There are only an estimated 3500 - 9000 individuals left in the wild today.
Summary of taxonomic history and morphology
The chimpanzees from the area between the Niger and Sanaga rivers have been known to science since the 19th century. The earliest description comes from Gray (1862) of a chimpanzee skin collected from Mount Cameroon by the explorer Captain Richard Burton, British consul to Fernando Po. Gray considered this specimen to be different from others he had seen, "in being covered with much more abundant and softer fur, and in the fur of the back being of a brown colour from the large brown tips to the blackish hair." (p. 180). He designated this Troglodytes vellerosus, making Pan troglodytes vellerosus available as a name for the chimpanzee subspecies from this area.
Matschie (1914, 1919) provided further descriptions of Nigeria-Cameroon chimpanzees by designating three new species, Anthropopithecus ellioti, A. oertzeni, and A. papio from the region. These would be considered junior synonyms of P. t. vellerosus. He differentiated the species by details of the size of the ears, orientation of cheek whiskers, prominence of brow ridges, size of upper lip and patterns of baldness on the forehead. The descriptions are based on a few skulls and the dried skins of one or two museum specimens, however, and although Matschie was aware of age changes and sex differences, he did not acknowledge much of this variation, or any degree of individual variation, in his descriptions. As such they are not very helpful in understanding the general appearance of the Nigeria-Cameroon chimpanzees.
Paul Matschie was one of the mavericks of mammalogy in the late 19th and early 20th centuries. He was an anti-evolutionist, beginning his 1919 paper by writing forcefully (translated from the German):
"Much has been written about whether men and apes are blood-relatives and whether a development of men from apelike ancestors, or both from a common stem form, can be admitted. By one who has for a long time worked in a large mammal collection, these questions can only be answered in the negative. He finds among mammals no transitions between different species, he knows that every one of an interrelated community, call it species, subspecies or race, possesses the same unvarying characters. In all cases where a mammal possesses the characteristics of two different forms, where one might superficially think it was a transition, further analysis reveals evidence that we have to do with hybridisation. Such hybrids occur only in areas in which the distributions of two species overlap; but they never form new races. Evidence for the origin of one mammal species from another is sought in vain".
In this paper in 1919, Matschie then goes on to talk about how human races are also distinct, all apparent transitions between them being due to hybridisation, and after describing briefly the differences between gorillas and chimpanzees, and something about age and sex variations within chimpanzees, and the history of chimpanzee taxonomy, he finally gets around to describing the species of chimpanzees which he thinks exist in West-Central Africa. In his view, there are nine between the Muni River and the mouth of the Congo, each with its own range (centred on a river valley), as well as the four in the Mount Cameroon-Cross River-Nigeria region. Associating the ranges with river valleys was part of his general model of mammalian distributions -- occasionally, he would come across a specimen which had been collected on a watershed, and he would draw attention to some aspect of asymmetry which, in his view, was a criterion of hybridisation. In the present case, he felt himself confidently able to associate the species he called Anthropopithecus ellioti with the valley of the Katsena, a tributary of the Niger, and Anthropopithecus oertzeni with the Cross River valley.
When the descriptions of all four of the "species" recognized by Matschie in the Mount Cameroon/Cross River district are taken together, common characteristics that emerge are the small ears and possibly thick fur (which would provide the etymology for vellerosus). But whether these and other features can be used to differentiate this subspecies from others in Africa remains to be verified by field primatologists, signalling the need for continuing research in this area. Recent comprehensive analyses of chimpanzee teeth and skulls reveal that the Nigeria-Cameroon chimpanzees have smaller teeth and skulls than surrounding chimpanzee populations, with higher incidence of infraorbital foramina in the facial region. Thus, in broad brushstrokes a picture is slowly emerging of this long-known, yet little-studied chimpanzee.
