Consequences of skin color and fur properties for solar heat gain and ultraviolet irradiance in two mammals

Summary In animals with fur or feather coats, heat gain from solar radiation is a function of coat optical, structural, and insulative characteristics, as well as skin color and the optical properties of individual hairs or feathers. In this analysis, I explore the roles of these factors in determining solar heat gain in two desert rodents (the Harris antelope squirrel,Ammospermophilus harrisi, and the round-tailed ground squirrel,Spermophilus tereticaudus). Both species are characterized by black dorsal skin, though they contrast markedly in their general coat thickness and structure. Results demonstrate that changes in coat structure and hair optics can produce differences of up to 40% in solar heat gain between animals of similar color. This analysis also confirms that the model of Walsberg et al. (1978) accurately predicts radiative heat loads within about 5% in most cases. Simulations using this model indicate that dark skin coloration increases solar heat gain by 5%. However, dark skin significantly reduces ultraviolet transmission to levels about one-sixth of those of the lighter ventral skin.Symbols and abbreviations: (unless noted, all radiation relations refer to total solar radiation) absorptivity of individual hairs - _C absorptivity of the coat - backward scattering coefficient reflectivity] of individual hairs - _C reflectivity of coat - _S reflectivity of skin - forward scattering coefficient transmissivity] of individual hairs - _C transmissivity of coat - _S transmissivity of the skin - transmissivity of the coat and skin - transmissivity of the coat to ultraviolet radiation - _S transmissivity of the skin to ultraviolet radiation - (1 – )² – ²] - h _C coat thermal conductance W/m²-°C] - h _E coat surface-to-environment thermal conductance W/m²-°C] - I probability per unit coat depth that a ray will be intercepted by a hair m^–1] - K volumetric specific heat of air at 20°C 1200 J/m³-°C] - l _C coat thickness m] - l _H hair length m] - d hair diameter m] - n hair density per unit skin area (m^–2] - Q _ABS heat load on animal's skin from solar radiation W/m²] - Q _I solar irradiance at coat surface W/m²] - r _E external resistance to convective and radiative heat transfer s/m] - r _C coat thermal resistance s/m]