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 1 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.
