Exaggeration of a sexually selected signal – not by female choice

JOHN H. CHRISTY; PABLO D. RIBEIRO

Coquimbo-La Serena, Chile

Congreso; The Crustacean Society Mid-Year Meeting 2007; 2007

The Crustacean Society

Sexual and natural selection on male courtship signals usually act in opposition: female preferences favor ever larger, more elaborate male signals while natural selection checks signal exaggeration.  Here we describe an apparent case of signal elaboration due to natural and intra-sexual selection, not a directional female preference. The signals are sand structures called hoods that courting male fiddler crabs Uca terpsichores sometimes build at the openings of their burrows.  Relative to male size hoods are the largest courtship structures built by male fiddler crabs. Males attract females to their burrows for mating, females prefer to approach males with hoods, hoods visually guide females´ movements to males´ burrows and the attractiveness of males with hoods increases with perceived predation risk.  Hence, hood building is sexual selected by a female preference. Males also use their own hoods to find their burrows when unable to do so using their non-visual path map. To determine whether hood size affects hood attractiveness to females and male burrow finding ability, we placed models of ½, ¾, full, and super-sized hoods on males´ burrows. We recorded the relative frequency that females approached males with the different models. We introduced errors in males path maps by displacing them on sliding platforms and recorded the time it took these males to relocate their burrows with the different model hoods. The female preference curve rose rapidly from males with no or ½ size model hoods to males with ¾ or average size hoods, after which there was no further increase in attractiveness.  In contrast the larger the hood model the more quickly and reliably males were able to relocate their burrows and safety.  Hence, directional natural and intra-sexual selection for burrow holding ability may explain why males build larger hoods that are not more attractive to females.