Sexual Differences in Morphology and Behaviour: Evolution And
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Sexual differences in morphology and behaviour: evolution and maintenance in fiddler crabs A thesis submitted to The Australian National University For the degree of Doctor of Philosophy Chun-Chia Chou October 2020 2 CONTENTS ABSTRACT 5 ACKNOWLEDGEMENTS 7 GENERAL INTRODUCTION 9 CHAPTER 1: Staying cool: the importance of shade availability for tropical ectotherms. 20 This chapter has been published as: Chou, C-C, Perez, DM, Johns, S, Gardner, R, Kerr KA, Head ML, McCullough EL, Backwell, PRY (2019) Staying cool: the importance of shade availability for tropical ectotherms. Behavioural Ecology and Sociobiology 73 (8): 106. Author contribution: CCC had overall responsibility for the project and was involved in every stage. In collaboration with PRYB, she conceived the original ideas and designed the methodology. All authors collected the data. CCC analysed the data, with advice from DMP. CCC wrote the manuscript; all authors read and edited drafts and gave approval for submission. CHAPTER 2: Effects of temperature on reproductive timing and hatching success in a 57 tropical fiddler crab. This chapter has been published as: Chou, C-C, Head, ML, Backwell, PRY. (2019) Effects of temperature on reproductive timing and hatching success in a tropical fiddler crab. Biol J Linn Soc., 128, 817–827. Author contribution: CCC and PRYB conceived the ideas and designed the methodology. CCC had overall responsibility for the project and was involved in every stage. CCC collected all the data. CCC analysed the data with help from MH and PRYB. CCC wrote the manuscript. MH and PRYB read and edited drafts, and gave approval for submission. 3 CHAPTER 3: Pre-mating parental investment: nutritional allocation in a fiddler crab. 88 This chapter has been prepared for publication in Functional Ecology as: Chou, C-C, Lee, SY. Pre-mating parental investment: nutritional allocation in a fiddler crab. Author contribution: CCC had overall responsibility for the project and was involved in every stage. She conceived the original idea; designed the methodology alongside SYL; collected and analysed all of the data; and wrote the manuscript. SYL edited drafts and gave final approval for submission. CHAPTER 4: Ultraviolet reflectance signals who you are but not how fit you are. 118 This chapter has been submitted for publication as: : Chou, C-C, Cosgrove, M, Backwell, PRY. Ultraviolet reflectance signals who you are but not how fit you are. In review in Behavioral Ecology Author contribution: CCC had responsibility for the project and was involved in all stages. CCC conceived the ideas and designed methodology with advice from PRYB. MC took and interpreted the UV photographs CCC collected the rest of the data; analysed the data and wrote the manuscript. PRYB and MC read and edited drafts, and gave permission for submission. FINAL DISCUSSION 141 APPENDIX: other studies undertaken during the PhD candidature 145 4 ABSTRACT In anisogamous mating systems, males and females play different roles. This results in sexual differences in morphology and behaviour. Although the importance of sexual selection is widely recognised, we still lack a clear understanding of how sexual differences in morphology and behaviour evolve and are maintained under different ecological stressors (e.g., nutritional processes, global warming, resource availability). Fiddler crabs represent an ideal system for studying this due to the remarkable morphological and behavioural differences between the sexes. Males are larger in size and have only one feeding claw since the other feeding claw has been enlarged as a specialisation for waving displays during courtship and for male-male combat. Females, on the other hand, possess two feeding claws; although they spend less time feeding than males, they spend a lot of time in both selecting a mate and in parental care. This thesis examines the evolution and maintenance of sexual differences in morphology and behaviour in two fiddler crabs: Austruca mjoebergi and Gelasimus borealis. In chapter 1, I found that temperature affected males and females differently. Males in sunny areas of the habitat spent far less time surface-active that males in shaded parts of the habitat. This has a strong detrimental effect on feeding and courtship time. Temperature and/or sun and shade had a negligible effect on female surface activity but the fertilisation and hatching success of clutches were correlated with environmental temperature. This indicates that the increased temperature can have impacts on the reproductive success of two sexes through different pathways. In chapter 2, I found that temperature also affected reproductive success. Although temperature had no effect on the timing of fertilisation or hatching, and larval development rate was not temperature- dependent, an increase in incubation temperature of 2-4 ˚C dramatically decreased hatching success. This suggests that this species of fiddler crab is unlikely to cope with predicted future temperature increases. 