Predation by red-jointed fiddler crabs on congeners: interaction between body size and positive allometry of the sexually selected claw

The enlarged (major) claw of male fiddler crabs is used in contests over breeding burrows and is waved to attract females. We recently discovered that males of the red-jointed fiddler crab, Uca minax, also use the claw to kill smaller-sized fiddler crabs, U. pugnax and U. pugilator, with which they co-occur in Atlantic coast salt marshes. Large U. minax males use walking legs or the enlarged claw to capture prey feeding on moist sand flats. On sand flats, small U. minax males and females are much less common than large males, suggesting that large males move onto sand flats to seek prey. Males of prey species use the major claw against attacking predators and, consequently, are more likely than females to escape. In laboratory experiments, large U. minax males were more likely to attack and kill small-clawed males and females than large-clawed males, consistent with a preference for more vulnerable, less threatening prey. The size of the major claw is a positive allometric function of body size. The allometric function varies little among species. Also, the mechanical advantage and indices of closing speed and closing force of the major claw, when corrected for body size, are not consistently greater in U. minax relative to prey species. Thus, predation by U. minax males may reflect the opportunity afforded by larger body size and positive allometric growth, which result in a major claw that is more massive than the prey it is directed against. Copyright 2003.