The interaction of behavior and reproductive cycles in patas monkeys

Reproductive cycles of females in a breeding group of 10 to 18 patas monkeys (Erythrocebus patas) were followed over 5 years using vaginal smear and lavage techniques. Two hundred hours of behavioral observations were made during the same period. The social organization centered on an inner circle of six females with a complex web of interrelationships; other females, and males, had a peripheral position linked to the central circle at one or two points only. Mating activity was periodic: either several or no females were receptive at one time. Most mating activity and most conceptions occurred between September and November but there was no strict association of mating periods with a particular season. Menstrual cycles in mating periods differed from those in nonmating periods in having heavier menstrual bleeding, more vaginal epithelial activity throughout the cycle, and midcycle leukocyte peaks were common. There was very little differentiation of stages of the menstrual cycle in terms of vaginal histology, but there was a clear midcycle peak in heterosexual interaction, especially during mating periods. Friendly and avoiding interactions between females did not change during their cycles or when they were pregnant, but did increase dramatically in early lactation. The return to receptivity postpartum was greatly influenced by the social environment. It seemed that sexual activity had a social facilitatory effect but was in turn inhibited by the presence of small infants. This interactive mechanism favors increased reproductive synchrony within the group.