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 could accurately predict conception in group-living female rhesus monkeys. Behavioral observations, 4 or 5 days per week in large, stable, social groups of monkeys, provided frequencies of female initiation of proximity, sexual solicitation, mounts, and ejaculations. Since a preovulatory peak in female 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 conception date was derived from ultrasound determination of fetal length. Estimates of conception based on female initiation of proximity with adult males were accurate for more than 90% of pregnancies, whereas observation of ejaculations by males predicted conception in fewer than 60% of pregnancies. Behavioral and ultrasound estimates of conception date were highly correlated and differed by less than I day on average. Accordingly, predictions of delivery date based on behavioral estimates of conception date were as accurate as those based on ultrasound-derived estimates. These data suggest that female-initiated sexual behavior can be used in rhesus monkeys as a practical, non-invasive tool for producing timed matings in social groups of monkeys, providing accurate estimates of conception date, gestational age, and predicted date of birth.