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

Selectively bred low-aggressive mice are frequently observed to freeze on social contact, despite the fact that this behavior was never a direct target of selection. To elucidate this finding, the present research aimed to identify 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 sequential 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 functioned 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 affiliative social interaction with social partners. The implications of these results for understanding how the differentiation of the NC900 and NC100 occurred within microevolution and development are discussed. Aggr. Behav. 27:463–475, 2001. © 2001 Wiley-Liss, Inc.