Male fiddler crabs defend multiple burrows to attract additional females

Males of many species defend resources to attract females. Surprisingly, defense of multiple female breeding sites (e.g., nests or burrows) appears to be rare, primarily reported in fish and birds. In fiddler crabs, burrows are a vital resource for reproduction and survival. Both sexes defend individual territories centered on a single burrow. We examined burrow acquisition and defense in Uca capricornis to test whether males defend multiple burrows as a novel strategy to acquire additional mates. When crabs were experimentally forced to acquire a new burrow, females often settled into an empty burrow near resident males. We documented more empty burrows around males than expected by chance and, in addition, larger males had a greater proportion of empty burrows in their immediate vicinity. We experimentally introduced crabs into empty burrows next to focal males: newly introduced males were soon evicted, whereas females were courted and stayed. These results suggest that male U. capricornis defend empty burrows as a strategy to obtain more mates. Intriguingly, however, U. capricornis tend to occur in socially monogamous pairs. This raises the possibility of sexual conflict within social pairs over the presence of additional females and that female-female competition might constrain male mating success.