MORETZ, J.A.; MORRIS, M.R.; Ohio University; Ohio University: A comparative analysis of the role of aggression in the evolution of vertical bars in northern swordtail fishes

Males of many swordtail species possess vertical bars, a pigment pattern that is found throughout the Xiphophorus genus and other poeciliid fishes. Males use these bars in both courtship and in male-male competition and in many species males are able to suppress and intensify their expression. Previous studies using swordtail fishes have shown that some species respond to the bars with decreased aggression while one species responded with increased aggression. In all cases observed to date, it appears that there is congruence between owning the trait and having a response. In this study, we used mirror image stimulation and bar manipulations to compare the aggressive responses of males to both their barred and barless images. We expanded our sample to include all nine species of northern swordtails, as well as one closely related species as an outgroup. We were interested in determining whether there was congruence between trait and response across the whole group and when present, if it was the same across the species. We found that for two of the three swordtail clades, males responded to the bars only if they were naturally barred and that all but one of these species did so with decreased aggression. In both the third clade and the outgroup, males did not respond to the bars, regardless if they had bars. In addition, males in the outgroup did not appear to be able to either suppress or intensify the expression of the bars. We suggest that these results demonstrate that both the vertical bars and the ability to manipulate their expression preceded the male responses to the bars. As a result, we suggest that other factors were likely responsible for the evolution of this trait and we will discuss alternative hypotheses for the evolution of the vertical bars.