Sexual behavior in old male rhesus monkeys: influence of familiarity and age of female partners.

Old male rhesus monkeys (Macaca mulatta) whose sexual behavior had declined over a 10-year period were studied. The same ovariectomized females, comparable in age to the males (about 20 years old), served as sexual partners during the 10 years. In the first experiment the old males were paired with unfamiliar females to determine whether changing sexual partners would reverse the decline in performance that had been observed. The unfamiliar females, also about the same age as the males, were ovariectomized and treated with estradiol before pairing, as were the familiar females. Although the males contacted the unfamiliar females more often than they contacted the familiar females, there were no other differences in sexual activity. The two groups of females did not differ in their behavior toward the males. The low level of sexual performance by the males with both familiar and unfamiliar females could have been the result of decreased sexual initiative and responsiveness associated with the advanced age of the females, so a second experiment was undertaken to test this possibility. The old males were paired with two different groups of unfamiliar, intact, cycling females. One group was young (about 4 years old), the other old (about 20 years). The sexual responses of the males to both young and old females were the same. The two groups of females did not differ in their behavior toward the males.