Female discrimination against sworded males in a poeciliid fish

Female poeciliid fish of the sister genera Xiphophorus and Priapella share a mating preference favouring a sword, despite phylogenetic evidence that the sword arose in Xiphophorus. This conclusion is based on the presence of a preference favouring a sword in three unsworded species and one sworded species. Here I investigate the response to a sword in the poeciliid fish Heterandria bimaculata by testing females with conspecific males that differ in the presence or absence of a sword. Female H. bimaculata were found to prefer unsworded males, thus sworded males were discriminated against. I also tested the strength of female response; females did not show a differential response to males based on sword length. The preference for unsworded males could indicate that when new male traits arise, biases might oppose trait evolution, even in cases in which similar male traits in related taxa are favoured. Explanations for the female preference for unsworded males are discussed.