Duration of social-investigatory behavior by mature male Long-Evans rats was used as a measure of individual recognition in a series of five experiments to assess social memory. In Experiment 1, the duration of social investigation during a second exposure to the same juvenile was directly related \o the length of the interexposure interval. In Experiment 2, males were exposed to the same or to a different juvenile 10 min after an initial 5-min exposure to a novel juvenile; reexposure to the same juvenile elicited significantly less social investigation than an exposure to a different juvenile. Experiments 3 and 4 demonstrated that following a 5-min introductory exposure, social memory of the juvenile male or female is relatively brief in comparison with that of mature males. Experiment 5 revealed a retroactive interference effect on recently acquired memory for an individual: Male subjects exposed to interpolated social experience engaged in significantly longer investigation of a juvenile stimulus than those with no interpolated social experience. The combined results suggest that the rat normally engages in spontaneous learning of individual identity and that social memory may be a significant aspect of complex social interactions.

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Male and female juvenile rats were individually exposed to nonplayful juvenile social stimuli in a novel test of play-soliciting behavior to examine hormonal and experiential determinants of sex differences. In Experiment 1, neonatally androgenized females engaged in play soliciting at a level equal to that of male controls and greater than that of nonandrogenized female controls. In Experiment 2, males and females were reared in unisexual and bisexual groups in order to compare long-term sex-related social experience effects on juvenile play soliciting. Males exposed only to other young males engaged in greater play soliciting than males exposed to both sexes; females, in contrast, were unaffected by sex of cagemates. Within rearing conditions, however, males engaged in greater play soliciting than females. The combined results suggest that perinatal gonadal androgen exposure effects on social play are prepotent and contribute essentially to sex differences in the initiation of social play behavior.