In 2009 it was shown by John Oates, Colin Groves, and Paulina Jenkins that the type specimen of of Pan troglodytes vellerosus had not been collected from Mount Cameroon, as had been generally assumed, but from Gabon. Therefore Oates et al. have chosen the name Pan troglodytes ellioti for these chimpanzees, based on Anthropopithecus ellioti which was named by Matschie from a specimen collected in Bascho in Cameroon and lodged in the Humboldt Museum, Berlin in 1905.
Summary of Population History
Chimpanzees are of broad interest, due to their close relationship with humans, their complex cultures and their broad distribution across forested regions of Africa. Chimpanzees belong to a single species, Pan troglodytes, that may be divided into as many as five subspecies based on geographical, genetic, morphological and behavioral data.
However, the taxonomy and recent population history of chimpanzees are active areas of research. The most complete phylogeographic analysis of wild chimpanzees is composed of genetic data from small, rapidly evolving regions of mitochondrial DNA that are widely used to illuminate the population histories of many organisms. These genetic data represent several hundred chimpanzees across Africa and suggest that the recognized chimpanzee subspecies are divided into two geographically and genetically defined groups: a western African group (P. t. verus and P. t. ellioti) and a central/eastern African group (P. t. troglodytes, P. t. schweinfurthii and P. t. marunguensis). Subsequent population genetic analyses from samples collected throughout Nigeria and Cameroon indicate that a significant phylogeographic break between these two groups occurs at the Sanaga River in central Cameroon, delimiting P. t. ellioti from P. t. troglodytes in southern Cameroon. This phylogeographic break at the Sanaga appears to be quite ancient in that the mtDNA lineages of chimpanzees restricted to either side of the Sanaga shared a last common ancestor approximately 600,000 years ago. However, the specific patterns of gene flow between chimpanzees near the Sanaga have not been well characterized. There is additional evidence for population subdivisions of chimpanzees in western Africa. Chimpanzees from Upper Guinea (P. t. verus) and those from the Gulf of Guinea region (P. t. ellioti) cluster together into two groups, whose distributions may be separated by either the Niger River or the Dahomey Gap.
Summary of Ecology and Behavior
Very little is known about the ecology and behavior of chimpanzees occupying the Gulf of Guinea biodiversity block. Similar to chimpanzees in other regions of Africa, in the Gulf of Guinea region, chimpanzees are found predominantly in moist forest, dry forests and forest galleries that extend into savannah woodlands. Dr. Volker Sommer (University College London) leads the only long term study of chimpanzees at Gashaka-Gumti National Park in eastern Nigeria. Gashaka-Gumti National Park in Nigeria is a mosaic habitat of woodland, lowland and gallery forest with 4-5 months of drought per year, and studies on chimpanzee behavior have been ongoing there since 2000. The home range and density of chimpanzees here are estimated at 26 km2 and 1.3 individuals/km2 respectively (Sommer et al., 2004). The Gashaka Gumti chimpanzees harvest arboreal ants and army ants using tools made from plant and tree material, and dip for honey with stick tools (Fowler and Sommer, 2007, Schoening et al., 2007, Sommer et al., 2012). The chimpanzees at Ngel Nyaki Forest Reserve, Nigeria, with a population density estimated at 1.67 individuals/km2 (Beck and Chapman, 2008) consumed fruits from 52 plant species and figs were an important fallback food, present in 61% of the fecal samples (Dutton and Chapman, 2015). The Ngel Nyaki population also engaged in subsistence tool use behavior, to acquire ants and honey from stingless beehives (Dutton and Chapman, 2014). Human activities were observed to influence chimpanzee nesting behaviors with ground nests common in areas with less hunting pressure around and within the Lebialem-Mone landscape in southwest Cameroon (Last and Muh, 2013). Chimpanzees in the Ebo Forest, Cameroon have an interesting tool use repertoire including nut cracking, termite fishing and honey dipping (Abwe and Morgan, 2008, Morgan and Abwe, 2006). Recent work in there has shown that chimpanzees use stone tools to access nuts from Coula trees (Coula edulis) and also use sticks to hunt for insects. Previously, nut cracking behavior had only been observed in P. t. verus populations west of the Nzo-Sassandra river in Ivory Coast.