Differential effects of a parasite on ornamental structures based on melanins and carotenoids

Models of sexual selection predict that ornamental coloration should be affected by parasites in order to serve as honest signals. Animals are commonly infested by a range of parasite species and often simultaneously display several ornaments. Thus the specific effect of a given parasite on ornaments is important for the understanding of the signal. Here we investigate experimentally the effect of an ectoparasite on carotenoid- and melanin-based traits in breeding great tits Parus major. In the experiment, nests were either infested with hen fleas, Ceratophyllus gallinae, or kept free of parasites. The color of the two traits and the size of the melanin-based breast stripe were assessed both in the year of experimental parasite infestation and during the following breeding season, after the annual molt. The size of the breast stripe of infested males and females significantly decreased, but increased significantly in uninfested males and females. The blackness of the breast stripe and the carotenoid-based plumage coloration was unaffected. Our experiment demonstrates that the size of the melanin-based breast stripe of adults depends on parasite infestation, suggesting that the trait can serve as an honest signal of previous parasite exposure.