EFFECTS OF SOLAR-RADIATION AND WIND-SPEED ON METABOLIC HEAT-PRODUCTION BY 2 MAMMALS WITH CONTRASTING COAT COLORS

We report the first empirical data describing the interactive effects of simultaneous changes in irradiance and convection on energy expendi ture by live mammals, Whole-animal rates of solar heat gain and convec tive heat loss were measured for representatives of two ground squirre l species, Spermophilus lateralis and Spermophilus saturatus, that con trast in coloration, Radiative heat gain was quantified as the decreas e in metabolic heat production caused by the animal's exposure to simu lated solar radiation, Changes in convective heat loss were quantified as the variation in metabolic heat production caused by changes in wi nd speed, For both species, exposure to 780 Wm(-2) of simulated solar radiation significantly reduced metabolic heat production at all wind speeds measured, Reductions were greatest at lower wind speeds, reachi ng 42 % in S, lateralis and 29 % in S, saturatus, Solar heat gain, exp ressed per unit body surface area, did not differ significantly betwee n the two species, This heat gain equalled 14-21 % of the radiant ener gy intercepted by S. lateralis and 18-22 % of that intercepted by S, s aturatus. Body resistance, an index of animal insulation, declined by only 10 % in S, saturatus and 13 % in S, lateralis as wind speed incre ased from 0.5 to 4.0 ms(-1). These data demonstrate that solar heat ga in can be essentially constant, despite marked differences in animal c oloration, and that variable exposure to wind and sunlight can have im portant consequences for both thermoregulatory stress experienced by a nimals and their patterns of energy allocation,