Sexual behavior in aging male rhesus macaques: Effects of test duration on sexual performance

The sexual behavior displayed by nine old (20-year and older) rhesus (Macaca mulatta) males in 10-min tests was compared to that displayed in 1-hr tests. The tests were part of a long-term study on the decline in male sexual activity that accompanies old age. The males were paired with 10 ovariectomized, estrogentreated females in two blocks of 10 tests; each male was tested once with each female in each test block. The percentages of males that achieved intromissions and ejaculated in the two test blocks were the same (P < 0.05). Although the percentage of tests in which males displayed these behaviors was higher in the longer-test block (P > 0.05), there was a significant positive correlation of performance in 10-min tests with performance in 1-hr tests. Assuming a random distribution of contacting, mounting, intromission, and ejaculation throughout the hour, we would have predicted a significantly lower number of these behaviors in the first 10 min of the 1-hr tests than we actually observed. The mean percentages of 1-hr tests with contacting, mounting, intromission, and ejaculation was significantly lower than that of 10-min tests conducted with the same males 11 years earlier. Thus, the decline in sexual performance was not an artifact of the limited (10-min) test duration.