SOCIAL PLAY WITH AN UNFAMILIAR GROUP IN WEANLING RATS (RATTUS-NORVEGICUS)

Early signs of aggressive behavior toward an intruder were examined in Sprague-Dawley rats. The interactions between groups of four 24-day-o ld pups (2 males, 2 females) and an unfamiliar male pup from another l itter were observed. Some patterns of social play were asymmetric. Com pared to the playmate from the group, the unfamiliar pup was most like ly to be underneath the other rats, either during play fighting or by crawling under a playmate. The single pup was most frequently involved in the following behaviors: on top (One animal climbs on top of anoth er animal placing its forepaws on the second animal), grooming, or cra wling under a playmate. The effects of relative birth weight, ambient temperature during the lactation period, relative body weight, and rel ative body temperature during the observation period were studied. Res ults show that pups born and raised in a warm environment interacted s ocially more than pups raised in a colder environment Pups lighter tha n the mean body weight of playmates tended to use submissive behaviors more than relatively heavier pups. Relatively heavy pups tended to us e play behaviors that might be considered more aggressive during the i nteraction. The results suggest that under appropriate conditions, ear ly evidence for dominance, i.e., asymmetry can be revealed. (C) 1997 J ohn Wiley & Sons, Inc.