The effect of against-background contrast on female preferences for a polymorphic colour sexual signal

It is thought that the more conspicuous a sexual signal, the more attractive it is; however, the role of the background in modifying the conspicuousness of a signal is not well understood, particularly for complex, highly polymorphic colour signals. The background against which a sexual signal is viewed is important because it can influence how an individual's colour pattern is perceived and therefore whether it is attractive to potential mates. Here, we tested the hypothesis that against-background contrast in hue (colour), luminance (brightness) and chroma (saturation) will influence the attractiveness of a male and we predicted that the more a male's colour pattern contrasts with the background, the more attractive he should be. We used mate choice tests to determine the effect of visual background contrast on female preferences in a freshwater fish, Poecilia reticulata, with a highly polymorphic male colour sexual signal. We placed three full-sibling males, with very similar colour patterns, in front of three different backgrounds which varied in their perceived contrast to females. We then gave females the opportunity to spend time with the male that they found most attractive. Contrary to our predictions, we found that females did not choose the males that contrasted the most with their background. More specifically, we found nonlinear relationships between female preference and the coefficient of variation (CV) of chromatic contrast, which represents the variation in colour patch saturation across the pattern; the higher the CV chroma, the more the colour patches within a pattern vary in saturation. Our results indicate possible trade-offs between natural and sexual selection and/or signatures of sensory bias. Against-background contrast may play a role in maintaining colour pattern polymorphism.