Direct and biometrical estimates of the closing force of major claws of the sand fiddler crab Leptuca pugilator (Bosc, 1801) (Decapoda: Brachyura: Ocypodidae): Support for the weakening combatant hypothesis

Abstract

Abstract Many conclusions concerning the functional biology of crab claws rely upon biometrical estimates of closing force, based upon measures of muscle cross-sectional area and mechanical advantage. Fiddler crab closing force patterns show variation with body size, claw size, location of the opposing claw tips, and physiological condition, so we have measured closing force of the sand fiddler crab Leptuca pugilator (Bosc, 1801) as a function of claw size, force exerted at claw tips, and at the commonly well-developed pollex tooth. Leptuca pugilator has an elongated claw with gracile dactyl and pollex. As predicted by biometrical proportions, closing force is greater at the pollex tooth than at the claw tip. The pollex tooth does shift with increasing claw size in relative position toward the claw hinge. Mechanical advantage at the pollex tooth and dactyl tip both decline with increasing claw length. But there is no difference in slope of log closing force as a function of log claw length between the pollex position and terminus of the dactyl, which demonstrates that force exerted at the pollex tooth has no impact on proportional change in closing force with increasing claw size. The log-log slope is ~0.9, reflecting the proportionally decreasing muscle cross-sectional area and lowering mechanical advantage with increasing claw size. For both the pollex tooth and the claw tip, mechanical advantage decreases very slightly with increasing claw size, but closing force proportionally decreases with increasing claw size, supporting the weakening combatant hypothesis for this species.