HORMONAL BASIS OF SEXUAL DIMORPHISM IN BIRDS - IMPLICATIONS FOR NEW THEORIES OF SEXUAL SELECTION

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.