Endogenous thermoregulatory rhythms of squirrel monkeys in thermoneutrality and cold

Whole body heat production (HP) and heat loss (HL) were examined to determi ne if the free-running circadian rhythm in body temperature (T-b) results f rom coordinated changes in HP and HL rhythms in thermoneutrality (27 degree s C) as well as mild cold (17 degrees C). Squirrel monkey metabolism (n = 6 ) was monitored by both indirect and direct calorimetry, with telemetered m easurement of T-b and activity. Feeding was also measured. Rhythms of HP, H L, and conductance were tightly coupled with the circadian T-b rhythm at bo th ambient temperatures (T-A). At 17 degrees C, increased HP compensated fo r higher HL at all phases of the T-b rhythm, resulting in only minor change s to T-b. Parallel compensatory changes of HP and HL were seen at all rhyth m phases at both T-A. Similar time courses of T-b, HP, and HL in their resp ective rhythms and the relative stability of T-b during both active and res t periods suggest action of the circadian timing system on T-b set point.