Postweaning social experience and adult aggression in rats.

One group of male rats was reared continuously in small, stable colonies of three animals each during a ten-week period beginning when they were 46 days old. A second group of animals was also reared in small groups of three animals each but membership within these colonies was randomly changed at weekly intervals. Thereafter, all subjects were housed in individual cages. A 24 hr test of the subjects' aggression toward an unfamiliar male conspecific at 130-133 days of age indicated that only the heaviest animal in each of the stable colonies was aggressive. In contrast, all animals in the random-composition group were aggressive toward intruders. Scores on a second test of intruder-directed aggression administered when the subjects were nine months old were significantly related to scores on the first aggression test. Subtle variations in social experience during development are critical in establishing long-term patterns of aggression toward conspecifics in laboratory rats.