOR COLOR BETWEEN THE SEXES IN PACIFIC TREE FROGS P a c i f i c Tree Frogs are polymorphic f o r c o l o r . I n d i v i d u a l s , although able to l i g h t e n and darken c o n s i d e r a b l y , cannot change t h e i r b a s i c c o l o r (Brattstrom and Warren 1955; Resnick and Jameson 1963). Models of the i n h e r i t a n c e of the major c o l o r s - — g r e e n , brown, and red have been developed by Resnick and Jameson (1963). Jameson and Peguegnat (1971) r e p o r t e d t h a t the polymorphism i s s t a b l e i n p o p u l a t i o n s s t u d i e d through out the s p e c i e s 1 range. The most i n t e n s i v e l y s t u d i e d p o p u l a t i o n i s one at J u l i a n , C a l i f o r n i a . Here, sampling o n l y males, Jameson and Peguegnat found t h a t i n samples taken dur i n g the breeding season the r e l a t i v e p r o p o r t i o n s of the morphs d i d not change from 1966 to 1968. However, the p r o p o r t i o n s changed c o n s i d e r a b l y d u r i n g each year; green males i n c r e a s e d i n the s p r i n g , and non-green males i n c r e a s e d i n the f a l l . On the b a s i s of these o b s e r v a t i o n s , Jameson and Peguegnat suggested t h a t the polymorphism i s s t a b l e because the " o v e r a l l s e l e c t i v e f o r c e s average out." In t h i s paper, I present evidence i n d i c a t i n g t h a t , at l e a s t i n my study ar e a s , the f a c t o r s m a i n t a i n i n g the c o l o r polymorphism can not be understood without studying both male 78 and female f r o g s . n e a r l y a l l P a c i f i c Tree Frogs i n Vancouver and Marion Lake are e i t h e r green or brown. Each of these morphs can be d i v i d e d i n t o two d i s t i n c t "sub-morphs": one has a broad red (or sometimes dark brown) mid-dorsal s t r i p e , and the other does not have the s t r i p e . However, s i n c e the s t r i p e d forms are r e l a t i v e l y r a r e , f o r a nalyses I have lumped each with the u n s t r i p e d form of the same g e n e r a l c o l o r . A. COLORS OF ADULT MALES AND FEMALES C o l l e c t i o n s of a d u l t f r o g s made i n March and A p r i l 1972 at v a r i o u s ponds i n Vancouver suggested that the p r o p o r t i o n s of the morphs d i f f e r between the sexes, the green morph being more common i n females, T h i s hypothesis was t e s t e d at Marion Lake. Methods Frogs were captured from a major breeding area (approximately 100 m by 40 m) on the east s i d e of Marion Lake i n 1972 and 1973. C a p t u r i n g , marking, and measuring procedures were as d e s c r i b e d on p. 16. 79 R e s u l t s and D i s c u s s i o n In both 1972 and 1973, the p r o p o r t i o n of green i n d i v i d u a l s was f a r higher i n females than i n males (Tables 1 and 2), thus c o n f i r m i n g the h y p o t h e s i s . Table 1. Numbers of brown and green a d u l t males and females captured at Marion Lake i n 1972. C o l o r Brown 107 7 S i g n i f i c a n t l y d i f f e r e n t , p<0.001 (Chi Sguare=15.83) Two p o s s i b l e e x p l a n a t i o n s f o r the c o l o r d i f f e r e n c e s between the sexes are: (1) The i n h e r i t a n c e of c o l o r d i f f e r s between the sexes; more green females than green males are produced. (2) D i f f e r e n t i a l s e l e c t i o n o c c u r s , with the green morph having g r e a t e r • • f i t n e s s " i n females than i n males. These two e x p l a n a t i o n s are not mutually e x c l u s i v e ; they c o u l d operate together to produce the observed d i f f e r e n c e s i n c o l o r . sex Green Males 68 Females 24 80 Table 2. Numbers of brown and green adult males and females captured at Marion Lake i n 1973. Sex Green Brown Males 45 87 Females 13 1 S i g n i f i c a n t l y d i f f e r e n t , p<0.001 (Chi Square =18.03) 81 B. COLORS OF JUVENILE MALES AND FEMALES - I f the d i f f e r e n c e s i n c o l o r between a d u l t males and females are due only t c d i f f e r e n t i a l s e l e c t i o n , assuming t h a t the s e l e c t i o n occurs e n t i r e l y during the a d u l t stage (which i s probably the o n l y time t h a t the sexes behave d i f f e r e n t l y , and t h e r e f o r e would be s u b j e c t to d i f f e r e n t s e l e c t i v e f o r c e s ) , equal numbers of the sexes should be found i n both morphs i n sub-adult animals. Conversely, i f the g e n e t i c hypothesis i s v a l i d , one would expect most green sub-adults t c be females and most brown sub - a d u l t s to be males. Methods Recently metamorphosed f r o g l e t s of both morphs were c o l l e c t e d on the shores of Marion Lake i n e a r l y August 1972. They were r a i s e d i n the l a b o r a t o r y to approximately 15 mm snout-vent l e n g t h , then d i s s e c t e d and