Sex steroid dependence of carotenoid-based coloration in female zebra finches.

Compared to male secondary sexual traits, the effect of testosterone (T) on female ornaments is understudied. In particular, it is unclear whether females experience different costs of T elevation than do males and how this affects the relationship between T and ornamentation in the two sexes. I experimentally and correlationally investigated the effect of T on the color of the carotenoid-based beak in female zebra finches (Taeniopygia guttata). Exogenous T administration elevated beak redness, indicative of sex-steroid sensitivity for this ornament. However, unlike in males, the relationship between T and color among unmanipulated birds was inversely U-shaped, with the most colorful females circulating intermediate T levels. This is consistent with the idea that females bear high costs of elevated T levels that weaken color intensity. Likely mechanisms for such an effect include: (1) the need to traffic carotenoids away from the beak to combat the immunosuppressive effects of high T, and (2) the loss of carotenoid stores in the body (e.g. from adipose tissue) due to a T-dependent increase in lipid metabolism (T-implanted females in this study decreased in body mass). Moreover, unlike what occurs in male zebra finches, T did not upregulate levels of plasma lipoproteins (carotenoid transporters) or plasma carotenoids in females; this provides further support that T controls beak-color intensity by different physiological mechanisms in the two sexes, with perhaps more of a localized role of sex-steroids at the beak in females.