Cyclic perineal swelling during the intermenstrual intervals of captive female pygmy chimpanzees ( Pan paniscus)

Abstract

The perineal swelling cycle and intermenstrual interval of pygmy chimpanzees ( Pan paniscus ) have not been well documented, and it is not clear that cither characteristic differs from those of common chimpanzees ( P. troglodytes ). Consequently, adult females in a breeding colony of P. paniscus were monitored for menses over four and one-half years, and the perineal swelling cycles were detailed for half that period. To test the hypothesis that there are no differences between the species of Pan in reproductive cyclicity of captive females, the results were compared with data collected from 22 captive P. troglodytes over an eight-year period by W. C. Young and R. M. Yerkes. The average duration of the intermenstrual interval of 49 days ( n =23; S.E.=2·9) for adult P. paniscus differed from an average of 34–36 days for adult P. troglodytes , but was similar to the 50-day interval of adolescent P. troglodytes . The pattern of the swelling cycle was different from any cycles exhibited by adult or adolescent P. troglodytes ; following the first sign of detumescence, the swelling retained significant tumescence for an average of 12 days. By altering slightly the definitions of each swelling phase, the durations of the phases subsequent to abrupt detumescence were found to resemble either the adult or the adolescent postswelling phases of P. troglodytes . Adult female P. paniscus exhibit marked swelling for approximately 75% of the intermenstrual interval in comparison to about 50% or less of the intervals exhibited by P. troglodytes . It follows that female P. paniscus are sexually attractive to males for a greater portion of the time, which may contribute to the differences in social organization reported for the two species in the wild. Aspects of the similarity in cyclicity between adult P. paniscus and adolescent P. troglodytes are congruous with interspecific morphometric analyses and the comparative anatomy of the female external genitalia, suggesting that some characteristics of P. paniscus may be neotenous with respect to P. troglodytes but not all distinctions can be accounted for by neoteny alone. The structural resemblance between the sexual swelling of female chimpanzees and the buttocks of human females suggests that female Australopithecinae may have been continuously attractive to males.