sexed. R e s u l t s and D i s c u s s i o n I was a b l e to sex a l l i n d i v i d u a l s u n e g u i v o c a l l y by the s t r u c t u r e of the gonads. No s i g n i f i c a n t d i f f e r e n c e i n the p r o p o r t i o n s of the morphs occu r r e d between the sexes (Table 3). These r e s u l t s are c o n s i s t e n t with the hypothesis t h a t the d i f f e r e n c e s i n c o l o r between a d u l t males and females are due 82 Table 3. C o l o r s of j u v e n i l e male and female f r o g s captured at Marion Lake. C o l o r Sex Green Brown Males 19 9 Females 19 12 Hot s i g n i f i c a n t l y d i f f e r e n t , 0.70>p>0.50 (Chi Square=0.30) only to d i f f e r e n t i a l s e l e c t i o n . The s e l e c t i o n need not operate d i r e c t l y on c o l o r , but s i n c e t h i s i s the most s t r a i g h t - f o r w a r d hypothesis, I w i l l accept i t f o r the remainder of t h i s paper. C. RELATIVE ABUHBANCE OF THE MORPHS AT THE TADPOLE STAGE P a c i f i c Tree Frogs are not polymorphic f o r c o l o r u n t i l metamorphosis; a l l t a d p o l e s are brown. Assuming, t h e r e f o r e , that the v i a b i l i t i e s of the two morphs are egual i n the tadpole stage ( i . e . that there i s no p l e i o t r o p h i c e f f e c t i n the tadpole s t a g e ) , and assuming that the polymorphism i s s t a b l e at Marion Lake (Tables 1 and 2 suggest that i t i s r e l a t i v e l y s t a b l e from 1972 to 1973), the r e l a t i v e v i a b i l i t i e s from the tadpole stage to the a d u l t stage of males and females of both morphs can be estimated by comparing the p r o p o r t i o n s of the morphs at the tadpole stage (determined by r a i s i n g 83 t a d p o l e s to metamorphosis i n the l a b o r a t o r y ) with the p r o p o r t i o n s f o r a d u l t s . Methods Tadpoles were c o l l e c t e d i n e a r l y August 1972 from the same area i n Marion Lake that the study of a d u l t s was done i n . They were r a i s e d i n the l a b o r a t o r y to metamorphosis, then scored f o r c o l o r . R e s u l t s and D i s c u s s i o n Of the 97 i n d i v i d u a l s s u c c e s s f u l l y r a i s e d to metamorphosis, 92 were green and only 5 were brown. These r e s u l t s r e p r e s e n t only a very rough estimate of the p r o p o r t i o n s of the morphs a t the tadpole stage. Even so, comparison with the p r o p o r t i o n s of the morphs i n a d u l t males and females (Tables 1 and 2) i n d i c a t e s that heavy s e l e c t i o n a g a i n s t green males occurs. Some s e l e c t i o n a g a i n s t green f r o g s (presumably both males and females) appears to occur s h o r t l y a f t e r metamorphosis. F r o g l e t s were captured on the shore of Marion Lake adjacent to the study area i n e a r l y September 1972, and even though green f r o g s appeared more conspicuous to both i n v e s t i g a t o r s (and t h e r e f o r e may have been captured out of p r o p o r t i o n to t h e i r a c t u a l abundance), a s i g n i f i c a n t l y higher p r o p o r t i o n of brown 84 individ u a l s was taken i n t h i s sample than i n the sample of tadpoles (Table 4). Table 4. Numbers of green and brown individuals i n f r o g l e t s raised i n the laboratory from tadpoles and i n f r o g l e t s captured on the lake shore. Sajti£le Green Brown Captured as tadpoles 92 5 Captured as f r o g l e t s 47 17 S i g n i f i c a n t l y d i f f e r e n t , p<0.001 (Chi Square=15.17) The se l e c t i v e pressures on the two morphs for the remainder of the juvenile stage are not known; however, since the behavior of males and females i s not known to be di f f e r e n t at t h i s stage, selection should occur equally on both males and females. 85 D. SELECTION IN ADULT HALES Since the r e l a t i v e p r o p o r t i o n s of the morphs i n s u b - a d u l t s about to become a d u l t s are not known, re g a r d i n g the presumed d i f f e r e n t i a l s e l e c t i o n during the a d u l t stage, one can only say t h a t e i t h e r brown males or green females or both are a t a s e l e c t i v e advantage over i n d i v i d u a l s of the other morph i n t h e i r r e s p e c t i v e sexes. An i n d i r e c t method f o r a s s e s s i n g the r e l a t i v e v i a b i l i t i e s of the two morphs i n a d u l t s i s to compare the s i z e d i s t r i b u t i o n s of the morphs. Assuming t h a t s i z e i s c o r r e l a t e d with age (recaptures i n 1973 of f r o g s marked i n 1972 i n d i c a t e t h a t i t i s ) and that the r e l a t i o n s h i p of s i z e to age i s the same f o r both morphs, i f one morph i s at a s e l e c t i v e advantage duri n g the a d u l t stage, f r o g s of that c o l o r should, on the average, be l a r g e r than those of the other morph. Methods For t h i s a n a l y s i s , I used the s i z e and c o l o r data f o r a d u l t males captured on the breeding grounds a t Marion Lake i n 1972 and 1973; data f o r females were i n s u f f i c i e n t f o r a n a l y s i s . 