Coat Color and Solar Heat Gain in Animals

Abstract

A recent analysis of the relation between animal coat color and solar heat gain stated there still is a question as to whether dark or light plumage is more effective in utilization of direct solar (Lustick et al. 1980). This may surprise some, since common experience indicates that dark surfaces acquire greater heat loads when exposed to solar radiation than do light surfaces. For animals such as birds or mammals that possess insulating coats of fur or feathers, however, recent studies have shown that the thermal consequences of coloration present the potential for widely varying effects that have not been generally appreciated by biologists. It is clear that an animal's surface coloration might importantly affect heat exchange with its environment and thus its thermoregulatory demands. Surface coloration arises from differential reflection or transmission of short-wave radiation (the longest wavelength visually perceived by animals typically is near 700 nm). That radiation not reflected or transmitted by the surface is absorbed, resulting in sensible heat generation. With the exception of bioluminescence, the only important natural sources of short-wave radiation are very hot objects, notably stars such as the sun. The intensity of solar radiation reaching the earth's surface varies, but under clear skies often reaches values of about 1000 W/m2 on a plane perpendicular to the solar beam. Roughly one-half of this energy lies in visible wavelengths and hence is of sufficient magnitude that coat color might significantly modify an animal's heat balance.