SEASONAL ADJUSTMENT OF SOLAR HEAT GAIN INDEPENDENT OF COAT COLORATIONIN A DESERT MAMMAL

Despite the apparent importance of solar radiation as a source of heat for free-living animals, there exists no substantial body of empirica l data describing physiological responses to solar radiation under the range of convective conditions likely to occur in nature. We therefor e quantified effects of simulated solar radiation and wind on metaboli c heat production in the rock squirrel, Spermophilus variegatus. This diurnal mammal inhabits the Sonoran Desert and seasonally replaces its pelage in a fashion in which it retains constant external appearance but incorporates optical and structural changes that are thought to si gnificantly alter heat-transfer properties of the coat. At a given win d speed, the presence of 950 W m(-2) of simulated solar radiation redu ces metabolic heat production by 15% (at a wind speed of 4 m s(-1)) to 37% (at a wind speed of 0.25 m s(-1)). Independent of effects of irra diance, metabolic heat production significantly increases with wind sp eed such that as wind speed is increased from 0.25 m s(-1) to 4.0 m s( -1), metabolic heat production is elevated by 66% (sunlight absent) or 88% (sunlight present). Previous analyses demonstrated that when expo sed to identical radiative and convective environments rock squirrels with summer pelages accrue solar heat loads 33%-71% lower than those e xperienced by animals with winter coats. This reduction of solar heat gain during the extremely hot Sonoran Desert summer apparently constit utes a previously unappreciated mode of thermal adaptation by seasonal adjustment of radiative heat gain without changes in the animal's app earance.