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The mountain gorilla (Gorilla beringei beringei) is one of the two subspecies of the eastern gorilla. There are two populations. One is found in the Virunga volcanic mountains of Central Africa, within three National Parks: Mgahinga, in south-west Uganda; Volcanoes, in north-west Rwanda; and Virunga in the eastern Democratic Republic of Congo (DRC). It is listed as critically endangered by the IUCN. The other is found in Uganda's Bwindi Impenetrable National Park. Some primatologists consider the Bwindi population in Uganda may be a separate subspecies,[3] though no description has been finalized. As of September 2016, the estimated number of mountain gorillas remaining is about 880.[4]

 

Gorilla taxonomy

Mountain gorillas are descendants of ancestral monkeys and apes found in Africa and Arabia during the start of the Oligocene epoch (34-24 million years ago). The fossil record provides evidence of the hominoid primates (apes) found in east Africa about 22–32 million years ago. The fossil record of the area where mountain gorillas live is particularly poor and so its evolutionary history is not clear.[5] It was about 9 million years ago that the group of primates that were to evolve into gorillas split from their common ancestor with humans and chimps; this is when the genus Gorilla emerged. It is not certain what this early relative of the gorilla was, but it is traced back to the early ape Proconsul africanus.[6] Mountain gorillas have been isolated from eastern lowland gorillas for about 400,000 years and these two taxa separated from their western counterparts approximately 2 million years ago.[7] There has been considerable and as yet unresolved debate over the classification of mountain gorillas. The genus was first referenced as Troglodytes in 1847, but renamed to Gorilla in 1852. It was not until 1967 that the taxonomist Colin Groves proposed that all gorillas be regarded as one species (Gorilla gorilla) with three sub-species Gorilla gorilla gorilla (western lowland gorilla), Gorilla gorilla graueri (lowland gorillas found west of the Virungas) and Gorilla gorilla beringei (mountain gorillas including, Gorilla beringei found in the Virungas and Bwindi). In 2003 after a review they were divided into two species (Gorilla gorilla and Gorilla beringei) by The World Conservation Union (IUCN).[5]

 

Physical description

Silverback of Ntambara group, in typical resting attitude.

The fur of the mountain gorilla, often thicker and longer than that of other gorilla species, enables them to live in colder temperatures.[8] Gorillas can be identified by nose prints unique to each individual.[9] Males, at a mean weight of 195 kg (430 lb) upright standing height of 150 cm (59 in) usually weigh twice as much as the females, at a mean of 100 kg (220 lb) and a height of 130 cm (51 in).[10] This subspecies is on average the second largest species of primate; only the eastern lowland gorilla, the other subspecies of eastern gorilla, is larger.[citation needed] Adult males have more pronounced bony crests on the top and back of their skulls, giving their heads a more conical shape. These crests anchor the powerful temporalis muscles, which attach to the lower jaw (mandible). Adult females also have these crests, but they are less pronounced.[9] Like all gorillas they feature dark brown eyes framed by a black ring around the iris. Adult males are called silverbacks because a saddle of gray or silver-colored hair develops on their backs with age. The hair on their backs is shorter than on most other body parts, and their arm hair is especially long. Fully erect, males reach 1.9 m (6 ft 3 in) in height, with an arm span of 2.6 m (8 ft 6 in) and weigh 220 kg (490 lb).[11] The tallest silverback recorded was a 1.94 m (6 ft 4 in) with an arm span of 2.7 m (8 ft 10 in), a chest of 1.98 m (6 ft 6 in), and a weight of 219 kg (483 lb), shot in Alimbongo, northern Kivu in May 1938. There is an unconfirmed record of another individual, shot in 1932, that was 2.06 m (6 ft 9 in) and weighed 218.6 kg (482 lb).

The mountain gorilla is primarily terrestrial and quadrupedal. However, it will climb into fruiting trees if the branches can carry its weight, and it is capable of running bipedally up to 6 m (20 ft).[citation needed] Like all great apes other than humans, its arms are longer than its legs. It moves by knuckle-walking (like the common chimpanzee, but unlike the bonobo and both orangutan species), supporting its weight on the backs of its curved fingers rather than its palms.[citation needed]

The mountain gorilla is diurnal, most active between 6:00 a.m. and 6:00 p.m.[citation needed] Many of these hours are spent eating, as large quantities of food are needed to sustain its massive bulk. It forages in early morning, rests during the late morning and around midday, and in the afternoon it forages again before resting at night. Each gorilla builds a nest from surrounding vegetation to sleep in, constructing a new one every evening. Only infants sleep in the same nest as their mothers. They leave their sleeping sites when the sun rises at around 6 am, except when it is cold and overcast; then they often stay longer in their nests.[12]

 

