1Evolutionary Anthropology, Duke University, 2Department of Human Evolutionary Biology, Harvard University, 3Department of Anthropology, University of New Mexico, 4Kibale Chimpanzee Project, Kibale National Park, Uganda
April 16, 2016 2:00, A 703/704
Sexual swellings appear in a variety of Old World monkeys and apes as a signal of female fertility and ovulation. In chimpanzees, this signal is exaggerated: the large anogenital swelling lasts for 10-15 days in a 35-day ovarian cycle. Estrogen induces swelling during the follicular phase while rising progesterone in the luteal phase leads to its collapse; ovulation is most likely to occur as detumescence approaches. Swellings often continue during pregnancy, when both estrogen and progesterone are high, and the mechanism of post-conception swellings remains enigmatic. We analyzed urinary estrone conjugates (E1C) and pregnanediol-3-glucuronide (PdG) in 372 samples from 26 pregnancies in 17 females from Gombe National Park, Tanzania, and the Kanyawara community of Kibale National Park, Uganda. We used a linear mixed model to test the effects of hormones on the level of swelling—none, partial, or full—on the day of sampling, while controlling for gestation day and repeated sampling within females. We found a negative effect of PdG on swelling (ES = -0.48, p < 0.0001), while E1C had a positive effect (ES = 0.19, p = 0.044). In accordance with preliminary analyses of records from Gombe and Kanyawara, older females were less likely to be swollen (ES = -0.02, p =0.037). These results demonstrate that estrogen and progesterone exert similar effects on swelling before and after conception. Ongoing research into the behavioral correlates of post-conception swellings will help to determine if they have adaptive value or occur as a byproduct of normal hormonal activity during pregnancy.
Long-term research at the Kibale Chimpanzee Project has been supported by National Science Foundation Grants BCS-0849380 and BCS 1355014, and the Leakey Foundation. Gombe research supported by Duke University.