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‐old pups (2 males, 2 females) and an unfamiliar male pup from another litter were observed. Some patterns of social play were asymmetric. Compared to the playmate from the group, the unfamiliar pup was most likely 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 another animal placing its forepaws on the second animal.), grooming, or crawling under a playmate. The effects of relative birth weight, ambient temperature during the lactation period, relative body weight, and relative body temperature during the observation period were studied. Results show that pups born and raised in a warm environment interacted socially more than pups raised in a colder environment. Pups lighter than the mean body weight of playmates tended to use submissive behaviors more than relatively heavier pups. Relatively heavy pups tended to use play behaviors that might be considered more aggressive during the interaction. The results suggest that under appropriate conditions, early evidence for dominance, i.e., asymmetry can be revealed. © 1997 John Wiley & Sons, Inc. Dev Psychobiol 30: 165–176, 1997