Claw-pinching force of sand fiddler crabs in relation to activity and the lunar cycle

Male sand fiddler crabs, Uca pugilator (Ocypodidae), possess a single enlarged claw that is used in ritualized contests over breeding burrows. Ability to close the claw forcefully, which increases with claw size and stoutness, may promote success in contests. As the density of males at breeding burrows changes across the lunar cycle, the frequency of contests may also change and affect the advantage associated with a powerful claw. At an Atlantic coast salt water marsh in northern Florida, we investigated claw size, claw stoutness, and pinching force with daily samples of foraging and courting males made across three lunar cycles. Foraging and courting males constituted the vast majority of surface-active males. Foraging males occurred in dense aggregations, droves, on moist, low-elevation sediments. Courting males, recognized by claw-waving displays, were at burrows in dry, high-elevation sediments where they fed relatively little. On average, courting males had greater claw-closing force than droving males but claw length and stoutness did not vary by activity. Claw-closing force, claw length, and claw stoutness of courting males were greatest on and near the days of new or full moons, when the densities of courting males peaked. No temporal patterns were observed for droving males. Mark/recapture of males revealed that claw power declined during courtship and increased while droving. Moreover, males switched from droving to courting as claw power increased and switched from courting to droving as claw power decreased. In the laboratory, the claw-closing force of unfed males decreased over time but rebounded quickly after males were given food. We suggest that males cycle between activities in response to strength and condition. However, it appears that males with larger, stouter claws are more likely to court at new and full moons, when the number of mating-receptive females peaks. •Claw pinching force is greater among courting than foraging males.•Claw pinching force declines while courting and improves while foraging.•Males switch from droving to courting once pinching ability is restored.•The density of courting males peaks at new and full moons.•Claw pinching force of courting males also peaks at new and full moons.