Antisymmetry in male fiddler crabs and the decision to feed or breed

Abstract

Summary In male Sand Fiddler Crabs, Uca pugilator, a major cheliped (with claw), used in intersexual displays and intrasexual contests, develops opposite a minor cheliped used for feeding. Cheliped size demonstrates antisymmetry because greater development is equally likely on the right or left side. The side with the major cheliped (major side) also has longer walking legs which may facilitate use of the claw. In contrast, eye stalk asymmetry is equally due to antisymmetry and fluctuating asymmetry. Fluctuating asymmetry is a subtle, non‐adaptive departure from the population‐level trajectory relating growth on major and minor sides. In a South Carolina (USA) marsh, cheliped and leg antisymmetries are greater and eye stalk asymmetry is less among males able to defer feeding in favour of breeding. However, the composition of up‐slope breeding and down‐slope feeding subpopulations changes across the lunar cycle. The number of mates sequestered in breeding burrows is positively correlated with cheliped and leg antisymmetry and negatively correlated with eye stalk asymmetry. Male fitness is a function of the product of time spent breeding and the number of mates per unit time while breeding. Both fitness components are predicted by relative cheliped antisymmetry and eye stalk fluctuating asymmetry, which are themselves significantly negatively correlated.