E&E PhD Exit Webinar: Sexual differences in morphology and behaviour: evolution and maintenance in fiddler crabs

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 represents an ideal system for studying this due to the remarkable morphological and behavioural differences between the sexes. Males are relatively larger in size and have only one feeding claw since the enlarged major claw is specialized for waving displayes during courtship and for male-male combat. Females, on the other hand, possess two feeding claws; they spend a lot of time in both selecting a mate and parental care. This thesis examines the evolution and maintenance of sexual differences in two fiddler crabs: Austruca mjoebergi and A. borealia.

I found that male fiddler crabs incur considerable behavioural and nutritional costs in maintaining the enlarged claw. The enlarged claw also 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 requires more nutrition than females because females allocated relatively more elemental nutrition into their gonads as a pre-mating parental investment.

Environmental temperature also affected males and females differently. Male in sunny areas of the habitat spend 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 successful of two sexes through different pathways.

This seminar will present the empirical evidence that ecological forces can shape the evolution of sexual differences in morphology and behavior in an invertebrate.