Male fiddler crabs prefer conspecific females during simultaneous, but not sequential, mate choice

Mate choice is potentially beneficial whenever prospective mates vary in quality, but when mates are encountered sequentially the cost of rejecting a current mating opportunity affects the net benefit of choosiness by lowering the mating rate. There is, however, no reduction in mating rate when choosing among potential mates that are encountered simultaneously. In general, mating with a heterospecific is costly as the resultant offspring are of low fitness. It is often argued that males, unlike females, will court and even mate with heterospecifics because the lost opportunity cost is minimal if they rarely encounter potential mates. In the fiddler crab Uca mjoebergi, we show that, in a natural situation, where females arrived sequentially males were equally likely to court conspecifics and heterospecifics. Females were released individually into the population, and nearly every male they passed performed a courtship waving display whether the female was conspecific or heterospecific. Taken alone, this result implies that males exhibit no species discrimination. However, in an experimental setting where males simultaneously viewed a conspecific and a heterospecific female, males waved faster and for longer at conspecific females, and attempted to mate more often with conspecifics. This indicates that U. mjoebergi males can discriminate between conspecific and heterospecific females and prefer to court conspecifics when given a choice. We used mate choice among rather than within species (to maximize variation in mate quality) to illustrate the need to distinguish between simultaneous and sequential mate choice when quantifying mating preferences.