Yolk testosterone shapes the expression of a melanin-based signal in great tits: an antioxidant-mediated mechanism?

Conspicuous traits produced by melanin deposition in integuments are often involved in visual communication. The information content of melanin-based signals is unclear as their expression is tightly controlled by genes and, apparently, is less dependent on individual condition. In birds, high heritabilities have been attributed to melanin-based plumages, often on the basis of egg-swapping manipulations (cross-fostering experiments). However, it is well known that female birds can differentially transfer testosterone to the egg yolk. Furthermore, high testosterone levels have been related to high oxidative stress. As we recently found that oxidative stress experienced during development influences the expression of melanin-based traits, here we manipulated the level of yolk testosterone in great tits (Parus major) to assess the influence of this maternal effect on the expression of the black breast stripe, a well-known melanin-based signal. We predicted that fledglings hatched from eggs with high testosterone levels will not only show larger black stripes but also experience changes in their antioxidant machinery. Indeed, the size of the black stripe of great tits hatched from testosterone-injected eggs was almost double that of controls. Furthermore, the same individuals showed a trend to higher levels of circulating antioxidants, which suggests an adaptive response against some testosterone-induced oxidative challenge.