Attractiveness of sand hoods built by courting male fiddler crabs, Uca musica: test of a sensory trap hypothesis

Courting male fiddler crabs, Uca musica, sometimes build sand hoods at the entrances of their burrows to which they attract females for mating. On average, females visit 17 males in as many minutes before they choose a mate, and they preferentially visit males with hoods. When moving between burrows, fiddler crabs of both sexes sometimes approach and temporarily hide against objects on the surface. Hence, mate-searching females may approach hoods because they resemble (mimic) other objects that crabs approach to reduce their predation risk. We conducted two experiments to test this sensory trap hypothesis. First, we determined whether sexually receptive and nonreceptive female U.musica and nonreceptive female U.stenodactylus, a species that does not build structures, spontaneously approach hoods (replicas), stones, pieces of wood and shells. As predicted by the sensory trap hypothesis, both species, irrespective of sexual receptivity, approached these objects and neither preferred hoods. Second, to determine whether female U.musica show a preference for hoods when they search for a mate, we recorded the frequency with which females approached males with natural hoods, hood replicas, wood, stones and shells. Again as expected, females approached males with these different structures at the same rates. We conclude that hoods are effective mimics of objects that females approach for safety whether they are searching for a mate or not. Males benefit by using this sensory trap because hoods make them more attractive, and receptive females may benefit when they approach hoods because they reduce their mate-search risk. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.