Female ornamentation in Malurus fairy-wrens: a hidden evolutionary gem for understanding female perspectives on social and sexual selection

Phylogeny, ecological environment, social organisation, and mating system are expected to affect degree of female ornamentation, either directly or indirectly, but our understanding of how female ornaments respond to these forces remains incomplete. This article evaluates the evolutionary history and adaptive significance of three putative ornaments - plumage colouration, bill colouration and tail-length - in female fairy-wrens. Despite considerable research on these traits in male fairy-wrens, they have yet to be studied in any detail in females. Phylogeographic analyses in combination with life-history data suggest that female plumage colouration and bill colouration are under active selection, independent of that experienced by males. Social organisation and mating system, as mediated by ecological environment, may shape degree of ornamentation in these traits among females. In contrast, tail-length appears to be driven directly by natural ecological selection imposed by environmental conditions, leading to parallel trait evolution among the sexes within each species. More refined comparative and population-level investigations of adaptive consequences and proximate mechanisms are future research priorities. The study of female fairy-wrens holds great promise to advance our collective understanding of how the ecological environment interacts with sexual selection and social competition to shape ornament evolution in complex social organisms.