The mountain gorilla (Gorilla beringei beringei) is one of the two subspecies of the eastern gorilla. It is listed as endangered by the IUCN since 2018. There are two populations. One population 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 population lives in Uganda's Bwindi Impenetrable National Park. Some primatologists speculate the Bwindi population is a separate subspecies. In 2019, about 1,063 mountain gorillas were estimated to live worldwide.

Evolution, taxonomy, and classification

Mountain gorillas are related to ancient monkeys and apes that lived in Africa and Arabia during the beginning of the Oligocene epoch (34–24 million years ago). Fossils found in East Africa show that apes lived there about 22–32 million years ago. However, the area where mountain gorillas live today has few fossils, so scientists do not fully understand their evolutionary history.

Around 8.8 to 12 million years ago, a group of primates that would later become gorillas separated from their shared 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 western gorillas about 1.2 to 3 million years ago. The genus was first called Troglodytes in 1847 but was renamed 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 near the Virungas), and Gorilla gorilla beringei (mountain gorillas in the Virungas and Bwindi). In 2003, the World Conservation Union (IUCN) changed the classification, dividing gorillas into two species: Gorilla gorilla and Gorilla beringei. Today, scientists agree that there are two species, each with two subspecies.

Characteristics

The fur of the mountain gorilla is often thicker and longer than that of other gorilla species. This helps them live in colder areas. Each gorilla has a unique nose print, which can be used to identify them.

Male mountain gorillas can stand 1.61–1.71 meters (5.3–5.6 feet) tall, have a body width of 1.38–1.63 meters (4.5–5.3 feet), an arm span of 2 to 2.7 meters (6.6 to 8.9 feet), and weigh between 120–191 kilograms (265–421 pounds). Female mountain gorillas are smaller, weighing 70–98 kilograms (154–216 pounds). This subspecies is smaller than the eastern lowland gorilla, the other type of eastern gorilla. Adult males have more noticeable bony ridges on the top and back of their skulls, giving their heads a cone-like shape. These ridges help attach strong muscles that connect to the lower jaw. Adult females also have these ridges, but they are less visible. All gorillas 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. The hair on their backs is shorter than on other parts of their body, and their arms have especially long hair. Fully grown males average 1.7 meters (5.6 feet) in height, with an arm span of 2.3 meters (7.5 feet) and weigh about 155 kilograms (342 pounds). The tallest silverback ever recorded was 1.95 meters (6.4 feet) tall, with an arm span of 2.7 meters (8.9 feet), a chest width of 1.98 meters (6.5 feet), and weighed 219 kilograms (483 pounds). This gorilla was shot in Alimbongo, northern Kivu, in May 1938. There is also a report of another gorilla, shot in 1932, that was 2.06 meters (6.8 feet) tall and weighed 218.6 kilograms (482 pounds). The heaviest silverback in captivity was a gorilla named N'gagi at the San Diego Zoo. He weighed 683 pounds (310 kilograms) at his heaviest, but weighed 636 pounds (288 kilograms) when he died in 1944.

Mountain gorillas are mostly ground-dwelling and walk on four legs. However, they may 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 fingers to support their weight.

Mountain gorillas are active during the day and spend much of their time eating because they need large amounts of food to stay healthy. They search 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, making 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, including the Virunga Mountains, at elevations from 2,200 to 4,300 meters (7,200 to 14,100 feet). Most groups live on the slopes of three inactive volcanoes: Karisimbi, Mikeno, and Visoke. The vegetation is thick at the bottom of the mountains and becomes less dense at higher elevations. The forests are often cloudy, misty, and cold. The mountain gorilla also sometimes lives in the border area near the Rwenzori-Virunga montane moorlands, which is at higher elevations than the Albertine Rift montane cloud forest.

Behaviour and ecology

The home range of a group of gorillas during one year is shaped by the availability of food and usually includes several different types of plant areas. George Schaller identified ten different zones, including: a bamboo forest at 2,200–2,800 m (7,200–9,200 ft); a Hagenia forest at 2,800–3,400 m (9,200–11,200 ft); and a giant senecio zone at 3,400–4,300 m (11,200–14,100 ft). Mountain gorillas spend most of their time in the Hagenia forest, where galium vines grow all year. All parts of the vine are eaten: leaves, stems, flowers, and berries. Gorillas travel to the bamboo forest during the few months when fresh shoots are available and climb into subalpine areas to eat the soft centers of giant senecio trees.

Mountain gorillas are mainly herbivores, meaning they eat mostly plants. About 85.8% of their diet includes leaves, shoots, stems, and pith from 142 plant species. They also eat bark and wood (6.9%), roots and their outer layers (3.3%), flowers (2.3%), fruit (1.7%), and small amounts of insects, fungi, galls, and gorilla dung (0.1%). In a one-year study in Bwindi Impenetrable Forest, adult male gorillas ate an average of 18.8 kg (41 lb) of food each day, while females ate 14.9 kg (33 lb).

