Evolutionary change in a receiver bias: a comparison of female preference functions

Female poeciliid ¢shes of the sister genera Xiphophorus and Priapella share a preference for males with swords, despite phylogenetic information suggesting that swords arose in Xiphophorus after the divergence of the two genera. This study examines the strength of sword and body-size preferences in a representa-tive of both genera. A comparison of the preference functions reveals that the strength of the preference favouring a sword in P. olmecae is signi¢cantly stronger than that in X. helleri. This result demonstrates that the pre-existing bias is not evolutionarily ¢xed, and that there has been change in the bias favouring the sword, in either the Priapella lineage, or the Xiphophorus lineage, or in both. Although females in both species prefer conspeci¢c males with swords, only X. helleri females also demonstrate a body-size prefer-ence. The preference functions for body size and sword length for X. helleri are not signi¢cantly di¡erent, whereas in P. olmecae the preference function for sword length is signi¢cantly stronger than for body size. These combined results indicate that an ancestral bias for body size cannot alone explain the pre-existing bias favouring a sword in P. olmecae