Whole body heat production (HP) and heat loss (HL) were examined to determine if the free-running circadian rhythm in body temperature (T_b) results from coordinated changes in HP and HL rhythms in thermoneutrality (27°C) as well as mild cold (17°C). Squirrel monkey metabolism (n = 6) was monitored by both indirect and direct calorimetry, with telemetered measurement of T_b and activity. Feeding was also measured. Rhythms of HP, HL, and conductance were tightly coupled with the circadian T_b rhythm at both ambient temperatures (T_A). At 17°C, increased HP compensated for higher HL at all phases of the T_b rhythm, resulting in only minor changes to T_b. Parallel compensatory changes of HP and HL were seen at all rhythm phases at both T_A. Similar time courses of T_b, HP, and HL in their respective rhythms and the relative stability of T_b during both active and rest periods suggest action of the circadian timing system on T_b set point.

body temperature; direct calorimetry; indirect calorimetry; metabolism; heat production; heat loss; nonhuman primates; conductance

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