Selection for conspicuous visual signals in a fiddler crab

Abstract: Sexual selection has an undeniable influence in the evolution of the spectacular diversity of courtship signals in the animal kingdom. A long history of study has pointed to mechanisms through which sexual selection can act: it can favor signals that are reliable indicators of species identity or effectively transfer mate quality information. In some species, these mechanisms have the potential to shape signal evolution. This is the case in fiddler crabs. Males court females by waving their sexually dimorphic claw. Females recognize conspecific males by the species-specific display, while intraspecific variants of the display appear to be indicators of male quality. We investigated which of these mechanisms prevail by using robotics to test female responses to waves of different heights in the fiddler crab, Austruca mjoebergi. We reveal that, although the studied species shares a sympatric habitat, females did not significantly more often approach the species’ average signal. We found evidence that more conspicuous, higher signals were more likely to attract females, although the effect was not particularly strong. We discuss our results in the light of other possible scenarios from which sexual selection can act in the evolution of signals. Significance statement: Sexual selection has strong role in the evolution of courtship signals. A large body of evidence has revealed that mating preferences may favor signals that indicate species identity or mate quality. We study which of these mechanisms is predominant in fiddler crabs. We use robotics to investigate female preference for a highly conspicuous and diverse characteristic of courtship signals. We found that females most likely do not use the signal for species recognition, but instead favor signals that stand out and indicate mate quality. Our results advocate that directional selection is likely to be predominant, but we suggest that a more complex mosaic of selective forces may influence the evolution of the high interspecific signal diversity in fiddler crabs.