CIRCADIAN-RHYTHMS OF THERMOREGULATION IN THE SQUIRREL-MONKEY (SAIMIRI-SCIUREUS)

Phase or amplitude differences between rhythms in heat production (HP) and heat loss (HL) have been suggested to account for the circadian rhythm in body temperature (T(b)). To describe the relationships among these rhythms in a primate, five unrestrained squirrel monkeys (1.0-1.3 kg) were studied using combined direct and indirect calorimetry, with telemetry of T(b) and activity, in a 24-h light-dark cycle (LD 12:12) at 25 +/- 0.5-degrees-C. Dry (D; sensible) HL, evaporative (E) HL, HP (oxygen consumption and CO2 production), T(b), and-activity were measured at 10-min intervals for a week. T(b), activity, HP, and HL displayed daily rhythms, peaking during the light period. Although the timing of peaks was not significantly different, the diurnal increase in T(b) was seen to result from a delayed increase in DHL, and possibly, EHL, relative to increased HP. The nocturnal decrease in T(b) was due to different time courses of decrease in HP and HL, with no clear lag in HL. The rhythm in T(b) therefore resulted from both phase and time course differences in HP and HL rhythms.