Mountain gorillas are highly social and live in groups that remain stable over time, held together by long-term relationships between adult males and females. Bonds between females are weaker. These groups do not defend specific areas, and the silverback (dominant male) protects the group rather than a territory. In the Virunga mountain gorillas, the average time a dominant silverback leads a group is about 4.7 years.

About 61% of groups consist of one adult male and several females, while 36% have more than one adult male. The rest are either lone males or groups made up entirely of males. Group sizes range from five to thirty individuals, with an average of ten. A typical group includes: one dominant silverback, who leads the group; one subordinate silverback (often a younger brother, half-brother, or son of the dominant silverback); one or two blackbacks (young males who act as sentries); three to four adult females who are bonded to the dominant silverback; and three to six juveniles or infants.

Most males and about 60% of females leave their birth group. Males usually leave at about eleven years old, often gradually moving to the edges of the group before leaving. They may travel alone or with other males for two to five years before forming a new group with females. Females typically leave their birth group at about eight years old, either joining an existing group or starting a new group with a lone male. Many females change groups multiple times before settling with a silverback.

The dominant silverback leads the group, choosing where to feed and protecting it from dangers. He also resolves conflicts and helps care for young gorillas if their mothers are absent. Young gorillas have been seen removing traps set by poachers.

When a silverback dies or is killed, the group may break apart. If there is no male descendant to take over, the group may split or adopt an unrelated male. A new silverback may harm the infants of the previous leader, but this has not been observed in stable groups.

Studies of mountain gorilla DNA show that their population has shrunk recently, leading to inbreeding. As a result, many gorillas have two copies of the same genes in 34% of their genome. This inbreeding may have removed harmful genes from the population.

Mountain gorillas are strong but usually gentle and shy. Aggression is rare in stable groups, but silverbacks may fight to the death when groups meet. Conflicts are often resolved through displays, such as chest-beating or charging, without physical contact.

A ritualized charge display has nine steps: (1) quickened hooting, (2) symbolic feeding, (3) standing on two legs, (4) throwing vegetation, (5) chest-beating with cupped hands, (6) kicking one leg, (7) running sideways on four legs, (8) slapping vegetation, and (9) thumping the ground with palms. Jill Donisthorpe reported that a male gorilla charged at her twice but turned away when she remained calm.

Midday rest is a time for social bonding. Grooming helps maintain relationships and keep fur clean. While less common than in other primates, females often groom their young.

Young gorillas play frequently and are more active in trees than adults. Play helps them learn social skills. Activities include wrestling, chasing, and rolling. The silverback and females often watch or join in.

Mountain gorillas recognize 25 distinct sounds, many used to communicate in dense forests. Grunts and barks are common during travel, while screams and roars warn of danger. Deep, rumbling sounds during feeding or resting show contentment.

Mountain gorillas avoid certain animals, like chameleons and caterpillars, and dislike water. They cross streams only if they can stay dry, such as by using fallen logs. They also avoid rain.

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 sent to the Berlin Zoological Museum, where Professor Paul Matschie (1861–1926) classified it as a new type of gorilla and named it Gorilla beringei after Captain von Beringe. 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 Albert National Park to protect the animals in the Virunga Mountains.

In 1959, George Schaller began a 20-month study of mountain gorillas. He later wrote two books: The Mountain Gorilla and The Year of the Gorilla. Before his research, little was known about mountain gorillas, but his work described how they live together, their life cycle, and their environment.

In 1967, Dian Fossey started an 18-year study of mountain gorillas. She made new discoveries, completed the first accurate count of the population, and helped protect the gorillas by organizing anti-poaching patrols. The Digit Fund, which she started, 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 Virunga Mountains. 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 grow in the Virungas and at Bwindi. Scientists now believe there are more than 1,000 mountain gorillas living in these areas.

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

A 2006 census at Bwindi found 340 gorillas, which was a 6% increase from 2002 and a 12% increase from 1997, when there were 320 gorillas. These estimates used traditional methods, like counting dung samples from gorilla nests. However, genetic testing during the 2006 census suggested there were about 300 gorillas in Bwindi. This difference shows how difficult it can be to accurately estimate population growth using less precise methods.

Computer models of gorilla populations in Bwindi and the Virungas show that gorillas who are used to humans (habituated) for research and tourism grow faster than those who are not. Habituation means gorillas behave normally around people after repeated, calm interactions. Habituated gorillas are better protected by staff and receive medical care for injuries or illnesses. However, researchers say some gorillas should remain unhabituated to reduce the risk of disease spreading. 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 teamwork at all levels, including protecting habitats, enforcing laws, and educating people.

