Tetrapod Zoology

We’ve had reason now and again to mention the unusual ape photographed at Yaounde Zoo (in Cameroon) a few times. I finally got round to digging out and scanning the only photo of the animal I’ve seen: it was taken by Peter Jenkins and Liza Gadsby and first appeared in the November 1996 issue of the Newsletter of the Internal Primate Protection League (IPPL). It was later published in issue 100 of Fortean Times.

Jenkins and Gadsby thought that the animal might be a gorilla-chimp hybrid. I can’t help but get this impression too, mostly because the eyes look gorilla-like while the rest of the animal is obviously chimp-like. Apparently little known is that there is a long history of debate over the existence of an alleged gorilla-like chimpanzee, known as the kooloo-kamba (an onomatopoeic reference to its call). W. C. Osman Hill was supporting the distinction of this form (as a Pan troglodytes subspecies) as recently as the late 1960s (Hill 1967, 1969). Supposedly, P. t. kooloo-kamba [originally Troglodytes kooloo-kamba Du Chaillu, 1860; sic: hyphens are not permitted in scientific names] has a gorilla-like nose, ‘an extremely prognathic face’, an entirely black face, and small, black ears. It also lives singly or in small groups, rather than in large troops. It was thought to inhabit Cameroon, Gabon and the former French Congo, and to live alongside chimps of the nominate subspecies (Hill 1967, 1969) [type specimen of P. t. kooloo-kamba (61.7.29.10 of NHM collection) shown below, from Elliot (1913)].

While Hill regarded the kooloo-kamba as a distinctive chimp subspecies, previous authors regarded it as a distinct species somehow ‘intermediate’ between chimps and gorillas, or as the product of gorilla-chimp hybridisation. Supposedly, several individuals were kept in captivity during the late 1800s and ealy 1900s, including ‘Mafuca’ of the Dresden Zoological Garden, and ‘Johanna’ of Barnum and Bailey’s circus collection. There’s a substantial literature on these animals. Some mammalogists said that they were gorillas, others than they were chimps, and others said that they were hybrids, or intermediates (see Shea (1984) and references therein). It has most recently been argued that ‘Mafuca’ was a Bonobo P. paniscus (see de Waal 1997), in which case at least some ‘kooloo-kambas’ were definitely not gorilla-chimp hybrids or intermediates at all.

It does now seem that the kooloo-kambas of the older literature reflect the fact that both gorillas and (especially) chimps are more variable in facial anatomy, body size and overall appearance than many primatologists were once willing to accept. Chimps of some populations, for example, are larger, darker-skinned, and superficially more ‘gorilla-like’ than many of the chimps first brought back to Europe, but this doesn’t mean that such animals are hybrids, or intermediates. Indeed, Shea (1984) concluded that the ‘kooloo-kambas’ present in osteological collections are either large male chimps, or small female gorillas. Various other controversial African apes – most notably the Pygmy gorilla Pseudogorilla mayéma (see Groves 1985) [shown here, photo ©, by B. Heuvelmans] – also tell us more about our poor understanding of variation, and don’t necessarily point to the presence of additional distinct taxa.

Having said all that, the possibility that some ‘kooloo-kambas’ or kooloo-kamba-like apes really were or are hybrids, or new taxa, does still exist. Could individuals like the Yaounde Zoo animal shown above really be hybrids? I don’t know: opinions gratefully received. Incidentally, if you’re wondering how the ‘Bili apes’ of DRC fit into all this, see the previous Tet Zoo comment here.

For previous Tet Zoo articles on apes and other primates see…

Refs – -

de Waal, F. 1997. Bonobo, the Forgotten Ape. University of California Press, Berkeley.

Elliot, D. G. 1913. A review of the primates, volume III: Anthropoidea (Miopithecus to Pan). American Museum of Natural History, New York.

Groves, C. P. 1985. The case of the pygmy gorilla: a cautionary tale for cryptozoology. Cryptozoology 4, 37-44.

Hill, W. C. O. 1967. The taxonomy of the genus Pan. In Starck, D., Schneider, R. & Kuhn, H. (eds) Neue Ergebnisse der Primatologie. Fisher, Stuttgart, pp. 47-54.

- . 1969. The nomenclature, taxonomy, and distribution of chimpanzees. In Bourne, G. H. (ed) The Chimpanzee, Vol. 1. Karger, Basel, pp. 22-43.

Shea, B. T. 1984. Between the gorilla and the chimpanzee: a history of debate concerning the existence of the kooloo-kamba or gorilla-like chimpanzee. Journal of Ethnobiology 4, 1-13.

Comments

#1 Sordes
December 31, 2009

I think one reason which speaks against kooloo-kamba being hybrids is that such events would probably occur only very rarely, and there would be little chance that such animals could form groups, but would possibly more probably be introduced in the gorilla or chimp families in which they were born (at least males) or live completely solitary.
I am always again surprised how variable especially chimps are. It´s not only the shape of their faces and heads, but also the colour of the skin which is extremely variable. The densitiy of hair is also very varying. I´ve seen chimps which had little more hair than (most) humans, whereas other ones were highly covered with hair. I once even saw in a documentation a really strange chimp which had hair and skin which were yellow-brown. I lived together with normal coloured chimps.
I have also a book with profile photos of man-apes, including chimps of course. The variability is really enormous, skull shape, nose, ears, skin colour, amount of hair, colour of the eyes, it´s really surprising.
BTW, I read by chance just yesterday an interesting report of a circus chimp of 75 kg which wrestled with a 110 kg Jiu Jitsu-champion at Japan several decades ago. Not surprisingly the chimp named “Pinz Charlie” won. If anybody is interested in the reference I can write it.
#2 Raymond Ho
December 31, 2009

Nice post! I think that these individuals probably display physical variations that are unknown to science. Although having them in zoos probably did not help disperse those genes into the natural gene pool.
#3 Jerzy
December 31, 2009

Thanks a lot for bringing up this photo!

For me, it looks like an average chimp. Brow ridges look rather prominent because it is seen from below, the face has crazed look, but nothing unusual for chimps. Brown eyes with dark “whites” are something normal in chimp – I see nothing gorilla-like here.

BTW – thank you for bringing the subject that mammal species are more variable than most zoologists think. We tend to see pictures of animals from the same locality again and again, and get false impression that every individual looks the same.
#4 Phillip IV
December 31, 2009

Could individuals like the Yaounde Zoo animal shown above really be hybrids?

