Social behavior and genital swelling in pregnant chimpanzees (Pan troglodytes)

Abstract

Previous reports of increased sexual activity associated with genital swelling during chimpanzee pregnancy have suggested that the tumescent pregnant female may be regarded as a regularly cycling estrous female. Analysis of additional data from seven pregnant and eight cycling females, however, indicated that tumescent pregnant females differed from cycling females in some social interactions with males. As in earlier reports, pregnant females were more receptive to male-initiated copulatory bouts. In addition, pregnant females initiated grooming bouts with males much less frequently than did cycling females, and males spent less time grooming and less frequently inspected the genitals of pregnant than cycling cagemates. This tendency to decrease social interaction during pregnancy is consistent with field reports that pregnant chimpanzees prefer the company of their offspring and other noncycling females. In addition, the genital swelling patterns of 40 chimpanzee pregnancies were analyzed. Length of gestation averaged 231.5 days. Although a high degree of individual variation existed, females were swollen an average of 41% of the days observed, yet reached maximum swelling only 8.7% of the time. Genital swelling was less frequent during the third trimester, averaging 50.5,47.9, and 22.6% tumescent days in each trimester. Young pregnant females showed maximum swelling more often than did older females. This finding is discussed with respect to field reports of intercommunity transfer of young, tumescent female chimpanzees.

Ovarian hormone concentrations and genital swelling patterns in female chimpanzees with Norplant implants

“…Genital swelling data were analyzed using three categories: no swelling, partial swelling, and full swelling. A full swelling was defined as maximal swelling of the labia minora and perianal tissues resulting in labial occlusion, a deeply recessed anus and a shiny taut appearance [following Wallis and Lemmon, 1986;Dahl et al, 1991]. Three facility employees were responsible for the daily recording of genital swelling data; however, one person recorded 75% of these data.…”

Genital swellings in silvered langurs: what do they indicate?

“…The opposite trend has been reported for swellings in pregnant chimpanzees. In chimpanzees, swellings are confined to the first third of the gestation period, and full swellings overall are rare (8.7% in captivity [Wallis & Lemmon, 1986] and 9.4% in the wild [Wallis & Goodall, 1993]), while they are rather common in pregnant silvered langurs (27.5% overall). It seems that swellings (particularly full swellings) to some degree ''advertise'' gestation.…”

“…We assume here that female dispersal occurs regularly in silvered langurs, because it has been documented in other species of the genus (see summary in Sterck [1998]). Originally, it was argued that swellings in general [Tenaza, 1989] and during the gestation period in particular [Wallis & Lemmon, 1986] may facilitate dispersal as long as a female with a swelling can be mistaken for a cycling female. However, this seems unlikely in silvered langurs, and the question for now must remain unanswered.…”

14 Chimpanzee Hunting Behavior

“…He noted that cycling females with conspicuous ano-genital swellings tended to receive meat from adult males more frequently than did females without these sexual swellings, and suggested that males shared meat with females in exchange for sexual access. Swollen females are attractive to males (Dixson 1998) as the swellings generally indicate approaching ovulation, although females will also show swellings when pregnant (Wallis and Lemmon 1986). Supporting evidence for this hypotheses was provided by Stanford (1998) who found that, at Gombe, the presence of a swollen female in a party of chimpanzees was the best predictor of a hunt occurring when encountering a group of red colobus, and reported five observations of females begging for meat from males, and only being given part of the kill after copulating.…”