Interaction between strains in the social relations of inbred mice.

An experiment involving trios of juvenile mice of two inbred strains (C57/BL/6 and 129/SV) was used to test for effects of interaction between strains on levels of exploratory/agonistic behavior and on the form of asymmetric social relations. Each trio consisted of two same-sex full siblings from the same litter and a third, unfamiliar individual (the "stranger") which was of the same sex as the siblings and was either of the same strain as the siblings or of the other strain. Levels of exploratory/agonistic behavior between the siblings and the stranger, both in the initial encounter and after 3 or more days, were higher in mixed-strain trios than in same-strain trios, and hierarchies formed on the basis of these behaviors were significantly more linear in mixed-strain trios than in same-strain trios. Although some absolute differences between the strains were detectable, the most marked behavioral differences were between mixed-strain and same-strain trios.