Fighting, nonagonistic social behavior, and exploration in isolation-reared rats

Sixty hooded Long-Evans rats were assigned to one of three conditions: isolation-rearing (from 14-30 days of age), brief isolation (from 28-30 days of age), and normal (no isolation). From day 14 to day 21, the isolation-reared animals were maintained in an incubator set at 35 degrees C to prevent hypothermia and were separated from each other by Plexiglas. They learned to nourish themselves by lapping milk that continuously flowed down one side of the enclosure. Thereafter, they were housed in standard hanging wire cages. Unlike previous studies of isolation at this age, there were no significant weight differences between the isolation-reared group and the two control groups. At 30 days of age, the three groups were compared on fighting (serious and play), nonagonistic social behaviors (following and crawling under):, and exploratory behavior (open-field ambulation). The isolation-reared group was significantly different from the normal group on all measures (isolates were higher on all measures except exploration). The isolation-reared animals showed significantly more serious fighting (aggression) than the brief isolation animals, whereas these two groups did not differ on other behaviors. This methodology allows for the study of isolation-rearing without the nutritional confounds found in previous research and shows an effect on aggression that is not accounted for by recent isolation. (C) 1999 Wiley-Liss, Inc.