In mid-Atlantic salt marshes, reproductively active male sand fiddler crabs, Uca pugilator, use a single greatly enlarged major claw as both a weapon to defend specialized breeding burrows from other males and an ornament to attract females for mating. Carrying the major claw, which can comprise as much as 40% of the total body mass, imposes significant energetic and locomotor costs to males; these costs are exacerbated by the location of breeding burrows in open areas high on the shore characterized by low food availability and high temperatures. Individual male mating success is positively correlated with both relative claw size and tenure in the breeding area. These observations suggest that male fiddler crabs face a potential trade-off: the very characteristics that make one successful in combat and attractive to females are accompanied by significantly greater energetic costs that may limit the total time one can participate in reproductive activity. We investigated whether variation in male breeding site tenure can be explained by a trade-off with claw size and associated energetic costs.

Results/Conclusions

In contrast to our expectations, an observational study following marked individuals found that males with relatively large major claws for their carapace width actually spent more time in the breeding area guarding a burrow and courting females, not less. Relative claw length was significantly positively correlated with body condition, suggesting that claw size is indicative of underlying male phenotypic quality. Finally, by experimentally manipulating males to be at one of three levels of condition before comparing the time spent in the breeding area by individuals in each group, we found that males in good condition also exhibited longer tenure. The relationship between body condition, relative claw length, and breeding area tenure therefore constitutes a positive feedback system that accords increased reproductive success (i.e., fitness) to high quality males. Our data are consistent with theory suggesting that sexually selected structures and behavioral displays that are costly to produce and maintain are honest signals of male quality to potential opponents and choosy females.