Sexual behavior, sexual swelling, and penile evolution in chimpanzees (Pan troglodytes)

Functional relationships between the penis, vagina, and cervix during copulation in the chimpanzee were studied. In 11 adult males, penile length during full erection ranged from 10.0-18.0 cm (mean +/- SD = 14.4 +/- 2.02). In 19 parous adult females examined during the early follicular phase of the cycle, or during lactation, vaginal depth (from the introitus to the os cervix) ranged from 9.8-16.5 cm (mean +/- SD = 12.6 +/- 1.69). However, when the sexual skin was maximally swollen, vaginal depth increased by up to 52% in some cases and ranged from 15.2-20.1 cm (mean +/- SD = 16.9 +/- 1.68). Under such conditions males vary in their ability to achieve maximal depth of intromission. Observations made on 6 females after natural matings revealed that in some cases males had deposited copulatory plugs in contact with the cervix, while in others plugs were lower down in the vagina. Direct observations under fluoroscopy using an artificial "penis" loaded with radio-opaque medium showed that the filiform tip can make contact with the os cervix. However, expulsion of radio-opaque medium under these conditions failed to demonstrate any immediate transfer of fluid through the cervix into the uterus. These studies indicate that, aside from its functions as a visual signal, the female chimpanzee's sexual skin swelling adds considerably to the distance males must negotiate during copulation to place spermatozoa at the cervical os. Evolution of the male's elongated, filiform penis may therefore be the result of sexual selection, to negotiate the long vagina of the female and to penetrate copulatory plugs deposited during previous copulations.