Mountain, or eastern gorillas, Gorilla beringei, are found in the Virunga volcanoes that separate the Democratic Republic of Congo from Rwanda and Uganda.
Biogeographic Regions: ethiopian (Native )
G. b. beringei (Matschie, 1903) is found in the Virunga Volcanoes region, an area of 440 km² straddling the border between Uganda (Mgahinga Gorilla National Park), Rwanda (Volcanoes National Park), and DRC (Virunga National Park), and also in the 330 km² Bwindi Impenetrable National Park in southwestern Uganda (Schaller 1963; Butynski 2001).
G. b. graueri (Matschie, 1914) is endemic to eastern DRC, and is found from the lowlands east of the Lualaba River and the Mitumba Range from Mount Tshiaberimu in the north of Virunga National Park, south to the Itombwe Massif, and formerly even further south in the area west of Fizi on the escarpment west of Lake Tanganyika (Schaller 1963; Butynski 2001; Mehlman 2008). The southern limit of the current Grauer’s Gorilla range has been extended by the discovery in late 2007 of a hitherto unreported population in the Hewa Bora region east of Kilembwe in Fizi District (J. Hart, pers comm.).
Gorillas are the largest primate, with average lengths of 150 cm for females and 185 cm for males. They are highly sexually dimorphic, with females weighing 70 to 114 kg and males averaging 160 kg. They have robust bodies, long muscular arms, short legs, massive heads, and males have large, sharp canine teeth. Mountain gorilla coats are silky and long, ranging in color from blue-black to brownish-grey. Mature males develop a large patch of silver or grey hair on their backs, giving them the name silverbacks. Males also have apocrine glands in their armpits that emit a strong odor when the animal is under stress.
Mountain gorillas differ from other gorillas in having longer hair, larger jaws and teeth, smaller nose, and shorter arms.
Range mass: 70 to 200 kg.
Range length: 150 to 185 cm.
Other Physical Features: endothermic ; homoiothermic; bilateral symmetry
Sexual Dimorphism: male larger
Mountain gorillas inhabit the montane cloud forest of the Virunga range. Occasionally they go into the afro-alpine meadows (4,000 m) where temperatures are subfreezing at night and there is little suitable food to forage on.
Range elevation: 4000 (high) m.
Habitat Regions: tropical ; terrestrial
Terrestrial Biomes: rainforest ; mountains
Habitat and Ecology
Habitat and Ecology
G. b. graueri is distributed from lowland tropical rainforest habitat through transitional forests to Afromontane habitat (500–2800 m). G. b. graueri has a different diet from that of G. b. beringei, largely due to differences in what plant species are present, but they also feed predominantly on herbaceous vegetation, as well as on fruit from many species (Ferriss et al. 2005; Yamagiwa et al. 2005).
Mountain gorillas occasionally eat invertebrates, but they are primarily folivorous. They eat the roots, leaves, stems, and pith of herbs, vines, shrubs, and bamboo. Their diet is supplemented by small amounts of bark, wood, roots, flowers, fruit, fungi, epithelium stripped from roots, galls, invertebrates, and gorilla dung.
Animal Foods: insects
Plant Foods: leaves; roots and tubers; wood, bark, or stems; fruit
Other Foods: fungus; dung
Primary Diet: herbivore (Folivore )
These animals may be important in structuring plant communities, as they feed heavily on vegetation.
These animals are very large, and live in regions where not many potential predators exist. It is not likely that they fall prey to any particular species with any regularity.
Life History and Behavior
All primates have complex patterns of communication. Gorillas are known to use vocalizations to communicate with one another. Tactile communication, in the form of grooming, play, and sexual contact, also occurs. Males emit a strong odor when stressed, which appears to function as a type of chemical communication. In addition to these, gorillas use body postures and facial expressions, as well as other visual signals, to communicate with one another.
Communication Channels: visual ; tactile ; acoustic ; chemical
Perception Channels: visual ; tactile ; acoustic ; chemical
Gorillas can reach ages of 40 to 50 years.
