title: Hormonal basis of sexual dimorphism in birds: implications for new theories of sexual selection.
abstract: It is widely assumed that the development of male secondary sexual traits in birds and mammals is testosterone-dependent. In birds, however, masculinity has dual origins. Male-type behaviour and morphology, such as spurs and wattles, are usually testosterone-dependent. However, showy male-type plumage is, generally, the neutral state of development. For example, castrating a peacock has no effect on his elaborate plumage whereas ovariectomizing a peahen causes her to develop showy male-type plumage. The surprising relationships between dimorphism and gonadal steroids in birds have important consequences for the current debate concerning the evolution of biological signals and, in particular, the immunocompetence-handicap principle.
authors.affiliation: Institute of Zoology, Zoological Society of London, Regent's Park, London, UK NW1 4RY; The Galton Laboratory, Dept of Genetics and Biometry, University College London, London, UK NW1 2HE.
authors.forename: I P
authors.initials: IP
authors.lastname: Owens
authors.forename: R V
authors.initials: RV
authors.lastname: Short