Ontogeny of defensive reactions in Norway rats.

Young rats of several ages were presented with stimuli (a caged domestic cat, a footshock, and a suddenly moving object) known to be aversive to adults and disruptive of behavior in mature animals. Twenty-day-old rats were relatively unaffected by these events, while rats aged 30 days and older tended to reduce their locomotion and freeze upon the presentation of these cues. These data are consistent with Bolles' hypothesis that shock-elicited responses are innate defensive reactions. We also suggest that the inefficient passive avoidance learning in juvenile rats may result from their deficit in shock-induced freezing.