The mountain gorilla (Gorilla beringei beringei ) is one of the two subspecies of the eastern gorilla. It is listed as endangered by the IUCN as of 2018.
There are two populations: One is found in the Virunga volcanic mountains of Central Africa, within three National Parks: Mgahinga, in southwest Uganda; Volcanoes, in northwest Rwanda; and Virunga, in the eastern Democratic Republic of Congo (DRC).The other population is found in Uganda's Bwindi Impenetrable National Park. Some primatologists speculate the Bwindi population is a separate subspecies, though no description has been finalized. As of June 2018, there were more than 1,000 individuals.
The fur of the mountain gorilla, often thicker and longer than that of other gorilla species, enables them to live in colder temperatures. Gorillas can be identified by nose prints unique to each individual
Males reach a standing height of 161–171 cm (63–67 in), a girth of 138–163 cm (54–64 in), an arm span of 2 to 2.7 m (6 ft 7 in to 8 ft 10 in) and a weight of 120–191 kg (265–421 lb). Females are smaller with a weight of 70–98 kg (154–216 lb).This subspecies is smaller than the eastern lowland gorilla, the other subspecies of eastern gorilla. 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. 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 can reach 1.7 m (5 ft 7 in) in height, with an arm span of 2.3 m (7 ft 7 in) and weigh 155 kg (342 lb). The tallest silverback recorded was 1.95 m (6 ft 5 in) tall 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 heaviest silverback recorded was a 267 kg (589 lb) 1.83 m (6 ft 0 in) tall specimen shot in Ambam, Cameroon.
The mountain gorilla is primarily terrestrial and quadrupedal. However, it will climb into fruiting trees if the branches can carry its weight. Like all great apes other than humans, its arms are longer than its legs. It moves by knuckle-walking, supporting its weight on the backs of its curved fingers rather than its palms.
The mountain gorilla is diurnal, spending most of the day eating, as large quantities of food are needed to sustain its massive bulk. It forages in the 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.
The mountain gorilla inhabits the Albertine Rift montane cloud forest, including the Virunga Mountains, ranging in elevation from 2,200 to 4,300 m (7,200 to 14,100 ft). Most groups live on the slopes of three of the dormant volcanoes: Karisimbi, Mikeno, and Visoke. The vegetation is very dense at the bottom of the mountains, becoming more sparse at higher elevations, and the forests are often cloudy, misty and cold.
The home range 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: bamboo forest at 2,200–2,800 m (7,200–9,200 ft); Hagenia forest at 2,800–3,400 m (9,200–11,200 ft); and the giant senecio zone at 3,400–4,300 m (11,200–14,100 ft). The mountain gorilla spends most of its time in Hagenia forest, where galium vines are found year-round. All parts of this vine are consumed: leaves, stems, flowers, and berries. It travels to the bamboo forest during the few months of the year when fresh shoots are available, and it climbs into subalpine regions to eat the soft centers of giant senecio trees.
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. 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.
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. 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 have bonded for life to the dominant silverback; and from three to six juveniles and infants.
Most males and approximately 60% of females leave their natal group. Males leave when they are about eleven 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. They may travel alone or with an all-male group for two–five years before they can attract females to join them and form a new group. Females typically emigrate when they are about eight 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 choose to settle down with a certain silverback male.
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. When the group is attacked by humans, leopards, or other gorillas, the silverback will protect them, even at the cost of his own life. He is the center of attention during rest sessions, and young gorillas 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. Young mountain gorillas have been observed searching for and dismantling poachers' snares.
When the silverback dies or is killed by disease, accident, or poachers, the family group may be disrupted. 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. 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. 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.
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%). In a year long study in Bwindi Impenetrable Forest adult males ate an average of 18.8 kg (41 lb) of food a day, while a females ate 14.9 kg (33 lb).
The mountain gorilla is threatened by habitat loss and poaching.
Loss of habitat is one of the most severe threats to gorilla populations. The forests where mountain gorillas live are surrounded by rapidly increasing human settlement. Through shifting (slash-and-burn) agriculture, pastoral expansion, and logging, villages in forest zones cause fragmentation and degradation of habitat. The late 1960s saw the Virunga Conservation Area (VCA) of Rwanda's national park reduced by more than half of its original size to support the cultivation of Pyrethrum. This led to a massive reduction in mountain gorilla population numbers by the mid-1970s. The resulting deforestation confines the gorillas to isolated deserts. Some groups may raid crops for food, creating further animosity and retaliation.The impact of habitat loss extends beyond the reduction of suitable living space for gorillas. As gorilla groups are increasingly isolated from one another geographically due to human settlements, the genetic diversity of each group is reduced. Some signs of inbreeding are already appearing in younger gorillas, including webbed hands and feet.
