Cross River Gorillas - a Neglected Subspecies

In 1904, Paul Matschie, a pioneer in mammalian taxonomy working at the Humboldt University Zoological Museum in Berlin described a new species of gorilla inhabiting the watershed of the Cross River in what was then German Cameroon, close to the border of British-governed Nigeria. Matschie named the species Gorilla diehli in honor of Mr. Diehl, an employee of the German Northwestern Cameroon Company, who had collected the gorilla skulls on which Matschie based his new species. According to Matschie the 1) short skull, 2) short molar row, 3) palate shape, 4) and skull base shape distinguished Gorilla diehli as a new species separate from Gorilla gorilla.
Matschie also noted in his description that one of the female skulls collected by Diehl from the same area was not G. diehli, but G. gorilla, and claimed both species existed together in the Cross River catchment area. The potential occurrence of two morphologically distinct gorillas from the same locality supported Matschie's claims that the two were distinct species. Without the intrinsic barriers to interbreeding that characterizes different species, two gorilla populations could not possibly inhabit the same isolated area and remain morphologically distinct.
Subsequent classifications by Rothschild in 1904 and Elliot in 1912 agreed that the Cross River gorillas were not a new species and demoted the population to the subspecies Gorilla gorilla diehli. Neither author examined the specimens described by Matschie, or tested Matschie's claim that two morphologically distinct gorillas inhabited the Cross River watershed. If Matschie's claim was true, G. g. diehli could not possibly be a subspecies.
Harold Coolidge's revision of the genus Gorilla in 1929 placed what was then recognized as G. g. diehli into the subspecies G. g. gorilla. He based his decision largely on anecdotal accounts of gorilla distribution, believing Cross River gorilla populations were continuous with those of other western lowland gorillas. Coolidge, like his earlier counterparts, failed however, to address Matschie's claims. Although Colin Groves in 1970 revised gorilla taxonomy and added a subspecies (Gorilla gorilla graueri) to the eastern gorilla populations, Matschie's claims remained unchallenged and Coolidge's taxonomy remained by and large the framework of the currently accepted classification. By now, the Cross River gorillas were known to occur in eastern Nigeria as well as southwestern Cameroon, and they had at least been recognized by Groves as a distinctive far-western population.
Working on primate distribution and behavior in West Africa for the past 30 years, John Oates had long ago recognized the Cross River watershed, the Cameroon highlands and Bioko island as an area of primate endemism. The Sanaga river to the south of this area seems to act as a barrier to primate migrations from the extensive forests of western equatorial Africa, which cover most of southern Cameroon, Gabon, Equatorial Guinea, northern Congo and southwestern Central African Republic and are inhabited by G. g. gorilla.
At least 6 Old World monkey species or subspecies appear to be unique to the Cross River-Cameroon Highland-Bioko area. These monkeys include: Preuss's red colobus (Procolobus badius preussi), the drill (Mandrillus leucophaeus), Martin's putty nosed guenon (Cercopithecus nictitans martini), the red-eared guenon (Cercopithecus eryhtrotis), Preuss's guenon (Cercopithecus preussi) and the crowned guenon (Cercopithecus pogonias pogonias). The area is also home to several distinctive prosimians. Given such endemism, John Oates had suspected Cross River gorillas could possibly be a unique taxon.
By the late 1970s, however, a general view had developed that the Cross River gorillas had been extirpated at least from Nigeria if not from Cameroon. But in 1983 surveys by Clement Ebin of the Cross River State Forestry Department obtained evidence of gorilla populations living in Nigeria's Mbe Mountains. Further surveys in Nigeria and Cameroon in the last decade have established the presently-known distribution of the Cross River gorillas which probably number no more than 200 individuals in 4 isolated populations but still exist.
John Oates began field studies on the Cross River gorillas in 1990, and Esteban Sarmiento has been focusing his attention on the museum specimens of these apes. Museum records in London and Berlin showed that there were at least 100 Cross River gorilla skulls in museum collections for us to compare. Many of the skulls at the British Museum of Natural History, London, however, lacked specific locality areas and/or were too damaged to be included in the study.
