GARCIA, I.L.: Fiddler crab posing: field tests of four hypothetical functions

On sunny summer days up to half of the Uca pugnax visible on exposed marsh surface in Lewes, DE exhibit a stereotypical posture. The crabs stand on the tips of their legs, backs oriented to the sun. They use four or fewer legs to hold their bodies off the ground while extending their claws and remaining legs away from the body. They can pose motionless for up to two hours and while in this posture seem less sensitive to movement around them, lingering, visible on the surface while other crabs dart into their burrows. If this behavior is a sexual display, then similar to the familiar claw waving display, its frequency of occurrence should parallel the lunar cycle. Behavioral surveys performed in the field throughout the lunar cycle show that the posing frequency does not parallel that of the waving display but instead increases as ambient temperatures rise. If this behavior facilitates molting (Jansen, 1970), then posing animals should molt more frequently than non-posers. I collected 72 crabs that were either posing, active on the surface or in nearby burrows and monitored them in the lab for 4 weeks. Burrowing animals molted at the highest rate (8%, compared to 4% for posing or active crabs). If posing serves to cool an animal, its temperature should be lower than others, while if the behavior serves to warm an animal, the opposite would be true. In the field, I measured the temperatures of crabs that were either posing for at least two minutes, otherwise active on the surface or in adjacent burrows. The posing crabs were significantly warmer (Friedman two-way ANOVA, N = 40 , P <.001). Thus, posing seems to provide a basking function. Higher body temperature could expedite digestion, gamete maturation or other physiological functions. However, these benefits must be considered in the context of the potential risk involved.