Habitat and ecology

Adult male feeding on insects in a rotting tree trunk

The mountain gorilla inhabits the Albertine Rift montane cloud forests and of the Virunga Volcanoes, ranging in altitude from 2,200–4,300 metres (7,200–14,100 ft). Most are found on the slopes of three of the dormant volcanoes: Karisimbi, Mikeno, and Visoke.[13] The vegetation is very dense at the bottom of the mountains, becoming more sparse at higher elevations, and the forests where the mountain gorilla lives are often cloudy, misty and cold.[14]

The mountain gorilla is primarily a herbivore; the majority of its diet is composed of the leaves, shoots and stems (85.8%) of 142 plant species. It also feeds on bark (6.9%), roots (3.3%), flowers (2.3%), and fruit (1.7%), as well as small invertebrates. (0.1%).[15] Adult males can eat up to 34 kilograms (75 lb) of vegetation a day, while a female can eat as much as 18 kilograms (40 lb).[citation needed]

The home range size (the area used by one group of gorillas during one year) is influenced by availability of food sources and usually includes several vegetation zones. George Schaller identified ten distinct zones, including: the bamboo forests at 2,200–2,800 metres (7,200–9,200 ft); the Hagenia forests at 2,800–3,400 metres (9,200–11,200 ft); and the giant senecio zone at 3,400–4,300 metres (11,200–14,100 ft).[12] The mountain gorilla spends most of its time in the Hagenia forests, where galium vines are found year-round. All parts of this vine are consumed: leaves, stems, flowers, and berries. It travels to the bamboo forests during the few months of the year fresh shoots are available, and it climbs into subalpine regions to eat the soft centers of giant senecio trees.[13]

 

Behaviour

Social structure

The mountain gorilla is highly social, and lives in relatively stable, cohesive groups held together by long-term bonds between adult males and females. Relationships among females are relatively weak.[16] These groups are nonterritorial; the silverback generally defends his group rather than his territory. In the Virunga mountain gorillas, the average length of tenure for a dominant silverback is 4.7 years.[17]

61% of groups are composed of one adult male and a number of females and 36% contain more than one adult male. The remaining gorillas are either lone males or exclusively male groups, usually made up of one mature male and a few younger males.[18] Group sizes vary from five to thirty, with an average of ten individuals. A typical group contains: one dominant silverback, who is the group's undisputed leader; another subordinate silverback (usually a younger brother, half-brother, or even an adult son of the dominant silverback); one or two blackbacks, who act as sentries; three to four sexually mature females, who are ordinarily bonded to the dominant silverback for life; and from three to six juveniles and infants.[19]

Most males, and about 60% of females, leave their natal group. Males leave when they are about 11 years old, and often the separation process is slow: they spend more and more time on the edge of the group until they leave altogether.[20] They may travel alone or with an all-male group for 2–5 years before they can attract females to join them and form a new group. Females typically emigrate when they are about 8 years old, either transferring directly to an established group or beginning a new one with a lone male. Females often transfer to a new group several times before they settle down with a certain silverback male.[21]

The dominant silverback generally determines the movements of the group, leading it to appropriate feeding sites throughout the year. He also mediates conflicts within the group and protects it from external threats.[14] When the group is attacked by humans, leopards, or other gorillas, the silverback will protect them even at the cost of his own life.[22] He is the center of attention during rest sessions, and young animals frequently stay close to him and include him in their games. If a mother dies or leaves the group, the silverback is usually the one who looks after her abandoned offspring, even allowing them to sleep in his nest.[23] Experienced silverbacks are capable of removing poachers' snares from the hands or feet of their group members.[24]

When the silverback dies or is killed by disease, accident, or poachers, the family group may be disrupted.[13] Unless there is an accepted male descendant capable of taking over his position, the group will either split up or adopt an unrelated male. When a new silverback joins the family group, he may kill all of the infants of the dead silverback.[25] Infanticide has not been observed in stable groups.

Analysis of mountain gorilla genomes by whole genome sequencing indicates that a recent decline in their population size has led to extensive inbreeding.[26] As an apparent result, individuals are typically homozygous for 34% of their genome sequence. Furthermore, homozygosity and the expression of deleterious recessive mutations as consequences of inbreeding have likely resulted in the purging of severely deleterious mutations from the population.

 

Aggression

Although strong and powerful, the mountain gorillas are generally gentle and very shy.[22] Severe aggression is rare in stable groups, but when two mountain gorilla groups meet, the two silverbacks can sometimes engage in a fight to the death, using their canines to cause deep, gaping injuries.[19] For this reason, conflicts are most often resolved by displays and other threat behaviors that are intended to intimidate without becoming physical. The ritualized charge display is unique to gorillas. The entire sequence has nine steps: (1) progressively quickening hooting, (2) symbolic feeding, (3) rising bipedally, (4) throwing vegetation, (5) chest-beating with cupped hands, (6) one leg kick, (7) sideways running four-legged, (8) slapping and tearing vegetation, and (9) thumping the ground with palms .[27] Jill Donisthorpe stated that a male charged at her twice. In both cases the gorilla turned away, when she stood her ground.

