The Western Lowland Gorilla (Gorilla gorilla gorilla) is the most widespread of all gorilla subspecies, yet remains the most at risk.
It inhabits the remote lowland rainforests and swamps of Central Africa, specifically in the Republic of Congo, Cameroon, Gabon, Equatorial Guinea, the Central African Republic, and parts of Angola.
Despite its wider distribution, this subspecies faces escalating threats, placing it on the IUCN Red List as Critically Endangered.
The population is estimated to have declined by more than 60% over the last 25 years. The primary drivers of this collapse include habitat degradation, poaching, and viral outbreaks such as Ebola. In some regions, entire gorilla groups have vanished in a single year due to disease transmission.
The absence of reliable population-wide census data makes conservation planning particularly difficult.
The Western Lowland Gorilla (Gorilla gorilla gorilla) has suffered exceptionally high levels of hunting- and disease-induced mortality (over 90% in some areas), which combined are estimated to have caused an overall decline of more than 60% over the last 20 to 25 years. Most protected areas have serious poaching problems and Ebola has hit almost half of the habitat under protected status. Both these threats are not readily mitigated.
Because gorillas are long-lived and their reproductive rates are extremely low (maximum intrinsic rate of increase about 3%, Steklis and Gerald-Steklis 2001), they are particularly susceptible to even low levels of hunting. Moreover, perhaps 20 to 30 years into the future, in addition to on-going habitat loss and degradation, climate change may well become another major threat. (2008 IUCN Red List, Walsh, P.D., Tutin, C.E.G., Baillie, J.E.M., Maisels, F., Stokes & E.J., Gatti, S).
Estimates from the 1980s placed the entire population, which occurs in seven Central African nations, at fewer than 100,000. Since then, scientists believed this number had dwindled by at least half, due to commercial hunting and disease, particularly outbreaks of the Ebola virus, which have extirpated gorillas from a great deal of otherwise intact forest. In early 2008, the estimate of the world’s population of critically endangered Western Lowland Gorillas received a boost with the discovery by WCS teams of large numbers of gorillas in remote and not easily accessible swamp forests in the northern part of the Republic of Congo, bringing the population estimates for north eastern Congo to 125,000. Current estimates of the total population are in the order of 150,000 -200,000 individuals.
The Eastern and Western Gorilla are separated by approximately 1,000 km (Garner & Ryder, 1996). Western and Eastern Gorillas can be distinguished by external features (Groves, 2002), together with clear geographic and morphological distinctions (Garner & Ryder, 1996). Western Lowland Gorillas live in the western Congolese forest, west of the Congo/Oubangi Rivers and south of the Sanaga River.
Scientific name
Gorilla gorilla gorilla (Savage & Wyman, 1847)
Common names
English – Western Lowland Gorilla, Coast Gorilla
French – Gorille de plaine de l’ouest
German – Westlicher Flachlandgorilla
Spanish – Gorila de llanura del oeste ou Gorila de planicie occidental
Description
The shape of the nose provides a point of difference between the eastern and western forms; the western species has an overhanging tip to its nose that is absent in the eastern forms. Western Lowland Gorillas have chestnut-brown, not black, hair on their heads though the extent is variable. Western Gorillas have more rounded faces and slender chests than Eastern Gorillas (Rowe 1996; Nowak 1999). Although predominantly terrestrial, they climb well, if cautiously. The Western Lowland Gorilla is the smallest and lightest of the four subspecies, and also the one in which sexual dimorphism is most pronounced (Gautier & al, 1999).
General Biology
Gorillas are mainly terrestrial. The gorilla’s large size and folivorous diet mean that the animals must spend long hours feeding everyday. Of all the great apes, the gorilla shows the most stable grouping patterns. The same adult individuals travel together for months and usually years at a time. It is because gorillas are mainly foliage eating that they can afford to live in these relatively permanent groups. Foliage, unlike fruit (especially the ripe fruits that the ape gut requires), comes in large patches than can in turn support large groups of animals. In West Africa, where fruits form a far higher proportion of the gorilla’s diet than in the East, gorilla groups tend much more often to split into temporary feeding subgroups than they do in east Africa, as animals range far apart searching for the relatively scarce ripe fruit. Sleeping subgroups have also been reported but are anecdotal and seem to occur in the process of permanent splitting of a multi-male group into two single male groups. Groups usually number 5 to 10 individuals, but some groups can count as many as 20 to 32 animals (Bermejo, 2004).
Gorillas are closely related to humans and are considered highly intelligent. It is possible that western gorillas have a food culture, with learned preferences passed on between individuals and generations (Nishihara, 1995). Use of tools has also recently been observed in the wild (Breuer, 2006).
