The Cross River Gorilla (Gorilla gorilla diehli) is a recently recognized subspecies of Western Gorilla living in the northern Cameroon-Nigerian border area (Sarmiento & Oates, 2000).
Origin of the species name
The American physician and missionary Thomas Staughton Savage first described the Western Gorilla (he called it Troglodytes gorilla) in 1847. The name was derived from a Greek translation of the word Gorillai ("tribe of hairy women"), described by Hanno the Navigator, a Carthaginian explorer who led a voyage (circa 480 BC) around the coast of West Africa. There is much academic debate over whether the ‘hairy women’ he saw is what we know as gorillas today.
Gorilla gorilla diehli (Matschie, 1904)
In 1904, Paul Matschie, a mammalian taxonomist 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. According to Matschie the 1) short skull, 2) short molar row, 3) palate shape, and 4) skull base shape distinguished Gorilla diehli as a new species separate from Gorilla gorilla.
Cross River Gorillas are very wary of humans, and this picture is one of very few ever taken of this species (Picture by J. Sunderland Groves)
Subsequent classifications by Rothschild in 1904 and Elliot in 1912 found that the Cross River gorillas were not a new species and demoted the population to the subspecies Gorilla gorilla diehli. 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. Although Colin Groves in 1970 revised gorilla taxonomy and added a subspecies (Gorilla gorilla graueri) to the eastern gorilla populations, 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 south-western Cameroon, and they had at least been recognized by Groves as a distinctive far-western population.
Working on primate distribution and behaviour in West Africa for the previous 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 are inhabited by Western Lowland Gorillas.
English – Cross River Gorilla
French – Gorille de Cross river, Gorille de Diehl
German – Cross-River-Gorilla
Spanish - Gorila del Cross River
Cross River Gorillas do not seem to be very easy to distinguish from Western Lowland Gorillas except that they differ significantly in their skull measurements and in particular in mean cheek tooth surface and the absence, or relatively poor development, of the sagittal crest in many males. These differences have been associated with shifts to more open habitats, and could also be associated with lower fruit abundance in habitats at high elevations, and/or periods of fruit scarcity during prolonged dry seasons. The extensive and unique montane forest ecosystem of the Obudu Plateau and other areas of Bamenda Highlands (Keay, 1979) may be a better representation of the habitat in which the taxon evolved.
Gorillas are mainly terrestrial. Their large body size and folivorous habits mean that the animals must spend long hours feeding everyday to maintain their body weight. Of all the great apes, the gorilla shows the most stable grouping patterns with the same adult individuals travelling together for months and usually years at a time. Foliage, unlike fruit, comes in large patches that can support large groups of animals. In west Africa, where fruit makes up a higher proportion of the gorillas’ diet than in the east, gorilla groups tend more frequently to split into temporary subgroups than they do in east Africa, as animals range far apart searching for fruit.
Cross River Gorillas inhabit low-lying and submontane tropical and subtropical broadleaf forests at elevations from 200 to 2,000 metres (Sarmiento & Oates, 2000, Sarmiento 2003, Oates et al., 2007). Sarmiento & Oates (2000) describe the habitat at lower elevation as moist, semi-deciduous forest. The forest has probably seen many generations of human disturbance and should therefore be considered an old secondary forest. Much of the forest, however, has not been recently disturbed, and large trees are relatively abundant in the areas furthest from human settlements. At higher elevation, above approximately 700m the composition and height of the forest canopy change; at these intermediate altitudes large mahoganies and Santiria trimera are frequently seen. Above 1,000 m there are distinctly montane elements in the flora, including Cephaelis mannii and Podocarpus milanjianus, and at the highest elevations (up to 2,000m) there is montane forest with smaller trees and abundant epiphytes. Much of the forest at higher altitude (1,500 to 1,800m), where the taxon possibly evolved or for which it is possibly best suited, has been converted to grassland by a long period of human occupation (cultivation, burning, cattle grazing) and so is no longer available.
Cross River Gorillas do not appear to have strong habitat preferences within their present range. They occur at altitudes between 100-2,000m but their present distribution seems to correlate more with human pressure and slope than with habitat types. In Nigeria they live primarily in the rugged terrain of the Afi and Mbe mountains and at the headwaters of the Asache and Mache rivers below the Obudu Plateau of Nigeria; in these areas there are many sheer rock faces or rocky outcrops throughout the forest. In Cameroon their nests are found in high concentrations only in a number of hilly areas in the Takamanda, Mone and Mbulu forests. This distribution may be the consequence of long term hunting pressure.
