ALLEN, B.J.*; LEVINTON, J.S.; State University of New York, Stony Brook; State University of New York, Stony Brook: Fiddler crab fitness: exercise and the cost of sex

Fiddler crabs (genus Uca) exhibit extreme sexual dimorphism. Males have a greatly enlarged major claw that comprises as much as 40% of the total body mass. Males with relatively larger claws have an advantage in male-male competition, and are more often chosen as mates by females. Sexually selected characters may have associated costs, and it has been suggested that the presence of the major claw might result in reduced foraging efficiency or increased risk of predation for male crabs. Empirical support for these hypotheses, however, has been mixed. Using a simple measure of performance efficiency, we tested the hypothesis that carrying the major claw represents an energetic cost to male fiddler crabs. We ran male sand fiddler crabs (Uca pugilator) on a motorized treadmill at a constant velocity, and measured time to fatigue. Crabs with a major claw tired significantly sooner than crabs without a major claw (which can be autotomized by the crab, and subsequently regenerated). Similarly, crabs carrying added weight tired sooner than crabs without an additional load. The relationship between weight and endurance is complex, and there appears to be a threshold weight associated with a marked decrease in crab performance. Crabs without a major claw crossed a threshold of endurance and could run for much longer periods of time. Our results suggest that claw size must be a reliable indicator of male fiddler crab quality, as only crabs in good condition would counteract the energetic cost of bearing such a large major claw. We are currently seeking to quantify this cost by comparing size-specific metabolic rates of male crabs with and without major claws.