Defining vertical bars in relation to female preference in the swordtail fish Xiphophorus cortezi (Cyprinodontiformes, Poeciliidae)

" Defining a male trait in relation to female preference is an important step towards determining the role that female preference has played in the evolution of a male trait. The pigment pattern `vertical bars' is a complex male trait that functions as a sexual signal in swordtail fishes. A previous study suggested that X. cortezi females had a preference for males with more vertical bars. In that study, however, bar number had been reduced on the stimulus males by removing the most posterior bars, which changed the distance over which the bars spanned, in addition to changing bar number. To determine whether X. cortezi females have a preference for greater bar span and/or number of bars, we tested for female preference in two experiments. First, we gave females a choice between males with a greater bar span or a smaller bar span, holding bar number and total pigmented area constant. Females spent signi®cantly more time with the males with the smaller bar span. This result, together with results from the previous study, suggest that females do not assess bar span alone, but possibly a composite component of the bars, such as bar frequency (number of bars/bar span). Secondly, we gave females a choice between six thinner bars or one wider bar of equal pigmented area. The six-bar treatment had a greater bar span in addition to more bars. We detected no preference for either treatment. These results suggest that the previously detected preference for more bars may actually re¯ect a preference for greater pigmented area. Finally, we present data on natural variation in the bars for X. cortezi and use principal components analysis to provide a composite definition of this trait "