Anogenital swelling in pregnant chimpanzees of Gombe National Park

Abstract

Previous studies show that anogenital swelling patterns of pregnant chimpanzees in captivity are correlated with the female's age during pregnancy and are associated with changes in social behavior. The purpose of the present study was to investigate whether the anogenital swelling patterns of free-ranging pregnant chimpanzees resemble those reported for captive chimpanzees. We reviewed the records of 47 pregnancies experienced by 20 female chimpanzees living in Gombe National Park from 1975 to 1992. Percentage scores for swelling during each trimester and for the overall occurrence of various levels of swelling were obtained by dividing the frequency by the total number of days the subject was observed (i.e., 28 days of swelling/165 days observation = 17.2% anogenital swelling for that pregnancy). Subjects exhibited anogenital swelling an average of 25.1% of the days observed during pregnancy, yet reached maximum only 9.4% of the time. Anogenital swelling occurred less frequently as pregnancy progressed, averaging 40.8, 27.3, and 2.7% in the first, second, and third trimesters, respectively. As seen in captive chimpanzees, young pregnant females at Gombe showed anogenital swelling more often than did older ones (r = −.65, P < .0001). These findings are consistent with reports of behavior associated with anogenital swelling (e.g., intercommunity transfer of young females exhibiting anogenital swelling).