Perhaps I’m being a bit naive here, but shouldn’t you be able to find out whether such hybrids are even possible via a series of in-vitro experiments?
#5 Jerzy
December 31, 2009

@Sordes – Grzimek cites few experiments on arm strength of zoo apes, and even young chimps were as strong as the very top human athletes.

Perhaps to be expected for animals which use arms for locomotion.
#6 Gunnar
December 31, 2009

Just to be picky: Hyphens ARE allowed in scientific names, but not in the ICZN. Botanists and other taxonomists who follow the ICBN, however, are free to use them as they like.
#7 Bob Michaels
December 31, 2009

First I`ve seen of this Ape. a Dna analysis is needed before any classification can be made.
#8 Mike Keesey
December 31, 2009

Sure looks like a common chimpanzee to me. A scary-looking chimp, but a chimp.

Can common chimpanzees even interbreed with bonobos, let alone gorillas? Can eastern and western gorillas interbreed? The only verified great ape hybrids I know of are Bornean × Sumatran orangutans (we have one at the L.A. zoo).
#9 anon
December 31, 2009

Re Gorilla-Chimp hybrids:

As I understand the current model, genera Homo and Pan are in the tribe or infrafamily Hominini, with the Hominini and Gorilla in the subfamily Homininae.

Also, anthropologists currently don’t even feel that there was any hybridization between the more closely related H. sapiens and H. neanderthalensis (current best guess is apparently ” ‘not’ H. sapiens neanderthalensis, though either way, Neanderthals and modern humans are more closely related to each other than gorillas and chimps are to each other).

In other words humans and chimps are more closely related to each other, while chimps and gorillas are comparatively less closely related to each other. Thus, genetically speaking, hybridization between humans and chimps would appear more likely than hybridization between chimps and gorillas.

I’m speaking here as an armchair anthropologist — would appreciate hearing from those who actually know something about this.
#10 Darren Naish
December 31, 2009

On balance, I would be happy to accept that the Yaounde Zoo animal was ‘just’ a weird looking chimp. Jenkins and Gadsby are both experienced primatologists (so far as I can tell from their publications), however, so there may be more to the story than mentioned here.

In response to Mike’s question (comment 8) about hybridisation: it’s recently been argued (based on the presence of supernumery teeth and the shape of cranial sutures) that Eastern lowland gorillas Gorilla beringei graueri arose via hybridisation between Mountain gorilla G. b. beringei and Western lowland gorilla G. gorilla gorilla. See…

Ackermann, R. R. & Bishop, J. M. 2009. Morphological and molecular evidence reveals recent hybridization between gorilla taxa. Evolution doi:10.1111/j.1558-5646.2009.00858.x
#11 Alan
December 31, 2009

One problem with identification of captive animals in African zoos is that they are almost all orphans of the bushmeat trade and have spent a large part of their early life being raised as pets in local villages on a very inadequate diet, which can seriously affect their physical development. Chimpanzees in zoos outside Africa are often not identified below the level of species, and may be hybrids of quite distinct taxa. The plan for the captive chimpanzee population is only to breed from identified P.troglodytes verus – hybrids or other taxa are on contraception (reversible means are used for non-hybrids).
#12 Mark Lees
December 31, 2009

The photo of the Yaounde ape looks rather intimidating – was it auditioning for a part in a horror film? Something about ‘possessed apes’ maybe?

I have seen other photos of that alleged hybrid in Yaounde Zoo. I found them on the internet several years ago (not sure how long, but had to be at least six years ago), but attempts to refind them now are proving unsuccesful so far. I do recall that some of them showed an animal that looked very much part chimp part gorilla. Assuming that the photo above is of the same animal, it really doesn’t do the case for its being a hybrid justice. Certainly to me it seemed a serious contender.

In the 1880s von Koppenfels claimed to have definite proof of gorilla x chimpanzee hybridization – he even claimed to have shot a male specimen that appeared to be such a hybrid. He stated that the hybrid was always the result of mating between male gorillas and female chimps – but I have no idea what he based this assertion on.

A few years ago (early 90s) I tried to find any credible accounts of ape hybridization. There were quite a few comments about gorilla chimp hybrids in zoos (unsurprisingly mostly in African zoos) – but in most cases there was very little information. A zoo in the southern USA is said to have had a hybrid result from mating between a gorilla and a chimp sometime in the 1940s or very early 1950s. There was no information as to whether the hybrid survived, or which parent was of which species. I have never managed to find any other information about this alleged hybrid, though I did find another reference to a supposed hybrid in the USA which seemed to refer to the same incident. If anyone knows any more about it I would love to know (even if its just to rule it out).

This does differ from most of the alleged hybrids in other zoos, which are not said to have been bred in captivity, but rather are wild caught and assumed to be hybrids based on appearance. It does seem surprising but there seem to have been quite a few animals in zoos or a taxidermy specimens that could not be identified with confidence as either gorillas or chimps.

The situation with Koolakamba is complicated – some evidence does point to it being possibly a form or subspecies of chimpanzee, other evidence tends to suggest it is just part of the range of variation within one of the accepted chimpanzee subspecies. I tend to favour the latter. While it may be a distinct form of chimpanzee, there seems little justification for viewing it as the result of hybridization.
#13 Mike Keesey
December 31, 2009

Thanks for the link to the Ackerman & Bishop paper — very interesting. Gorilla beringei graueri are definitely a bit strange-looking: http://pin.primate.wisc.edu/fs/sheets/images/239lg.jpg
#14 Nathan Myers
January 1, 2010

I was positively boggling at the respectful repetition in the article of suggestions of a hybrid chimp/gorilla, wondering for some time if we were being fun of. It’s a relief to read in the comments that such a notion is as wild as I had thought. That’s not to say I think it would be impossible (I’ve heard of some odd felines), but I was surprised not to see it treated with deep skepticism.
#15 Anthony Docimo
January 1, 2010

In the first pic of this thread you posted, I thought I was looking at a photo of Oliver.

