Costs of bearing a sexually selected ornamental weapon in a fiddler crab

Summary1 Sexually selected structures with dual function of combat and display are likely to be honest signals of male quality to opponents and mates, but should be costly to produce and maintain. 2 Male fiddler crabs use a single greatly enlarged claw as both a weapon in agonistic contests with other males and an ornament to attract females for mating. Given the extreme size of this structure (up to half the total body mass), there is surprisingly little evidence for costs as predicted by theory. 3 We experimentally investigated several potential costs of the large claw to male sand fiddler crabs Uca pugilator . Mass‐specific metabolic rates were significantly higher and treadmill endurance capacity significantly lower for males bearing an intact major claw compared with those without. 4 In contrast, presence of the claw did not affect maximal sprint speeds, suggesting that the massive structure does not compromise the ability of male crabs to evade predators. These counterintuitive results conform to recent theoretical models of energy costs of locomotion. 5 Our study provides empirical support for a key assumption of sexual selection theory – energetic and endurance‐related locomotor costs incurred while bearing this ornamental weapon act in opposition to sexual selection favouring larger claws.