 

Volcanoes National Park (French: Parc National des Volcans) lies in northwestern Rwanda and borders Virunga National Park in the Democratic Republic of Congo and Mgahinga Gorilla National Park in Uganda. The national park is known as a haven for the mountain gorilla. It is home to five of the eight volcanoes of the Virunga Mountains (Karisimbi, Bisoke, Muhabura, Gahinga and Sabyinyo), which are covered in rainforest and bamboo. The park was the base for the zoologist Dian Fossey.

 

History

Children on a farm near Volcanoes National Park

The park was first gazetted in 1925, as a small area bounded by Karisimbi, Visoke and Mikeno, intended to protect the gorillas from poachers. It was the very first National Park to be created in Africa. Subsequently, in 1929, the borders of the park were extended further into Rwanda and into the Belgian Congo, to form the Albert National Park, a huge area of 8090 km2, run by the Belgian colonial authorities who were in charge of both colonies.[1] In 1958, 700 hectares of the park were cleared for a human settlement.[2]

After the Congo gained independence in 1960, the park was split into two, and upon Rwandan independence in 1962 the new government agreed to maintain the park as a conservation and tourist area, despite the fact that the new republic was already suffering from overpopulation problems. The park was halved in area in 1969.[citation needed] Between 1969 and 1973, 1050 hectares of the park were cleared to grow pyrethrum.[2]

The park later became the base for the American naturalist Dian Fossey to carry out her research into the gorillas. She arrived in 1967 and set up the Karisoke Research Centre between Karisimbi and Visoke. From then on she spent most of her time in the park, and is widely credited with saving the gorillas from extinction by bringing their plight to the attention of the international community. She was murdered by unknown assailants at her home in 1985, a crime often attributed to the poachers she had spent her life fighting against.[3] Fossey's life later was portrayed on the big screen in the film Gorillas in the Mist, named after her autobiography. She is buried in the park in a grave close to the research center, and amongst the gorillas which became her life.

The Volcanoes National Park became a battlefield during the Rwandan Civil War, with the park headquarters being attacked in 1992. The research centre was abandoned, and all tourist activities (including visiting the gorillas) were stopped. They did not resume again until 1999 when the area was deemed to be safe and under control. There have been occasional infiltrations by Rwandan rebels from the Democratic Forces for the Liberation of Rwanda in subsequent years, but these are always stopped quickly by the Rwandan army and there is thought to be no threat to tourism in the park.

 

Flora

Vegetation varies considerably due to the large altitudinal range within the park. There is some lower montane forest (now mainly lost to agriculture). Between 2400 and 2500 m, there is Neoboutonia forest. From 2500 to 3200 m Arundinaria alpina (bamboo) forest occurs, covering about 30% of the park area. From 2600 to 3600 m, mainly on the more humid slopes in the south and west, is Hagenia-Hypericum forest, which covers about 30% of the park. This is one of the largest forests of Hagenia abyssinica. The vegetation from 3500 to 4200 m is characterised by Lobelia wollastonii, L. lanurensis, and Senecio erici-rosenii and covers about 25% of the park. From 4300 to 4500 m grassland occurs. Secondary thicket, meadows, marshes, swamps and small lakes also occur, but their total area is relatively small.

 

Fauna

The park is best known for the mountain gorilla (Gorilla beringei beringei). Other mammals include: golden monkey (Cercopithecus mitis kandti), black-fronted duiker (Cephalophus niger), buffalo (Syncerus caffer), spotted hyena (Crocuta crocuta) and bushbuck (Tragelaphus scriptus). There are also reported to be some elephants in the park, though these are now very rare.[4] There are 178 recorded bird species, with at least 13 species and 16 subspecies endemic to the Virunga and Ruwenzori Mountains.[5]

 

Tourism in the park

Young gorilla grabs tourist at Volcanoes National Park

The Rwanda Development Board (RDB) runs several activities for tourists, including:[6]

Gorilla visits - as of January 2015, there are ten habituated gorilla groups open to tourists, allowing for a total of 80 permits per day. Tourists report at the park head office by 7:00 for a pre-tracking briefing. Once tourists meet the gorillas they spend an hour with them.

Golden monkey visits.

Climbing of Karisimbi volcano - this is a two-day trek with overnight camping at an altitude of 3,800 m.

Climbing of Bisoke volcano - one day.

Tour of the lakes and caves.

Visiting the tomb of Dian Fossey.

Iby’Iwacu cultural village tour

The majority of revenue from tourism goes towards maintaining the park and conserving the wildlife. The remainder goes to the government and (around 10%)[citation needed] to local projects in the area to help local people benefit from the large revenue stream generated by the park.

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