The mountain gorilla (Gorilla beringei beringei) is one of two groups within the eastern gorilla species. It is classified as endangered by the IUCN as of 2018.

There are two groups of mountain gorillas. One group lives in the Virunga Mountains, which includes Mgahinga in southwestern Uganda, Volcanoes in northwestern Rwanda, and Virunga National Park in eastern Democratic Republic of Congo. The other group lives in Uganda’s Bwindi Impenetrable National Park. Some scientists think the Bwindi group may be a different subspecies. As of 2019, there are about 1,063 mountain gorillas worldwide.

Evolution, taxonomy, and classification

Mountain gorillas are descendants of ancient monkeys and apes that lived in Africa and Arabia during the Oligocene epoch (34–24 million years ago). Fossils from East Africa show that hominoid primates (apes) lived there about 22–32 million years ago. However, the fossil record in the area where mountain gorillas live is not very complete, so their evolutionary history is not fully understood.

Around 8.8 to 12 million years ago, the group of primates that would become gorillas split from their common ancestor with humans and chimpanzees. This is when the genus Gorilla first appeared. Mountain gorillas have been separated from eastern lowland gorillas for about 10,000 years. These two groups also split from their western relatives about 1.2 to 3 million years ago. The genus was first named Troglodytes in 1847, but later changed to Gorilla in 1852. In 1967, scientist Colin Groves suggested that all gorillas belong to one species (Gorilla gorilla) with three subspecies: Gorilla gorilla gorilla (western lowland gorilla), Gorilla gorilla graueri (lowland gorillas west of the Virungas), and Gorilla gorilla beringei (mountain gorillas, including Gorilla beringei, in the Virungas and Bwindi). In 2003, after a review, the World Conservation Union (IUCN) classified gorillas into two species: Gorilla gorilla and Gorilla beringei. Today, scientists agree that there are two species, each with two subspecies.

Characteristics

The thick and long fur of the mountain gorilla helps them live in colder areas. Each gorilla has a unique nose print, like a fingerprint, that can be used to identify them.

Male mountain gorillas stand between 1.61 and 1.71 meters tall, which is about 5.3 to 5.6 feet. Their body width ranges from 1.38 to 1.63 meters, or 4.5 to 5.3 feet. Their arms span 2 to 2.7 meters, or 6.6 to 8.9 feet, and they weigh between 120 and 191 kilograms, or 265 to 421 pounds. Female mountain gorillas are smaller, weighing between 70 and 98 kilograms, or 154 to 216 pounds. This subspecies is smaller than the eastern lowland gorilla, the other type of eastern gorilla. Adult males have large, bony crests on the top and back of their skulls, giving their heads a cone-like shape. These crests support strong muscles that connect to the jaw. Adult females also have these crests, but they are smaller. Like all gorillas, they have dark brown eyes surrounded by a black ring. Adult males are called silverbacks because they develop a gray or silver-colored patch on their backs as they age. Their back hair is shorter than other body parts, and their arms have long hair. Fully grown males are about 1.7 meters tall, or 5.6 feet, with arms that span 2.3 meters, or 7.5 feet, and weigh about 155 kilograms, or 342 pounds. The tallest silverback ever recorded was 1.95 meters, or 6.4 feet, with an arm span of 2.7 meters, or 8.9 feet, a chest width of 1.98 meters, or 6.5 feet, and a weight of 219 kilograms, or 483 pounds. This gorilla was shot in Alimbongo, northern Kivu, in May 1938. There is an unconfirmed report of another gorilla, shot in 1932, that was 2.06 meters, or 6.8 feet, tall and weighed 218.6 kilograms, or 482 pounds. The heaviest silverback in captivity was a gorilla named N'gagi, who lived at the San Diego Zoo and died in 1944. He weighed up to 683 pounds, or 310 kilograms, in his lifetime, but weighed 636 pounds, or 288 kilograms, when he died.

Mountain gorillas are mainly ground-dwelling and walk on all fours. However, they climb trees if the branches are strong enough to support their weight. Like all great apes except humans, their arms are longer than their legs. They move by walking on their knuckles, using the backs of their curved fingers instead of their palms.

