Approach of females to magnified reflections indicates that claw size of waving fiddler crabs correlates with signaling effectiveness

Male sand fiddler crabs, Uca pugilator, wave a claw to attract females to a breeding burrow. The effect of claw size on the likelihood of attracting mate-seeking females is little studied although in some other species females preferentially approach larger males. We used paired mirrors to reflect different sized images of the same male in a South Carolina (USA) back-beach habitat. Use of mirrors controlled for waving rate (but not velocity), waving motion, claw color, and claw shape. Female choice was attributed to instances in which a female contacted one of two mirrors. Paired mirrors were inclined toward one another in an arena defined by blinds and containing a single male. Two reflections of the male were visible to females moving approximately 50 cm toward the mirrors. The male was behind a small internal blind and not directly visible. In one-half of the trials, a non-magnifying mirror was placed at the bottom of mirrors so that only the elevated claw was magnified. Thus, body and burrow size and apparent distance were controlled. Receptive females preferred the larger reflection whether or not the body of the male was magnified, suggesting the importance of claw size. Non-receptive females did not exercise a choice. Control arenas, without a male, rarely attracted females. The results suggest that females choose on the basis of claw size. Selection on females may favor response to larger-clawed males because use of the claw in contests between males over burrows maintains the honesty of claw size as a signal of burrow quality.