Status: wild: 50 (high) years.
Mountain gorillas are polygynous; the dominant male in each group has exclusive access to all the females in the group.
Mating System: polygynous
Reproductive rates are slow and a female may leave only 2 to 6 offspring over a 40 year life-span. Males that have a harems of 3 to 4 females increase their reproductive output by fathering 10 to 20 offspring over 50 years. These animals don't mature sexually until well into their teens.
Mating behavior is initiated by the female, with a series of slow and hesitant approaches to the male. A female is receptive only during estrus, and she will cease to ovulate for several years after giving birth. The length of the estrous cycle of a female mountain gorilla is 28 days, and there is no visible external menstrual flow.
A single, dependent young is born after a eight and a half month gestation period. Weaning often doesn't occur until three years of age, and juveniles may remain with mothers for years after that. Females are sexually mature by 10 years of age, but males are unlikely to start breeding before 15 years. Reproductive output for females is about one surviving offspring every 8 years (survival implying reaching breeding age).
Breeding interval: The interval between reproductive events depends upon infant survival. Females are capable of producing an infant every 4 to 5 years.
Breeding season: These animals breed throughout the year.
Average number of offspring: 1.
Average gestation period: 8.5 months.
Range weaning age: 36 to 48 months.
Average age at sexual or reproductive maturity (female): 10 years.
Average age at sexual or reproductive maturity (male): 15 years.
Key Reproductive Features: iteroparous ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization ; viviparous
Females provide most of the parental care in this species. Females nurse and carry their young for about 4 years. They also play with the young, teach them, and groom them.
The role of males in parental care is less direct, although no less important. Males protect the females and the young within their social group from potentially infanticidal rival males who may take control of the group.
Parental Investment: altricial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Male, Female); pre-weaning/fledging (Provisioning: Female, Protecting: Male, Female); pre-independence (Protecting: Male, Female); extended period of juvenile learning
Mountain gorillas are highly endangered. This is due both to habitat destruction and severe poaching pressures. Gorilla species are subjected to heavy pressure from poaching for body parts and for young animals collected for zoos and private collections, generating illegal income. The civil war occurring in the region they inhabit has only added to their plight, increasing mortality through accidents and the breakdown of patrol units against poachers.
US Federal List: endangered
CITES: appendix i
IUCN Red List of Threatened Species: critically endangered
IUCN Red List Assessment
Red List Category
Red List Criteria
- 2000Critically Endangered
- 1996Critically Endangered
- 1996Critically Endangered
- 1994Endangered(Groombridge 1994)
- 1990Endangered(IUCN 1990)
- 1988Endangered(IUCN Conservation Monitoring Centre 1988)
- 1986Endangered(IUCN Conservation Monitoring Centre 1986)
IUCN Red List Assessment
Red List Category
Red List Criteria
The Mountain Gorilla subspecies is found in only two isolated subpopulations in Rwanda, Uganda, and the DRC. The Virunga subpopulation was estimated at 380 individuals in 2003, an increase from 320 in 1989 (Gray et al. 2006). Approximately half of the subpopulation is mature individuals (Kalpers et al. 2003; Gray et al. 2006). However, all population growth in the Virungas between 1989 and 2003 has been limited to one sector of the population, the four gorilla groups in perhaps ecologically the richest area, which is also relatively well protected (Kalpers et al. 2003; Gray et al. 2006). Not only do unhabituated (and therefore less well-protected) groups have a lower ratio of juveniles to adults, but the current rate of growth of the whole population is lower than that during the 1980s (Kalpers et al. 2003). Additionally, a resurgence in poaching and killing of gorillas (approximately 3% of the entire Virunga subpopulation in 2007: see below) directly limits population growth and emphasizes the fragile nature of this small population.
