Sex-specific thermal constraints on fiddler crab behavior

Sexual selection via endurance rivalry favors increased tenure at the breeding site but can be costly. Here, we investigated thermal constraints on endurance rivalry in fiddler crabs, a group well suited for studies of interactions between sexual and natural selection due to ritualized mating behaviors and extreme sexual dimorphism. Fiddler crab males display in thermally stressful high-intertidal breeding areas. Retreating into burrows, an effective thermoregulatory mechanism, requires a cessation of courtship behaviors. We predicted that males should spend more time on the surface than females and that their ability to do so should decrease with increasing body temperature. In contrast, the amount of time females spend on the surface should be little affected by body temperature. We tested these predictions in the fiddler crab Uca panacea by manipulating body temperatures and monitoring surface activity and habitat choice in large outdoor tanks during the breeding season. Males showed consistently higher surface activity than females. Effects of body temperature on surface activity were sex-specific, occurring in males but not in females, indicating sex-specific thermal constraints on surface behavior and supporting our hypothesized thermal constraints on endurance rivalry.