Mountain gorillas are active during the day and spend much of their time eating because they need a lot of food to stay healthy. They forage for food in the early morning, rest in the late morning and around noon, and forage again in the afternoon before resting at night. Each gorilla builds a nest from plants to sleep in, creating a new one every evening. Only baby gorillas sleep in the same nest as their mothers. They leave their nests when the sun rises, usually around 6 a.m., unless it is cold and cloudy, in which case they may stay in their nests longer.

Distribution and habitat

The mountain gorilla lives in the Albertine Rift montane cloud forest, which includes the Virunga Mountains. These areas range in elevation from 2,200 to 4,300 meters (7,200 to 14,100 feet). Most gorilla groups are found on the slopes of three inactive volcanoes: Karisimbi, Mikeno, and Visoke. The vegetation is thicker at the bottom of the mountains and becomes thinner at higher elevations. The forests are often cloudy, misty, and cold. Occasionally, the mountain gorilla also lives in areas near the Rwenzori-Virunga montane moorlands, which are at higher elevations than the Albertine Rift montane cloud forest.

Behaviour and ecology

The area where a group of gorillas lives for a year depends on where food is available and usually includes different types of plant areas. 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 eaten: 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 primarily a herbivore; the majority of its diet is made up of the leaves, shoots, stems, and pith (85.8%) of 142 plant species. It also eats bark and wood (6.9%), roots and their epithelium (3.3%), flowers (2.3%), and fruit (1.7%), as well as insects, fungi, galls, and gorilla dung (0.1%). In a one-year study in Bwindi Impenetrable Forest, adult males ate an average of 18.8 kg (41 lb) of food a day, while females ate 14.9 kg (33 lb).

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 not territorial; the silverback generally defends his group rather than his territory. In the Virunga mountain gorillas, the average length of time a dominant silverback stays in charge is 4.7 years.

About 61% of groups are made up of one adult male and several females, and 36% contain more than one adult male. The rest are either lone males or groups of only males, usually one mature male and a few younger males. Group sizes range from five to thirty, with an average of ten individuals. A typical group includes: one dominant silverback, who is the group's leader; another subordinate silverback (often a younger brother, half-brother, or adult son of the dominant silverback); one or two blackbacks, who act as sentries; three to four sexually mature females, who are bonded for life to the dominant silverback; and three to six juveniles and infants.

Most males and about 60% of females leave their birth group. Males usually leave when they are about eleven years old, and the process of leaving can be slow: they spend more time on the edge of the group until they leave completely. They may travel alone or with an all-male group for two to five years before they can attract females to join them and form a new group. Females typically leave their birth group when they are about eight years old, either joining an existing group or starting a new one with a lone male. Females often move between groups several times before settling with a silverback.

The dominant silverback usually decides where the group moves, leading it to places where food is available throughout the year. He also helps resolve conflicts within the group and protects it from outside dangers. When the group is attacked by humans, leopards, or other gorillas, the silverback will protect them, even if it means risking his life. He is the center of attention during rest times, and young gorillas often stay close to him and include him in their games. If a mother dies or leaves the group, the silverback often takes care of her abandoned offspring, even allowing them to sleep in his nest. Young mountain gorillas have been seen searching for and destroying poachers' traps.

When the silverback dies or is killed by disease, accident, or poachers, the group may become unstable. If there is no male descendant who can take over, the group may split or adopt an unrelated male. When a new silverback joins the group, he may kill all the infants of the previous silverback. Infanticide has not been seen in stable groups.

Analysis of mountain gorilla genomes shows that a recent drop in their population size has caused a lot of inbreeding. As a result, individuals are usually homozygous for 34% of their genome sequence. The effects of inbreeding, such as increased homozygosity and the expression of harmful recessive mutations, may have led to the removal of the most harmful mutations from the population.

Although strong and powerful, mountain gorillas are generally gentle and shy. Serious aggression is rare in stable groups, but when two groups meet, the silverbacks may fight to the death, using their canines to cause serious injuries. Conflicts are usually resolved through displays and other threatening behaviors that aim to scare without causing physical harm.

A ritualized charge display is unique to gorillas. The sequence has nine steps: (1) progressively quickened hooting, (2) symbolic feeding, (3) rising on two legs, (4) throwing vegetation, (5) chest-beating with cupped hands, (6) one leg kick, (7) sideways running on four legs, (8) slapping and tearing vegetation, and (9) thumping the ground with palms. Jill Donisthorpe said a male charged at her twice. In both cases, the gorilla turned away when she stood her ground.