While the G. b. beringei subpopulation in Bwindi Impenetrable National Park was believed to have increased from about 300 gorillas in 1997 to 320 individuals in 2003 (McNeilage et al. 2006), a census in 2006 that combined genetic analysis of the entire population with traditional census methods revealed that there are only approximately 300 individuals in Bwindi (Guschanski et al. in review). These new results do not lead to the conclusion that the population has declined in size; instead, due to the ‘sweep census’ method used, it is not possible to put error estimates around the population estimates and therefore it is difficult to assess how the population size has been changing over time.
In total, the subspecies G. b. beringei has only approximately 680 individuals remaining in two isolated populations.
G. b. graueri
In 1995, the population of G. b. graueri was estimated at 16,900 animals (Hall, Saltonstall et al. 1998; Hall, White et al. 1998). In the last decade, it is believed that the total population has declined dramatically, as the lowland populations have been progressively fragmented and reduced (Hart and Liengola 2005; Hart et al. 2007). Many populations have disappeared in the last 30 years (comparing Schaller 1963 and Hall, Saltonstall et al. 1998); for example, Itombwe lost about half of its subpopulations between 1960 and 1996 (Omari et al. 1999). Their habitat continues to become fragmented and discontinuous; the current occupancy range for Grauer’s Gorilla is estimated at 21,600 km², a decline of 25% from surveys completed in 1959 (Mehlman 2008). However, data are lacking to determine the extent of decline, apart from in the uplands of Kahuzi-Biega National Park, where the population dropped from an estimated 270 in 1996 to only 170 animals in 2000 (WCS 2000).
G. b. beringei
For the Mountain Gorilla, G. b. beringei, the Virunga Volcanoes region and Bwindi Impenetrable NP are surrounded by some of the highest human densities in Africa (Plumptre and Williamson 2001; CIESIN and CIAT 2005) in countries with some of the fastest increasing human populations in Africa (World Resources Institute 2007). So many people with so much need for land pose significant threats to both subpopulations of Mountain Gorilla. While a key conservation strategy for both subpopulations of Mountain Gorillas is tourism, there is concern about the risk of disease transmission and disturbance to the gorillas, both of which could jeopardize these conservation programmes (e.g. Butynski and Kalina 1998; Homsy 1999). Transmission of infectious disease agents has been proven among habituated wild gorillas, people, domestic animals and other wild animals. Although not yet documented in gorillas, human-origin viral respiratory disease has recently been shown to cause high mortality among habituated chimpanzees (Köndgen et al. 2008). However, overall, Mountain Gorillas visited by researchers and tourists have consistently done better than those not visited, due to the level of protection afforded to areas and groups that are monitored daily (Harcourt 1986; Weber 1993). For instance, in both the 1981 census and 2003 census of the Virunga gorillas, the ratio of immatures to adults were higher in gorilla groups visited by researchers or tourists than in groups not visited (Harcourt et al. 1983; Gray et al. 2006), at 0.4 vs. 0.6 juveniles per non-silverback adult in 2001 (Gray et al. 2006, Table 3). Nevertheless, the threats remain considerable, and intensive conservation activities must continue.
The Virunga subpopulation of G. b. beringei suffered numerous impacts from more than a decade of war and instability in the region (Plumptre and Williamson 2001). Threats included incursions by militia, habitat destruction for firewood and farmland, illegal cattle grazing, illegal timber extraction, and illegal hunting, including snares set for other mammals such as antelope that can injure or kill gorillas. In 2004, 15 km² was deforested for conversion to farmland (NASA 2005) and recently there has been a sharp increase in timber extraction for the illegal production of charcoal. There has also been a resurgence of poaching for the illegal pet trade and bushmeat (Kalpers et al. 2003) and since 2003, 12 orphans (both Grauer’s and Mountain Gorillas) have been confiscated and taken into the care of veterinarians. In 2007, at least eight gorillas were shot dead in three incidents in Virunga NP (Williamson and Fawcett 2008). These losses amount to about 3% of the Virunga subpopulation. Since September, the Mikeno sector, where DRC’s Mountain Gorillas are found, has been under rebel control and park authorities have been prevented from monitoring the gorillas. The failure of the 2008 Peace Conference means that the region remains volatile and the gorillas vulnerable, despite the efforts of international NGOs and UN observers.
