The Role of the Visual Train Ornament in the Courtship of Peafowl, Pavo

The role of the visual train ornament in the courtship of peafowl, Pavo cristatus by ROSLYN DAKIN A thesis submitted to the Department of Biology in conformity with the requirements for the degree of Master of Science Queen’s University Kingston, Ontario, Canada September, 2008 Copyright © Roslyn Dakin, 2008 ii ABSTRACT The peacock (Pavo cristatus) has long been considered the quintessential example of a sexually selected animal, and in the last two decades, peafowl have provided widely-cited evidence for female mate choice as well as the genetic benefits of mate preferences for ornamented males. However, previous studies have failed to reach a consensus with respect to the importance of various signaling modalities in peafowl courtship. In this thesis, I repeat two previous studies of peacock train morphology and I describe the use of light by males during their courtship displays, to clarify the role of visual signaling. I confirm previous reports that removing a large number of eyespots decreases male mating success, yet I find substantial variation in mating success among normal males that cannot be explained by eyespot number. I show that these two apparently conflicting results are not contradictory, since the removal treatment modifies males beyond the normal range of eyespot number. Next, I describe the two display behaviours used by males during courtship. When males perform their pre-copulatory “train-rattling” display, they are oriented at about 45° relative to the sun on average, with females directly in front. This directional pattern suggests that train-rattling is involved in the communication of a visual signal. The “wing-shaking” display, on the other hand, is performed with females positioned behind males, and is always elicited when a model female is positioned on the shaded side of a male. The wing-shaking display may therefore allow males to control female viewing geometry. These results indicate that mate choice in peafowl is complex, and that visual signaling is important despite recent claims to the contrary. Females may avoid males missing a large number of eyespots via iii a threshold-based mechanism, while choosing among full-trained males based on some other (possibly visual) cue. iv CO-AUTHORSHIP This thesis conforms to the “Manuscript Format” outlined in the Department of Biology Guide to Graduate Studies. Chapters 2 and 3 are co-authored by my supervisor, Dr. Robert Montgomerie, who contributed to the experimental design as well as the analysis and presentation of results. v ACKNOWLEDGEMENTS First, I thank my supervisor Bob Montgomerie for guidance and support over the last three years – I have benefited tremendously from it. I also acknowledge my committee members, Steve Lougheed and Paul Martin, who provided a number of insights. Second, I thank the directors and staff of the Los Angeles Arboretum (LAA), Assiniboine Park Zoo (APZ), Toronto Zoo (TZ) and Bronx Zoo (BZ) for allowing me to chase after their peafowl. This work would not have been possible without the logistic support of Dr. Mark Wourms (LAA), Dr. Bob Wrigley and Dr. Gord Glover (APZ), Dr. Christine Sheppard (BZ), and Dr. Tom Mason and Dr. Graham Crawshaw (TZ). The keeper and veterinary staff from the Assiniboine, Toronto and Bronx Zoos were particularly helpful in handling the birds – especially Janice McCarthy and David McLelland. Funding for this research was provided by an NSERC Canada Graduate Scholarship. Field work was supported by NSERC grants to R Montgomerie (Discovery and equipment), and the Society for Canadian Ornithologists (Fred Cooke award). I am indebted to Rob Ewart and Jason Clarke for hard work (and good company) in the field – I couldn’t have done it without you. Lori Parker helped with field work in Winnipeg and provided encouragement. Dr. Norman Johnson provided a peahen from his farm, and Vanya Rowher did a fantastic job of preparing the experimental mount. Finally, I am grateful to my family and friends for their support, especially those in the Biology department for many inspiring discussions, and to Charlie, for help in more ways than I can list here. vi TABLE OF CONTENTS Abstract …………………………………………………………………….…… ii Co-authorship ……………………………………………………………....…... iv Acknowledgements ………………………………………………………….….. v Table of Contents …………………………………………………………….…. vi List of Tables ………………………………………………………………….... viii List of Figures …………………………………………………………………... ix CHAPTER 1. General Introduction …………………………………………….. 1 References …………………………………………………………….. 15 CHAPTER 2. Train morphology, ornamentation and mate preferences in peafowl ………………………………………………………………………….. 22 Abstract ……………………………………………………………….. 22 Introduction ………………………………………………………….... 22 Materials and Methods ………………………………………………... 