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 tra its in birds and mammals is testosterone-dependent. In birds, however, masculinity has dual origins. Male-type behaviour and morphology, suc h as spurs and wattles, are usually testosterone-dependent. However, s howy male-type plumage is, generally, the neutral state of development . For example, castrating a peacock has no effect on his elaborate plu mage whereas ovariectomizing a peahen causes her to develop showy male type plumage. The surprising relationships between dimorphism and gon adal steroids in birds have important consequences for the current deb ate concerning the evolution of biological signals and, in particular, the immunocompetence-handicap principle.