Effects of Ejaculations By Males On the Sexual Invitations of Female Rhesus Monkeys (Macaca Mulatta)

The effects of ejaculation on female sexual invitations were examined in 6 pairs of adult rhesus monkeys of opposite sexes observed during 1 hr mating tests (552 tests), whereby each of 3 females was tested on alternate days with one of 2 male partners (6 males). Ovariectomized females were given subcutaneous silastic implants of testosterone to maintain female sexual motivation, and the sexual activity of males was altered by the administration and withdrawal of intravaginal oestrogen. Plasma hormone determinations confirmed that females remained under constantly high androgenic and low oestrogenic stimulation throughout the study. Comparison of the number of sexual invitations in pairs of successive tests showed that tests without ejaculation following those in which ejaculations occurred were associated with increased numbers of invitations; conversely, ejaculatory tests following non-ejaculatory tests were associated with decreased numbers of invitations. In contrast, there were no changes in pairs of tests with ejaculations in both tests or ejaculations in neither test. Data from sequences of 4 successive tests showed that where an ejaculatory test was followed by 3 non-ejaculatory tests, the increase in invitations during the latter was short-lived: by the third non-ejaculatory test invitations had declined to levels similar to those in the initial ejaculatory test. In contrast, no changes were observed during 3 successive ejaculatory tests following a test without ejaculation. It was therefore concluded that the occurrence of ejaculation in a test affected invitational behaviour in the next two non-ejaculatory tests. Changes in ejaculatory performance during blocks of 10 or more tests would therefore have no predictive value for changes in invitational behaviour unless the distribution of ejaculatory tests is taken into account; this was confirmed by withdrawing intravaginal oestrogen. As expected, a marked decline in mean numbers of ejaculations per test in 3 animal pairs was not associated with consistent or significant changes in the mean numbers of female sexual invitations per test. The circumstances under which changes in invitational behaviour may be used as an index for a hormone-dependent change in female sexual motivation, and the possible nature of the consummatory event for which invitations are appetitive behaviours, are discussed.