Early separation of young rats from the mother and the development of play fighting.

Play fighting was studied in rats which were either normally reared (separated from the mother on day 30) or prematurely separated from the mother on day 15. The animals were tested in pairs on days 33, 36 and 39 of age and play fights were recorded. Play fighting behavior increased markedly after three days of social isolation. In all pairs of normally reared animals (controls, 30S) the rats with the higher pinning scores on the first day also had the higher frequency of pinning on the second and third day of the test. The same trend in the distribution of pinning was found in pairs where the normally weaned animals (30S) played with the early weaned ones (15S). The latter, however, were always pinned by the former (30S) and had significantly lower pinning scores. Such a stable distribution of pinnings did not develop during play fights in pairs of early weaned animals (Group 15S).