Thermogenesis during ultrasonic vocalization by rat pups isolated in a warm environment: a thermographic analysis.

Ultrasonic vocalizations, emitted by rat pups when separated from their mother, littermates, and home cage, have been used as a measure of isolation distress. Recently, we demonstrated that cold exposure is the primary component of isolation that induces the vocalization. We were unable, however, to suppress all ultrasound production when transferring pups to a thermoneutral (35 degrees C) environment. Using an infrared thermography system that allows us to estimate noninvasively heat production by brown adipose tissue, we found that pups transferred from the home nest to a 35 degree C test chamber exhibited sizable levels of heat production while they were vocalizing. Moreover, both heat production and ultrasound emission decreased over the 15-min test. Next, we used extreme care to minimize thermal, and therefore respiratory, stimulation of pups before, during, and after the transfer procedure. We found that such precautions prevented both heat production and ultrasound emission following transfer. These results indicate that infant rats' thermal sensitivities are far greater than previously suspected.