Natural Habitat and Range
The Western Lowland Gorilla occupies a broad but fragmented range across Central Africa.
Its habitat primarily consists of tropical lowland rainforest, swamp forest, and seasonally flooded riverine systems.
Despite their widespread presence, populations remain isolated by rivers, roads, and human settlements.
Western Lowland Gorillas inhabit dense primary rainforest, swamp forest, thicket, secondary vegetation, forest edges and clearings, riverine forests and abandoned cultivated fields within or adjacent to forest. They have occasionally been observed nesting along savannah-forest edges or in the savannah itself, but do not live permanently in these savannah habitats. They have also been observed feeding in seasonally fruiting trees in coastal forests.
Western Lowland Gorillas inhabit primary and secondary lowland tropical forests at elevations from sea-level to 1,300 m. Gorilla occurrence and density seems to be positively correlated with terrestrial herbaceous vegetation, particularly monocotyledonous plants (e.g. gingers and palms). The average rainfall in their area of distribution is around 1,500 mm with the greatest amount of rain falling between August and November and diminishing during December through March (Poulsen & Clark 2004). Western Gorillas appear to be absent from areas close to human settlement and disturbed secondary forests, avoiding utilized roads and plantations (Tutin & Fernandez, 1984). Gorillas favour areas where edible herbs are more abundant, which is often the case in moderately disturbed or secondary forest.
In the Odzala-Koukoua NP in Congo, Western Lowland Gorillas occupy a large variety of habitats. Here they primarily live in open-canopy forest with a rich understorey of vegetation of Marantaceae. This forest type is dominant in the northeastern part of the park. The ground vegetation is dominated by an almost impenetrable thicket of Marantaceae species, including Haumania liebrechtsiana, Megaphrynium macrostachyum, and Sarcophrynium spp.
Western Lowland Gorillas are also found in more closed-canopy primary forests. In and around the northern part of the Odzala-Koukoua NP there are more than 100 forest clearings. They have a particularly sodium-rich herbaceous vegetation and are known as saline or bais. Gorillas are known to visit these clearings on a regular basis to feed on plants from families such as Cyperaceae and Asteraceae. Swamp forests are now considered important habitats and feeding areas for western gorillas, supporting them in high densities both in the wet and the dry season (Fay et al., 1989). High densities of gorillas in swamp habitat in northern Congo were recently reported at the International Primatological Society Congress (Stokes et al. 2008). In Northern Congo western gorillas favour swamp forests where Raphia, a palm used both for food and nest construction (Blake et al, 1995), is common. In southwestern Central African Republic the distribution of gorillas seems to be influenced by the availability of Afromomum spp. (Carroll, 1988).
Country-by-Country Range Overview
Angola (status unknown)
The Western Lowland Gorilla is found in the northern part of the Cabinda exclave of Angola in the Mayombe forests. Only a few dozen individuals remain.
Cameroon (Critically Endangered)
The nominate form (Gorilla gorilla gorilla) is distributed in Cameroon southeast of the Sanaga River. The vast majority of Cameroon’s gorillas (several thousand) are found in the humid tropical forests of the southeast, with strongholds in the Lobeke, Boumba-Bek and Nki National Parks. There is a small population of gorillas in the Ebo forest, north of the Sanaga, but from genetic information it is suspected that these, and any others north of the Sanaga, may be members of a separate subspecies already known from the Cameroon-Nigeria border, the Cross River Gorilla (G. g. diehli).
Central African Republic (Critically Endangered)
The gorilla is found in the southwestern corner of Central African Republic. It is present in Dzanga-Ndoki National Park and in the Dzanga-Sangha Reserve (ca 5,000 km²). This area is contiguous with good habitats in the Congo (Nouabale-Ndoki National Park and its buffer zone (Sangha Region).
Republic of the Congo (Critically Endangered)
The Western Lowland Gorilla still occurs in the Republic of Congo north of the Equator, particularly in the well-protected areas of the Nouabale-Ndoki National Park and its buffer zone, the Lac Tele Community Reserve (especially in the patches dominated by Raphia swamp forests), and the area east of the Odzala National Park known as the Ngombe-Pikounda area. The Odzala National Park itself, once the stronghold of many thousands of apes, has suffered from the Ebola outbreaks of the early 2000’s in which large numbers of apes have died. Recent surveys by WCS (2007-2008) indicate that perhaps half of the world population of Western Lowland Gorillas lives in the forests of northern Congo.
