Building a dishonest signal: the functional basis of unreliable signals of strength in males of the two-toned fiddler crab, Uca vomeris

Candice L. Bywater, Frank Seebacher, Robbie S. Wilson

Males of many species use signals during aggressive contests to communicate their fighting capacity. These signals are usually reliable indicators of an individual's underlying quality, however, in several crustacean species, displays of weapons do not always accurately reflect the attribute being advertised. Male fiddler crabs possess one enlarged claw that is used to attract females and to intimidate opponents during territorial contests. After the loss of their major, claw males can regenerate a replacement claw that is similar in size but considerably weaker. As this inferior weapon can still be used to successfully intimidate rivals, it represents one of the clearest cases of unreliable signalling of strength during territorial contests. We investigated the functional mechanisms that govern signal reliability in the two-toned fiddler crab, Uca vomeris. Male U. vomeris exhibit both reliable and unreliable signals of strength via the expression of original and regenerated claw morphs. We examined the morphological, biomechanical and biochemical characteristics of original and regenerated claws to establish the best predictors of variation in claw strength. For a given claw size, regenerated claws have less muscle mass than original claws, and for a given muscle mass regenerated claws were significantly weaker than original claws. The mechanical advantage was also lower in regenerated claws compared with original claws. However, the activity of three catabolic enzymes did not differ between claw types. We concluded that the structural and physiological predictors of force production influence the frequencies of reliable and unreliable signals of strength in U. vomeris. This study furthers our understanding of the proliferation of unreliable signals in natural populations.