Peri-ovulatory synchronisation of behaviour in male and female rhesus monkeys

Abstract

RHESUS monkeys, like many other primates, do not restrict their sexual activity to the ovulatory period, and copulation may occur at any time during the menstrual cycle^1,2. Thus, in the laboratory, the male's ejaculatory frequency is generally highest near mid-cycle (although different pairs show different patterns of mounting and ejaculation³) while, in the wild, sexual activity seems to be more or less restricted to the middle part of the cycle^4,5. It is difficult to judge the stage of the cycle in troops of feral rhesus monkeys, but during the period of maximum sexual activity females actively approach males and form temporary consort bonds which involve mutual grooming and frequent copulations⁶. To obtain an independent and objective measure of female sexual motivation, we have used an operant conditioning technique in which the female was required to press a lever for access to her male partner. This has made it possible to relate changes in female motivation precisely to changes in the circulating levels of ovarian hormones. We have also demonstrated a synchronisation and maximisation of female sexual motivation with male ejaculatory performance during the peri-ovulatory (fertile) period of the menstrual cycle.