More at ease in bipedal posture than common chimp.
"The aspects of Pan paniscus anatomy most readily perceived as paedomorphic
are the face-jaw complex and the neurocranium. The bonobo skull resembles that
of the adolescent chimpanzee (Coolidge, 1933), and in comparison to adult chimpanzees,
bonobos have smaller mandibles, reduced prognathism, and a more globular cranium
(Johnson 1981)." (Blount, Ben G. (1990) Issues in bonobo (Pan paniscus) sexual
behavior. American Anthropologist 92: 707)
"The high male-female affinity and reduced sexual differentiation in Pan paniscus
social organization may be a behavioral link to the morphological characteristics
(Shea 1984). Paedomorphism of a chimpanzee-like skull and consequent lessened
sexual dimorphism reduce those aspects of the anatomy associated with aggressive
display and activity, namely the heavier and more robust facial-jaw features.
Sex as a source of aggressive competition would favor increased dimorphism,
but sex as a tension-reducing mechanism would favor decreased dimorphism and
promote nonaggressive social proximity. Extended receptivity and flexibility
in sexual behavior, as a consequence of paedomorphism, would serve similar ends
by reducing tension and promoting sociability." (Blount, Ben G. (1990) Issues
in bonobo (Pan paniscus) sexual behavior. American Anthropologist 92: 708)
"Dahl (1985) has shown that the morphology of the external genitalia in adult
female P. paniscus closely resembles that of juvenile rather than adult P. troglodytes.
In addition, he argues (Dahl, 1986) that certain aspects (although not all)
of the swelling and menstrual cycles of P. paniscus resemble those of juvenile
P. troglodytes, particularly the length of the swelling phase and the intermenstrual
interval. (Shea, B.T. (1989) Heterochrony in human evolution: the case for neoteny
reconsidered. Yearbook of Physical Anthropology 32: pp. 92)
"In all situations, females exhibit choice. First of all, females decide which
males to mate with in the group situation. Although involved in a "flurry of
sexual activity," a fertile female surrounded by males still decides if she
will respond to a male invitation (Goodall, 1986). In addition, consorships
are formed only with female cooperation." (Small, Meredith F. (1989) Female
choice in nonhuman primates. Yearbook of Physical Anthropology 32: pp. 122)
"As noted below for he human case, discussions of behavioral features in terms
of neoteny and paedomorphosis are often problematic. Nevertheless, it is worth
pointing out that several authors (e.g. Kuroda, 1979, 1980; Dahl, 1986) have
described some of the behavioral features of P. paniscus as paedomorphic and
resembling juvenile patterns of P. troglodytes. Examples here include play behavior,
food sharing frequency, decreased social differentiation by sex, ventro-ventral
copulation, and the characteristic genito-genital (GG) rubbing of females. Several
of these behaviors may be directly linked to the morphological features of the
juvenilized external genitalia discussed above (see Dahl, 1985, 1986). I have
argued elsewhere (Shea, 1983a, 1984) that the most important link among the
morphological and behavioral distinctions between P. paniscus and P. troglodytes
is the reduced social differentiation by sex. The most notable morphological
change associated with neotenic facial growth in P. paniscus is the marked reduction
in sexual dimorphism of the gnathic and total facial region. Although the canine
teeth of P. paniscus are significantly sexually dimorphic, they are much less
so than is the case in P. troglodytes, and the same holds true for a comparison
of dimorphism in facial dimensions between the species (Fenart and Deblock,
1972, 1973, 1974). While we have much to learn regarding the intriguing behavioral
distinctions between these two chimpanzee species (see Wrangham, 1986, for one
recent discussion), it seems likely that the reduced sexual dimorphism in the
facial region of P. paniscus is related to social factors, such as lowered male-male
and male-female aggression, increased female bonding, increased food sharing,
and perhaps aspects of sexual behavior." (Shea, B.T. (1989) Heterochrony in
human evolution: the case for neoteny reconsidered. Yearbook of Physical Anthropology
32: pp. 93-4)
Also see Shea (1984)
Shea, B. T. (1984). An allometric perspective on the morphology and evolutionary relationships between pygmy (Pan paniscus) and common (Pan troglodytes) chimpanzees. In R. L. Sussman (Ed.), The pygmy chimpanzee (pp. 89-125). New York: Plenum.