Dian Fossey divided conservation efforts into three categories:

• Active conservation includes patrolling wildlife areas to stop poachers, enforcing laws, counting gorillas in breeding areas, and protecting their limited habitat.
• Theoretical conservation aims to grow tourism by improving roads, upgrading park facilities, and habituating gorillas near park boundaries so tourists can visit.
• Community-based conservation involves protecting biodiversity with the help of local people.

Collaborative management has helped reduce conflicts in Bwindi Impenetrable National Park. The park was created in 1991 without consulting local communities, which limited their access to resources and caused economic problems. This led to forest fires and threats to gorillas. To address this, three plans were made: allowing controlled resource use, sharing tourism revenue, and creating a trust fund for community development. These steps reduced tensions, and surveys show more people now support the park. There have been no more fires, and fewer snares in the area.

While community-based conservation works well, active and theoretical conservation also overlap. For example, Rwanda’s national parks improved ranger salaries, training, and equipment in 2002. These changes were funded by tourism, which brought in about $8 million in 2008 from 20,000 visitors.

According to UNESCO’s director, Audrey Azoulay, "Species conservation works best when local communities are central to the plan." In Rwanda, 10% of tourism revenue goes to communities, supporting 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 income for the Uganda Wildlife Authority, covering park costs and contributing to the national budget. Ranger-led tourist visits also help count gorilla populations.

Other conservation methods include creating corridors between isolated areas to help gorillas move safely.

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 surrounded by areas where human settlements are growing quickly. Activities like slash-and-burn farming, expanding grazing areas, and logging cause the forests to break into smaller pieces and become damaged. In the late 1960s, the Virunga Conservation Area (VCA) in Rwanda’s national park was reduced by more than half its original size to grow a plant called Pyrethrum. This caused a large drop in mountain gorilla numbers by the mid-1970s. The loss of trees forces gorillas to live in isolated areas. Some gorilla groups may steal crops for food, which causes more conflict between humans and gorillas. Habitat loss affects more than just the space gorillas have to live. As gorilla groups become more separated by human settlements, the genetic diversity within each group decreases. Signs of inbreeding, such as webbed hands and feet, are already appearing in young gorillas.

Mountain gorillas are not usually hunted for meat, but they are often injured or killed by traps and snares meant for other animals. Their body parts, such as heads, hands, and feet, are sold to collectors. Baby gorillas are sometimes sold to zoos, researchers, and people who want them as pets. Taking baby gorillas often causes the loss of at least one adult, as gorilla groups will fight to protect their young. Virunga gorillas are especially at risk from illegal trade for the pet market. Young gorillas can be worth between $1,000 and $5,000 on the black market. Poachers may kill or injure other gorillas to capture infants. Gorilla groups that survive often break apart. One example is the "Taiping 4," where a Malaysian Zoo received four wild-born baby gorillas from Nigeria for $1.6 million using fake documents. Poaching for meat is also a serious problem in areas with political instability. Most African great apes live in regions with long-term insecurity, where laws are not properly enforced. For example, in January 2007, mountain gorillas were killed in Bikenge, Virunga National Park, in a well-documented case.

Even though mountain gorillas live in national parks, they are also at risk from well-meaning people. Gorilla groups that are visited by tourists and locals face the risk of disease spreading between humans and gorillas, despite rules requiring people to stay at least seven meters away from gorillas. Mountain gorillas share a similar genetic makeup with humans and have immune systems that are not adapted to human diseases. This makes them vulnerable to illness. Studies suggest that infectious diseases, mainly respiratory ones, cause about 20% of sudden deaths in mountain gorilla populations.

A successful ecotourism program that reduced human-gorilla interaction led to a 76% increase in four sub-populations in Rwanda between 1989 and 2000. However, seven commonly visited sub-populations in the Democratic Republic of Congo (DRC) saw a nearly 20% decline between 1996 and 2000. Disease transmission is not only from humans but also from domestic animals and livestock through contaminated water. Research has found that parasites like Cryptosporidium sp., Microsporidia sp., and Giardia sp. are genetically the same in livestock, humans, and gorillas, especially near the Bwindi Impenetrable Forest in Uganda.

Rwanda, Uganda, and the Democratic Republic of Congo have faced political instability and war in recent decades. Studies suggest that times of war and unrest harm gorilla habitats and populations. Increased human contact, whether aggressive or passive, raises gorilla mortality and lowers their ability to reproduce. Direct effects of conflict include the placement of mines in Volcanoes National Park, which killed many gorillas. Logging also increased as refugees fleeing cities cut down trees for wood. During the Rwandan genocide, poaching increased due to the breakdown of laws and lack of consequences for illegal activities.

Other sources

• African Wildlife Foundation, awf.org
• Adams, D.; Carwardine, M. (1991). Last Chance to See. London: Pan Books. ISBN 0-330-32002-5.
• Groves, C. (2001). Primate Taxonomy. 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.