The effects of testosterone on antibody production and plumage coloration in male house sparrows (Passer domesticus)

Many bird species have patches of colour in their plumage, contrasting with their basic coloration, which are used to display and signal status to con specifics. These are called 'badges of status', because they are believed t o be low-cost signals of social status. For a signalling system to be evolu tionarily stable, cheating must be controlled. The conventional view is tha t there is frequent testing. which uncovers cheats. Recently, the immunocom petence handicap hypothesis (ICHH) suggested that signals may be dependent on testosterone fur their development, with a cost being imposed through im mune suppression. We report experiments on house sparrows (Passer domesticu s) which show that testosterone significantly influences the size of the bi b (a 'badge of status'). The ultimate effect of the testosterone manipulati on was to impair antibody production, as predicted by the ICHH. However, te stosterone manipulations also changed the levels of the 'stress hormone' co rticosterone. The level of corticosterone was also related to the degree of immunosuppression. After controlling for the effect of corticosterone, tes tosterone enhanced the birds' ability to produce antibodies, counter to the ICHH. The hypothesis therefore must be modified. We suggest that testoster one has a dual effect: it leads to immunosuppression through a mechanism in volving corticosterone but, conversely, leads to increased immunocompetence probably via dominance influencing access to resources.