Effect of wind and solar radiation on metabolic heat production in a small desert rodent, Spermophilus tereticaudus.

To understand better how complex interactions between environmental variables affect the energy balance of small diurnal animals, we studied the effects of the absence and presence of 950 W m(-)(2) simulated solar radiation combined with wind speeds ranging from 0. 25 to 1.00 m s(-)(1) on the metabolic rate and body temperature of the round-tailed ground squirrel Spermophilus tereticaudus. As wind speed increased from 0.25 to 1.00 m s(-)(1), metabolic heat production increased by 0.94 W in the absence of solar radiation and by 0.98 W in the presence of 950 W m(-)(2) simulated solar radiation. Exposure to simulated solar radiation reduced metabolic heat production by 0.68 W at a wind speed of 0.25 m s(-)(1), by 0.64 W at 0.50 m s(-)(1) and by 0.64 W at 1.00 m s(-)(1). Body temperature was significantly affected by environmental conditions, ranging from 32. 5 degrees C at a wind speed of 1.0 m s(-)(1) and no irradiance to 35. 6 degrees C at a wind speed of 0.50 m s(-)(1) with 950 W m(-)(2 )short-wave irradiance. In addition, several unusual findings resulted from this study. The coat of S. tereticaudus is very sparse, and the observed heat transfer of 5.68+/-0.37 W m(-)(2 ) degrees C(-)(1) (mean +/- s.e.m., N=11) is much higher than expected from either allometric equations or comparative studies with other rodents of similar mass. Solar heat gain was remarkably low, equalling only 10 % of intercepted radiation and suggesting a remarkably high regional thermal resistance at the tissue level. Animals remained normally active and alert at body temperatures as low as 32.5 degrees C. These findings suggest a unique combination of adaptations that allow S. tereticaudus to exploit a harsh desert environment.