Are there different requirements for trace elements in eumelanin- and pheomelanin-based color production? A case study of two passerine species

Melanin is the most common pigment in animal integuments including bird plumage. It has been shown that several trace elements may play roles in the production and signaling function of melanin-colored plumage. We investigated coloration and content of various metal elements in the rectrices of two insectivorous passerines, Common Redstarts (Phoenicurus phoenicurus) and Blackcaps (Sylvia atricapilla), which have eumelanin- and pheomelanin-based coloration, respectively. We hypothesized that 1) the two species would differ in concentrations of metals important in melanin synthesis (Ca, Fe, Cu, Zn), 2) differences in metal concentration levels would be related to feather coloration. Our study confirmed the first prediction and provides the first evidence that selected elements may play a greater role in pheomelanin than in eumelanin synthesis. Concentrations of three elements considered as important in melanin synthesis (Ca, Fe, Zn) were 52% to 93% higher in rusty colored Common Redstart feathers compared to the dark gray Blackcap feathers. However, element concentrations were not correlated with feather coloration or sex in either species. Our study suggests that, of the two melanin forms, pheomelanin synthesis may bear higher costs associated with the acquisition of specific elements or limited elements may create trade-offs between ornamentation and other physiological functions. Our findings warrant further investigations designed to better understand the roles of macro- and microelements in the synthesis of both forms of melanin.