Only a handful of direct sightings of Cross River Gorillas have been made, and almost all information on their ecology and behaviour derives from observations of sleeping nests, feeding trails and reports by local hunters. Nest clusters suggest that group size is typically small (fewer than 6 weaned individuals) although much larger groups do occur. At Afi mountain nesting patterns suggest that a group as large as 20 individuals will sometimes divide into smaller foraging parties. No attempts to habituate them have been made given that the animals are so few and vulnerable to hunting.
Adult females in a social group are mostly unrelated, and the social ties that exist between them are weak. Groups typically contain one adult male (a silverback). In contrast to many other primates, it is the bond between each individual female and the silverback, rather than bonds between the females that hold the group together. Upon reaching maturity, both the males and females leave the natal group. The females usually join another group or a lone young adult male, whereas the males remain solitary until they can attract females and establish their own groups (Parnell, 2002). Cross River Gorillas are wary of humans as a result of hunting.
Distribution (current and historical)
Cross River Gorillas are restricted to a limited area (<10,000 km²) of southwest Cameroon and neighbouring parts of Nigeria, between 5°55’-6°25’N and 8°50’-10°00’ E. The Cross River Gorillas are thought to have ranged into the relic montane forests of the Obudu plateau (1,500-1,700m elevation) until the recent past (Harcourt et al., 1989). There are now eleven known discrete localities where Cross River Gorillas live. Recent genetic studies suggest that gorillas at 10 of these localities constitute one population, divided into three sub-populations, which still occasionally exchange individuals (Bergl and Vigilant 2007). Potential gorilla habitat still connects all of these localities, although sometimes tenuously (Bergl & Vigilant, 2007; Oates et al., 2007).
Cross River Gorillas form the most northern and western of all gorilla populations and are separated from the nearest Western Lowland Gorilla population to the south by approximately 250 km. The Cross River area and the nearest outpost of western equatorial African forest occupied by Western Lowland Gorillas are separated by the grasslands and fragmented forests of the Cameroon highlands, and the relatively densely settled lowlands of western Cameroon, effectively isolating the Cross River Gorillas from the other west African gorilla populations.
Evaluation and evolution of populations
Accurate population estimates for gorillas are often difficult to establish, because of their small population size and their vast range. Population censuses and estimates of gorillas are commonly carried out on the basis of nest site counts (e.g. Kühl et al., 2008). The Cross River Gorilla has probably had a restricted range for some time. The 1966-1970 Nigerian civil war and lack of information led, by the late 1970s, to a general view that Cross River Gorillas had been extirpated at least from Nigeria, and maybe from Cameroon as well.
In 1983 surveys by Clement Ebin of the Cross River State Forestry Department obtained evidence of gorilla populations living in Nigeria's Mbe Mountains. Estimates were very low, with only around 100-200 believed to be remaining in the wild. Further surveys in Nigeria and Cameroon in the 1990s suggested that there were probably no more than 200 individuals in four isolated sub-populations. Following long-term surveys established in the late 1990s (Sunderland-Groves & Maisels, 2003) it is now thought that up to 300 of these animals survive. Within Cameroon they are more widespread than previously thought (Oates et al. 2007). Although the discovery of new localities is encouraging, some of these localities are quite isolated, and therefore pose conservation challenges. The Cross River Gorilla is listed as Critically Endangered (IUCN 2008, CR A4cd) and is found in 11 localities on the Nigerian-Cameroon border, most of them connected by large tracts of continuous forests.
Some patterns of seasonal movement are observed. Hunters who frequent the forests below the Obudu Plateau report that gorillas use higher elevations in the wet season and retreat to valley bottoms in the dry season (Oates et al., 1990). Most observations at other locations suggest that the gorillas tend to maintain a relatively stable range across the seasons (Sunderlands, comm pers.). Recent field surveys in border locations suggest that Cross River Gorillas still regularly cross the border between Nigeria and Cameroon. Genetic data suggest movements between several of the areas in which Cross River Gorillas are found (Bergl & Vigilant, 2007). Gorillas do not display territorial behaviour, and neighboring groups often have overlapping ranges (Bermejo, 2004, Doran et al., 2004). A group usually favours a core area within the home range but seems to follow a seasonal pattern depending upon the availability of ripening fruits.
