Thermogenic, respiratory, and ultrasonic responses of week-old rats across the transition from moderate to extreme cold exposure.

Previously, it was reported that week-old rats exposed to air temperatures that elicited submaximal levels of heat production (designated moderate cold exposure) remained asleep and did not vocalize (Blumberg & Stolba, 1996). In contrast, pups exposed to air temperatures that elicited maximal levels of heat production (designated extreme cold exposure) woke up and emitted ultrasonic vocalizations. We now report on the physiological and behavioral responses of pups in the transitional region between moderate and extreme air temperatures. Small decreases in air temperature across the transition resulted in pronounced decreases in physiological temperature and concomitant increases in ultrasound production. In a second experiment, it was shown that during moderate cold exposure respiratory frequency increased as air temperature decreased but, as extreme air temperatures were reached, respiratory frequency was maximized as ultrasound production began. The results from these two experiments illustrate how air temperatures that differ by as little as 2 degrees C can differentially modify the physiological and behavioral responses of neonates.