Male spacing and female choice in a fiddler crab

The aggregation of courting males is widespread among animal taxa, yet we do not understand why males congregate and therefore intensify local competition for female attention. The most commonly invoked theoretical explanation is that females preferentially approach clustered males due to the many benefits they would gain, and clustered males would therefore have higher mating success. However, although theoretical explanations of aggregation formation are well advanced, empirical studies are scarce, especially in invertebrates. In fact, there is little evidence that females do prefer to approach clustered displayers over spaced displayers. Here we address this question by using robotic crabs to test female preferences in fiddler crabs (a visually displaying species) and show that females do not preferentially approach clustered males. We suggest that if this pattern is more widespread, the most commonly invoked explanation of courting aggregations is of limited use. We offer explanations for the strong clustering behavior we observe in this species and discuss the implications of this finding for the theoretical underpinnings of this research field.