Of the two long-term ecological studies on Cross River Gorillas at Afi Mountain in Nigeria and at Kagwene Mountain in Cameroon, the mean daily range at both sites was roughly 1 km. Annual range at Afi was 31km² and at Kagwene approximately 19km². The remaining populations are now confined to highland areas within a larger area of more-or-less continuous forest. This large forest block is becoming fragmented in some areas. Transboundary protected areas and corridors between the isolated populations have been proposed as important conservation measures.
Conservation status, by range state
Nigeria (Critically Endangered): In Nigeria there are probably three localities and a fourth is shared with Cameroon. There are estimated to be approximately 75-110 individuals remaining in Nigeria (Oates et al., 2007).
Cameroon (Critically Endangered): Results from surveys undertaken in 2000 and 2001 indicated that there may be up to 180 individuals remaining on the Cameroon side of the border. Recently the number of Cross River Gorillas in Cameroon has been estimated at 125-185 individuals (Oates et al., 2007). Although surveys to clarify gorilla distribution are still ongoing and this figure may be subject to change, these results confirm that the Cross River Gorilla population is indeed larger than previously believed.
Actual and potential threats
The major threats affecting or having affected Cross River Gorilla populations are (1) habitat loss or modification, (2) direct killing (for the bushmeat trade, traditional medicine etc.) and (3) the population is at risk due to its very small size and its highly fragmented distribution. Between 1998 and 2002, conservation efforts undertaken by the local people in collaboration with the Cross River Gorilla Research Project (Cameroon) and the Ministry of Forestry and Wildlife (MINFOF) project PROFA have markedly reduced gorilla hunting in these areas. However, other threats to the gorillas such as encroachment into their preferred habitat will certainly have an effect in further isolating sub-populations (J. Groves, 2002). If lowland forest corridors cannot be secured and gorillas are deterred from reaching gorilla groups in other highland sites, inbreeding and loss of genetic variation may imperil isolated groups.
Degradation and decline of habitats
Habitat loss is a major threat to gorillas as forests are rapidly being lost to local illegal logging and subsistence agriculture. In 2000, it was estimated that 135,170 km² of forest remained in Nigeria, with an average annual decrease of forest cover of about 4,000 km² or 2.6 percent. There are logging concessions in almost all forest reserves in Nigeria, although not all are being actively logged. Much illegal logging also occurs. By 1987, around 24 percent of Nigeria’s protected land area had already been converted into farmland, plantations and bush-fallow. In Nigeria and in Cameroon the expansion of agriculture, oil palm plantations, and road networks has led to the widespread degradation and fragmentation of habitat.
Hunting has historically threatened the survival of Cross River Gorillas. In 1989, it was suggested that in Nigeria twice as many were killed each year as were being born (Harcourt et al., 1989). At that time a single gorilla carcass could fetch as much as twice the monthly salary. About 15 communities hunted in the gorilla’s range, and in 1986 one of these alone was reported to have killed eight gorillas. The hunting of gorillas is now much reduced. This is largely due to increased conservation in Nigeria, beginning with a Nigerian Conservation Foundation (NCF) project, followed by the Okwangwo programme of the WWF and, most recently, by a Wildlife Conservation Society (WCS) programme.
The bushmeat trade
Much recent concern has been focused on the bushmeat trade. Forest is being converted for crop production and livestock in many parts of Africa. Where new routes are opened up for timber or mineral extraction, exploitation of forest animals for food use (bushmeat) rises in order to support the incoming labour force and to export bushmeat to urban markets. Although bushmeat has been traditionally important in many regions, the impact of bushmeat hunting is now much greater because it is expanding rapidly with access to remote areas becoming easier. Also, new markets have grown to serve rising demand among urban populations, where it is considered a delicacy. Gorilla meat forms only a small proportion of the commercial bushmeat trade, but the impact on ape populations is disproportionately great because of their slow reproductive rate and the social consequences of killings of silverbacks (infanticide may ensue when nursing mothers join a new male).
Other forms of direct exploitation
Gorillas have been killed for consumption, but in Cameroon local tradition dictates that gorilla meat cannot be sold. Bones are used in traditional medicines in both Cameroon and Nigeria and skulls are typically retained as a trophy. Hands or feet do not seem to be particularly valued. Infants have been sold (Nyango at the Limbe Wildlife Center is an example) but it seems to be more of an opportunistic event.