As of 2018, the mountain gorilla was listed as endangered on the IUCN Red List. Conservation efforts have led to an increase in the overall population of the mountain gorilla (Gorilla beringei beringei ) in the Virungas and at Bwindi. The overall population is now believed to be at more than 1,000 individuals.
In December 2010, the official website of Virunga National Park announced that "the number of mountain gorillas living in the tri-national forested area of which Virunga forms a part, has increased by 26.3% during the last seven years - an average growth rate of 3.7% per annum." The 2010 census estimated that 480 mountain gorillas inhabited the region. The 2003 census had estimated the Virunga gorilla population to be 380 individuals, which represented a 17% increase in the total population since 1989, when there were 320 individuals. The population has almost doubled since its lowest point in 1981, when a census estimated that only 254 gorillas remained.
The 2006 census at Bwindi indicated a population of 340 gorillas, representing a 6% increase in total population size since 2002 and a 12% increase from 320 individuals in 1997. All of those estimates were based on traditional census methods using dung samples collected at night nests. Conversely, genetic analyses of the entire population during the 2006 census indicated there only were approximately 300 individuals in Bwindi. The discrepancy highlights the difficulty in using imprecise census data to estimate population growth.
According to computer modeling of their population dynamics in both Bwindi and the Virungas, groups of gorillas who were habituated for research and ecotourism have higher growth rates than unhabituated gorillas. Habituation means that through repeated, neutral contact with humans, gorillas exhibit normal behavior when people are in proximity. Habituated gorillas are more closely guarded by field staff and they receive veterinary treatment for snares, respiratory disease, and other life-threatening conditions. Nonetheless, researchers recommended that some gorillas remain unhabituated as a bet-hedging strategy against the risk of human pathogens being transmitted throughout the population.The main international non-governmental organization involved in conservation of mountain gorillas is the International Gorilla Conservation Programme, which was established in 1991 as a joint effort of the African Wildlife Foundation, Fauna & Flora International, and the World Wide Fund for Nature. Conservation requires work at many levels, from local to international, and involves protection and law enforcement as well as research and education.
Dian Fossey broke down conservation efforts into the following three categories:
A collaborative management process has had some success in the Bwindi Impenetrable National Park. The forest was designated a national park in 1991; this occurred with little community consultation and the new status prohibited local people from accessing resources within the park as well as reducing economic opportunities. Subsequently, a number of forest fires were deliberately lit and threats were made to the gorillas. To counteract this, three schemes to provide benefits from the existence of forest communities and involving the local community in park management were developed. They included agreements allowing the controlled harvesting of resources in the park, receipt of some revenue from tourism, and establishment of a trust fund partly for community development. Tension between people and the park has thus been reduced and now there is more willingness to take part in gorilla protection. Surveys of community attitudes conducted by CARE show a steadily increasing proportion of people in favour of the park. Moreover, there have been no cases of deliberate burning and the problem of snares in these areas has been reduced.
While community-based conservation bears out individual analysis, there are significant overlaps between active and theoretical conservation and a discussion of the two as halves of a whole seems more constructive. For example, in 2002, Rwanda's national parks went through a restructuring process. The director of the IGCP, Eugène Rutagarama, stated that "They got more rangers on better salaries, more radios, more patrol cars and better training in wildlife conservation. They also built more shelters in the park, from which rangers could protect the gorillas". The funding for these types of improvements usually comes from tourism - in 2008, approximately 20,000 tourists visited gorilla populations in Rwanda, generating around $8 million in revenue for the parks. In Uganda too, tourism is seen as a "high value activity that generates enough revenue to cover park management costs and contribute to the national budget of the Uganda Wildlife Authority." Furthermore, tourist visits which are conducted by park rangers also allow censuses of gorilla sub-populations to be undertaken concurrently.
In addition to tourism, other measures for conservation of the sub-population can be taken such as ensuring connecting corridors between isolated areas to make movement between them easier and safer.
