Effects of solar radiation and wind speed on metabolic heat production by two mammals with contrasting coat colours.

We report the first empirical data describing the interactive effects of simultaneous changes in irradiance and convection on energy expenditure by live mammals. Whole-animal rates of solar heat gain and convective heat loss were measured for representatives of two ground squirrel species, Spermophilus lateralis and Spermophilus saturatus, that contrast in coloration. Radiative heat gain was quantified as the decrease in metabolic heat production caused by the animal's exposure to simulated solar radiation. Changes in convective heat loss were quantified as the variation in metabolic heat production caused by changes in wind speed. For both species, exposure to 780 W m-2 of simulated solar radiation significantly reduced metabolic heat production at all wind speeds measured. Reductions were greatest at lower wind speeds, reaching 42% in S. lateralis and 29% in S. saturatus. Solar heat gain, expressed per unit body surface area, did not differ significantly between the two species. This heat gain equalled 14-21% of the radiant energy intercepted by S. lateralis and 18-22% of that intercepted by S. saturatus. Body resistance, an index of animal insulation, declined by only 10% in S. saturatus and 13% in S. lateralis as wind speed increased from 0.5 to 4.0 ms-1. These data demonstrate that solar heat gain can be essentially constant, despite marked differences in animal coloration, and that variable exposure to wind and sunlight can have important consequences for both thermoregulatory stress experienced by animals and their patterns of energy allocation.