Democratic Republic of Congo (formerly considered Probably Extinct)
The Western Lowland Gorilla was until very recently considered as probably extinct in the Mayombe forests (Bas-Congo, extreme western DRC). Recent reports (Redmond, 2006) suggest that a small (transboundary) migrating population of a few tens of animals might survive.
Gabon (Critically Endangered)
Western Lowland Gorillas used to occur throughout Gabon’s forests, but in the last two decades their distribution has shrunk alarmingly and about half of the population has disappeared. The northeast of the country was hit by Ebola haemorrhagic fever between 1996-2003 and almost all apes in that area died. In addition they are absent from all areas around larger towns and are quasi-absent in most of the northwest of the country due to hunting pressure.
Equatorial Guinea (Critically Endangered)
The Western Lowland Gorilla still occurs in Rio Muni, the continental part of the country but no recent surveys have taken place. In the 1990s, gorillas occurred in Monte Alen National Park just across the border from the Monts de Cristal in Gabon (Garcia & Mba 1997). In 1999 gorillas were still present in Parque Nacional de Los Altos de Nsork. During a survey by Larison et al (1999), no sign of gorillas was recorded at Rio Campo or Montes Mitra, although, local people claimed they were present.
Actual and potential threats
In evaluating threats to gorillas in western equatorial Africa it is useful to think on two time scales. In the short term, by far the most serious threats are poaching and disease epidemics. In the longer term, habitat loss and disturbance will increase as a threat and are likely to become as serious a threat as hunting and disease. All these traditional threats may be exacerbated by climate change and possible changes in rainfall patterns and temperatures, which may result in significant changes in forest cover.
Although still widely distributed across a large forested region, and occurring in numerous Protected Areas, the Western Lowland Gorilla is listed as Critically Endangered because of a series of cumulative threats of increasing scale: poaching and commercial hunting is identified as a major real or potential threat for every site of the Gorilla’s range. Diseases, and Ebola hemorrhagic fever epidemics in particular, are identified as a potent actual threat and a potential future threat to gorillas (particularly where they are found at high densities, which facilitates transmission). Logging has emerged as one of the greatest future threats, especially in concessions surrounding protected areas, the roads and transport opportunities created by industrial logging leading to a massive increase in commercial bushmeat hunting. Commercial and artisanal mineral exploitation affect some areas and require specific responses.
Direct exploitation
Western Lowland Gorillas are hunted for their meat, for sale to private collections, for trophies and for traditional ritual or medical purposes. Although this is illegal everywhere, the regulations in most range states are poorly enforced. The intensity of hunting of Western Lowland Gorillas varies throughout their range. Factors affecting bushmeat hunting are local taboos, poor enforcement of legislation and the wide availability of ammunition and guns. Logging roads allow greater access to remote areas, making the entire fauna more vulnerable to hunting in areas under exploitation or previously exploited.
The bushmeat trade
In the forested region of western Africa, bushmeat hunting for subsistence is a major threat to western gorillas. Although bushmeat in general has always been culturally and nutritionally important in many regions, the impact of bushmeat hunting is now more widespread and serious. Hunting is increasing rapidly with improved access to remote areas, and new markets are developing to serve a rising demand among urban human populations, where bushmeat 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 silverbacks being killed (infanticide may ensue when nursing mothers join a new male).
There are no estimates of either the overall Western Lowland Gorilla population losses due to hunting, nor of its impact on population trends, although the negative impact of hunting on other gorilla populations is well known. In the northeast of Congo (Motaba River region) it has been estimated that 5 percent of the Western Lowland Gorilla population is killed each year by hunters, despite the low density of local human populations. This level of offtake is unsustainable (Kano & Asato, 1994). In contrast, effective controls of hunting are in place in Nouabaé-Ndoki NP (Congo) and its buffer zone, in Odzala – Koukoua NP (Congo) and in the Dzanga Sector of Dzanga-Ndoki NP (Cameroon) where gorillas are rarely hunted (Blom et al., 2001). The controls are the result of successful collaboration between national administration and international agencies or NGOs.
Other forms of direct exploitation
In the past, gorillas have been killed for their heads, hands and feet, which were sold to collectors. This probably grew out of expatriates showing interest in gorilla body parts on sale in traditional medicine stalls (Redmond, 1989). Infants were sold to zoos, researchers and people who wanted them as pets. Both these trades – live infants and body parts for traditional medicine or amulets – continue to this day, either as a by-product of bushmeat hunting or sometimes, when a trader commissions hunters to kill a gorilla and/or capture an infant, in the erroneous belief that there is a market for great apes.
