Reduction of sexual interaction in rhesus monkeys by a vaginal action of progesterone

Abstract

PROGESTERONE acting in the female seems to reduce sexual interaction in several primate species. It has been reported that sexual activity declines during the luteal phase of the menstrual cycle (when circulating progesterone levels are maximal), in monkeys^1–3, lowland gorillas⁴ and humans⁵, and that administering progesterone to ovariectomised, oestrogen-treated rhesus monkeys has a similar effect⁶. A major unresolved question is how progesterone causes these changes in behaviour. One possibility is that progesterone acts on the female's central nervous system, causing her to accept or solicit fewer male mounts. Another is that progesterone somehow alters the vagina thereby changing non-behavioural cues (such as a smell⁷ or tactile qualities) which contribute to her sexual attractiveness. Here we present evidence favouring the second mechanism, since the reduction in sexual interaction caused by systemic administration of a physiological dose of progesterone to female monkeys could be reproduced by instilling very small amounts of this hormone directly into the vagina.