Why do vultures have bald heads? The role of postural adjustment and bare skin areas in thermoregulation

Griffon vultures (Genus Gyps) have large areas of bare skin on their body, and by changing their posture they can vary the extent to which these are covered by feathers. We used a mathematical model to estimate the amount of bare skin exposed in the postures adopted in cold and hot conditions. Measurements of heat flow through different parts of museum skins, which differed in their feather density, were then used together with the estimates of proportions of body surface covered by each feather density type, to determine rates of heat loss from the whole body. Postural change can cause the proportion of body surface composed of bare skin areas to change from 32% to 7%, and in cold conditions these changes are sufficient to account for a 52% saving in heat loss from the body. We suggest that the bare skin areas in griffon vultures may play an important role in thermoregulation.