Investigation of the role of postnatal testosterone in the expression of sex differences in behavior in infant rhesus macaques (<em>Macaca mulatta</em>).

In several primate species, males have been shown to exhibit a surge in circulating testosterone during the early postnatal period. This surge has been postulated to play a role in the development of sex differences in behavior. In this study, the role of postnatal testosterone in infant behavioral development was investigated in socially living rhesus macaques. Seven male infants were treated with a GnRH agonist, avorelin, from the first week of life onwards. Ten female infants were exposed to testosterone by implantation of capsules containing testosterone. The behavioral development of these and control infants was recorded from birth to 6 months of age. The sexually dimorphic patterns of play and mounting were not affected by manipulation of postnatal testosterone in either male or female infants. Similarly, most mother-infant interactions were not affected by the hormonal manipulation of infants. Mothers of testosterone-treated females were found to take more responsibility for moving into and out of arm's reach of their infants than mothers of some other groups of infants; however, this measure did not normally differ between mothers of male and female infants. Manipulation of the postnatal testosterone surge does significantly affect penile growth and development, but does not affect the expression of infant sex differences in behavior nor greatly affect the development of the mother-infant relationship in rhesus macaques. (C) 1999 Academic Press.