Bonobo × Chimpanzee

Pan paniscus × Pan troglodytes

EUGENE M. MCCARTHY, PHD GENETICS
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Bonobo
Bonobo, or Pygmy Chimpanzee
(Pan paniscus)

Chimpanzee
Common Chimpanzee
(Pan troglodytes)

Some karyologic evidence:
Pearson (2012, p. 280) says that the primary karyologic difference between the common chimpanzee and the pygmy chimpanzee, which consists “of a pair of small acrocentrics in the former being replaced by a metacentric chromosome in the latter (Hamerton et al., 1963), has been ascribed to the presence of extremely large fluorescent short arms and satellites on the P. paniscus chromosome 23 (HSA 22).” He then states that his laboratory had recently examined an individual that was supposedly a pygmy chimpanzee (P. paniscus) “and found that only one P. paniscus chromosome 23 had fluorescent satellites (Fig. 8), indicating that this was either a polymorphic difference within the species [i.e., that some P. paniscus individuals have this fluorescent version of chromosome 23 and that others don't] or that the animal examined was a hybrid between P. troglodytes and P. paniscus.” He then goes on to say that “There is no evidence to suggest the latter.” But it is not easy to see why, given his finding, that he would think there is no such evidence. The very fact that the individual in question possessed two types of chromosome 23 (one characteristic of common chimpanzees, the other characteristic of pygmy chimpanzees) is very strong evidence indeed. In fact, in many studies involving other pairs of organisms less closely related to humans (and where there is therefore less bias) such evidence is taken as sufficient in itself to establish that the specimen in question is a hybrid.
leopon
A leopon (leopard × lioness)


ape-human hybrid
A probable human-chimpanzee hybrid born to a 16-year-old girl in Vichy, France in 1897.


sheep-pig hybrid
Sheep-pig hybrids?


zeedonk
A zeedonk (zebra × donkey)

Hybridization between chimpanzees (Pan troglodytes) and bonobos, also known as pygmy chimpanzees, (Pan paniscus) has occurred in captivity (Vervaecke and van Elsacker 1992, Vervaecke 2002; Vervaecke et al. 2004).

In 1979, a circus director in France bought what he believed to be a male chimpanzee. It performed in acts and regularly mated with two female chimpanzees. This male ape, however, turned out to be a bonobo. Between 1991 and 2000, seven hybrids were produced from these matings, most or all of which survived. By 2000, the oldest was working in the circus act in place of his retired father.

Apparently, natural hybridization between these two apes has not as yet been reported. The ranges of the bonobo and the chimpanzee are separated by the Congo and the Lualaba rivers, so their ranges are essentially allopatric. But contact may occur upstream of Boyoma Falls (formerly known as Stanley Falls) where the river can easily be crossed, at least by humans, during times of low water. Fenart and Deblock (1973, p. 11) note that there is one point there where it is possible to cross by leaping from rock to rock at low water, but that a survey of that area revealed no evidence of natural hybridization. Untrained apes cannot swim and are afraid of water. Moreover, the two rivers in question are crocodile infested. So it seems likely that little opportunity for hybridization exists even in this one region of potential contact.

In a brief abstract of a presentation given at the Spring 2003 meeting of the Primate Society of Great Britain, Vervaecke et al. (2004) state the following: “From historical accounts it appears that natural populations of bonobos and chimpanzees have been allopatric since their

phylogenetic separation. There are no accounts of hybridisation under natural conditions [since they do not normally come into contact with each other under natural conditions]. There is, however, evidence for interbreeding between bonobos and chimpanzees in captivity. We briefly (6.5 h) observed and filmed a small group of four bonobo-chimpanzee hybrids (2 males, aged 10 and 9, and 2 females aged 10 and 8). There was clear individual variation in degree of expression of typical chimpanzee or bonobo features. In each individual, there were anatomical features reminiscent of both of the parental species: the specific bonobo-like pink coloration of the lips, the hairstyle and slender body build, the chimpanzee-like dorsal position of the female genitals, more sturdy body build and prognathism. The behaviours could be categorized less exclusively. We observed mostly female oriented interactions, including grooming, play, approaches and intersexual ventro-dorsal sex. There was one female coalition against the males. The individuals lip-smacked while grooming and performed swaggering displays. The hoots were like chimpanzee pant-hoots with an introduction, build-up and climax in a higher pitched bonobo tone and more ‘e’ than ‘o’ sounds. Studies on Pan in the seventies and eighties emphasised discontinuity between the two species, contrasting the male-dominated, aggressive nature of the chimpanzee to the female-oriented, peaceful nature of the bonobo. In the nineties and in present studies, the continuity among Pan is increasingly being documented. The existence of hybrids challenges our tendency for binary thinking and points at the conceptual relativity of the species gap.

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