(didn’t a DNA sample from Oliver determine that his parentage *was* mixed – but because one of his chimp parents was from a lesser-known genetic population)
#16 Anthony Docimo
January 1, 2010

ps: !Happy New Year!
#17 Sordes
January 1, 2010

Theoretically I see no reasosn why hybrdis between chimps and gorillas would be completely impossible. There are much stranger hybrids on record, between species which are not only much more different in a physical or social way, but which are also much more distantly related. Of course nobody can say 100% for sure that such a hybridication would be genetically impossible or possible untill we would have a poof in form of a confirmed hybrids or no hybrid after many confirmed matings between the two species. The distant relationship don´t have necessarily to rule out a possible hybridisation. Sometimes already comparably closely related species fail to produce hybrids, but in other cases animals like babirussas and domestic pigs successfully produce hybrids.
BTW, has anybody current photos of the babirussas-hybrids from the zoo at Denmark? I know only some photos of them when they were still piglets.
#18 Sven DiMIlo
January 1, 2010

Weirdest hybrids of which I have personal knowledge:
Blanding’s (Emys (Emydoidea) blandingii) x Wood turtle (Glyptemys (Clemmys) insculpta).
I have seen both the hybrids and their DNA tests. Alas, they are not mine so I cannot publish. I probably shouldn’t have said even this much.
Point being: weird hybrids can indeed occur! You think a horny male gorilla would hesitate for even a second if he found himself with a receptive female chimp?
#19 Sven DiMIlo
January 1, 2010

I should add that the owner of said turtles is waiting until they can be sexed, wood turtles having genetic sex determination and Blanding’s temperature. Should be interesting.
#20 anon
January 1, 2010

@ Sordes: “There are much stranger hybrids on record, between species which are not only much more different in a physical or social way, but which are also much more distantly related.”

Can you please give some examples?

My own cursory research doesn’t seem to show any mammalian hybrids between parents more distantly related than the genus level.

I also think that accounts of hybrids from sources distant from the reputable scientific mainstream should be regarded with great caution — as we’ve seen repeatedly here at Tet Zoo, non-scientists (and even many scientists), when confronted with unfamiliar or “funny looking” animals, are likely to produce very creative theories about their provenance.
#21 Darren Naish
January 1, 2010

Yeah, I’m with Sordes and others (with regard to Nathan’s comment # 14): I don’t, personally, find the idea of a chimp x gorilla hybrid at all ridiculous. For previous discussion on ‘extreme’ hybrids go here. Some mammalian hybrids are very distant relatives, representing lineages that have been separated for (in cases) 10, or 20, or more, million years. True, gorilla x chimp hybridisation is very unlikely to ever happen in the wild. But things are very different in captivity.
#22 Cameron
January 1, 2010

Anon #20:

genus level

No such thing, genera are relative.

when confronted with unfamiliar or “funny looking” animals, are likely to produce very creative theories about their provenance

= phylogenetic roulette. While this is disturbingly common in informal (forums, blog comments) and “grey” literature, I seriously doubt experts in the subject would be… subject to it.
#23 Jerzy
January 1, 2010

@Mark Lees
“A few years ago (early 90s) I tried to find any credible accounts of ape hybridization.”

I hope you found out several hybrids of bonobo male x two chimp females in Belgian circus.

“A zoo in the southern USA is said to have had a hybrid result from mating between a gorilla and a chimp sometime in the 1940s or very early 1950s.”

Could you elaborate? Gorillas, especially adults, were still very rare in zoos at that time. The first captive birth of a gorilla was Colo in Columbus Zoo in 1956. So I find very strange that such breeding would not receive more publicity and could not be traced – of course, if it existed for real.

“It does seem surprising but there seem to have been quite a few animals in zoos or a taxidermy specimens that could not be identified with confidence as either gorillas or chimps.”

Well, if there are specimens they could be verified easily by DNA analysis.

I consider gorilla x chimp hybrid theoretically possible, because there are surprisingly many intergeneric hybrids of primates (several baboon x macaque spp., baboon x mangabey, douc langur x proboscis monkey etc.). So I think it is a topic worth of interest.
#24 Jerzy
January 1, 2010

Out of interest, this is fragment of message from zoo-biology mailing list. I hope the author doesn’t mind, just in case I modified it.

“The Common chimpanzee x Bonobo hybrids have been described in a French circus.

Here is a scientific publication about them :
VERVAECKE (H.), STEVENS (J.) & VAN ELSACKER (L.), 2004 : Pan continuity : bonobo-chimpanzee hybrids. Folia Primatologica 75(1) : 59.

Hilde Vervaecke also gave a detailed account of her encounters with these animals in the French circus (including a photo), in her (excellent) book (unfortunately published only in Dutch) :

VERVAECKE (H.), 2002 : De bonobo’s : schalkse apen met menselijke trekjes. Louvain, Davidsfonds : p. 134-138.

In short, in 1979 (i.e. before France ratified CITES) a circus director bought what he thought was a male chimp. It performed in circus acts and regularly mated with two female Common chimpanzees, before a visitor managed to convince the director that he was in fact a Bonobo and not a Chimpanzee. Between 1991 and 2000, seven hybrids were born, the oldest of which was by 2000 working in the circus act as a successor to his retired father.

As far as I know, most of the hybrids are still alive today. At least until recently, they frequently featured in advertising etc.. In 2007, some of them also acted in a French tv drama production about the life of Ham (the first (?) chimp in space) – here is a website about this film which contains some photos of the animals, see :
http://programmes. france2.fr/ ham/index. php?page= article
< http://programmes. france2.fr/ ham/index. php?page= article&numsite= 88&id_artic
le=172&id_rubrique= 90> &numsite=88& id_article= 172&id_rubrique= 90
“

#25 Noni Mausa
January 1, 2010

Trying to get a time-frame for this photo, it seems that it’s not from the 50s as I first guessed on the basis of its pinky-bluey smeared kind of coloration. But from other references, it appears the picture dates from the early to mid-90s. In which case the animal may still be alive in captivity and available for a quick blood test or tissue sample.
#26 Kelly Miller
January 1, 2010

There are a number of examples of family-level hybrids in the Delphinidae. A bottlenose/false killer whale hybrid (who is fertile) and a bottlenose/rough-toothed dolphin hybrid come up pretty quickly on a Google search.
#27 Graham King
January 1, 2010

Darren, today’s title is IMHO among your best! Certainly an attention-grabber. And raising multiple questions (I have just read or re-read several of your previous articles and links.)

I find these two topics fascinating: individual variability within species and also hybridisation between different species (or even between different genera).

Our own human experience (broadened beyond the parochial by travel and now by TV) shows how greatly individuals of one species can differ in appearance, build and behaviour. And our selective breeding of domestic animals and pet varieties (including dwarf, long- or short-coated or hairless, smooth or wiry coat, tailless, shortlegged etc) also shows this. Why should we think other (wild) species hold less variability? If they do seem to, is that largely due to:
hugeness of our human (and domesticated animal) populations (large gene pools with associated richness of permutations now possible); our propensity to alter our environment to suit our wishes rather than we and our animals having to be ‘fit’ within narrow limits for our environment;
and our inadequate familiarity with (enough individuals of) most other (wild) species?

