Both hypoxia and milk deprivation diminish metabolic heat production and ultrasound emission by rat pups during cold exposure.

Rat pups (7-9 days of age) were made cold and hypoxic simultaneously while interscapular temperature, rectal temperature, and ultrasound emission were monitored. These hypoxic pups cooled faster than control pups, which indicates decreased thermogenesis and decreased oxygen consumption, and produced less ultrasound. In a separate experiment, pups deprived of milk for 24 hr cooled faster and also produced less ultrasound than did nondeprived littermates. Further analyses revealed that those pups that cooled the slowest (and thus used the most oxygen) vocalized the most, both among control animals as well as across the two manipulated groups. This finding suggests that ultrasound emission covaries with thermogenesis. The observed pattern is opposite to that predicted by traditional communication hypotheses of rat pup vocalizations and favors understanding the sounds as symptoms of laryngeal braking.