Dietary mineral content influences the expression of melanin-based ornamental coloration

Many animals develop bold patches of black or brown coloration that are derived from melanin pigments and serve as sexual or social signals. At present, there is much debate among behavioral ecologists over whether melanin-based color signals are costly to produce. Studies that have manipulated crude aspects of nutrition (i.e., total food intake) or health have generally found melanin-based plumage ornaments to be less responsive to such factors than other types of extravagant color (e.g., carotenoid or structural based). However, a recently advanced hypothesis argues that limited minerals in the diet, such as calcium (Ca), zinc (Zn), and iron (Fe), may serve to increase melanin pigment production and maintain signal honesty. Here, I experimentally tested whether variation in the calcium content of the diet affects the color and extent of melanin-based plumage in male zebra finches (Taeniopygia guttata). Calcium supplementation increased the size, but not darkness, of the black breast plumage patch in fledgling and adult males; however, sexually selected, carotenoid-based red beak coloration was not affected by the diet manipulation. These results are the first to support the idea that acquisition of minerals from the diet is a unique, limiting factor for the expression of ornamental melanin coloration in animals.