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The external genitalia of female pygmy chimpanzees.

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THE ANATOMICAL RECORD 211:24-28 (1985)
The External Genitalia of Female Pygmy
Chimpanzees
JEREMY F. DAHL
Yerkes Regional Primate Research Center and Department of Anthropology, Emory
University,Atlanta, GA 30322
ABSTRACT
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 (I? troglodytes). It is
suggested that the simple, structural relocation of the clitoris from the normal
condition noted in adult I? troglodytes makes possible the homosexual, intergenital
rubbing observed in l? 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 I? paniscus without
the evolution of bipedalism and the consequent reorientation of the genitalia seen
in Homo sapiens.
Although the pygmy chimpanzee (Pan paniscus) has
been recognized for more than 50 years (Coolidge, 1933;
Schwarz, 1929), it has only recently been studied owing
to the inaccessibility of its natural habitats and its rarity in captivity. Interpretation of morphological studies
has been diverse with respect to the conclusion that I?
paniscus is “a true pedomorphic species” (Coolidge, 1933,
p. 56) and, hence, resembles the common ancestor of the
PadGorilla lineage with Homo (cf., Latimer et al., 1981;
McHenry and Corruccini, 1981; Shea, 1983; Zihlman,
1979; also see Johnson, 1981).On the other hand, behavioral studies clearly show that striking resemblances
exist between the sexual and communicative behaviors
of I? paniscus and H. sapiens (Savage et al., 1977; Savage-Rumbaugh and Wilkerson, 1978). In these contexts,
detailed observations on the appearance of the external
genitalia of captive l? paniscus were initiated in 1981 as
a n integral part of a comprehensive study on the behavior and reproductive characteristics of the species. The
structure of the external genitalia was found to differ
from that reported previously (Savage and Bakeman,
1978; Savage-Rumbaugh and Wilkerson, 1978). Details
of this structure are presented here with the objective of
clarifying the variability within Pan and the significance of the condition in I? paniscus with respect to that
in H. sapiens.
The external genitalia of the common chimpanzee P a n
troglodytes) have been described previously (Bingham,
1928; Schultz, 1927) and photographed in living individuals (Graham, 1970; Yerkes, 1943; Zuckerman and Fulton, 1934). The vulva1 aperture is surrounded by the
labia minora, which become highly tumescent and conspicuous during the ovarian cycle. The clitoris is enfolded anteriorly by the prepuce and attached posteriorly
0 1985 ALAN R. LISS, INC.
to the frenulum. Like the labia minora and other perineal tissue, the prepuce also becomes tumescent, but the
frenulum undergoes little change so that the clitoris
becomes embedded in the tumescent prepuce. Ontogenetic variation in these structures has been ascertained
by Wislocki (1936)(see also von Harms, 19561, who has
illustrated the labia majora surrounding the labia minora in a fetus and the relatively larger frenulum in a
juvenile female; the labia majora are absent in adult I?
troglodytes. Because of this ontogenetic variation, in the
present study the structures in I? paniscus were compared to those in both juvenile and adult I? troglodytes.
This comparison may be particularly significant in the
context of the work of Shea (1983), who suggests that
variation among the great apes may be a result of
heterochrony .
MATERIALS AND METHODS
Subjects
The subjects were four adult pygmy chimpanzees (Pan
paniscus) housed a t the Yerkes Regional Primate Research Center Field Station near Lawrenceville, Georgia. One individual, Matata, arrived a t the Yerkes
Center on a lend-lease basis from the Government of
Zaire and the other three animals, Linda, Laura, and
Lorel, were on breeding loan from the San Diego Zoo.
Matata and Linda were wild-caught, and Laura and
Lorel are the captive-born offspring of Linda. At the
beginning of the observations, Linda and Matata were
approximately 30 years and 10 years, respectively; Laura
was 14 years; and Lorel was 12 years.
Received March 8, 1984; accepted July 19, 1984.
GENITALIA OF FEMALE PAN PANISCUS
Fig. 1. Photographs of the external genitalia of female Pan paniscus (see Fig. 2 and text for explanation).
