Development of ultrasonic calls in rat pups follows similar patterns regardless of isolation distress.

Ultrasonic vocalizations (USVs) emitted by rat pups have been extensively studied in the context of isolation from their mother and littermates. The heightened call rate of isolation-induced USVs indicates increased anxiety, whereas other acoustic parameters, such as frequency and call duration, are thought to be useful in reflecting developmental changes, which by weaning have developed into their adult form. However, it is possible that stress can affect the quality as well as the quantity of calls, and that as the pups become more mobile, the effects of the stress may change. Therefore, in the present study rats were tested in a test arena either in isolation or with littermates, so as to assess the effects of isolation stress on the developmental changes reported in the literature. Rat pups of both sexes were tested on multiple days from postnatal day (PND) 7 to weaning. Overall, the main changes in the frequency, duration, and types of calls were accounted for by age. The effects of isolation were minor, as were sex differences. This study indicates that the development of USVs in infant rats is a robust process and seemingly resistant to the effects of isolation-induced stress.