5 In chapter 3, I found that male fiddler crabs incur considerable behavioural and nutritional costs in maintaining the enlarged claw. The enlarged claw costs males in terms of feeding time: the feeding rate of male fiddler crabs is significantly lower than females (that have two feeding claws). To compensate for this, males have increased the size of their single feeding claw and spend more time feeding that females. I found that, overall, males did not require more nutrition than females because females allocated relatively more elemental nutrition into their gonads as a pre-mating parental investment. In chapter 4, I found that ultraviolet (UV) signals are monomorphic (occur in both males and females). Although females prefer males with UV signals (as opposed to no UV signals), the amount of UV reflection had no effect on female preferences. UV signals also showed no effect on the outcome of territorial fights between males. The finding suggests that the UV signal does not reflect individual quality but provides some critical information (possibly species recognition) during mate choice process. Together, the findings of this thesis present empirical evidence that ecological forces can shape the evolution of sexual differences in morphology and behaviour in an invertebrate. Temperature, food quality/availability and ultraviolet colouration can all affect the behaviours and morphologies of male and female fiddler crabs. This thesis adds to the growing body of knowledge that will allow a thorough understanding of the behaviour and ecology of this key mangrove species. 6 ACKNOWLEDGEMENTS Finishing my PhD has not been an easy task and thus I would like to thank those who helped, supported, did favours, and encouraged me. Each act of kindness has contributed to my ability to finish this thesis. I firstly want to thank Pat, my PhD supervisor. She is such an amazing person and was a powerful influence during my PhD life. There is no way that I could have completed my PhD without her. We had countless meetings during my PhD candidature; I never felt anxious about having a meeting with Pat even though sometimes I might be struggling with my experiments. I knew she would always be here to help me through. Knowing this provided me with the confidence to invest all my energy into research on the ideas that most interest me. This allowed me to see where my limits lay, which is a rather satisfying experience. It has allowed me to think clearly about my growth research-wise and life-wise, but more importantly, it has given me a deep understanding of my true identity and my limits. Having an opportunity to test one’s limit is rare and most people do not get this opportunity. Therefore, I am truly grateful to the opportunity of working with Pat and to the flexibility she creates in the lab. I learned a lot from Pat and enjoyed every single moment working with her during this four-year journey. My family and friends have also played an important role throughout my PhD. My family are very positive and respected my decision to study in Australia. Thanks to modern technology, we were able to have regular weekly video chats; we sometimes fought, sometimes argued about very small things, and we shared the sad and happy moments, just like we used to do before I went to Australia. This made me feel that nothing had changed and we stayed very close with each other despite the physical distance. I was very lucky to meet a lot of amazing people and became friends with them during my studies in Australia. They were from several groups: my colleagues from EE, my Taiwanese friends, and my badminton teammates. Working with the EE big family is always a pleasant experience. The atmosphere is very friendly, harmonious, and cooperative. New members are made to feel comfortable 7 and helped to settle in. As I adapted to the new environment and got to know more people, I started making solid friendships that will last a lifetime. I am very appreciative to have a group of people who possess the same goals and who inspire and encourage each other, and have the same language so that they are able to understand things I am struggle with. I especially want to thank Hee-Jin and Jessie for constantly being the source of my motivation and positivity during my PhD life. They are my true peers and I always felt awesomely energetic after talking to them. I also like to thank Wes for his kind help with proofreading the thesis. I received great encouragement from my old friends back in Taiwan and I also established solid friendship with some new Taiwanese friends at the ANU. Most of us were in a similar life stage and coming from the same cultural background accelerated the development of our friendships. They were not only good traveling partners; they were also always there to share the bitter and sweet aspects of international study.