86 R e s u l t s and_Discussion For both 1972 and 1973, brown males were l a r g e r than green males (Tables 5 and 6), suggesting that brown males are at a s e l e c t i v e advantage as a d u l t s . Table 5. Mean snout-vent l e n g t h s (mm) of green males and brown males, Marion Lake, 1972. Green_Males (n=68) Brown Males (n= 107) 35.25 37.97 S i g n i f i c a n t l y d i f f e r e n t , p<0.01 Table 6. Mean snout-vent l e n g t h s (mm) of green males and brown males, Marion Lake, 1973. Green_Kaj.es (n=45) Brown, Males (n=87) 37.15 38.31 S i g n i f i c a n t l y d i f f e r e n t , p<0.01 When does the s e l e c t i o n a g a i n s t green a d u l t males occur? 87 The most obvious answer, i n t u i t i v e l y , i s during the breeding season. Individual males may spend several months on the breeding grounds. At Marion Lake the general coloration of the breeding grounds i s brown, especially early in the breeding season before plants begin to produce new growth. Therefore, i f i t i s of survival value to be c r y p t i c , brown males should be at an advantage. By the same argument, the high proportion of green females might be accounted for by the fact that they come onto the breeding grounds for only one or two days each, and then presumably return to the forest bordering the,lake where i t may not be advantageous to be brown. To test the hypothesis that brown males are at a se l e c t i v e advantage during the breeding season, I examined the data for males captured in 1972 and 1973 to determine i f the proportion of brown males increased during the breeding season. No such trend was found (Tables 7 and 8). These r e s u l t s contradict the hypothesis but not unequivocally. It i s possible that green males come onto the breeding grounds l a t e r than brown males (this strategy would place green indivi d u a l s on the breeding grounds when they would be more c r y p t i c ) . If so, the net selection against green males might be masked. 88 Tabl e 7. Numbers of green and brown a d u l t males caught d u r i n g one-week p e r i o d s a t Marion Lake i n 1972. Data f o r each week i n c l u d e r e c a p t u r e s . Number Week Green Brown 1-8 May 8 27 9-15 May 18 27 16-22 May 28 39 23-29 May 13 14 30 May-6 June 3 12 7-14 June 1 2 9 Not s i g n i f i c a n t l y d i f f e r e n t , 0.20>p>0.10 (Chi-Sguare=8.89; df=5) 1 8-day p e r i o d E. SUMMARY AND DISCUSSION P a c i f i c Tree Frogs at Marion Lake are e i t h e r green or brown. In a d u l t s , the green morph i s much more common i n females than i n males; t h i s seems to be e n t i r e l y the r e s u l t of d i f f e r e n t i a l s e l e c t i o n , which probably occurs d u r i n g the a d u l t stage of the l i f e c y c l e . The green morph appears to be at a s e l e c t i v e disadvantage dur i n g the f r o g l e t stage i n both females and males, From the f r o g l e t stage to the a d u l t stage, green females appear to have v i a b i l i t i e s e g u a l to or g r e a t e r than brown females, but during 89 the same p e r i o d , the green morph d e c l i n e s d r a m a t i c a l l y i n males. These r e s u l t s i n d i c a t e that green ' males are at a s e l e c t i v e disadvantage both as j u v e n i l e f r o g s and as a d u l t s . T h e r e f o r e , the o v e r a l l s e l e c t i v e f o r c e s on males do not "average out" as Jameson and Pequegnat (1971) claimed f o r the J u l i a n , C a l i f o r n i a p o p u l a t i o n . The green morph seems to p e r s i s t i n Marion Lake males only because a very high p r o p o r t i o n of green females occurs i n the breeding p o p u l a t i o n . Table 8. Numbers of green and brown a d u l t males caught duri n g one-week pe r i o d s at Marion Lake i n 1973. Data f o r each week i n c l u d e r e c a p t u r e s . Number Week Green Brown 11-17 A p r i l 4 9 18-24 A p r i l 10 14 25 A p r i l - 1 May 12 38 2-8 May 9 14 9-15 May 11 24 16-22 May 11 9 Not s i g n i f i c a n t l y d i f f e r e n t , 0.30>p>0.20 (Chi-Sguare=7.21; df=5) 90 LITERATURE CITED Brattstrom, B. H. and J. W. Warren. 1955. Observations on the ecology and behavior of the P a c i f i c Tree Frog, Hjrla r e g i l l a . Copeia 1955:181-191. Jameson, D. L. and S. Peguegnat. 1971. Estimation of r e l a t i v e v i a b i l i t y and fecundity of color polymorphisms in anurans. Evolution 25:180-194. Resnick, L. E. and D. L. Jameson. 1963. Color polymorphism in P a c i f i c Tree Frogs. Science 142:1081-1083. 

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