Polymorphism, mate choice and sexual selection in the Gouldian finch ( Erythrura gouldiae )

Mate choice can result in both assortative mating and directional sexual selection, but few studies have addressed both processes simultaneously. Here we test several hypotheses regarding the possible role of female mate choice in maintaining the face-colour polymorphism of, and affecting directional sexual selection in, the Gouldian finch. These endangered Australian finches are highly sexually dimorphic and are genetically polymorphic for face colour: there are black-, red- and gold-faced individuals. First we showed that Gouldian finches tend to pair positive-assortatively by face colour morph in aviaries. In a laboratory experiment, we tested whether female mate choice is assortative by face colour. Overall, females neither preferred males of the same or of different face colour morphs as themselves. We found weak evidence for positive assortative female choice at one of the two loci involved in determining face colour. Next, we tested whether females showed frequency-dependent mate choice, and found that they preferred neither rare nor common male morphs. In order to test for directional sexual selection on males by female mate choice, we examined the correlations between male morphological traits and attractiveness to females. We found that tail pin length and bill size are correlated with male attractiveness, and may be under sexual selection. Thus, whilst female mate choice may be an important process in determining the evolution of male morphology, and potentially sexual dimorphism, it does not appear to be the primary force behind the assortative mating pattern among the face colour morphs.