The external genitalia of female pygmy chimpanzees.

The external genitalia of four adult female pygmy chimpanzees (Pan paniscus) were examined during a 2-year period. It was found that the labia majora are retained in adults of this species and that, when tumescent, the labia minora effectively relocate the frenulum and clitoris so that they point anteriorly between the thighs. When detumescent, the configuration of the labia minora and clitoris resembles that of immature common chimpanzees (P. troglodytes). It is suggested that the simple, structural relocation of the clitoris from the normal condition noted in adult P. troglodytes makes possible the homosexual, intergenital rubbing observed in P. paniscus, when ventroventral juxtaposition of the individuals permits eye-to-eye contact. In addition, this change probably increases sexual stimulation of the female during heterosexual, ventroventral copulations. The possibility of intense, face-to-face sexual interactions has been achieved by P. paniscus without the evolution of bipedalism and the consequent reorientation of the genitalia seen in Homo sapiens.