The abduction of infants generally involves the death of at least two adults, as members of a group will fight to protect their young. Gorilla infants are susceptible to post-traumatic stress and poor nutrition; as a result, mortality rates of 80 per cent have been reported among confiscated infants (Redmond, 1989). These losses create a startling multiplier effect, because if four out of five infants die, and each infant is captured by killing two adults, this means that every infant that survives probably represents a loss of 15 gorillas to the population (5×2 dead adults, four dead infants and one live one).
Diseases
Disease is a potentially devastating threat to gorillas and other great apes. Gorillas are susceptible to many of the same diseases as humans, such as Ebola, common colds, pneumonia, smallpox, chicken pox, tuberculosis, measles, rubella, yellow fever and yaws. Outbreaks of the Ebola virus since 2000 have claimed thousands of great apes in Africa. Ebola haemorrhagic fever is a highly infectious, usually fatal disease that affects humans and non-human primates, such as monkeys, gorillas and chimpanzees. Many scientists believe Ebola is transmitted to humans through the butchering and handling of primate bushmeat. This disease has been confirmed in six African nations: the Democratic Republic of Congo, the Republic of Congo, Gabon, Sudan, Ivory Coast, and Uganda.
Ebola haemorrhagic fever is an incurable human disease that kills about 80 percent of its human victims. This virus has an even higher mortality rate of 95-99 percent among western gorillas (and chimpanzees). Recent Ebola epidemics in western equatorial Africa have affected the gorilla in Gabon and Congo. Ebola outbreaks are thought to have strongly contributed to the decline of great ape populations in Gabon, where four outbreaks are known to have occurred. Farther East, declines in Western Gorilla populations attributed to Ebola have also been reported in the Lossi Gorilla Sanctuary of Congo (Walsh et al., 2003; Bermejo et al., 2006), and has lately decimated a gorilla population in the Odzala-Koukoua NP (Caillaud et al., 2006; Devos et al., 2008).
Another potential general threat to gorillas is exposure to human diseases (e.g Graczyk et al., 2001; Graczyk et al., 2003; Mudakikwa, 2001) particularly for habituated gorillas that come into contact with humans, in areas of gorilla tourism (Butynski, 2001). Gorilla tourism exposes gorillas to humans, sometimes close enough for droplet infection by sneezing or coughing, and hence to any diseases that humans may be carrying, some of which the gorillas may never have been exposed to before (Homsy, 1999).
At present, this threat is not significant for the Western Lowland Gorilla. The Western Lowland Gorilla has proved to be difficult to habituate, particularly as the dense vegetation of its habitat does not allow it to be tracked easily (Williamson & Feistner, 2003). Gorilla tourism is therefore not as well established as it is with eastern gorillas. However the discovery that Western Lowland Gorilla could easily be seen at bais has increased the likelihood of successful gorilla tourism and could, if unregulated, lead to increased contacts with humans as happens in Rwanda or Uganda. Strict regulations concerning tourist group size, health, visiting hours and behaviour must be adopted as soon as possible in areas where Western Lowland Gorilla tourism is being, or planned to be established.
Degradation and decline of habitats
Throughout the Western Lowland Gorilla’s range, the forests on which it depends for survival are being cut down for timber and to make way for agriculture. As the global demand for palm-oil continues to rise, both for food and bio-fuels, land to be converted into oil palm plantations is now being sought in Africa. Until recently, there has been relatively little habitat degradation over much of the Congo Basin, with low conversion to agricultural land. As late as the 1980s West and Central African timber was considered to be of low commercial value, which limited the pressure, posed by selective logging. This situation changed dramatically during the 1990s.
By 2000, more than half of Gabon’s forests had been allocated as logging concessions (Anon, 1999). In Cameroon, over 170,000 km² of the country’s forests have either been logged or allocated for logging. Satellite images have revealed that networks of new logging roads have now spread into what had previously been considered the least accessible forests in the country (Minnemeyer et al., 2002). Logging roads and access routes fragment forest and improve access for hunters. Forest fragmentation poses a potential threat to Western Lowland Gorillas in that it can block transfers between groups and access to seasonal food resources.
Impact of Conflict
The impacts of wars and political conflicts, particularly well documented for the Mountain and Eastern Lowland Gorilla, could have affected the Western Lowland Gorilla in a similar way. Civil wars and unrest increase hunting levels by exacerbating poverty and dependence on natural resources, particularly among displaced peoples and refugees. Angola, the Republic of Congo, Central African Republic and the DRC have all suffered periods of instability in recent decades. In addition to influxes of refugees, the forests have served as hiding places and retreats for rebel forces, leading to disturbance and hunting. This is a common phenomenon in times of war, particularly in forests close to international borders.