The midday rest period is important for building and strengthening relationships within the group. Mutual grooming helps keep social bonds strong and keeps hair clean of dirt and parasites. Grooming is not as common in gorillas as in other primates, although females often groom their young.

Young gorillas play often and are more active in trees than the large adults

Research

In October 1902, Captain Robert von Beringe (1865–1940) shot two large apes during an expedition to set the borders of German East Africa. One ape was recovered and sent to the Berlin Zoological Museum, where Professor Paul Matschie (1861–1926) classified the animal as a new type of gorilla and named it Gorilla beringei after the man who shot it. In 1925, Carl Akeley, a hunter from the American Museum of Natural History who wanted to study gorillas, persuaded Albert I of Belgium to create the Albert National Park to protect the animals of the Virunga Mountains.

George Schaller began observing mountain gorillas for 20 months in 1959. He later wrote two books: The Mountain Gorilla and The Year of the Gorilla. Before his research, little was known about the lives of mountain gorillas. His work described their social groups, life patterns, and environments.

Dian Fossey started an 18-year study of mountain gorillas in 1967. She made new discoveries, completed the first accurate count of gorilla populations, and created conservation efforts, such as patrols to stop poaching. The Digit Fund, which Fossey founded, continued her work and was later renamed the Dian Fossey Gorilla Fund International. The Fund’s Karisoke Research Center monitors and protects mountain gorillas in the Virungas. Close study and protection of Bwindi mountain gorillas began in the 1990s.

Conservation

As of 2018, the mountain gorilla was listed as endangered on the IUCN Red List. Conservation efforts have helped the mountain gorilla population (Gorilla beringei beringei) grow in the Virungas and at Bwindi. The total population is now believed to be more than 1,000 individuals.

In December 2010, the official website of Virunga National Park reported that the number of mountain gorillas in the tri-national forested area, which includes Virunga, had increased by 26.3% over seven years, or an average of 3.7% each year. The 2010 census estimated 480 gorillas in the region. The 2003 census had counted 380 gorillas, which was a 17% increase since 1989, when there were 320. The population has nearly doubled since its lowest point in 1981, when a census estimated only 254 gorillas remained.

The 2006 census at Bwindi counted 340 gorillas, a 6% increase since 2002 and a 12% increase from 320 in 1997. These estimates used traditional methods, such as counting dung samples from night nests. However, genetic analysis during the 2006 census suggested there were about 300 individuals in Bwindi. This difference shows the challenges of using less accurate census data to track population growth.

Computer models of gorilla population trends in Bwindi and the Virungas show that gorillas habituated to humans for research and tourism have higher growth rates than unhabituated gorillas. Habituation means gorillas become used to humans through repeated, calm interactions. Habituated gorillas are better protected by field staff and receive medical care for injuries, diseases, and other dangers. However, researchers suggest some gorillas should remain unhabituated to reduce the risk of disease spreading to the population. The main international group working to protect mountain gorillas is the International Gorilla Conservation Programme, created in 1991 by the African Wildlife Foundation, Fauna & Flora International, and the World Wide Fund for Nature. Conservation requires efforts at all levels, including protection, law enforcement, research, and education.

Dian Fossey divided conservation efforts into three categories:

• Active conservation includes regular patrols to remove poacher tools and weapons, strict law enforcement, census counts in breeding and ranging areas, and habitat protection.
• Theoretical conservation focuses on growing tourism by improving roads, renovating park facilities, and habituating gorillas near park boundaries for visitors.
• Community-based conservation involves protecting biodiversity through collaboration with local communities.

A collaborative management approach has helped reduce conflict in Bwindi Impenetrable National Park. The park was established in 1991 with little input from local people, limiting their access to resources and economic opportunities. This led to forest fires and threats to gorillas. To address this, three initiatives were created: controlled resource harvesting, sharing tourism revenue, and a trust fund for community development. These steps reduced tensions and increased support for gorilla protection. Surveys by CARE show more people now favor the park, and deliberate fires and snares have decreased.

While community-based conservation is effective, active and theoretical conservation efforts often overlap. For example, in 2002, Rwanda’s national parks improved ranger training, equipment, and facilities. These upgrades were funded by tourism revenue, such as the $8 million generated by 20,000 visitors in 2008.