Re hybridisation, I guess two scenarios may favour the formation of hybrids – or rather, may make more likely attempted cross-specific matings (which may or may not result in viable fertile hybrids):
one I will call ‘the desperation scenario’, meaning restricted availability of conspecifics (eg in zoos or small/scattered populations) but with the simultaneous availability of ‘near-match’ nonspecifics;
the other I will call ‘the confusion scenario’, meaning individuals who lack adequate formative socialising experience with their own species – orphaned young, loners, captured/raised by humans, etc – and so may not have formed such clear impressions of ‘like’ and ‘unlike’ self as to readily select prospective mates by species.
The two categories of course are not exclusive, and such may occur either in the wild or in captivity.

Of course in captivity (zoo or lab) it is easier for us humans to monitor what occurs.

A complicating factor though may be that in captivity, even normal within-species matings may be impaired if the environment or social sample there is inadequate.

Re Sordes’ comment #1 “I think one reason which speaks against kooloo-kamba being hybrids is that such events would probably occur only very rarely, and there would be little chance that such animals could form groups, but would possibly more probably be introduced in the gorilla or chimp families in which they were born (at least males) or live completely solitary”..
I am unclear what he means.
But if a male gorilla and female chimp (just say) mated, producing a hybrid, and remained together long-term as a family group, further matings of the parents could produce a group of hybrid siblings whose own social experience would give them a broadened concept of ‘kin’ and thus likely favour them having a receptivity to mating, thmselves, across that species divide. They might also mate within their own family group. Thus even if such a hybridisation pairing were rare, once it occurred, it would surely tend to repeat and recur?
Of course such perpetuation would require hybrids to be not only viable, but fertile and accepted by available mates.

I also wonder whether all the different species we now recognize as different would also be recognized as different if known only from fossils.. where plumage and behaviour enable us to distinguish (or trick us into seeing?) visibly-different but morphologically-similar species, would these not be lumped as one species if known only from fossils?
Also though – may we mistakenly be splitting some fossil animals into different species and genera (especially those poorly-known from partial remains or few individuals).. through ignorance of what (possibly considerable) morphological variation may have been normal within some extinct species?
#28 Alan Kellogg
January 2, 2010

Speaking of chimps, how are the discussions towards establishing P. t. schwienfurthi as it’s own species, Pan schwienfurthi, going?
#29 David Marjanović
January 2, 2010

Louvain

Leeuwen in Dutch.

schwienfurthi

Ei, not ie; “eye”, not “ee”.
#30 Ranjit Suresh
January 3, 2010

Of course, if gorilla-chimp hybrids have occurred, then it raises the question of whether human-chimp or human-gorilla hybrids might also be possible.

Soviet biologist Ilya Ivanov failed in his effort to create human-chimp hybrids, but he was working in the 1920′s with limited resources. I wonder if it might be feasible today.

Link:
http://en.wikipedia.org/wiki/Ilya_Ivanovich_Ivanov_(biologist)
#31 asigurari auto
January 3, 2010

I like monkeys when I watch I see myself how many million years ago or even earlier. When you think that we could be us who remained in the forest industry and those they could have evolved. I’m terrified.
#32 David Marjanović
January 3, 2010

they could have evolved

They did. They didn’t somehow stand still. They are not identical to any common ancestors of us with them.
#33 Adam
January 3, 2010

re: #30
What, someone has actually tried that? I had never heard of it, but I’m not surprised. Hm.
#34 Dartian
January 4, 2010

Sordes:

I lived together with normal coloured chimps.

I do hope there’s a typo in that sentence…

Jerzy:

it looks like an average chimp. Brow ridges look rather prominent because it is seen from below, the face has crazed look, but nothing unusual for chimps. Brown eyes with dark “whites” are something normal in chimp – I see nothing gorilla-like here.

Also (at least as far as one can tell from that photo), the ears seem to be too large for a gorilla but just about the right size for a chimp.

Mark:

He stated that the hybrid was always the result of mating between male gorillas and female chimps – but I have no idea what he based this assertion on.

Pure guesswork? (Incidentally, I wouldn’t be sure that in a gorilla – chimpanzee pairing the gorilla would necessarily be the one who’s calling the shots. Gorillas may have the brawn, but chimps have the ‘tude.)

A zoo in the southern USA is said to have had a hybrid result from mating between a gorilla and a chimp sometime in the 1940s or very early 1950s. There was no information as to whether the hybrid survived, or which parent was of which species. I have never managed to find any other information about this alleged hybrid, though I did find another reference to a supposed hybrid in the USA which seemed to refer to the same incident. If anyone knows any more about it I would love to know (even if its just to rule it out).

I’ve never heard of that alleged hybrid and can thus provide no specific information, but I’d like to point out, along the lines of Jerzy in comment #23, that well into the 20th century, all great ape species had a pretty poor survival record, and an even worse breeding record, in western zoos (in other words: until fairly recently, almost all great apes in captivity were wild-caught). Gorillas, in particular, proved hard to maintain in captivity; they tended to succumb to various human-transmitted diseases and die at an early age. Before the 1950ies, just having a gorilla reach a mature age in an American or a European zoo was pretty remarkable in itself. An interspecific hybrid with a chimpanzee, had there been one, ought to have made global headlines.
#35 Mike Keesey
January 4, 2010

Jerzy’s link is malformed, so here’s an alternative: http://bit.ly/4JQa7Q

Caroline does indeed look like what I’d expect a bonobo-common chimpanzee hybrid to look like.

Funnily enough, that same site mistakenly labels another chimpanzee as “Le Gorille”: http://bit.ly/8kaSJS
#36 Graham King
January 5, 2010

Darren in #21:

For previous discussion on ‘extreme’ hybrids go here. Some mammalian hybrids are very distant relatives, representing lineages that have been separated for (in cases) 10, or 20, or more, million years.

This raises a point: is length of separation in itself a deciding factor? Sure, that indicates longer oportunity for differences to have arisen and maybe accumulated; BUT some lineages appear to have remained essentially the same over relatively huge geological spans (Coelacanth, Limulus, various insects in amber) while others vary significantly and observably even in the short-term.

Surely the feasibility, or not, of hybridisation depends on the exact nature of their differences? Some potential mutational loci may be pretty irrelevant to the question of inter-fertility (build, hairiness or pigmentation, say) – though not perhaps irrelevant to mate-selection! – while some loci may be crucial to it.
Two species’ outward morphologies could be obviously different in a number of ways (bottle-nose dolphin and false killer whale hybrid mentioned by Kelly in #26) yet not preclude a successful cross, while between outwardly-more-similar species some single invisible quirk of physiology or developmental biology arising as a difference between them might make cross-fertilisation impossible or render their hybrids non-viable or infertile.