Shea suggests that the pygmy,contrasted with common is a more derived and specialized
species. The implication is that a more similar morphological and behavioral
profile to humans is an artifact of neoteny and not a close evolutionary relationship.
This of course was the first guess by Coolidge, as BS cites:...It is further
thought that the less specialized, which usually means the more primitive or
juvenile forms, approach more closely to an ancestral state in the evolution
of a given animal than do the more highly developed adult forms. If this is
true, the paniscus, a true paeadomorphic species, which shows definitely juvenile
characteristics in an adult state, is the most important of the chimpanzees
in a study of the phylogeny and relationships of this high order of anthropoid
apes...1933, p.56.
BS also cites Tuttle as one of several who suggested that pan paniscus (pp -
bonobo) derived from pan troglodytes (pt - common chimp) as a neotenous relative.
BS gives data showing age vs weight plots reveal that pp is smaller than pt(m&f)
"Pan p is clearly smaller species "above the waist" than P t.
In is only in absolute or relative hindlimb length that the pygmy chimp is not
diminuitive. p.100. BS also provides other data on anatomical comparisons, pointing
out that there are also differences in pts which is smaller than ptt.
BS suggests hindlimb difference in pp is a result of "biomechanical scaling"
..that relative shortening of the hindlimbs is required to maintain functional
equivalence in climbing behavior as body size increases...104.
BS supports arguments that juvenilzation correlates with less sexual dimorphism
and predicts less behavioral differences between male and female pan p chimps.
122 "The face and teeth are presumably differentilally affected (i.e. more
paedomorphic than the rest of the body) due to their prominent role in sociosexual
recognition and differentiation..123" "Elsewhere I have shown that
the paedomorphic skull of P. pan results from neoteny, or decreased rates of
shape change in the skulll relative to overall body size. (in Science, 222,
512-2,1983.)"
Lower relative testosterone levels in male bonobos vs male chimps?
Read Eens, M., Elsacker, L., Heistermann, U., Mohle, U., & Sannen, A. (2003). Urinary testostereone metabolite levels in bonobos: A comparison with chimpanzees in relation to social system. Behviour, 140(683-693.).
Abstract: Bonobo (Pan paniscus) social structure is characterized by partial female dominance, in contrast to the male dominated chimpanzee (Pan troglodytes) society. Furthermore, female bonobos exhibit more overt aggressiveness and a prolonged period of proceptivity during menstrual cycle compared to chimpanzees. Since dominance, aggressiveness and proceptivity are suggested to relate to high T levels, we expected T concentrations of bonobo females to be high. To test this, urinary T metabolite concentrations (measured by immunoreactive 5androstane-17-ol-3-one) were determined, which reliably reflect T status in both species. We predicted a larger overlap between T metabolite concentrations of both sexes in bonobos compared to chimpanzees. Our results support this hypothesis. However, interspecific comparison for each sex revealed substantially lower T metabolite concentrations in bonobo males than in chimpanzee males, while female bonobos displayed levels about equal to those of chimpanzee females. It thus appears that low T metabolite levels in bonobo males are responsible for the large overlap instead of high T metabolite levels in bonobo females. In conclusion, this study suggests that quantitative sex differences in T levels may be predictive of social system. However, partial female dominance in bonobos is probably not mediated by high female T levels. Rather it appears that T levels of mainly male bonobos are adapted to social organization. We hope that these preliminary results ignite further research in this novel area.
Recent references
The Smart and Swinging BONOBO. (cover story)
Authors: Raffaele, Paul
Source: Smithsonian; Nov2006, Vol. 37 Issue 8, p66-75, 9p, 1 map, 7c
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