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During early ontogeny, stimuli that pose a threat to an animal change. Unrelated adult male rats may kill young rats, bur infanticide ends around weaning. Predation, on the other hand may increase during early ontogeny when mts 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 mts 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. (C) 2001 John Wiley & Sons, Inc

Activity. --- Adult. --- Analgesia. --- Animal. --- Animals. --- Cat odor. --- Cat. --- Control. --- Cues. --- Defensive behavior. --- Defensive immobility. --- Defensive responses. --- Defensive. --- Dentate gyrus. --- Developmental-changes. --- Emotional motor system. --- Fear. --- Immobility. --- Increase. --- Infanticide. --- Level. --- Male conspecifics. --- Male rat. --- Male-rats. --- Male. --- Neurobiological basis. --- Odor. --- Ontogeny. --- Periods. --- Predation. --- Predator odor. --- Predator. --- Rat. --- Rats. --- Rattus-norvegicus. --- Response. --- Responses. --- Risk. --- Stimuli. --- Stress-induced analgesia. --- Stress. --- Weaning. --- Young-rats. --- Young.