Circulatory Adaptations to Cold in Birds
The maintenance of high body temperatures in homeothermic animals is to a large extent dependent on an effective insulation like feathers, fur, or subcutaneous fat. However, in most animals the insulative covering is more or less incomplete, and in birds the beak, eyes, feet, and underside of wings are generally naked or poorly feathered. Together with the upper part of the respiratory tract, these surfaces constitute potential heat loss areas and have therfore been characterized as thermal windows (Schmidt-Nielsen, 1983). There is an obvious advantage in reducing heat loss from the naked extremities in a cool environment, but this is complicated by the circulating blood which will enevitably carry heat to these areas from the body core by means of internal convection. In contrast, a surplus of heat flow to unprotected areas may be of vital importance in order to prevent the tissue from freezing during exposure to subzero ambient temperature.
KeywordsHeat Exchanger Vasoactive Intestinal Polypeptide Giant Fulmar Reduce Heat Loss Arteriovenous Anastomosis
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