I say ‘impossible’ but one does hear of the occasional mule that has reproduced; and as a character in Jurassic Park says, “Life will find a way..”
#37 David Marjanović
January 6, 2010

The coelacanths have undergone quite a bit of molecular evolution…
#38 Blackbird
January 9, 2010

The photo appears to be as a very usual looking mature chimpanzee, but I’d love to see more photos. Nothing unusual on eye colour or skin colour. Young chimps have pale skin but it gets dark after they mature, while very young gorillas have already dark almost black skin. Curiously, bonobos have also dark faces from a young age, but the area round their lips and chins tends to be red or light colour. A beautiful resource to see the diversity of ape facial features is the book by James Mollison “James and other Apes” (http://www.amazon.co.uk/James-Other-Apes-Mollison/dp/0954689437/ref=sr_1_1?ie=UTF8&s=books&qid=1263061779&sr=8-1) a gallery of ape portraits, from captive specimens, what Mollison do to have them pose for for him like that, even very young individuals? Chimps have the paler irises, from honey to chestnut color. Surprisingly, orangutans have the darker irises, bonobos and gorillas are in between. Of interest could be that there is a shocking photo of a blue eyed 2 year old bonobo (Fizi). There seems to be a lot of variation in the extent to white in the cornea.
#39 Jelle Zijlstra
January 11, 2010

David (#29): Leuven, not Leeuwen. Leeuwen means “lions” in Dutch.

On a more substantial matter, Richard Van Gelder of the AMNH argued in the 1970s that species capable of hybridizing should be considered congeneric (Am. Mus. Novitates 2635, http://hdl.handle.net/2246/5474). It led him to quite some extensive synonymizations–all cercopithecines, all felids but the cheetah. He cites a number of surprising hybridizations (jaguar-puma, for example), but rejects a few even better ones (rat-rabbit, roe-sheep, moose-cattle). The criterion was later rejected mostly because the ability to hybridize is plesiomorphic.
#40 David Marjanović
January 11, 2010

<headdesk>

Sorry. I must have confused it with Leeuwarden or something… or with the fact that it actually is called Löwen, “lions”, in German.

(We also call Hradec Králové in the Czech Republic Königgrätz, even though there’s “queen” in there, not “king”. Someone must have misunderstood Czech grammar…)

The criterion was later rejected

Alain Dubois from the museum here in Paris still advocates it, or at least did in a paper a few years ago…

Anyway, this same argument must be made against the “Biological Species Concept”s.
#41 Christopher Taylor
January 11, 2010

The criterion was later rejected mostly because the ability to hybridize is plesiomorphic.

Not so much because it’s plesiomorphic (there are still a lot of researchers out there unconcerned by such things) as because it’s wildly impractical. Conducting sufficient hybridisation tests for the millions of species on the planet is simply not possible.
#42 Mike Keesey
January 12, 2010

Not to mention the problem of what happens when species A can interbreed with species B and species B with species C, but species A can’t interbreed with species C.
#43 Jelle Zijlstra
January 14, 2010

David and Christopher are right, I was overly definite about this argument there (although it is noted in “Mammal Species of the World” under Canidae in reference to Van Gelder’s classification).

Mike, Van Gelder’s proposals were all based on very spotty hybridization records. I think his synonymizing all of Cercopithecinae was based on a chain that went from one genus to the other with only one split. One wonders what happens when one tries to hybridize the two ends of the chains.
#44 Angela
January 16, 2010

Hi Darren,
I found this post very interesting and the comment discussion about hybridization was interesting. I’m not a professional by any means so please forgive me if I get my facts mixed up here, but I have one question.
From Dawkins’ The Ancestors Tale I see that Gorillas and Chimpanzees evolved into separate species 7 and 6 million years ago respectively. Is a million years of separate evolutionary development not enough to make hybridization impossible between the two species?

Thank you for a great blog and for taking the time to answer my question (even if the answer is screamilgly obvious to you)

Angela
#45 Lars Dietz
January 16, 2010

Van Gelder actually was not the first to propose this. Ernst H. L. Krause proposed the same thing for plants in the early 1900s, which led him, among other things, to synonymize all Brassicaceae in a single genus Crucifera. Reichenbach also mentioned this idea in 1853, but was against it because it would lead to extreme lumping (see here: http://tinyurl.com/yak7mrv , page 14 and 15, in German). He even mentioned a turkey-legged goose that was allegedly the result of hybridization. I would rather say it was the result of creative taxidermy.
#46 David Marjanović
January 16, 2010

Is a million years of separate evolutionary development not enough to make hybridization impossible between the two species?

That depends on which mutations actually happen, not on the time. More time just makes it more probable. And which mutations need to happen depends on what kind of organism it is…

Usually it takes several million years in mammals, I think.
#47 Jerzy
January 16, 2010

Handbook of the Birds of The World proposed in passing that species which can hybridize can be placed in one family, not genus. American wood-quails were given own family Odontophoridae because they never hybridized with other Galliformes. Sounds more sensible for me.

Problem with hybridization is that most interesting information comes from the early 20. century. When you read something shocking, then you can hardly verify this eg. chicken-Lyrebird cross – Galloanserae-Passeriformes. Also, hybridization is not yes-no. Often it took breeders many animals over the years, so that the hybrid survived. Apparently, hybridizing birds was some fashion of bird keepers in early 20.century. Most amazing mammal hybrids come from zoos at that time which only kept one animal of each species.

