Synchrony of estrous swelling in captive group-living chimpanzees (Pan troglodytes)

Abstract

Recent findings suggest that menstrual synchrony may occur in humans and is more dependent on factors of close personal association than simple prox imity. Part I of this study analyzed the estrous cycles of 10 captive group living chimpanzees (Pan troglodytes) housed in two separate groups in the same building. Each group contained at least one male. The mean discrepan cies for (1) the approximate day of ovulation (last day of maximum swelling) and (2) the day of onset of estrous swelling were determined for all possible pairs within groups and compared to the mean discrepancies of pairs between groups. With respect to the probable day of ovulation, the mean discrepancy of animals caged together was 8.0 days and that of animals caged separately was 10. 7 days. Comparing the onset of estrous swelling, the mean between groups was 10.4 days, whereas females caged together averaged only 5.7 days. A repeated-measures analysis of variance indicated a significant group effect in the onset of estrous swelling (F = 5.68,p < 0.05), whereas there was no significant difference between the animals caged together and those caged separately with respect to the last day of maximum swelling (F = 1.37, NS). Part II of the study compared the onset of the first postpar tum estrus (PPE) to the onset of estrus in cagemates. During a 6-year period, 13 females exhibited 33 PPEs while in social contact with cycling females. Of 86 paired comparisons, the PPE onset of 43 (50%) occurred with in 6 days of the swelling onset of a cagemate. The results suggest that, as in other species (including humans), the social contact and time spent together influence the synchrony of chimpanzee estrous cycles, especially in terms of swelling onset. The synchrony of estrous swelling and possible mediation by olfactory and/or hormonal cues are discussed as they relate to chimpanzee social structure and female reproductive strategy.