Christoph P. Wiedenmayer

Abstract: Exposure to a deadly threat, an adult male rat, induced the release of corticosterone in 14‐day‐old rat pups. The endocrine stress response was decreased when the pups were reunited with their mother immediately after exposure. These findings demonstrate that social variables can reduce the consequences of an aversive experience.

We examined how the experience of a threatening stimulus alters subsequent behavior in a situation where the immediate threat is absent. A small huddle of 12‐day‐old rats was exposed to a potentially infanticidal adult male rat for 5 min. During male exposure, pups were significantly more immobile than control pups. Thirty, 60, and 180 min after male exposure, the pups were isolated for 5 min from litter and dam in an unfamiliar environment. When isolated, pups that had been previously exposed to the male emitted significantly fewer ultrasonic vocalizations than controls, but did not differ in immobility. Low levels of vocalization were apparent 30 and 60 min after male exposure and were not evident at 180 min. The pups seemed to have adjusted their behavior to a potential male threat in a different context for a limited period of time. © 2003 Wiley Periodicals, Inc. Dev Psychobiol 42: 386–391, 2003.

During early ontogeny, stimuli that pose a threat to an animal change. Unrelated adult male rats may kill young rats, but infanticide ends around weaning. Predation, on the other hand, may increase during early ontogeny when rats begin to extend their activity range. We investigated the developmental course of two defensive responses, immobility and analgesia, in young rats exposed to an adult male rat or to predator cues. Preweaning 14-day-old rats became immobile and analgesic when exposed to the male and showed immobility but not analgesia when exposed to cat odor. On Day 26, around weaning, the presence of the male rat no longer induced immobility and analgesia whereas cat odor produced higher levels of immobility and analgesia compared to control and male-exposed animals. This developmental change in responsivity may reflect the differences in the risk of being harmed by a male or a cat during different periods of ontogeny. © 2001 John Wiley & Sons, Inc. Dev Psychobiol 39: 1–7, 2001