BTW2 – There is now an accidental mangabey-madrill hybrid living behind the scenes in Brookfield zoo. Copenhagen zoo recently produced casually 3 babirussa-domestic pig crosses. I saw the photo on zoochat.com website, and they look strange – one is mostly white, one is in white-brown patches, one is more brown. I wonder if they are proper hybrids at all, or some vvery strange genetic phenomenon went there.
#48 Simon Tonge
January 19, 2010

Darren

I read this post somewhat belatedly so am late responding to it. The animal in the photograph is clearly a chimpanzee, the only odd thing about it is that it appears to be a strange green colour (at least on my computer screen!)and has slightly madder than usual eyes. Quite why experienced primatologists like Jenkins and Gadsby would think it a hybrid is puzzling. From reading other peoples responses it is quite clear that there is no credible evidence whatsoever that chimps and gorillas have ever hybridised. When you consider that one (gorilla) is a polygamous, strongly sexually-dimorphic primate with very discreet sexual behaviour, and the other is promiscuous, non-dimorphic and with pronounced sexual swellings in the female it seems obvious that natural mating between the two is most unlikely. Chimps and bonobos are certainly more likely to breed together, as are either of those with human beings, which would be a much more interesting line of speculation!
#49 Deano
February 5, 2010

The crossing of species borders: bonobo-chimpanzee hybridisation in captivity.
Hilde Vervaecke�,�; Jeroen Stevens �,�; Linda Van Elsacker�,�
� University of Antwerp, Belgium
� Centre for Research and Conservation, Royal Zoological Society of Antwerp, Belgium
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. There is, however, evidence for interbreeding between bonobos and chimpanzees in captivity. Studies on Pan in the seventies and eighties emphasised discontinuity between the two species, contrasting bonobo versus chimpanzee anatomy and behaviour. In more recent studies the continuity among Pan is increasingly being documented. In this respect, the hybrids form an interesting testcase. We will define several species-specific parental traits and study their expression in seven different hybrids. We illustrate their morphology and behaviour with photographic and video material. The hybrids have been naturally procreated by a bonobo father and two chimpanzee mothers. It will become apparent that the hybrids show individual variation in degree of expression of typical chimpanzee- or bonobo features. The existence of hybrids poses an interesting challenge to the traditional biological species concept. Their existence also challenges our tendency for binary thinking.
#50 Deano
February 5, 2010

Reference for Chimp – Bonobo hybrid.

1992

Article
Vervaecke Hilde, van Elsacker Linda.- Hybrids between common chimpanzees (**Pan troglodytes**) and pygmy chimpanzees (**Pan paniscus**) in captivity.- In: Mammalia, 56:4(1992), p. 667-669
#51 onix
March 6, 2010

its interesting, the topics are interesting and i read through all the comments. judging from the comments chimp – gorilla hybridization is exremely rare at best, all of us are interested but none of us know of a definite example. i think jane goodwin in her book about bonobo’s says: the idea that bonobo’s (that in her study show more human-like social traits then chimps) and humans could hybridise she calls ‘ a challenge at times she wondered if she should take it on herself.”
i dunno why i mention this, i have also in the predigital era seen a few old vague photographs of some /thing/one that looked much like such a hybrid chimp-human, altho reading that near hairless chimps exist makes that photographs more dubious. (it looked really weird, but seemed to represent a young specimen). next what i think of is the probably beknown to you, icebear- brown bear hybrids that these days have been observed. for those the theory is the severe limitions on their natural environments invoked mating that could have taken place during 10000s of years but only occurs now. so if such (pan-gorilla)hybrids can exist, i would not be surprised if they were found in small groups at instances, since any such limitation of environments concerning 2 surviving chimp and gorilla tribes in a small area would facilitate relatively (and unnaturally) frequent meetings.
also (altho someone already mentions chimps have the ‘tude’, i have been told by an african person, one that liked to be in the bush and saw chimps firsthand and often, that (free) male chimps take a serious interest in female humans, consider them sexually attractive, so indeed it struck me as illogical someone would say a such hybrid would be the result of a male gorilla getting to a female chimp. when look you at their lifestyles it is quite obvious a female gorilla could be interested in a male chimp, especially when she were alone, and that male gorilla are except fysically hardly capable, to copulate with a female chimp, very much regulated in their love affairs by longlasting grouping relations. a gorilla male eg. has a favourit, that is usually his oldest woman, chimps otoh compete over matings in a group. chimps are also known to have sneak sex (copulation with no dominant groupmembers witnessing them). anyway, somehow without more definite descriptions for now the argumants that chimps are more varied and often malnoursihed when kept in captivity appear to turn the table against a hybrid. i know about two phenomenons that i also relate, the first is that ornag utans on borneo have recently been found in a 100s time higher then average density in some food poor mountainous refugee (for wich they apparently opt to live there together themselves , notwithstanding they might be shot in any to humans negotiable terrain. and that whole tribes of chimpansee with untill that unknown behaviours (and thus perhaps also looks) have even been found still recently (i think it files under chimpansee’s using spears?!.
when you look at the paleogical taxonomical babirussa record for example the theory that humans influenced the feircness of their looks is qualling. as to say that any chimp tribe with more fierce looks (this ones ancestors..) or behaviour (spear throwing) stood only minor change of survival in extremely unaccesible habitats..
#52 Phil Regal
March 19, 2010

Claims about hybridization should be viewed with skepticism, but NOT because this is impossible. Consider for example that there are plenty of domestic cattle genes in some herds of American bison. There are no laws of hybridization.It is hit or miss. We should be skeptical rather because history shows that when scientists and others see a specimen that is unusual they speculate that it is a hybrid or a mutation. That sounds more scientific than to say “we don’t know.” And one does not need the evidence that one would need to claim that it is a new species. In the case of the koolookamba there was fairly decent evidence that it was maybe a new species or distinctive population of something or other.

The big problem is that the zoological context for thinking about odd ape specimens is dismal. “Experts” do not in fact know as much as they think they do. I have looked over a lot of bones of bonobos and chimpanzees in museum cabinets and it is a mess. There is simply not the material for proper scientific analysis and systematics. It is commonly not clear where specimens were actually collected, rather than probably sent from shipping points off on some river in the jungle where specimens from different localities were apparently being assembled. It is not clear how long they might have been in captivity and their skeletons influenced by diet or health. This is the worst data base for doing proper systematic and taxonomic work that I have seen for any animals of scientific significance. It begs proper study and the implications are enormous. Consider how much medical and behavioral research has been done on chimpanzees.

Take behavior for example. We have lots of field studies on the most accessible East African Chimpanzee populations but most captive chimpanzees are verus from West Africa. So the differences in behavior between for example the chimps in Netherlands wildlife parks and the field studies in East Africa could be in part genetic differences rather than as has been assumed differences due to captivity. if some or most of the Netherlands chimps are eventually identified and turn out to be West African, as one should suspect. But the careless assumption has been that a chimp is a chimp and only ecology explain differences.

The problem of arm-waving taxonomy was manifest with the bonobo for decades. Too many “authorities” simply wrote it off as an odd chimp population and ignored considerable scientific evidence to the contrary.

Back to the koolookamba. I have seen lot of opinions and some poor photographs but it appears that we simply do not know and that the matter cannot be settled by for example speculating on whether bonobos and chimps or chimps and gorillas can cross.

In my experience it is better to keep scientific mysteries open than to sweep them under the rug with facile explanations.

