Mating in the rhesus monkey (Macaca mulatta) after conception and its relationship to oestradiol and progesterone levels throughout pregnancy.

Behavioural and hormonal data were collected throughout pregnancy in captive rhesus monkeys. Heterosexual pairs observed in daily time-limited mating tests showed two distinct periods of increased sexual interaction, as measured by the incidence of ejaculation. One period was coincident with the preovulatory oestradiol peak, while the other occurred between the 6th and 10th weeks of pregnancy. The positive relationship found during the menstrual cycle between the ratio of circulating oestradiol: progesterone and degree of sexual activity continued for the first 8 weeks of pregnancy. However, sexual activity ceased in the second half of pregnancy despite a continued high oestradiol: progesterone ratio.