26 Results ……………………………………………………………….... 30 Discussion …………………………………………………………….. 34 References …………………………………………………………….. 38 CHAPTER 3. Peacocks orient their displays towards the sun ………………….. 49 Abstract ……………………………………………………………….. 49 Introduction ………………………………………………………….... 50 Materials and Methods ………………………………………………... 54 Results ……………………………………………………………….... 60 Discussion …………………………………………………………….. 64 References …………………………………………………………….. 68 vii CHAPTER 4. General Discussion …………………………………………….... 79 References …………………………………………………………….. 84 SUMMARY …………………………………………………………………….. 87 APPENDIX ……………………………………………………………………... 88 viii LIST OF TABLES Table 2.1 Previous studies of the number of eyespot feathers in peafowl ….……. 40 Table 2.2 Comparisons of the train morphology and behaviour of control males from three populations …………………………………………….….... 41 Table 2.3 Regular arrangement of eyespots in rows on the trains of four normal adult peacocks ……………………………………………………..…… 42 Table 2.4 Effect of eyespot removal on male behaviour and mating success ……. 43 Table 2.5 Relations between train morphology and mating success for control males ……………………………………………………………….…... 44 Table 3.1 Display responses of males to a model female presented under different experimental contexts ……………………………………….................. 71 Table 3.2 Directionality of male display responses to a model female …………... 72 Table A.1 Sampling for model presentation experiment from Chapter 3 ……….... 88 ix LIST OF FIGURES Figure 1.1 The highly complex train ornament of a peacock …………………... 20 Figure 1.2 Close up view of a single eyespot feather …………………………... 21 Figure 2.1 Four specialized upper-tail covert feathers in the peacock’s train ….. 45 Figure 2.2 Train morphology of two normal adult peacocks that were not captured, and two eyespot removal treatment peacocks ………….…. 46 Figure 2.3 Effect of removing the 20 outermost eyespot feathers on male mating success ……………………………………………………………….. 47 Figure 2.4 Relations between the number of eyespots and male mating success, and between train length and male display rate ……………………... 48 Figure 3.1 Daily time budget for peacocks showing the average percent of time males devote to each activity at each level in the hierarchy ……….... 73 Figure 3.2 Diagram from above of the 60° sectors around a displaying peacock, used to identify the position of the female relative to the male, and three angles of interest ……………………………………………….. 74 Figure 3.3 Sequence of events over typical male display bouts ……………….... 75 Figure 3.4 Position of female observers relative to the male during display, and male movement during the wing-shaking and train-rattling displays .. 76 Figure 3.5 Sun-male angle when females were absent ………………………….. 77 Figure 3.6 Sun-male angles and sun-female angles during the wing-shaking and train-rattling displays ……………………………………………….... 78 1 CHAPTER 1. General Introduction For millennia, people have marveled at the intricate complexity of the peacock’s tail (Figure 1.1). The sheer size of the tail – or more correctly, the train – is particularly baffling. It contains roughly 200 elongated feathers, 150 of which are tipped with an iridescent multicoloured eyespot, so named because of an eye-like pattern of concentric ellipses (Figure 1.2). Many have considered peacocks “the most splendid of living birds” (Darwin 1871), and the current distribution of feral populations throughout the western world is no doubt due to man’s fascination with their glittering train (Jackson 2006). Yet the fact that has been so obvious to generations of human admirers – that the sole purpose of the train is its beauty – is one that has posed some problems for evolutionary biology. From Darwin (1871) asking how natural selection could allow such extravagance to more recent questions about how variation in ornamental traits could be maintained (Williams 1975, Borgia 1979), the peacock’s train has been considered the quintessential example of a sexually selected trait. Questions about the evolution of such ornamental traits have led to a number of insights with broad relevance, such as the reasons that animal signaling systems might remain stable despite the benefits of deception (Maynard Smith and Harper 2003). These studies have also contributed to our understanding of the causes and consequences of mate choice (Andersson 1994), and may even be relevant to the generation of biological diversity, since the main difference between many closely- related species often involves ornamental traits similar to the peacock’s train (Darwin 1871). 2 “A Taste for the Beautiful” Peacocks were a problem for Charles Darwin. The complexity of the eyespot feathers made him sick with worry (Darwin 1860 in Burkhardt et al. 1993), especially since he was sure that the size of the train would

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