Sexes of a monomorphic species differ in preference for mates with a novel trait

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Abstract
We investigated the different roles of the sexes in the origination of novel traits in the sexually monomorphic Javanese mannikin Lonchura leucogastroides. We introduced a red feather as an evolutionarily novel trait in both sexes and tested their preferences for prospective mates with this trait. Males rejected females bearing the red feather and preferred to court unadorned females. In contrast, females partly preferred adorned males. Specifically, previously unattractive males gained in attractiveness and could increase their reproductive success when bearing the ornament, whereas previously attractive males lost in attractiveness, but this did not affect their reproductive success. We introduced two other novel traits in males and investigated the females' response to these in mate choice tests. Each of the three new traits interacted with the natural attractiveness of males. The more attractive a male was before ornamentation, the more it lost in attractiveness after ornamentation and vice versa. Thus, the position of the trait did not affect the interaction. Because males rejected adorned females and females partly preferred adorned males, novel traits might evolve by intersexual selection in males rather than in females. This can lead to a sexual dimorphism with conspicuous traits in males. Our study reveals a new insight into the mechanism of the evolution from monomorphism to dimorphism with ornamental traits in males.

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