Effect of environmental temperature on body temperature and metabolic heat production in a heterothermic rodent, Spermophilus tereticaudus.

This study quantifies the thermoregulatory ability and energetics of a mammal, the round-tailed ground squirrel Spermophilus tereticaudus, that can relax thermoregulatory limits without becoming inactive. We measured body temperature and metabolic rate in animals exposed for short periods (1 h) to air temperatures ranging from 10 to 45 degrees C and for long periods (8 h) to air temperatures ranging from 10 to 30 degrees C. Within 45 min of exposure to air temperatures ranging from 10 to 45 degrees C, the mean body temperatures of alert and responsive animals ranged from 32.1 degrees C (T(air)=10 degrees C) to 40.4 degrees C (T(air)=45 degrees C). This thermolability provided significant energetic savings below the thermoneutral zone, ranging from 0.63 W (18 %) at 10 degrees C to 0.43 W (43 %) at 30 degrees C. When exposed for 8 h to air temperatures between 10 and 30 degrees C, animals varied their body temperature significantly over time. At all air temperatures, the lowest body temperature (maintained for at least 1 h) was 31.2 degrees C. The highest body temperatures (maintained for at least 1 h) were 33.6 degrees C at 10 degrees C, 35.3 degrees C at 20 degrees C and 36.3 degrees C at 30 degrees C. The energetic savings realized by maintaining the minimum rather than the maximum body temperature was 0.80 W (25 %) at 10 degrees C, 0.71 W (33 %) at 20 degrees C and 0.40 W (47 %) at 30 degrees C. This study demonstrates in several ways the ability of this species to adjust energy expenditure through heterothermy.