Cross River Gorilla, Gorilla gorilla diehli - Top 25 Most Endangered Primates

Illustration © 2007 by Stephen Nash. Cross River Gorilla
Gorilla gorilla diehli Matschie, 1904
Nigeria and Cameroon
(2000, 2002, 2004, 2006)

The Cross River gorilla (Gorilla gorilla diehli) is the most western and northern form of gorilla, and is restricted to the forested hills and mountains of the Cameroon-Nigeria border region at the headwaters of the Cross River. It is separated by about 300 km from the nearest population of western lowland gorillas (Gorilla gorilla gorilla), and by around 200 km from the recently-discovered gorilla population in the Ebo Forest of Cameroon. The most recent surveys suggest that between 200 and 300 Cross River gorillas remain. Groups of the gorillas concentrate their activities in 11 localities across a 12,000 km² range, but genetic research has found evidence that despite their scattered distribution these subpopulations continue to maintain contact through the occasional migration of individuals.
There are currently two protected areas within the Cross River gorillas’ range in Nigeria: the Afi Mountain Wildlife Sanctuary and the Okwangwo Division of Cross River National Park. In Cameroon, the Kagwene Gorilla Sanctuary is in the process of gazettement, and planning has begun for the creation of a Takamanda National Park. Beyond those protected areas, about half of the remaining Cross River gorillas occur in community-managed forests and a forest reserve (Mone River in Cameroon). There are many human settlements around the forests where the gorillas occur, and some villages are even enclaved within Okwangwo and Takamanda.
The encroachment of farms, dry-season fires set to clear forest or improve pasture, and development activities, such as roads, are continuing threats to the integrity of gorilla habitat. However, large tracts of lower elevation forest remain between the localities where the gorillas are presently concentrated and if these areas can be protected, the animals could expand their range and their population. Genetic evidence suggests that the population of Cross River gorillas was much larger in the past, and that a dramatic decline has occurred over the last 200 years, almost certainly due to the introduction of hunting with firearms. After several years of awareness-raising by conservationists and researchers, hunting of Cross River gorillas for bushmeat has been reduced to a low level but it is still a potential threat, as are wire-snare traps set for other animals.
A conservation action plan for Cross River gorillas has been prepared, based on the deliberations of a workshop held in Calabar, Nigeria, in April 2006, organized by the Wildlife Conservation Society and funded by WWF’s African Great Ape Programme and the U.S. Fish and Wildlife Service’s Great Ape Conservation Fund (Oates et al. 2007). Among some of the key recommendations made by this workshop are for education and awareness efforts to be expanded, a transboundary conservation committee to be created, and new surveys to be launched in areas that are predicted by remote-sensing image analysis to support gorillas. The workshop recommended completion of the process to create protected areas at Kagwene and Takamanda, as well as the establishment of conservation systems at Mbe (Nigeria) and at Mone River, Mbulu and Bechati-Fossimondi (Cameroon). All these areas require management plans to be developed and implemented.
About 250 km south from the Cross River population, a small isolated population of gorillas occurs in a small portion of the 1,500 km² forest straddling the Ebo River in southwestern Cameroon, approximately 50 km north of the Sanaga River. Field research undertaken by the Zoological Society of San Diego suggests that five or fewer gorilla groups survive in Ebo, which is also inhabited by 10 other diurnal primates, including highly threatened forms such as the drill (Mandrillus leucophaeus), Preuss’s red colobus (Procolobus pennantii preussi), and the Gulf of Guinea chimpanzee (Pan troglodytes vellerosus).
The taxonomic affinities of the Ebo gorillas are still unclear. Based on measurements of a single skull, they may be most closely related to the gorillas of the inland plateau of Cameroon (south of the Sanaga River), rather than to Cross River gorillas. The Ebo gorilla population only became known to the outside world in 2001; they have been afforded little protection in the past, and the forest’s primates are under extreme pressure from bushmeat hunting, given the proximity of the forest to the main urban centers in Cameroon. The Zoological Society of San Diego established a research station there in 2005, and with the full-time presence of researchers, along with technical assistance from WWF’s Cameroon Coastal Forests Program, the conservation status of the Ebo Forest has improved and the area is currently being gazetted as a national park.