Let me add some exasperation — Why on earth do we not have better photographs of these odd creatures? I could dig out a multivolume review of chimp biology that has a photo of a koolookamba from Holloman AFB. Terrible photo, and again, capturing a sinister pose. I could not easily track down the actual specimen. Maybe someone here with the time and interest can and could let us know. There might even be blood samples somewhere.

“1997 Holloman Air Force Base announces the closure of its chimpanzee facility; 30 chimpanzees are sent to Primarily Primates (a sanctuary in Texas) and the remaining 111 are sent to the Coulston Foundation, despite offers from other sanctuaries to care for them.”

and
http://www.releasechimps.org/harm-suffering/research-history/air-space/

“A final betrayal: delivery to toxicologist Fred Coulston

By the 1960s, the U.S. Air Force stopped using chimpanzees. Rather than sending them to sanctuaries to protect them from further experimentation, the Air Force leased many out to biomedical laboratories. Most were leased to Fred Coulston, who promoted their use for testing chemicals and drugs. He eventually founded the Coulston Foundation, a facility that would become infamous for its poor conditions and repeated violations of the Animal Welfare Act.

Some Air Force chimpanzees were left to languish in confinement at Holloman Air Force Base. Born free 20 years earlier in the African jungle, these chimpanzees would spend the next several decades in the confinement of laboratory cages. A lucky few were rescued by animal protection groups and given permanent sanctuary.”
#53 Phil Regal
March 19, 2010

This discussion prompted me to seek a bit more recent information. Next is something interesting that I found by googling. See what a mess we are dealing with!! Phil

“Koolakamba

Elaine Jean Struthers, Ph.D.
Coulston Foundation

This information or bibliography may be out-dated. Please see PrimateLit for current references.

Any discussion of the great apes must eventually encompass the mysterious Koolakamba. Speculation will then ensue as to its importance and even its existence. Over the past several years, while I have worked at what is currently the Coulston Foundation Holloman AFB site, I have followed this discourse with interest since this facility has been home to a few individuals identified as Koolakamba. If we accept the premise of the existence of the Koolakamba as a distinct entity then we must ask; is it a subspecies of the chimpanzee, a gorilla-chimp hybrid, or perhaps representative of individual variation? If we do not accept the premise of its existence then we must assign it to its place in the traditional folk mythology of the indigenous peoples of West Africa, as well as the more contemporary mythology of the international clan of primatologists.

Several articles and related research projects which have considered the question of the so-called hybrid ape, or Koolakamba, are available. The initial references appear to be from the descriptive work of DuChaillu from 1860, 1861, 1867, and 1899; some of which was republished in 1969 (Explorations and Adventures in Equatorial Africa). There may have been at least one prior reference to the Koolakamba in a French work by Franquet (1852, as cited by Shea, 1984). DuChaillu refers to the ape as Koolakamba based upon his description of words used by the indigenous peoples (Commi, Goumbi, and Bakalai[sic]) in the region of the Ovengi River of West Central Africa, modernly the areas of Cameroon and Gabon. The people allegedly referred to the ape as “Kooloo” because that is what its unique vocalization, quite unlike the vocalizations of other apes in the region, sounded like to them. “Kamba”, according to DuChaillu, is a Commi word meaning to “speak” (DuChaillu, 1899).

DuChaillu differentiates between four ape-types in his work, these are the Gorilla, the common chimpanzee, the nshiego mbouve (Troglodytes calvus), and the Koolakamba (DuChaillu 1861 and 1969). He provides a detailed physiological description of each variant species as well as illustrations of the important morphological features. The physical characteristics described for Koolakamba include a short and broad pelvic structure, large supraorbital ridge, high zygomatic ridges, less prominent “muzzle”, dentition in which the upper and lower incisors meet squarely forming a grinding surface, and a larger cranial capacity than that of the common chimpanzee. Much of what DuChaillu records is essentially ethnographic. He includes the indigenous names and lore relevant to the ape, and reveals his own cultural foibles in the writing. His works are classic period pieces with wonderfully descriptive text and presumably accurate illustrations, but limited quantitative (mostly anthropometric) data.

DuChaillu’s summation employed folk taxonomy in identification of apes in the wild. It has been asserted elsewhere (Shea, 1984) that the system of folk differentiation, unlike the extant European system embedded in DuChaillu’s worldview, identified individual variation as a type, rather than subspecies variation as a type. Classification of the Koolakamba as a unique entity, it is suggested (Shea, 1984), may be due to DuChaillu’s misinterpretation of folk taxonomy. Perhaps, then, the Koolakamba really only represents the wide range of individual variation found in the Lower Guinea chimpanzees (Pan troglodytes troglodytes) of Gabon and Cameroon.

The most comprehensive chimpanzee taxonomy was undertaken by Osman Hill (Hill, 1967; Hill, 1969). Hill’s classic work was apparently based in large part upon qualitative observations made at the Holloman AFB site of what is today the Coulston Foundation. Hill contradicts DuChaillu’s description on some points, and in fact there seem to be enough discrepancies between the two descriptions to indicate that the respective writers are not discussing the same entity, as Shea(1984) points out in his excellent and intensive treatment of the Koolakamba debate. One example is ear size. DuChaillu described the ear size of the Koolakamba as large whereas Hill indicates that the ears are small, close set to the head, and similar in appearance to that of the gorilla. Another point of inconsistency between the two authors is the facial structure which Duchaillu described as broad and flat, while Hill depicts it as extremely prognathic. However, Hill agrees with DuChaillu on one very important point, he too classes the Koolakamba as a unique subspecies rather than as an individualistic anomaly. Whether the Koolakamba is a real or mythical entity still seems to be mostly a matter of scholarly debate, an artfully done history of that debate up to 1984 has been written by Brian Shea (Journal of Ethnobiology, v.4 #1, 1984).

A number of researchers have also investigated the possible molecular identity of the Koolakamba as a true subspecies. Contemporary research methodologies can perhaps allow a more definitive explanation of the status of the Koolakamba. Work done by Ferris (et al., 1981a,b) used testing of serum to identify mitochondrial DNA restriction endonuclease polymorphism patterns which indicated distinctions between chimpanzee subspecies. The Koolakamba was not identified as unique in the Ferris research (perhaps due to not having any designated Koolakambas in the sample pool?). This work was later expanded by other research (Davidson, 1986) examining electrophoretic variation in serum esterases. The sample pool in Davidson’s work were all derived from the Holloman colony and included two designated Koolakambas. No discernible differences were noted for the Koolakamba subjects. A subsequent research project initiated by Gene McCarthy (then of the University of Georgia) in 1991 proposed to survey genetic markers, both mitochondrial and protein, in chimpanzees. In particular the project sought to settle, once and for all, the question of whether or not the Koolakamba was genetically distinct. Due to sampling difficulty, which arises from identification problems and small numbers of accessible subjects, the project has not yet come to culmination.

