RHYTHMIC CHANGES IN THE COPULATORY FREQUENCY OF RHESUS MONKEYS (MACACA MULATTA) IN RELATION TO THE MENSTRUAL CYCLE AND A COMPARISON WITH THE HUMAN

Summary. The frequency of ejaculation in thirty-two pairs of rhesus monkeys studied during seventy-five menstrual cycles was significantly higher (P<0-001) in the follicular than in the luteal phase of the cycle: this pattern was closely similar to the changes in the incidence of sexual intercourse reported in a human population. Although many Old World monkeys and apes do not show well-defined periods of oestrus or 'heat', and will copulate throughout the female's menstrual cycle, there are indications that a sexual rhythm persists in higher primates and that increased frequencies of copulation occur near the expected time of ovulation. Laboratory studies on pairs of rhesus monkeys have frequently shown increased numbers of mounts by the male near mid-cycle and a sharp decline in mounting activity early in the luteal phase (Michael, Herbert & Welegalla, 1967; Michael & Welegalla, 1968). The precise pattern of changes observed differed in different pairs and with different cycles and, in some, rhythmic changes were entirely absent. In general, changes in the various indices of sexual behaviou