Sex differences in play behavior

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 It could be debated whether the neural determinants of sex differences in play behavior are different from those involved in the expression of aggressiveness and fear.   At any rate,  there is little debate about the greater propensity of male mammals (including humans) for « rough and tumble » play (Graves, 1978; Pellegrini, 1989; Rosenthal, 1983).  In humans, this male trait appears to be manifest in a widely differing cultures (Freedman et al, 1979).  The human male infant even appears to be slightly precocious than the female in his motor development,  probably because of this gender difference (Deldime & Vermeulen, 1997;  Reinisch et al, 1991).  Sex differences in social play in lower mammals, like rats, appear to be more quantitative than qualitative, referring to frequency and not the form of the behaviors  -but in higher mammals there are some clear qualitative differences as well.  For example, 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 (Erhardt, 1984). 

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 (Meany & McEwen, 1986). Though the amygdala has not yet been investigated in this manner in monkeys as far as I could discern,  prenatal androgenization of female macaques has been reported to increase juvenile rough and tumble play (Goy et al, 1988).    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 (Hines & Kaufman, 1994).

In one investigation the effects of altering neonatal levels of progestins on the later development of social play behaviour was studied in rats. Progestin levels were raised in experiment one by administering injections of either progesterone or medroxyprogesterone acetate. Exposure to either hormone led to reduced levels of social play in juvenile rats of both sexes, confirming earlier reports of lowered levels of play following medroxyprogesterone obtained via maternal milk. In a second experiment, endogenous progestin levels were lowered by administration of the antiserum to progesterone. The prediction that this should result in raised levels of juvenile play was supported for males, but not for females.  Females in by contrast showed a decrease in play.  The reduction of play behavior by progestin in male rats has been reported by other investigators as well. 

Theories concerning the function of sex differences in social play often emphasize either the social or motor learning functions. However, such differences may reflect socio-biological and developmental cascades that are, in some way, initiated by perinatal hormonal events and then further modulated by hormones throughout development.