Melanins, metals, and mate quality

Many animals display large patches of black or brown color in their integument, which contain melanin pigments and function as important sexually selected indicators of mate quality. To date, however, the particular means by which melanin-based color ornaments are costly to produce and serve as honest advertisements have remained elusive. Here, I propose a novel biochemical mechanism by which melanin-based coloration may serve as a reliable indicator of quality, involving the interplay of metal ions. In animals and other biochemical systems, a series of scarce macro- and microminerals (e.g. Ca, Zn, Cu, Fe) obtained from the diet act as critical regulatory factors in the biosynthesis of eumelanin and phacomelanin pigments, but can also be toxic to the body when accumulated in high concentrations. As large polymers with many functional (carboxyl) groups, melanin granules also bind these metals and store them in pigmented cells. Thus, by sequestering large deposits of melanin, often in dead tissue (as in hair or feathers), animals may directly reveal dietary access to these rare elements and the physiological protection they have afforded themselves from initially beneficial, but eventually damaging, high mineral concentrations.