Threats to the Bwindi subpopulation of G. b. beringei include illegal use of forest resources (poaching, pit-sawing, firewood collection, etc.), encroachment and demand for land, human-induced fires, invasive exotic species and human-wildlife disease transmission (McNeilage et al. 2006). The forest is also recovering from high levels of timber extraction, gold-mining, encroachment and poaching that occurred prior to designation of National Park status in 1991.
G. b. graueri
In eastern DRC, Grauer’s Gorillas face substantial threats to their survival: agriculture and pastoral activities are leading to massive loss and fragmentation of forest habitat (as noted already, the current occupancy range for Grauer’s Gorilla is approximately 21,600 km², a decline of 25% since 1959; Mehlman 2008); widespread illegal mining activities in the forests increase demand for bushmeat, including consumption of gorillas; and illegal capture of infants (and concomitant killing of group members), which has increased substantially since 2002. Ongoing political unrest and military activity, including occupation of national parks, and killing of gorillas for food, have compounded the problems (Hall, Saltonstall et al. 1998; Plumptre et al. 2003; Yamagiwa 1999, 2003). At present, there is no commercial logging in the Grauer’s range, but there are continuous low-level extractive activities (charcoal production, bamboo harvesting and wood cutting), which put further stress on the habitat (J. Hart pers. comm. 2007). As some of the country emerges from civil war, new concessions for timber, minerals, and possibly petroleum will pose conservation challenges for the future (Caldecott and Miles 2005, Ch. 16).
The Eastern Gorilla is listed under Class A of the African Convention and Appendix I of CITES. The subspecies is found only within National Parks. These protected areas all have active national programmes for conservation management, assisted by international NGOs. Although the protected areas are relatively well monitored, measures of the impacts of illegal activities on the gorillas should continue.
Most subpopulations of G. b. graueri are found in protected areas, where international NGOs are supporting rehabilitation and conservation programmes, such as in Kahuzi-Biega NP, Maïko NP, Tayna Nature Reserve and Kisimba-Ikobo Nature Reserve. However, due to the presence of armed militia groups in some areas, conservation activities sometimes require assistance from the United Nations Mission in the Congo (MONUC). Efforts are underway to establish up-to-date distribution, abundance, and threats to improve conservation management. It is important to identify key populations of G. b. graueri and continue to provide active protection. Work must continue to document the post-conflict distribution, abundance and conservation status of Grauer’s Gorilla throughout its range. Efforts must also be made to support and maintain active protection for Grauer’s Gorilla where it is already established, while simultaneously developing and mobilizing conservation activities in the more remote and inaccessible sectors of its range.
Relevance to Humans and Ecosystems
There is continuing political pressure to convert the remaining gorilla reserves into areas for farming or commercial use. Due to the high population density, many people feel that the land would be better put to commercial use.
Gorillas may be visited by ecotourists, enhancing local economies.
Positive Impacts: body parts are source of valuable material; ecotourism ; research and education
The eastern gorilla (Gorilla beringei) is a species of the genus Gorilla and the largest living primate. At present, the species is subdivided into two subspecies. The eastern lowland gorilla (G. b. graueri) is the most populous, at about 5,000 individuals. The mountain gorilla (G. b. beringei) has only about 700 individuals. In addition, scientists are considering elevating the Bwindi gorilla population (which numbers about half of the mountain gorilla population) to the rank of subspecies.
Taxonomy and phylogeny
There are at least two subspecies of the eastern gorilla: the mountain gorilla (Gorilla beringei beringei) of the volcanic slopes of Rwanda, Uganda and eastern Democratic Republic of Congo; and the eastern lowland gorilla (Gorilla beringei graueri) of the lowlands of eastern Democratic Republic of Congo and Uganda. A small population from the Bwindi Impenetrable Forest in southern Uganda and adjacent areas in Congo differs genetically from the other subspecies, and is therefore often considered as a separate, yet undescribed, subspecies.
