ELEVATED PREDATION RISK CHANGES MATING-BEHAVIOR AND COURTSHIP IN A FIDDLER-CRAB

The fiddler crab, Uca beebei, lives in individually defended burrows, in mixed-sex colonies on intertidal mud flats. Avian predation is comm on, especially of crabs unable to escape into burrows. Mating pairs fo rm in two ways. Females either mate on the surface at their burrow ent rance ('surface mating') or leave their own burrow and sequentially en ter and leave ('sample') courting males' burrows, before staying in on e to mate underground ('burrow mating'). We tested whether perceived p redation risk affects the relative frequency of these mating modes. We first observed mating under natural levels of predation during one bi weekly, semi-lunar cycle. We then experimentally increased the perceiv ed predation risk by attracting grackles (Quiscalus mexicanus) to each half of the study site in two successive biweekly cycles. In each exp erimental cycle, crabs were significantly less likely to mate on the s ide with more birds. Moreover, on the side with elevated predation ris k, the number of females leaving burrows to sample was greatly reduced relative to the number of females that surface-mated. Males waved les s and built fewer mud pillars, which attract females, when birds were present. We discuss several plausible proximate explanations for these results and the effect of changes in predation regime on sexual selec tion.