Distress vocalizations in infant rats: what's all the fuss about?

Ultrasonic vocalizations emitted by infant rodents are typically characterized as cries of distress. There are two contexts that are known to reliably elicit ultrasound production: extreme cold exposure and administration of clonidine, an alpha 2 adrenoceptor agonist. Noting that these two contexts both entail pronounced decreases in cardiac rate, we have hypothesized that the vocalizations are acoustic by-products of a physiological maneuver, the abdominal compression reaction (ACR), that increases venous return to the heart when return is compromised. As a critical test of this hypothesis, we measured venous pressure near the right atrium in 15-day-old rats after clonidine administration. Consistent with the ACR hypothesis, emission of ultrasound was accompanied by large and reliable increases in venous pressure and, therefore, venous return. These results provide strong, direct support for the ACR hypothesis and, by doing so, underscore the potential pitfalls of anthropomorphic interpretations of the vocalizations of infant rats.