The role of sexual selection in the loss of sexually selected traits in the swordtail fish Xiphophorus continens

Abstract

Swordtail fish have been studied extensively in relation to several exaggerated male traits that have evolved due to sexual selection. However, one species of swordtail fish, Xiphophorus continens, lacks all of these traits. Males in this species are relatively small, lack swords and do not possess any of the pigment patterns found throughout the swordtails and platyfish. In this study we compared the mating behaviour of X. continens to the mating behaviours of other swordtail fish to determine whether a relaxation of sexual selection may have played a role in the loss of large male size and the pigment pattern vertical bars. We found that X. continens lacked the courtship behaviours present in other swordtail species, and that female mating preference for male size and male response to the pigment pattern vertical bars were significantly weaker than in other swordtails we have tested. In addition, results of staged male–male interactions suggested that neither male size nor vertical bars are under strong selection due to male–male competition. However, we did detect a polymorphism in female preference for vertical bars, suggesting that the response of female X. continens to vertical bars has been evolutionarily retained even though the trait was lost. Based on what is known about female preference for body size and bars, and male response to body size and bars across the northern swordtails, we hypothesize that the loss of large male size was due to a relaxation of sexual selection, whereas the loss of vertical bars was not.