The Royal College of Surgeons Hunterian Museum in London had been bombed during World War II and the Cross River gorilla skulls housed there had been destroyed. The same fate was believed to have befallen the Berlin specimens including those described by Matschie. But the bombs that fell on the Humboldt University Zoological Museum (the damage of which can still be seen today) only destroyed paperwork and records, the specimens were still intact. Renate Angermann, curator of mammals, had recognized the importance of this collection and had already located and recatalogued all the Cross River gorilla specimens by the time Esteban Sarmiento was ready to analyze them.
Comparison of skull measurements of non-Cross River western gorillas (approximately 55 males and 30 females) to Cross River gorillas (40 males and 20 females) including all those specimens referred to by Matschie in his description supported most of Matschie's claims. Cross River gorillas have shorter skulls, shorter molar rows, narrower palates, and a relatively broader skull base than the other western gorillas. In addition, Cross River gorillas have smaller cheektooth surface areas, smaller gapes and much smaller braincase volumes than do the other western gorillas. Cross River gorillas also have a suite of characteristic non-metric cranial traits, which collectively are not seen in any other gorilla population.
Statistical analyses on the skull measurements distinguished both male and female Cross River gorillas from other western gorillas and correctly assigned all individual specimens to their respective population. The statistical analyses, however, also proved that Matschie was wrong. The single female skull from the Cross River area, which Matschie claimed to be Gorilla gorilla, was indistinguishable from the other Cross River gorillas. It was clear, therefore, that the Cross River watershed was not inhabited by two morphologically distinct gorilla species.
In combination the skull measurements suggested that Cross River gorillas are probably smaller in body size than G. g. gorilla. Long bone and vertebral measurements from the single male and single female skeleton available, however, indicate a body size similar to that of G. g. gorilla. In fact, the single female skeleton measured is one of the largest females in our western gorilla sample. Moreover, Cross River gorillas do not seem to differ in long bone proportions (upper limb to lower limb, arm to forearm, and thigh to leg ratios) from the other western lowland gorillas.
Measurements of the male hand and foot segments show that this animal has comparatively shorter hands and feet. Unfortunately, with only two skeletons it is not possible to determine if such body and limb segment proportions are representative of the whole population.
Our attempts to associate the morphological differences we found in Cross River gorillas with their ecology is confounded by considerable differences in the habitats presently occupied by the four Cross River gorilla populations. We suspect many of their unique cranial characters may be associated to a diet that, historically, has been made up of smaller drier and harder foods than usually consumed by the other western gorillas. Such a diet could be a corollary of the relatively drier habitats and colder temperatures that exist at the northern latitudes they inhabit.
Data currently being analyzed by Kelley McFarland from her 1996-1999 field studies on the Afi Mountain population of these gorillas may help to throw some new light on these ecological questions.
Regardless of what environmental variables their morphological differences are precisely associated with, it is clear that these differences separate Cross River gorillas as a taxon distinct from other western gorillas. Because Cross River gorillas are more than 250 km northwest of the nearest western gorilla population and no other morphologically distinct gorilla shares its range, we are convinced that their differences can be best summarized by placing them in the subspecies Gorilla gorilla diehli. Our scientific study supporting a subspecies distinction of Cross River gorillas will appear in an upcoming American Museum of Natural History Novitates publication.

Esteban E. Sarmiento und John F. Oates

Dr. Esteban Sarmiento is a functional anatomist in the American Museum of Natural History. His primary research interest involves the skeletons of hominoids. He has taught in the USA, South Africa and Uganda.
Prof. John F. Oates is a member of the Conservation Committee of the ISP (International Primatological Society) and of the Steering Committee of the IUCN Primate Specialist Group. He has been studying the ecology of tropical forest primates since 1964.

Cross River overview

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