25
26
J.F. DAHL
--
_
.
a
B
ma
mi
f
D
E
F
I
C
._.--
@f
mi
r
era1 margin of the genitals (Figs. 1, 2A,B). When the
sexual swelling was partially tumescent, the area between the labia majora and the labia minora was continuous except for a small, raised ridge immediately dorsal
to the vulval aperture (Figs. 1, 2E). When maximally
tumescent, the labia majora appeared as additional lobular structures located dorsolateral to the labia minora
and ventral to the prepuce (Figs. 1,2C,D),but were less
conspicuous in tumescent, younger individuals.
Labia minora
When completely detumescent (Figs. 1, 2A,B), the labia minora appeared as wrinkled, relatively large structures which formed the borders of the vulval aperture.
These labia arose dorsally from under the labia majora
and terminated ventrally in the frenulum to the posterior and in the prepuce to the anterior. The labia minora
were greatly enlarged during tumescence and became
elongated so that the frenulum and clitoris were relocated. The frenulum projected beyond the most anterior
border of the prepuce and pointed forward between the
thighs (Figs. 1, 2C,D). When viewed laterally, with the
subject standing quadrupedally (Figs. 1, 2C), the dorsal
third of the labia minora formed a flat, vertical surface
facing posteriorly. Ventrally, the labia curved to the
anterior through 90” so that the frenulum was horizontal with the ground. The prepuce also became swollen
so that the apex of the labia minora was displaced ventrally (Figs. 1, 2E). The curvature of the labia minora
and frenulum was often such that the frenulum pointed
upward as well as forward.
Clitoris
Fig. 2. Annotated drawings of the photographs in Figure 1. A) Posterioanterior view of Laura’s perineum showing the demarcation of
the labia into majora and minora; B) as in A, but at complete detumescence; C) lateral view of Linda’s hindquarters (left side) during
quadrupedal locomotion showing tumescent perineum and the curvature of the labia minora; D) lateral view of Linda’s perineum (right
side) when she was sitting and leaning to the left; E) lateroventral
view of the sex swelling of Matata showing the juxtaposition of the
prepuce and frenulum, and the ridge dorsal to the vulval aperture; F)
partially tumescent genitals of Laura showing the contribution of the
prepuce to the swelling and the location of the clitoris (view from the
anterior and slightly to the side). a, Anus; c, clitoris; f, frenulum; ma,
labia majora; mi, labia minora; p, prepuce; r, ridge; t, thigh v, vulval
aperture.
Procedure
Observations of the perineum were conducted over a
2-year period on Linda, Laura, and Lorel, and over a 4month period on Matata. Detailed notes were taken on
the appearance of the vulval aperture and its surrounding tissues, and drawings were made as their appearance changed through time; similar attention was given
to the anus. From time to time, black-and-white photographs, color photographs, and video records were also
made.
RESULTS
Labia majora
When completely detumescent, the labia majora of
Pan paniscus were evident as a clearly demarcated,
wrinkled fold of tissue located at the dorsal and dorsolat-
The clitoris was cylindrical and partially embedded in
the anterior surface of the recurved labia minora. It was
surrounded dorsally by the prepuce and inserted ventrally along the anterior surfaces of the frenulum (Figs.
1,2F). A glans was not discernible.
DISCUSSION
Schultz (1927) reviewed his own and others’ findings
on the external genitalia of the great apes and concluded, “During postnatal life the labia majora undergo
a marked reduction in the three large apes. . . . It is
certain that these structures are laid down in fetal life,
but, whereas in man they persist throughout growth, in
the anthropoids they undergo a process of atrophy, which
in many cases leads sooner or later to their complete
disappearance” (p. 26). The results of the present study
reveal that the labia majora of Pan paniscus are retained in adults and, therefore, resemble the condition
in adult Homo sapiens. The significance of this characteristic with respect to earlier thinking on human evolution has been evaluated by Bingham (1928). It is now
possible to suggest that the presence of the labia majora
in adult Z? paniscus is a shared, underived characteristic
of the Hominoidea and that their relative “atrophy” in
adult 19 troglodytes is probably a derived condition.
