Sex differences in Japanese macaques (Macaca fuscata): effects of prenatal testosterone on juvenile social behavior.

The aim of this study was to assess, in a nonhuman primate, the extent to which exposure to androgen during the prenatal period interacts with early social experience to affect the display of male or female patterns of behavior. Pregnant females from a large age-graded, heterosexual group of Japanese macaques (Macaca fuscata) were implanted about the 40th day of gestation with Silastic packets of testosterone. The packets were removed on the 100th day of gestation, and the females were allowed to give birth in their outdoor corral. An unplanned procedural change, by the surgeon who did the implants, created two groups of prenatally androgenized females: a high-dose group (N = 3), and a low-dose group (N = 4). The anatomical differentiation of these groups differed in that the high-dose group had small penises and no vaginas while the low-dose group had enlarged clitorises and patent vaginas. The behavior of these two groups of females was compared with that of normal males (N = 6), prenatally androgenized males (N = 6), and normal females (N = 5) from birth to 2 years of age. There were no differences between treated and normal males, but there were sex differences between males and normal females in the frequency of mounting, playing, displaying, and grooming. The high-dose group of prenatally androgenized females differed from normal females on only one measure: increased frequency of mounting. The low-dose group mounted other juveniles more frequently than did the normal females, but the difference was not statistically significant. We concluded that mounting behavior was most sensitive to the prenatal hormone environment because it showed the largest sex difference in normal animals. Given the small sample sizes, within-group variability could have obscured possible hormonal effects on other behaviors where sex differences were less dramatic.