Other threats
There is a lack of fundamental knowledge regarding numbers and distribution, and insufficiency in biological information crucial to assessing population viability and mechanisms of population decline. Accidental entrapment in wire snares used to trap other wild animals is also an ever-present threat. International trade in live gorillas and gorilla parts, which used to be a major threat, has declined since the gorilla was listed in Appendix I of CITES, but cases are still reported.
Anti-poaching and law enforcement
The taxon is legally protected in all the seven countries of its distribution. The killing or sale of live or dead gorillas or of their body parts, and the disturbance of them in the wild is illegal throughout their range. Nevertheless, poaching and the illegal capture of live individuals are an important problem in all of these countries. Great efforts must be made by the range states to enforce their laws concerning gorillas, including not only effective anti-poaching and seizure of gorilla parts or live individuals on the ground, but follow-up of the legal process through arrest, the passage of each case through the tribunal, and eventually to prosecution of all cases judged guilty.
Maintaining transboundary links for gorillas
Although current habitats are not as fragmented in Central Africa as in West Africa or the Albertine Rift, they will become so in the next few decades. Particular attention to protecting existing transboundary protected areas, the creation of new ones, and the protection of intact forest corridors between range states will allow the long-term maintenance of genetic flow between gorillas living in neighbouring countries.
Additional remarks
Western Lowland Gorillas are a keystone species in their forest habitat, and so their protection is essential to the long-term management of the Congo basin, now recognized as a globally important factor in inter-continental weather patterns and for maintaining climate stability. The COMIFAC Convergence Plan and the efforts of the Congo Basin Forest Partnership, which aim for the coordinated management of the whole Congo Basin Ecosystem, must recognize that gorilla conservation is integral to their programme of work. Now that payment for the eco-system services provided by the Congo Basin to the rest of the planet – including carbon sequestration and storage, rainfall generation and bio-diversity – is being seriously considered, it is essential that the ecological role of gorillas is taken into account.
Regulatory Provisions
International
CMS: Gorillas listed on Appendix I of the CMS since 2005.
CITES: The Gorilla has been on the Appendix I of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) since 1975.
ACCNNR : Gorilla is also listed in A class in the African Convention on the Conservation of Nature and Natural Resources in 1969.
The Gorilla Agreement: a treaty under the CMS, negotiated in October 2007, came into force 1st June 2008.
National
National laws for control of hunting and capture exist in all countries with gorilla populations, but a lack of funds and inaccessibility make widespread enforcement of this legislation rare.
In Angola, biodiversity protection and management are under the responsibility of the Ministry of Urban Affairs and Environment, but in practice the Forestry Development Institute (IFD) remains in overall charge of the forest sector, sharing with the National Directorate of Agriculture and Forest (DNAF) responsibilities of policy formulation and guidance. Poor enforcement of laws, poaching, harvesting and settlements inside Protected Areas are other problems.
In Cameroon, the country’s forestry, wildlife and fishery regulations list gorillas as a Category A species, which are fully protected by law against hunting, capture or sale. Protected areas such as national parks and wildlife reserves are established under the auspices of the “Direction de la Faune et des Aires Protégées” (DFAP) of the Ministry of Environment and Forestry (MINEF), which is also responsible for the protection of the country’s biodiversity in general.
In the Central African Republic, the Ministry of the Environment, Waters, Forests, Hunting and Fishing is responsible for wildlife conservation and the use of natural resources in CAR. These are governed by Ordinance n°84.045 (1984) and Law n°90.003 (1990). Great Apes are listed in Category A as ‘completely protected’ under Ordinance N° 84.045.
In the Republic of the Congo, the Ministry of Forest Economy and the Environment (MFEE) is responsible for wildlife conservation and regulated use, including the management of protected areas.
In Gabon the Ministry of Water and Forests is responsible for the management of natural resources. Gabon set up a wildlife management service and an anti-poaching service. Gorillas (and chimpanzees) are now fully protected species under Gabonese law.
In Equatorial Guinea, conservation issues are administered by the Instituto Nacional de Desarrollo Forestal y Gestion del Sistema de Areas Protegidas (INDEFOR), within the Ministry of Infrastructure and Forests. Many of the priority populations identified by GRASP for Western Lowland Gorillas are at least partly protected and occur within (proposed) National Parks, Biosphere Reserves or Community Managed Nature Reserves, but many are also, at least partly, in logging concessions.