The functions of freezing in the social interactions of juvenile high- andlow-aggressive mice

Selectively bred low-aggressive mice are frequently observed to freeze on s ocial contact, despite the fact that this behavior was never a direct targe t of selection. To elucidate this finding, the present research aimed to id entify the possible functions freezing may serve in social interactions. It was hypothesized that freezing may modify social interactions through self -regulatory mechanisms and/or via its modulating effects on the actions of social partners. These hypotheses were evaluated with respect to the sequen tial changes observed over the course of a 10-min dyadic test in freezing, social reactivity, and approaches among juvenile (24-30-day-old) mice from the NC900 and NC100 high-and low-aggressive lines. Analyses of the patterns of social interactions between subjects and partners revealed two primary results. First, freezing was more than an expression of fear; it also funct ioned as a regulator of emotional arousal, as suggested by the substantial reduction of reactive behaviors seen in animals that showed high levels of freezing. Second, freezing functioned to facilitate high levels of affiliat ive social interaction with social partners. The implications of these resu lts for understanding how the differentiation of the NC900 and NC100 occurr ed within microevolution and development are discussed. (C) 2001 Wiley-Liss , Inc.