NAVARA, K.J.; HILL, G.E.; MENDON�A, M.T.: Are there immunological costs to carotenoid display in songbirds?

The recent discovery that carotenoids enhance immune function in mammals has led to the hypothesis that carotenoid pigmentation is an honest signal of immunocompetence in birds. In mammals, carotenoids have been found to increase B and T-lymphocyte proliferation, and also to act as free-radical scavengers, thereby protecting white blood cells. However, extrapolating these results to an avian system may be premature. Many birds have high, sexually dimorphic levels of circulating carotenoids, which exceed those amounts found to be beneficial in mammals. We experimentally tested whether captive American Goldfinches use carotenoids for ornamental display at the expense of immune function. In our experiment, we supplied molting male goldfinches with varying diets, which contained very low, approximately normal, or very high doses of the carotenoid pigments, lutein and zeaxanthan. We then assessed the cell-mediated and humoral immune responses, changes in steroid hormones known to affect immune function (testosterone and corticosterone), as well as the response of the birds to parasitic infection. We saw no effect of carotenoid diet on immune function, and steroid levels varied with degree of sickness resulting from parasitic infection.