The sexual differentiation of social play.

Sex differences in social play are quantitative and not qualitative, referring to frequency and not the form of the behaviors. Whereas increased perinatal exposure to exogenous testosterone masculinizes social play, experimental manipulations of androgen levels after this period (i.e. following critical periods for neuronal differentiation) apparently have no effect on the expression of social play. This effect appears to involve, at least in part, androgen receptor occupancy in the amygdala. In the rat, there is a prominent sex difference in nuclear-bound androgen receptors in the amygdala during the sensitive period for the masculinization of play-fighting. Moreover, testosterone implants directly into the amygdala during this period masculinize social play in females. Progesterone exposure reduces play-fighting in male rats, as does corticosterone. This latter effect may be mediated by corticosteroid receptors in the limibic brain. Perinatal androgen exposure may also be important in humans, since girls born with congenital adrenal hyperplasia diagnosed and treated at birth still show male-like patterns of play. Theories concerning the function of sex differences in social play emphasize either the social or motor learning functions. Juvenile male primate social rank correlates with number of peer social interactions, which predominantly take the form of play-fighting. Females on the other hand appear to spend less time play-fighting and spend more time waiting and competing for interactions with infants, i.e. play-mothering, whereby they acquire the motor skills necessary for handling infants. Such differences may reflect socio-biological and developmental cascades that are, in some way, initiated by perinatal hormonal events.