It has been suggested that the Koolakamba type represents a hybrid ape, perhaps a cross between gorilla and chimpanzee. The notion of hybridism between apes has been a quintessential topic of debate for numerous years. Evidence indicates that at least some ape hybridization (lesser apes) is indeed possible (Myers & Shafer, 1979; and Wolkin & Myers, 1980). The Atlanta Zoo housed two female Siamangs with a male Gibbon, and in 1975 one of the females gave birth to a hybrid offspring (Wolkin & Myers, 1980). This “Siabon” was later transferred to Georgia State University. A second hybrid was later born (in 1976) to the same pair, but it did not survive past the neonatal period. That great apes can produce hybrid offspring, then, is probable. That the Koolakamba represents a form of hybridized ape is at least plausible. Though we may not be able to confirm the existence of any such hybrids at present, it may be reasonable to reserve a category for such a hybrid and label that category “Koolakamba”.

For many years, the Coulston Foundation Holloman AFB site has been alleged to have Koolakambas among its chimpanzee population. These references have been based on acquisition records, Hill’s inventory (1967), and records of the geographical origin of wild caught subjects. Hill published a photo, probably taken around 1964, of what was allegedly a male Koolakamba. Although I have an excellent reproduction of the photo and have worked within the colony for almost 10 years, and while I have conducted thorough anecdotal interviews with the long term staff, no one recognizes the particular individual in the photo. I am quite certain he either died long ago or was moved to some other facility. The animal in the photo is quite distinctive looking, however, and we do possess a female (Jennifer) who was born here in 1970 who looks a great deal like him. Jennifer’s dam died in 1979 and was one of the very early acquisitions of the AF, but she was not noted to be a Koolakamba or otherwise unusual. The sire is unknown because at that time the chimpanzees were still housed in the open free-ranging consortium facility. We have often speculated that it might be possible that the male pictured in the Hill article could be Jennifer’s sire.

An article by Don Cousins (Acta Zoologica v.75, 1980) specifically identifies two Alleged Koolakambas at the Holloman site; Sevim, and Minnie currently 39 years old. Sevim was a female, she died of natural causes in 1983, but left a number of offspring. Minnie is something of a local celebrity, and has resided here since 1957. She currently is employed as a foster mother after several years as a prominent member of the breeding contract colony. She assists us in raising the infants that for various reasons have had to be reared in the nursery during early infancy. Currently she is raising her own natural offspring, Lil’ Minnie, who may represent a Koolakamba type as she looks exactly like her illustrious mother. Minnie certainly fits the description of the Koolakamba both pysically and behaviorally that DuChaillu offers in his works. Minnie frequently likes to walk bipedally, she is extremely gregarious and smart preferring toys that require manual dexterity and finesse to manipulate (e.g. baby activity boards, feeding puzzles, etc.). Minnie also has a well-defined aggressive streak and she traditionally holds an Alpha position in whatever social configuration she is placed.

There have been other chimpanzees among our colony that morphologically and behaviorally (re: DuChaillu) we hazarded might be of the Koolakamba type, but we could not really confirm them as such. It is often difficult to identify the geographical place of origin (based on accession records) for the few remaining wild-caught members of the Holloman founder population. Often such records were intentionally misleading due to irregularities in customs laws prior to the CITES law governing transport of chimpanzees from their native countries. Records sometimes indicate which country the animals were shipped from but that is not necessarily synonymous with the country in which they were actually caught. This further obscures our ability to guess if individuals might be members of the Koolakamba tribe. When an acquisition record indicates place of origin as Gabon or Cameroon, it is worth examining the subject in question to assess morphological features, but the designation of “Koolakamba” remains subjective.

Another organization that may be helpful in maintaining records for individuals identified as Koolakamba is ISIS. Rick Lukens, research associate at ISIS and formerly network analyst at the Holloman AFB site, is aware of the Koolakamba issue and is careful to note any references to the subspecies in records he handles. We at the Coulston Foundation are keenly interested in the Koolakamba debate and will be pleased to field any inquiries regarding this unique variation of Pan.

Bibliography

Cousins, D., 1980: On the Koolakamba – a Legendary Ape. Acta Zoologica et Pathologica Antverpiensia, 75: 79-93.
DuChaillu, P.B., 1860: Descriptions of Five New Species of Mammals Discovered in Western Equatorial Africa. Proceedings Boston Society Natural History, 7:296-304, 358-367.
Davidson, William S., 1986: Variations in the Serum Esterases of Chimpanzees. Comp. Biochem. Physiol., Vol. 83B, 3:697-699.
DuChaillu, P.B., 1861, and 1969: Explorations and Adventures in Equatorial Africa. Murray, London; reprinted in 1969 by Negro Universities Press, New York.
DuChaillu, P.B., 1867: A Journey to Ashango-Land and Further Penetration into Equatorial Africa. Murray, London.
DuChaillu, P.B., 1899: Stories of Gorilla Country. Harper and Brothers Publishers, N.Y. and London.
Ferris, S.D., Brown, W.M., Davidson, W.S., and Wilson, A.C., 1981: Extensive Polymorphism in the Mitochondrial DNA of Apes. Proceedings National Academy of Sciences, U.S.A., 78:6319-6323.
Hill, W.C. Osman, 1967: The Taxonomy of the Genus Pan.Neu Ergebnisse der Primatologie, eds. D. Starck, R. Schneider, and H.J. Kuhn, pp.47-54. Stuttgart:Fischer.
Hill, W.C. Osman, 1969: The Nomenclature, Taxonomy, and Distribution of Chimpanzees; in The Chimpanzee, 1:22-49. Karger, Basel and N.Y.
Myers, Richard H., and Shafer, David A., 1979: Hybrid Ape Offspring of a Mating of Gibbon and Siamang. Science, 205:308-310.
Shea, B.T., 1984: Between the Gorilla and the Chimpanzee: a History of Debate Concerning the Existence of the Kooloo-kamba or Gorilla-like Chimpanzee. Journal of Ethnobiology, 4:1-13.
Tuttle, R.H., 1986: Apes of the World. Noyes Publications, Park Ridge, N.J.
Wolkin, Joan R., and Myers, Richard H., 1980: Characteristics of a Gibbon-Siamang Hybrid Ape. International Journal of Primatology 1: 203-221.”