Social influences on circulating levels of cortisol and prolactin in male talapoin monkeys

Three captive groups of adult talapoin monkeys (Miopithecus talapoin), each with four males and four or five ovariectomized, estrogen-treated females, were observed. Behavioral interactions were recorded and levels of cortisol and prolactin were measured. In each group, males formed a linear dominance order, determined by the direction of spontaneous aggression; highest male rank was associated with frequent socio-sexual interactions and lowest rank with infrequent socio-sexual behavior. The first study compared males' cortisol and prolactin titers when all males were either housed with estrogen-treated females, or singly caged. The two lowest-ranking males of each group had elevated cortisol levels when group-housed; prolactin levels did not reflect these changes. In the second study, males of one group interacted with females when only the lowest-ranking, or the highest-ranking, female was made attractive (i.e., received estradiol), while the other females were present, but not estradiol-treated. Across these conditions (1) frequencies of some socio-sexual and aggressive behaviors changed, (2) cortisol levels increased in all males, and (3) prolactin levels decreased in three of four males. Dissociation of changes in cortisol and prolactin titers suggests that these hormones may be differentially responsive to social modifications, not simply reflecting a single intervening variable, such as "stress."