Hormonal Influences on Sexual Dichromatism Across the Avian Tree of Life: A Primer

Color differences between the sexes (dichromatism) in birds are a hallmark example of a secondary sexual character that is frequently associated with sexual signaling and mate choice. Previous work has shown that the physiological and genetic mechanisms that underlie presence or absence of secondary sexual characters in birds are highly diverse across the avian clade. Hormones such as estrogen and testosterone are known to have important impacts on the development and regulation of plumage coloration across an individual's lifetime, but the distribution of what we know about hormones and dichromatism, relative to the distribution of dichromatic taxa, is confined to a few well-studied taxa. Here, we review current knowledge of hormonal influences on dichromatism and reconstruct the ancestral states of dichromatism across over 8800 species of birds to highlight where further research is needed. We find that, though two decades have passed since the last major review of hormonal influences on dichromatism, our working knowledge of how hormones influence dichromatism is still functionally and taxonomically limited. Moreover, there is increasing evidence that non-sex hormones such as cortisol and thyroid hormone may have multifarious impacts on dichromatism, including interactive effects on other influential hormones. By combining proximate and ultimate perspectives, we highlight the diversity of hormonal influences on dichromatism in the avian tree of life and put forward ideas for future study.