DOMINANCE AND AGE-RELATED-CHANGES IN THE PLAY FIGHTING OF INTACT AND POST-WEANING CASTRATED MALE-RATS (RATTUS-NORVEGICUS)

The play fighting behaviour of male rats (Rattus norvegicus) castrated at weaning was compared to that of intact controls during the juvenil e and post-pubertal phases of development. Following puberty, both the castrated and intact animals exhibited an age-related change in their play fighting; the frequency of initiating play fighting decreased an d juvenile patterns of playful defense were replaced by more adult-lik e patterns. As these changes occurred even in the absence of the puber tal surge of gonadal hormones, they were more likely to result from th e organizational effects of gonadal hormones in the perinatal period t han the activational effects of these hormones at puberty. Although th e castrated animals exhibited the age-related changes in behaviour, th ey did not exhibit the asymmetries in play associated with dominance r elationships. As demonstrated in previous studies, in pairs of intact rats, the animal that attacks the most and uses more juvenile defenses during play fighting and weighs the least is typically the subordinat e. In the castrates, asymmetries in weight and playful defense are not related to play frequency, indicating the absence of a dominance rela tionship. Although the characteristic changes in male play fighting at puberty are independent of the activational effects of gonadal hormon es, dominance relationships and their associated changes in play fight ing are dependent on these hormones. Therefore, in the perinatal perio d gonadal hormones most likely organize the age related changes in pla y behaviour, whereas post-pubertally gonadal hormones activate dominan ce relationships and thus, indirectly modify play fighting by affectin g dominance-associated assymetries in behaviour. (C) 1996 Wiley-Liss, Inc.