The labia minora in adult Pan paniscus also resemble
those in immature Z? troglodytes. In 1936, Wislocki
showed that the frenulum of a juvenile female Z? troglodytes is extended by the body of the labia minora so
that it protrudes beyond the prepuce (when the clitoris
is elevated [Wislocki, 1936, Plate 25bJ).This is in agreement with the observations on adult €? paniscus (Figs.
27
GENITALIA OF FEMALE PAN PANISCUS
Fig. 3. Photographs of the external genitalia of Pun puniscus (left)and Pun troglodytes (right)
(see Fig. 4 and text for explanation).
A
V
a
Fig. 4. Annotated drawings of the photographs in Figure 3. The frenulum of each individual
is in the same plane. The vulval aperture and anus can be seen in the common chimpanzee but
not in the pygmy chimpanzee (see text). The frenulum in Pun troglodytes is embedded in the
enlarged prepuce, but in Pun puniscus the frenulum extends beyond the prepuce and curves
upward. Abbreviations as in Figure 2.
1, 2). Retention of the immature configuration of the
labia minora into adulthood in I? paniscus becomes
functionally significant a s a result of tumescence. In
adult I? troglodytes the clitoris is maintained in approximately the same plane as the vulval aperture when
tumescent, but in I? paniscus it is relocated between the
hindlimbs (Figs. 3, 4). This permits pairs of female I?
paniscus to stimulate each other’s frenulum in the ventroventral position. Ventroventral genital rubbing
among females has been described for wild individuals
by Kitamura (19841, Kuroda (1980), and Thompson-Handler et al. (1984), and has been observed frequently in
the subjects of the present study. Ventroventral copulations among heterosexual pairs are also more frequent
among I? paniscus than I? troglodytes (Kano, 1984; Patterson, 1979; Savage and Bakeman, 1978; Savage-Rumbaugh and Wilkerson, 1978). It is possible that the
relocation of the clitoris in I? paniscus increases sexual
stimulation of the female by the male. Kano (1984) reported that females always initiated ventroventral copulation and, in certain instances, employed skillful
tactics to achieve this position. It would appear that I?
paniscus shares with humans the ability to engage in
intense, face-to-face sexual interactions without the ev-
28
J.F. DAHL
olution of bipedalism and the resultant reorientation of
the external genitalia.
Savage-Rumbaugh and Wilkerson (1978) reported the
presence of a glans in both female and male Pan paniscus, but during the present study no glans could be
discerned in either the male or females in the Yerkes
group. Moreover, Izor et al. (1981) have provided a detailed account of the male penis and stated, “Nowhere
on the external surface is there any indication of a
transition from corpus penis to glans penis” (p. 219). A
glans penis is also absent in l? troglodytes, and Izor et
al. interpret this to be a shared, derived characteristic,
indicating close phylogenetic proximity between the two
species of Pan. Contrary to Savage-Rumbaugh and
Wilkerson (1978),therefore, it is concluded here that the
genitals of female P paniscus are homologous to those
of males in the absence of a glans. In this respect, pygmy
chimpanzees are not “. . . more like H.s. sapiens than
any other living ape in every aspect of sexuality,” a s has
been claimed (Savage-Rumbaugh and Wilkerson, 1978,
p. 341).
In a study of the skull and body proportions of Pan,
Shea (1983)notes that “shuttling” of the developmental
trajectories of various body regions may provide new
adaptive morphological configurations with minimal genetic changes. This argument can be applied to the
interpretation of variation in the external genitalia of
Pan. The configuration of the external genitalia of female l? paniscus appears to be a de novo characteristic
derived by paedomorphism via neoteny, and it is not a n
indication of the arrangement in the common ancestor
of PadGorilla and Homo.
the Congo River. Am. J. Phys. Anthropol., 18t1-59.
Graham, C.E. (1970) Reproductive physiology of the chimpanzee. In:
The Chimpanzee, Vol. 3. G.H. Bourne, ed. S. Karger, Basel, pp.
183-220.
Izor, R.J., S.L. Walchuk, and L. Wilkins (1981) Anatomy and systematic significance of the penis of the pygmy chimpanzee, Pan paniscus. Folia Primatol., 35:218-224.
