Effects of Age and Season on Sexual Behavior and Plasma Testosterone and Dihydrotestosterone Concentrations of Laboratory-Housed Male Rhesus Monkeys (Macaca mulatta)1

Sexual behavior and plasma testosterone and dihydrotestosterone levels were examined in twelve male rhesus monkeys of different ages and at various times of the year. Plasma testosterone was found to vary significantly throughout the year. High concentrations of testosterone were measured in the fall and winter seasons, whereas low concentrations were measured in the spring and summer. A similar seasonal pattern was noted in the sexual behavior of these animals. Statistically significant reductions in the incidence of ejaculation and the frequency of yawning were recorded in the spring compared with the fall months. It appeared that changes in sexual behavior and circulating testosterone were associated. Four of our males were more than 20 years old. The sexual activity of these animals was significantly less than that of younger males, but comparisons of circulating testosterone and dihydrotestosterone levels revealed no differences between the old and young animals. Thus, the endocrine testes of the older monkeys appeared to be functioning normally and the lower sexual activity observed in these animals could not be accounted for by differences in total testosterone or dihydrotestosterone.