Members of a social group should attempt to maximize their fitness by maintaining an optimal group composition. Allowing an immigrant into the group may be beneficial or costly depending on the characteristics of the immigrant as well as the composition of the group. Therefore, we examined behavioral interactions between pine voles to test three functional hypotheses proposed to explain behavior of residents toward non-residents: the resource defense, mate defense, and benefit of extra-pair copulation hypotheses. To test these, we examined the effects of age, sexual experience and sex of non-residents as well as the effects of sex of residents on the behavior of residents. Neither male nor female residents showed affiliative behavior toward non-residents. Residents were more aggressive toward non-residents than vice versa. The frequency of same-sex aggression was greater than opposite-sex aggression for male residents and this aggression was directed at adult male non-residents to a greater degree than at subadult males. Resident males were least aggressive toward adult females. We found no differences in the behavior of females toward subadults, sexually naive adult non-residents or sexually experienced adult non-residents. Females also displayed similar amounts of aggression toward male and female non-residents. Therefore, for males, aggression may function in defense of a mate while for females, aggression functions in resource defense. For both sexes, aggression is likely to play a role in the regulation of group composition.