FEMALE PREFERENCE FOR SWORDS IN XIPHOPHORUS-HELLERI REFLECTS A BIAS FOR LARGE APPARENT SIZE

Swordtail fish (Poeciliidae: genus Xiphophorus) are a paradigmatic cas e of sexual selection by sensory exploitation, Female preference for m ales with a conspicuous ''sword'' ornament is ancestral, suggesting th at male morphology has evolved in response to a preexisting bias. The perceptual mechanisms underlying female mate choice have not been iden tified, complicating efforts to understand the selection pressures act ing on ornament design. We consider two alternative models of receiver behavior, each consistent with previous results. Females could respon d either to specific characteristics of the sword or to more general c ues, such as the apparent size of potential mates. We showed female sw ordtails a series of computer-altered video sequences depicting a cour ting male, Footage of an intact male was preferred strongly to otherwi se identical sequences in which portions of the sword had been deleted selectively, but a disembodied courting sword was less attractive tha n an intact male, There was no difference between responses to an isol ated sword and to a swordless male of comparable length, or between an isolated sword and a homogenous background. Female preference for a s worded male was abolished by enlarging the image of a swordless male t o compensate for the reduction in length caused by removing the orname nt. This pattern of results is consistent with mate choice being media ted by a general preference for large males rather than by specific ch aracters, Similar processes may account for the evolution of exaggerat ed traits in other systems.