Effects of neonatal testicular suppression with a GnRH antagonist on social behavior in group-living juvenile rhesus monkeys.

Twenty-four male and eight female 1-year-old rhesus monkeys (Macaca mulatta) were observed for social interaction with other yearlings and with their mothers. The males comprised three groups which differed in the level of neonatal androgen exposure. One group received the GnRH antagonist Antide during their first four neonatal months (Antide n = 8), which suppressed pituitary LH secretion resulting in peak neonatal T levels < 0.7 nmol/liter. A second group received Antide treatment combined with a long-lasting testosterone replacement (Ant/And n = 8), which resulted in peak neonatal T levels of 29.1 +/- 3.8 nmol/liter. The third group (Vehicle n = 8) received the Antide and androgen vehicles and had intermediate peak T levels of 5.2 +/- 1.0 nmol/liter. Behavior of males was compared to that of unmanipulated control females living in the same social group (Control Female n = 8) when androgen levels were uniformly low (< 0.7 nmol/liter) in all male groups. Subjects received 12 weekly 30-min focal observations by an observer blind to the neonatal treatments. Marked sex differences were found in several categories of sociosexual behavior. All three groups of males engaged in significantly more sexual and play behavior than females, with the exception of quiet solitary play, which females exhibited significantly more frequently In addition, females exhibited significantly more interest in infants than did any male group. There were no differences between groups in agonistic behavior or time spent in contact with other individuals, but females spent significantly more time than any male group in proximity to other animals. Both females and Antide males initiated proximity and followed animals significantly more frequently than Ant/And males, but not Vehicle males. Proximity durations with mothers initiated and terminated by yearlings were longer for females than for any male group and for Antide males than for Ant/and males. Antide males were groomed significantly longer than any other group. These results demonstrate effects of neonatal testosterone exposure on social behavior in yearling rhesus. Suppression of neonatal T did not affect sexually dimorphic patterns of play and sexual behavior, but altered the character of interactions with their mothers. Whether this reflects a delay in the development of maternal independence or a fundamental alteration in patterns of social interaction remains to be resolved.