Ontogeny of behavioral inhibition induced by unfamiliar adult male conspecifics in preweanling rats.

Previous studies showed that when socially isolated at 22 degrees C, postnatal day 14 rats, but not younger day 7 rats, reduce their emission of ultrasonic vocalizations when exposed to an unfamiliar adult male rat, a naturalistic threat. Because ultrasound production is associated with factors such as age and body temperature, this study examined in age-appropriate thermoneutral temperature ranges whether preweanling rats of different ages are equally capable of inhibiting their emission of ultrasounds when threatened. In Experiment 1, 7- and 14-day-old rats were socially isolated and exposed to unfamiliar anesthetized adult male rats in a thermoneutral environment. Only 14-day-old rats significantly reduced their emission of ultrasounds. This reduction in ultrasound production was accompanied by freezing. In Experiment 2, additional ages were examined under identical test conditions. At 3, 6, and 9 days of age, pups frequently emitted ultrasounds when exposed to the anesthetized male rat. However, at 12 days of age, rat pups responded to the anesthetized male rat by freezing and significantly reducing their emission of ultrasounds. Results indicate clearly that under the present testing conditions the ability of rat pups to inhibit ultrasounds and freeze when threatened is not present at birth but emerges by the end of the second postnatal week.