According to UNESCO Director Audrey Azoulay, "Species conservation works best when local communities are central to strategies. Biodiversity protection must meet community needs." In Rwanda, 10% of gorilla tourism revenue, about €10 million, is returned to communities for schools, roads, and water supplies. In 1980, there were 250 mountain gorillas; by 2025, there are 1,063, with 80% living in the Virunga Mountains of the DRC, Rwanda, and Uganda.

In Uganda, tourism is a key source of revenue for park management and the Uganda Wildlife Authority’s budget. Ranger-led tourist visits also help count gorilla sub-populations.

Other conservation methods include creating corridors between isolated areas to allow safer movement for gorillas.

Threats

The mountain gorilla is in danger because of habitat loss and poaching. Habitat loss is one of the biggest threats to gorilla populations. The forests where mountain gorillas live are being taken over by human settlements. People use methods like slash-and-burn farming, expanding grazing areas, and logging, which break up and damage the forest. In the late 1960s, the Virunga Conservation Area in Rwanda’s national park was reduced by more than half to grow Pyrethrum, a plant used for insecticides. This caused the mountain gorilla population to drop sharply by the mid-1970s. Deforestation forces gorillas into small, isolated areas. Some gorilla groups may steal crops for food, which causes conflicts with humans. Habitat loss not only reduces living space but also separates gorilla groups from each other, lowering genetic diversity. Signs of inbreeding, such as webbed hands and feet, have already been seen in young gorillas.

Mountain gorillas are rarely hunted for bushmeat, but they are often injured or killed by traps meant for other animals. Their body parts, like heads and hands, are sold to collectors. Baby gorillas are sometimes sold to zoos, researchers, or people who want them as pets. When baby gorillas are taken, at least one adult usually dies trying to protect the young. Virunga gorillas are especially vulnerable to illegal trade for pets. Young gorillas can be worth $1,000 to $5,000 on the black market. Poachers may kill or injure other gorillas to capture infants. Survivors often leave their groups. One example is the "Taiping 4," where four baby gorillas were sent to a Malaysian zoo using fake documents for $1.6 million. Poaching for meat is also a problem in areas with political instability, where laws are not enforced. In 2007, mountain gorillas were killed in Bikenge, Virunga National Park, as part of a well-documented case.

Even though mountain gorillas live in national parks, they are still at risk from humans who mean well. Gorillas that are visited by tourists or locals face a risk of disease spreading from humans. Rules require people to stay at least seven meters away from gorillas to prevent this, but diseases still occur. Mountain gorillas have a genetic makeup similar to humans and immune systems not adapted to human diseases. This makes them vulnerable. Researchers say that infectious diseases, especially respiratory illnesses, cause about 20% of sudden deaths in gorilla populations.

A successful ecotourism program that limited human-gorilla interaction helped four sub-populations in Rwanda grow by 76% between 1989 and 2000. However, seven sub-populations in the Democratic Republic of Congo (DRC) declined by nearly 20% from 1996 to 2000. Disease transmission is not only from humans but also from domestic animals and livestock through polluted water. Studies show that parasites like Cryptosporidium sp., Microsporidia sp., and Giardia sp. are the same in livestock, humans, and gorillas near the Bwindi Impenetrable Forest in Uganda.

Political instability and war in Rwanda, Uganda, and the DRC have harmed mountain gorillas. Research suggests that times of war reduce gorilla populations by increasing human activity, raising mortality rates, and lowering reproduction. Direct impacts from conflict include mines placed in Volcanoes National Park, which killed gorillas, and increased logging by refugees fleeing cities. During the Rwandan genocide, poaching increased due to the breakdown of laws and lack of consequences.

Other sources

• African Wildlife Foundation, awf.org
• Adams, D.; Carwardine, M. (1991). Last Chance to See. Published in London by Pan Books. ISBN 0-330-32002-5.
• Groves, C. (2001). Primate Taxonomy. Published by Smithsonian. ISBN 1-56098-872-X.
• Harcourt, A. H. (1979). "Social relationships among adult female mountain gorillas." Animal Behaviour. 27 (1): 251–264. doi: 10.1016/0003-3472(79)90145-3. S2CID 53159816.