The eastern lowland gorilla and mountain gorilla were previously thought to be two of the three subspecies of one single species, the gorilla (Gorilla gorilla). However, genetic research has shown that the two eastern subspecies are far more closely related than the western subspecies: the western lowland gorilla (G. gorilla gorilla), which justified the separate classification. The two eastern subspecies are now classified as G. beringei.
The eastern gorilla is a large hominid with a large head, broad chest, and long arms. It has a flat nose with large nostrils. The face, hands, feet and breast are bald. The fur is mainly black, but adult males have a silvery "saddle" on their back. When the gorilla gets older, the entire fur becomes grayish, much like the gray hair of elderly people. This is why the older males are sometimes called silverbacks. The eastern lowland gorilla has a shorter, thicker, deep black fur, while the mountain gorilla has a more bluish color. The mountain gorilla is slightly smaller and lighter than the eastern lowland gorilla, but still larger and heavier than the western lowland gorilla and the Cross River gorilla. Males are much larger than females. A full-grown adult male Eastern gorilla typically weighs 140–205.5 kg (309–453 lb) and stands 1.7 m (5.6 ft) upright and a female typically weighs 90–100 kg (200–220 lb) and stands 1.5 m (4.9 ft) tall. The tallest silverback recorded was a 1.94-metre (6.4 ft) individual shot in Alimbongo, northern Kivu in May 1938. The heaviest gorilla recorded was a 1.83-metre (6.0 ft) silverback shot in Ambam, Cameroon which weighed about 266 kilograms (586 lb), although the latter area is within the range of the western gorilla, far outside that of the eastern gorilla.
Distribution and ecology
The eastern gorilla occurs in the lowland and mountain rainforests and subalpine forests of eastern Democratic Republic of the Congo, southwestern Uganda and Rwanda, within the triangle between the Lualaba River, Lake Edward and Lake Tanganyika. The eastern gorilla prefers forests with a substrate of dense plant material.
Eastern gorillas are herbivorous, with a heavily foliage based diet . They have smaller home ranges than western gorillas as foliage is more abundant than fruit. They are diurnal but the majority of foraging occurs in the morning and late afternoon. At night they build nests by folding over vegetation, usually on the ground.
Eastern gorillas live in stable, cohesive family groups, led by a dominant silverback male. Eastern gorillas tend to have larger group sizes than their western relatives, numbering up to 35 individuals. There is no distinct breeding season and females give birth only once every 3-4 years due to the long period of parental care and a gestation period of 8.5 months. Newborn gorillas have greyish-pink skin and can crawl after 9 weeks; they are not fully weaned until 3.5 years. Males defend their females and offspring using their large size in intimidating displays involving charging and chest-beating.
The eastern gorilla is the rarer though less threatened of the two gorilla species. The hunt for bushmeat and the decline in suitable habitat as a result of intensifying forestry and the development of agriculture form the most important threats for the species. In some national parks, expeditions in search for mountain gorillas are a popular tourist attraction. This has both advantages (environmental awareness, financial benefit) and disadvantages (disturbance of natural behavior) for the conservation of the gorillas.
As opposed to the western lowland gorilla, the eastern gorilla is seldom found in zoos. The Antwerp Zoo is the only zoo outside the native range of the species that has eastern lowland gorillas (two older females). Outside the native range, the mountain gorilla is not held in captivity at all. Small groups consisting of animals confiscated from poachers are kept in the Democratic Republic of Congo: Eastern lowland gorillas at the Gorilla Rehabilitation and Conservation Education (GRACE) center in Tayna Nature Reserve, and mountain gorillas at the Senkwekwe Center in Virunga National Park.
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- "Gorillas on Thin Ice". United Nations Environment Programme. 15 January 2009. Retrieved 19 May 2010.: "The Eastern Lowland Gorilla population in the DRC has plummeted dramatically over the last 10 years, with probably only about 5,000 of the formerly 17,000 animals remaining."
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- Senkwekwe Orphan Mountain Gorilla Center. Retrieved 16 August 2013