Sexual differences in morphology and behaviour: evolution and maintenance in fiddler crabs
Creators: name: Chou, Chun-Chia
Publisher: name: The Australian National University
Year: 2020

description: 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 (in italic font) and Gelasimus borealis (in italic font). 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 degree Celsius dramatically decreased hatching success. This suggests that this species of fiddler crab is unlikely to cope with predicted future temperature increases. 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 mate 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.