Physiology &Behavior, Vol.52, pp. 1-5, 1992 0031-9384/92 $5.00+ .00 Printedin the USA. Copyright© 1992PergamonPressLtd. Sex Differences in the Play Fighting Activity of Golden Hamster Infants ROGERIO F. GUERRA, l MAURO L. VIEIRA, EMILIO TAKASE AND SIMONE GASPARETTO Universidade Federal de Santa Catarina, Departamento de Psicologia, Laboratrrio de Psicologia Experimental, Campus Universitrrio da Trindade, 88049, Florianrpolis, S.C., Brasil Received 13 December 1990 GUERRA, R. F., M. L. VIEIRA, E. TAKASE AND S. GASPARETTO. Sex differences in the play fighting activity of golden hamster infants. PHYSIOL BEHAV 52(1) 1-5, 1992.--Play fightingof homosexual and heterosexual dyads in golden hamsters was studied. The behaviors of male-male (MM), female-female (FF), male-female (MF) and female-male (FM) dyads were recorded from the 25th to the 31st day of age. Play fightingactivity of heterosexual dyads was depressed since MF and FM dyads exhibited a significantly high physical contact time and engaged in less play fighting,with a significantly lower time spent on such activity. Alternation ratio in the pinning posture was very high and subjects did not differ in the mean number of active (top) and passivepinning(under) in each dyad, showingan inexistenceof sex dominance. In most cases, differences in the performances of MM and FF dyads were not significant. We have concluded that play fighting in golden hamsters may be predominantly modulated by environmental stimuli (sex of the partner) rather than by endogenous stimuli (androgen level). Play fighting sex differences androgen pinning postures locomotion Physical contact Golden hamster PLAY fighting occurs in many species of rodents. This behavior is very easy to discern, but its definitions and functions are very difficult to characterize (4,10,19,25). In fact, when playing, infant rodents may behave in many ways as they do during serious encounters. In rats, play fighting may be discerned from real fighting since snout or oral contact is directed to the partner's nape of the neck, whereas during real fighting bites are directed to the opponent's rump (20). In golden hamsters, the infants attack and defend the cheeks and cheek pouches during play fighting, whereas they attack and defend the rump and lower flanks during real fighting (22). Play fighting begins around the 15th day of age and becomes more frequent as the infant grows older (5,7). Sex differences in play-fighting behavior have already been mentioned for many species of mammals (13). In fact, play en- counters between juvenile male rats are more frequent than be- tween females, and also males are more likely to engage with other males than with females (12) and to initiate more play than do females (8). Male rats engage in more rough-and-tumble than do females of the same age, and female pups submitted to a testosterone treatment during their early development exhibit an increased play-fighting behavior comparable to that of males (15). It has already been concluded that androgen level of infant rats appears to promote the sexual differentiation of some CNS structures during early development which, in turn, mediate sex differences in play fighting, independent of later testosterone levels (11,14). The present article examined sex differences in play-fighting activity of homosexual and heterosexual dyads of golden hamster infants. METHOD Subjects Eighty juvenile golden hamsters (Mesocricetus auratus) were used as subjects. The animals were obtained from the stock of the Laborat6rio de Psicologia Experimental at the Universidade Federal de Santa Catarina. Equipment The animals were housed in polypropylene home cages (40 X 30 X 17 cm). Chronometers and standard sheet notes were utilized during the experimental sessions for recording of be- haviors. Procedure On the 24th day of age, the infants were removed from the mother's cage and seperately housed in 40 x 30 × 17 cm home cages in four different groups of male-male (MM), female-fe- male (FF), male-female (MF) and female-male (FM). Each group was formed by 20 subjects, or 10 dyads, each dyad was formed by infants of different litters. The focal-infant (FInf) of each dyad was notched in its right ear and the partner-infant (PInf) in its left ear. The animals were observed from the 25th Requests for reprints should be addressed to Dr. Rogerio F. Guerra.