Another potential general threat to gorillas is exposure to human diseases, particularly for habituated gorillas that come into contact with humans, in areas of gorilla tourism (Butynski, 2001). At present, this threat is not great for the Cross River Gorilla, but an evaluation of habituation for ecotourism has recently been completed at Afi Mountain Wildlife Sanctuary (Andrew Dunn, pers. comm.). Strict rules will be needed to regulate tourist contacts with gorillas (Homsy, 1999). Similarly, the WCS Global Health Program is also helping evaluate and suggest ways to minimize the risks of disease transmission between humans and livestock on the one side and the gorillas of the Kagwene Gorilla Sanctuary in Cameroon on the other.
Beside severe impacts on human populations, several outbreaks of the Ebola virus since 2000 might have claimed thousands of great apes in Africa. Ebola hemorrhagic fever is a severe, often fatal disease that affects humans, gorillas and chimpanzees. Many scientists believe the disease is spread through the butchering and handling of primate bushmeat. So far Cross River Gorillas have not been affected by Ebola.
Road development will possibly split the Cross River Gorilla populations in Mone Forest Reserve and the proposed Takamanda National Park. Accidental entrapment in wire snares used to trap other wild animals can also threaten gorillas, this threat needing to be assessed in the Nigeria-Cameroon border region. The potential isolation of some localities and Cross River Gorilla populations have given rise to concerns about inbreeding, but recent genetic data suggest that exchange between subpopulations persists (Bergl & Vigilant, 2007) and that genetic diversity remains at an acceptable level (Bergl et al. 2008). International trade in live gorillas and gorilla parts has declined since the gorilla was listed in Appendix I of CITES.
International trade in live gorillas and gorilla products, formerly a significant threat to the species, has greatly declined since the gorilla was listed on Appendix I of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) in 1977.
Nigeria has ratified the African Convention on the Conservation of Nature and Natural Resources, the
Convention of International Trade in Endangered Species of Wild Fauna and Flora (CITES), and the Convention on Biological Diversity (CBD) as well as the Convention on Migratory Species (CMS). In Nigeria the Endangered Species Act of 1985 is the legal instrument through which the international treaties are enforceable. Law protects all wildlife in National Parks. In Cameroon, forestry and wildlife regulations list gorillas as Category A species, which are fully protected against hunting, capture, or sale. Protected areas such as national parks and wildlife reserves are established under the Direction de la Faune and des Aires Protégées (DFAP) of the Ministry of Forestry and Wildlife (MINFOF).
National protection status
National laws for the control of hunting and capture exist in all countries with gorilla populations, but lack of funds and complex terrain make wide enforcement of this legislation rare. As most Cross River Gorillas occur within forest reserves, wildlife sanctuaries and national parks, they and their habitat have some degree of protection. However, some localities currently lack any formal protection status. A transboundary protected area has been proposed which would unite the Okwangwo Division of Cross River National Park with Cameroon’s (proposed) Takamanda National Park.
International protection status
All gorillas are listed in CITES Appendix I since 1975, and all Range States are Parties. The gorilla is listed on Class A of the African Convention on the Conservation of Nature and Natural Resources (1969).
The Cross River gorilla is part of taxon Gorilla and as such listed on Appendix I of the Convention on Migratory Species (CMS).
Additional protection needs
- Establishment of a transboundary protected area for the Takamanda-Okwangwo complex
- Upgrading the protection status of the Takamanda Forest Reserve
- Developing land-use plans for the Takamanda-Mone-Mbulu area in Cameroon
- Including a network of protected areas and corridors and a plan for the conservation of Afi-Mbe- Okwangwo area in Nigeria
- Including some formal conservation status for the Mbe Mountains (most likely a community wildlife sanctuary)
- Maintenance of forested connections between gorilla habitats
- Strengthening protection and law enforcement measures for all Cross River Gorilla populations
- Maintaining and expanding basic research into the ecology, distribution and population biology of the Cross River Gorilla
- Building the capacity of relevant institutions in Nigeria and Cameroon (including Government departments, universities, NGOs)
- Strengthening and expanding conservation education and awareness programmes at all levels
- Incorporating local community needs into the development of management strategies
- Including the study of alternative livelihoods options
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