Infant bystanders modulate the influence of ovarian hormones on female socio-sexual behaviour in free-ranging rhesus macaques

It has long been established that one of the driving factors underlying changes in female socio-sexual behaviour across the ovarian cycle is variation in the hormones oestrogen and progesterone. However, the effect that the social environment, and specifically con-specific bystanders, exerts on social relationships is far less clear. Here we explore the modulating effects of infant bystanders on relationships between female ovarian cycling and socio-sexual behaviour in free-ranging rhesus macaques during the 6-month mating season on Cayo Santiago, Puerto Rico. We used non-invasive hormone assessment to time ovulation in females, and analysed measures of social and sexual behaviour with respect to a 2-day ovulation window. Rates of copulation and ejaculation varied relative to ovulation, with female-male sexual interactions peaking around ovulation. Moreover, the presence of an infant bystander affected these rates, with fewer sexual interactions occurring for a given day with respect to ovulation when infant bystanders were more frequently in close proximity to the female. Other bystander categories (adult females, adult males, and adult female & infant groupings) did not have the same effect on female mating behaviour. These results suggest that mother-offspring conflict might manifest not only as direct interactions between mother and infant (e.g., weaning or carrying conflict), but also through indirect interactions.