Testosterone and carotenoids: an integrated view of trade-offs between immunity and sexual signalling.

Allocation tradeoffs with the immune system can enforce honesty on sexual signals that act as indicators of individual quality. Such tradeoffs can be brought about by (1) the dual action of testosterone, which stimulates sexual signals but also suppresses immune functions, and/or (2) competition for carotenoids, which can be deposited as ornamental pigments or used as antioxidants in support of immune functions. Recent studies integrate these two mechanisms by showing that testosterone treatment in male birds upregulates circulating lipoproteins, plasma carriers for carotenoids, and increases bio-availability of carotenoids, which enhances carotenoid-based sexual coloration and immune responsiveness. (c) 2007 Wiley Periodicals, Inc.