THERMOREGULATION IN MACACA-MULATTA - THERMAL BALANCE STUDY

An indirect calorimetric study was performed on four male, unacclimated, unanesthetized, rhesus monkeys (Macaca mulatta), weighing 3.1-5.5 kg, over an ambient temperature (T(a)) range of 15 to 40 °C. Oxygen consumption, CO2 production, mean weighted skin temperature (T(sk)), rectal temperature (T(re)), respiratory evaporative water loss (E(resp)), and total evaporative water loss were measured continuously after equilibration at each T(a). T(sk) changed linearly with T(a), whereas T(re) was maintained within the range of 37.9-40.0°C at T(a)'s of 15-40°C. The lower critical T(a) (LCT) for the increase in metabolic heat production (M) was 24.7°C, which corresponds to the critical T(sk) of 33.5°C; M was elevated so that at 15°C (88 W/m2) was approximately 1.7 times that at 25°C (55 W/m2). Above the upper critical T(a) (UCT) of 30.6°C, heat losses were facilitated by evaporative heat loss due to eccrine sweating (E(sw)) so that at 40°C, E(sw) (49 W/m2) was 4 times insensible water loss in the thermal neutral zone (TNZ) (12.8 W/m2); whereas E(resp) (4.8 W/m2)increased only 2.1 times TNZ levels (2.3 W/m2). Panting was not observed. The critical T(re) and T(sk) for the onset of sweating are 38.0 and 34.7°C, respectively. Whole-body conductance has a minimum value of 10.5 W/(m2·°C) below the LCT, and increased to 72 W(M2·°C) at 40°C. These data indicate that in Macaca mulatta thermoregulation is achieved in the TNZ (24.7-30.6°C) by vasomotor control; above the UCT, E(sw) is the major avenue of heat loss, and the role of E(resp) in dissipating M is relatively insignificant. Below the LCT, thermal balance is maintained by an increase in M, attributable to shivering thermogenesis.