Squirrel monkeys (Saimiri sciureus) were exposed in the far field of a horn antenna to both brief (10-min) and prolonged (90-min) periods of 2,450-MHz continuous microwaves. Ambient temperature (Ta) was constant at 15, 20, or 25 degrees C. Microwave power density ranged from 2.5 to 10 mW/cm2, representing a range of whole-body energy absorption from 0.4 to 1.5 W/kg. Reliable reductions in metabolic heat production (M), calculated from oxygen deficit in the monkey's expired air, were initiated at all Ta by 10-min whole-body exposures to power densities of 4 mW/cm2 (2 monkeys) or 6 mW/cm2 (1 monkey) and above. The magnitude of M reduction was linearly related to microwave intensity above the threshold level. Termination of microwaves was followed by a rapid M rebound. The change in M produced by a given power density was nearly the same in Ta = 15 and 20 degrees C. During 90-min exposures at Ta = 20 degrees C, the vigorous M reduction to microwave onset adapted slowly, ensuring continual precise regulation of internal body temperature. Thus cold-exposed endotherms readily compensate for microwave-induced body heating by reducing endogenous heat production.
