The mechanisms of color production in black skin versus red skin on the heads of New World vultures

A determination of how the color of animal integument is produced is a starting point for investigations into the function and evolution of coloration. The mechanisms that give rise to the color of bare skin of New World vultures are largely unexplored. Here, we investigate the source of color production in the bare skin of Turkey Vultures (Cathartes aura) and Black Vultures (Coragyps atratus). Using UV–vis reflectance spectroscopy, we found evidence that hemoglobin is the primary pigment responsible for the red coloration of the bare skin on the heads of Turkey Vultures, and that eumelanin is responsible for the black coloration of the bare skin on the heads of Black Vultures. Light microscopy of incisional skin samples further supported these mechanisms of color production by revealing the presence of numerous blood vessels near the surface of the Turkey Vulture skin, and a high concentration of melanosomes in the skin of Black Vultures. Using high performance liquid chromatography (HPLC), we detected carotenoids within the skin of both species with significantly higher total concentrations of carotenoids in the skin of Turkey Vultures compared to the skin of Black Vultures. The carotenoids detected were dietary carotenoids that typically produce yellow coloration when accumulated in integument and were present in low concentrations. We hypothesize that the dietary carotenoids present do not contribute to the color of the skin, but rather help to compensate for the lack of melanosomes found in Turkey Vulture skin. The presence of additional carotenoids may act as an antioxidant to minimize UV damage when the bare Turkey Vulture head skin is exposed to direct sunlight for prolonged periods of time when soaring and scavenging for food.