Johnson, S.C. (1981)Bonobos: Generalized homonid prototypes or specialized insular dwarfs? Curr. Anthropol., 22363-375.
Kano, T. (1984)Reproductive behavior of the free-ranging pygmy chimpanzees (Pan paniscus) of Wamba, on the right bank of the Upper
Luo Luo River in the Zaire Basin. (Unpublished manuscript).
Kitamura, K. (1984)Genitc-genital contacts in the pygmy chimpanzee
(Pan paniscus. (Unpublished manuscript).
Kuroda, S. (1980)Social behavior of the pygmy chimpanzee. Primates,
21:181-197.
Latimer, B.M., T.D. White, W.H. Kimbel, D.C. Johanson, and C.O.
Lovejoy. (1981)The pygmy chimpanzee is not a living missing link
in human evolution. J. Hum. Evol., 10:475-488.
McHenry, H.M., and R.S. Corruccini (1981) Pan paniscus and human
evolution. Am. J. Phys. Anthropol., 54:355-367.
Patterson, T.L. (1979)The behavior of a group of captive pygmy chimpanzees (Pan paniscus). Primates, 2Ot341-354.
Savage, E.S., and R. Bakeman (1978)Sexual morphology and behavior
in Pan paniscus. In: Recent Advances in Primatology, Vol. 1, Behavior. D.J. Chivers and J. Herbert, eds. Academic Press, New
York, pp. 613-616.
Savage, E.S., B.J. Wilkerson, and R. Bakeman (1977) Spontaneous
gestural communication among conspecifics in the pygmy chimpanzee (Pan paniscus). In: Progress in Ape Research. G.H. Bourne,
ed. Academic Press, New York, pp. 97-116.
Savage-Rumbaugh, E.S., and B.J. Wilkerson (1978) Socio-sexual behavior in Pan paniscus and Pan troglodytes: A comparative study.
J. Hum. Evol., 7:327-344.
Schultz, A.H. (1927) Studies on the growth of gorilla and of other
higher primates with special reference to a fetus preserved in the
Carnegie Museum. Mem. Carneg. Mus., 11(111:1-86.
Schwarz, E. (1929) Das Vorkommen des Schimpansen auf den linken
Kongo-Ufer.Rev. Zool. Bot. Afr., 16(4):425-426.
Shea, B.T. (1983)Paedomorphosis and neotony in the pygmy chimpanzee. Science, 222521-522.
ACKNOWLEDGMENTS
Thompson-Handler, N., R.K. Malenky, and N. Badrian (1984) Sexual
behavior of Pan paniscus under natural conditions in the Lomako
This research was supported by U.S. Public Health
Forest, Equateur, Zaire. In: The Pygmy Chimpanzee: Evolutionary
Service grant RR-00165(Division of Research Resources,
Biology and Behavior. R.L. Susman, ed. Plenum Press, New York
pp. 347-366.
National Institutes of Health). The work would not have
been possible without the collaboration of Dr. Be- von Harms, J.W. (1956)Fortflanzungsbiologie. In: Primatologia, Vol. 1.
H. Hofer, A.H. Schultz, and D. Starck, eds. S. Karger, Basel, pp.
nirschke and the San Diego Zoo. Thanks are extended
562-716.
to Drs. F.A. King, R.D. Nadler and K.G. Gould for their Wislocki, G.B. (1936) The external genitalia of the simian primates.
Hum. Biol., 8~309-347.
support of this project, and to Frank Kiernan for his
Yerkes, R.M. (1943)Chimpanzees: A Laboratory Colony. Yale Universkillful photographic input.
sity Press, New Haven.
Zihlman, A.L. (1979)Pygmy chimpanzee morphology and the interpreLITERATURE CITED
tation of early hominids. S. Afr. J. Sci., 5165-169.
Binnham. H.C. (1928) Sex develooment in apes. Comp.
Mon- Zuckerman, S., and J.F. Fulton (1934) The menstrual cycle of the
- Psvchol.
Primates. Part VII. The sexual skin of the chimpanzee. J. Anat.,
igr., 5(1):1-165.
69(1):38-46.
Coolidge, H.J. (1933) Pan paniscus: Pigmy chimpanzee from south of
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