Selection for sexually dimorphic traits and signal diversity in fiddler crabs

Abstract

The evolution of diverse sexual signals and traits in animals is a promising yet challenging field in behavioural ecology. Sexual features are under the influence of a multitude, and often cryptic, mechanisms. In addition, the composition and strength of selection varies for each study species. Fiddler crabs are ideal subjects for investigating the evolution of diverse sexual traits. Males possess one sexually dimorphic enlarged claw. The structure is used as a weapon in conflicts over territory and signaled in the form of wave displays as courtship to females and warning to intruders. The displays are diverse on the interspecific level, characterized by species-specific movement patterns. In this thesis, I explore the forces behind the evolution of sexually dimorphic claws and the great diversification of fiddler crab signals.

In the first two chapters of my thesis I search for a clearer understanding of sexual selection in shaping the species-specific diversity of wave displays in fiddler crabs. First, I investigate if wave displays are cues for species identity in sympatric populations. I use robotic crabs in my experiments and verify that females are able to choose the conspecific over a heterospecific wave movement. In chapter II, I direct my focus to female natural choices and identify their preferences on wave displays and claw size. This final approach allows me to reveal other selective mechanisms, such as constraints from natural selection in male signaling effort.

In the second part of my thesis, I expand my approach to the level of social context and its effects on fiddler crab sexual traits. In chapter III, I look into the laterality of the sexually dimorphic claw and examine the different fight endurances when opponents are same- or different-handed. I point out how distinct handedness ratios in fiddler crab populations set contrasting selective pressures in male-male coalitions. In chapter IV, I explore differential social situations in the production of wave signals, and take a step further to explore the social contexts that stimulate the emergence of a rare signal phenomenon, the synchronous waving.

Finally, in chapter V, I look into signal diversity from a wider perspective by applying comparative approach to understand the evolution of the wave displays. I gather structural and temporal information of the wave displays of 28 species to predict the likelihood and evolutionary paths of presenting waves in synchrony. Lastly, I indicate how the phenomenon may stem from the particularities of a species mating systems and ecological adaptations. Show more