Testosterone and sexual signalling in male house sparrows (Passer domesticus)

Female preference for mates with elaborated ornaments has often been explai ned on the basis that exaggerated secondary sexual traits might reflect ind ividual quality and females might gather direct and indirect benefits in ma ting with such males. Sexual signals must however also entail costs to be r eliable indicators of male quality. Androgens have been suggested as a phys iological link between sexual signals and individual quality for several re asons, including their immunosuppressive, effect. In this study, we tested two hypotheses linked to the hormonal basis of sexual signal expression. Fi rst, we investigated whether testosterone is correlated with the size of th e black feather bib on the throat of male house sparrows (Passer domesticus ) - a trait involved in intra- and inter-sexual selection. Second, we teste d whether testosterone affects the seasonal exposure of the trait. Observat ional work conducted in 1998 showed that the testosterone level was positiv ely correlated with badge size both in spring and in the subsequent fall, a fter molt. In 1999, we experimentally reduced spring testosterone levels us ing silastic implants filled with cyproterone acetate, an antiandrogen. Mal e house sparrows implanted with cyproterone acetate showed reduced exposure of the badge, because the white tips of the black feathers of the badge wo re off later than in control males implanted with empty silastic tubes. Thi s result suggests that testosterone can be causally involved in the express ion of a secondary sexual trait in house sparrows, at least in terms of its seasonal exposure.