Laboratory measures of whole-animal performance are expected to correlate with morphological indices and success in activities that contribute to fitness. Males of the sand fiddler crab, Uca pugilator Bosc (Brachyura Ocipodidae), possess a single enlarged claw that is used in ritualized contests for possession of breeding burrows. Contests may escalate to pinching actions, suggesting claw closing force as a relevant performance indicator. When a claw is lost, it is regenerated. Within a western Atlantic salt marsh, regenerated claws are less massive and operate at reduced mechanical efficiency relative to original claws, suggesting a less powerful weapon. In the laboratory, the closing force of regenerated claws is less than that of original claws of the same length or mass. Perhaps as a consequence, males with regenerated claws are unlikely to use pinching behaviors and usually lose contests against males with original claws. Males with regenerated claws are less likely to possess breeding burrows that females find acceptable and tend to possess burrows during non-peek breeding times, when the frequency of contests is low. As crabs grow, claw power is sacrificed for greater length. This may facilitate advertising burrow ownership to passing females through claw waving displays. The reduced mass of regenerated claws may permit males to resume effective waving sooner than if they regenerated a stout claw, but at the cost of competency in contests.