The bigger you are, the weaker you are: Multispecies evidence for the weakening combatant hypothesis

Abstract

As fiddler crabs increase in size, closing force weakens proportionally because claw growth is not isometric. As a result, mechanical advantage decreases at the tip with increasing claw length. Closing force at an interior tubercle is greater at the tip, and a previous study of two species claimed evidence for compensation by relative movement of tubercles toward the hinge as size increases. I reconsidered this hypothesis with a biometric analysis of eight species from five genera and all three major biogeographic regions. I tested test two null hypotheses: 1) as claw length increases, mechanical advantage did not increase at the tubercle, relative to the claw tip, and 2) closing force did not increase at the tubercle, relative to the claw tip, as claw size increased. Both null hypotheses are largely supported, which in turn supports the weakening combatant hypothesis. In all cases, log force as a function of log claw length at an interior tubercle is 1.40, which is less than 2, the slope value expected if there was complete compensation. This is only slightly greater than the slope at the dactyl tip, which is 1.21. Thus, as a fiddler crab gets larger, with an incremental increase of claw length its closing force proportionally decreases, supporting the idea of a weakening combatant. But it gains a large showy claw with relatively narrow pincers and relatively less mass required to swing around while waving. This is indeed a beautiful weapon, but not for sheer closing force.