Peak occurrence of female sexual initiation predicts day of conception in rhesus monkeys (Macaca mulatta)

The present study investigated whether peaks in female sexual initiation co uld accurately predict conception in group-living female rhesus monkeys. Be havioral observations, 4 or 5 days per week in large, stable, social groups of monkeys., provided frequencies of female initiation of proximity, sexua l solicitation, mounts, and ejaculations. Since a preovulatory peak in fema le sexual initiation is likely linked to the preovulatory oestradiol surge, we used the third day after a peak in behavior as the behavioral estimate of conception date, For each pregnancy, an independent estimate of concepti on date was derived from ultrasound determination of fetal length. Estimate s of conception based on female initiation of proximity with adult males we re accurate for more than 90% of pregnancies., whereas observation of ejacu lations by males predicted conception in fewer than 60% of pregnancies. Beh avioral and ultrasound estimates of conception date were highly correlated and differed by less than 1 day on average. Accordingly, predictions of del ivery date based on behavioral estimates of conception date were as accurat e as those based on ultrasound-derived estimates. These data suggest that f emale-initiated sexual behavior can be used in rhesus monkeys as a practica l, non-invasive tool for producing timed matings in social groups of monkey s, providing accurate estimates of conception date, gestational age, and pr edicted date of birth.