Jacqui Sunderland-Groves, John F. Oates & Bethan Morgan

References
Bassey, A. E. and J. F. Oates (eds.). 2001. Proceedings of the International Workshop And Conference on the Conservation of the Cross River Gorillas, Calabar, Nigeria, April 6–9, 2001.
Bergl, R. A. 2006. Conservation Biology of the Cross River Gorilla (Gorilla gorilla diehli). PhD thesis, Graduate Center, City University of New York, New York.
Bergl, R. A. and L. Vigilant. 2007. Genetic analysis reveals population structure and recent migration within the highly fragmented range of the Cross River gorilla (Gorilla gorilla diehlii). Molec. Ecol. 16: 501–516.
Groves, C. P. 2005. A note on the affinities of the Ebo Forest gorilla. Gorilla Journal (31): 19–21.
Groves, J. L. 1999. Gorillas in the Takamanda Forest Reserve. Gorilla Journal. (18): 17–18.
Groves, J. L. 2001. Gorillas of Takamanda, Mone and Mbulu forest, Cameroon. Gorilla Journal (22): 27–29.
McFarland, K. L. 2007. Ecology of Cross River Gorillas (Gorilla gorilla diehli) on Afi Mountain, Cross River State, Nigeria. PhD thesis, Graduate Center, City University of New York, New York.
Morgan, B. J. 2004. The gorillas of the Ebo Forest, Cameroon. Gorilla Journal (28): 12–14.
Morgan, B. J. and J. L. Sunderland-Groves. 2006. The Cross-Sanaga gorillas: The northernmost gorilla populations. Gorilla Journal (32): 16–18.
Morgan, B. J., C. Wild and A. Ekobo. 2003. Newly discovered gorilla population in the Ebo Forest, Littoral Province, Cameroon. Int. J. Primatol. 24(5): 1129–1137.
Oates, J. F., D. White, E. L. Gadsby and P. O. Bisong. 1990. Conservation of gorillas and other species. Appendix 1 to Cross River National Park (Okwangwo Division): Plan for Developing the Park and Its Support Zone. World Wide Fund for Nature, Godalming, UK.
Oates, J. F., K. L. McFarland, J. L. Groves, R. A. Bergl, J. M. Linder and T. R. Disotell. 2003. The Cross River gorilla: Natural history and status of a neglected and critically endangered subspecies. In: Gorilla Biology: A Multidisciplinary Perspective, A. B. Taylor and M. L. Goldsmith (eds.), pp.472–497. Cambridge University Press, Cambridge, UK.
Oates, J. F., R. A. Bergl and J. M. Linder. 2004. Africa’s Gulf of Guinea Forests: Biodiversity Patterns and Conservation Priorities. Advances in Applied Biodiversity Science (6): 1–90. Center for Applied Biodiversity Science, Conservation International. Washington, DC.
Oates, J. F., J. Sunderland-Groves, R. Bergl, A. Dunn, A. Nicholas, E. Takang, F. Omeni, I. Imong, R. Fotso, L. Nkembi and L. Williamson (eds.). 2007. Regional Action Plan for the Conservation of the Cross River Gorilla (Gorilla gorilla diehli). Wildlife Conservation Society, New York, and IUCN/SSC Primate Specialist Group, Arlington, VA.
Sarmiento, E. E. 2003. The Cross River gorilla: The most endangered gorilla subspecies. Primate Conserv. (19): 65–72.
Sarmiento, E. E. and J. F. Oates. 2000. Cross River gorillas: A distinct subspecies, Gorilla gorilla diehli Matschie, 1904. Am. Mus. Nov. (3304): 1–55.
Sunderland-Groves, J. L. and J. F. Oates 2003. Priority conservation actions for Cross River gorillas in Cameroon and Nigeria. Report, Wildlife Conservation Society, New York.
Sunderland-Groves, J. L. B. and Jaff. (eds.). 2004. Developing a Conservation Strategy for the Cross River Gorilla. Proceedings of the 2nd International Workshop and Conference on the Conservation of the Cross River Gorilla. Wildlife Conservation Society.
Sunderland-Groves, J. L., F. Maisels and A. Ekinde. 2003. Surveys of the Cross River gorilla and chimpanzee populations in Takamanda Forest Reserve, Cameroon. In Takamanda: the Biodiversity of an African Rainforest. J. A. Comiskey, T. C. H. Sunderland and J. L. Sunderland-Groves (eds.), pp. 129–140. Smithsonian Institution Monitoring and Assessment of Biodiversity Program Series 8. Washington DC: Smithsonian Institution.

Suggested citation:
Sunderland-Groves, J., Oates, J. F. and Morgan, B. 2007. Cross River Gorilla, Gorilla gorilla diehli Matschie, 1904. In: Primates in Peril: The World’s 25 Most Endangered Primates 2006–2008, R. A. Mittermeier et al. (compilers), pp.11-12. Unpublished report, IUCN/SSC Primate Specialist Group (PSG), International Primatological Society (IPS), and Conservation International (CI), Arlington, VA.