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Asymmetry of Leg Size and Differential Leg Usage in the Sand Fiddler Crab, Uca Pugilator

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Abstract In fiddler crabs, males develop profound left-right asymmetry as a large claw grows opposite a small feeding appendage. Females are symmetrical, with paired small feeding appendages. The claw, used in signaling and contests, may favor adaptive responses in walking legs. This study of sand fiddler crabs, Uca pugilator, investigates differences in leg size and proportion: 1) between major (with the claw) and minor (without the claw) sides, using 404 males from which a pair of legs was removed, and 2) between males and females, using 1085 adults from which a single leg was removed. Among males, only the first walking leg was longer on the major side. This was due to the much greater length of the merus, a proximal long segment that is held parallel to the ground. The merus was longer on the major side in all walking legs. This may permit the claw to be extended away from the body without loss of balance when males either engage in contests over breeding burrows or wave the claw to attract females to burrows. More distal segments, the long carpus + manus, held approximately perpendicular to the ground, and the dactyl, anchoring the leg to the ground, were longer on the minor side, resulting in longer minor-side legs for the second to fourth pairs of legs. Female legs were proportioned like those on the minor side, with a long carpus + manus relative to the merus. However, rear legs of females are relatively long, which may facilitate holding an egg mass above the surface. In males, minor-side legs were more massive than major side legs. This suggests a greater work load on the minor side, as during contests that escalate to interlocking of claws, where minor side legs are important in pushing the opponent. Legs of females were more massive than those of males. The greater mass of female legs is not necessary to support an egg mass, which weighs less than the claw of a similar-sized male. The stouter legs of females may facilitate digging out of breeding burrows.

Affiliations: 1: (DKM:; AEP: Department of Biology, P. O. Box 8042, Georgia Southern University, Statesboro, Georgia 30460, U.S.A


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