Meeting Abstract

39.10  Jan. 6  Fiddler crab claws are both beautiful and powerful weapons: a paradox resolved CHRISTY, J. H.*; DENNENMOSER, S.; Smithsonian Tropical Research Institute christyj@si.edu

Male fiddler crabs have one greatly enlarged claw that they wave to attract females for mating and that they use as a weapon to fight other males. Longer claws probably are more effective visual signals and attractive to females. Male fiddlers increase the length of their large claw by lengthening their dactyl (movable finger) and polex (fixed finger) disproportionately relative to the size of their manus, which contains the closing muscle. Consequently, as males grow, the closing force they deliver at the tip of their claw decreases relative to their size. J. Levinton and colleagues have called this the paradox of the weakening combatant: beauty begets weakness. We studied claw morphology and fighting in two tropical fiddler crabs Uca beebei and Uca terpsichores and found a resolution to this paradox. When fighting males contracted their intertwined claws the forces were delivered not at the tips of the claws but at tubercles on the inner margins of the dactyl and polex. These tubercles contacted the inner and outer surfaces of the manus and often left small puncture wounds providing a record of where the forces were delivered. As claws grow these tubercles remain relatively close to the apex of the gape so that the mechanical advantage governing the delivery of forces at these points stays constant or decreases only slightly relative to claw length. Consequently the closing force at these tubercles, the product of the mechanical advantage and the cross-sectional area of the closer muscle, increased much more rapidly with male size than did the forces at the tip of the claw. Through differential growth of the dactyl and polex distal to the tubercles where the forces of contraction are applied, male fiddler crabs make large claws that are both beautiful and powerful weapons.