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

Female poeciliid fishes 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 representative of both genera. A comparison of the preference functions reveals that the strength of the preference favouring a sword in P. olmecae is significantly stronger than that in X. helleri. This result demonstrates that the pre-existing bias is not evolutionarily fixed, 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 conspecific males with swords, only X. helleri females also demonstrate a body-size preference. The preference functions for body size and sword length for X. helleri are not significantly different, whereas in P. olmecae the preference function for sword length is significantly 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.