Testosterone is involved in acquisition and maintenance of sexually selected male plumage in superb fairy-wrens, Malurus cyaneus

Testosterone has been proposed as a physiological link between the level of sexual signalling and male condition. Bright plumage is one of the most no ticeable sexual signals and is often used by females as a basis for mate ch oice. Yet bright male plumage is not necessarily testosterone dependent. We investigated the role of testosterone in the moult into seasonal nuptial p lumage in male superb fairy-wrens. Early pre-nuptial moult is under intense intersexual selection and males can acquire the bright plumage any time be tween autumn and the next spring. Testosterone was always undetectable or v ery low in males in dull eclipse plumage. During the pre-nuptial moult, bot h the number of males with detectable testosterone and average testosterone levels increased sharply. High testosterone was more correlated with nupti al plumage than with presence of the cloacal protuberance (indicative of sp erm storage). Subcutaneous testosterone implants always induced the pre-nup tial moult within 2-3 weeks after implantation, even well outside the natur al time range of moulting. Moreover, removal of the implants before the nup tial plumage was completed, arrested the moult process. The evidence sugges ts that development of the nuptial plumage is testosterone dependent, altho ugh we cannot exclude that testosterone exerts its action after conversion to a metabolite such as oestrogen. Once the nuptial plumage was completed, all males maintained substantially elevated testosterone, sometimes months before the onset of breeding. These high levels could be necessary to maint ain the plumage, and/or are involved in courtship displays. The results are discussed with respect to potential costs involved in acquiring and mainta ining the nuptial plumage.