MOUNTAIN GORILLA

Gorilla beringei beringei

Geographic Distribution and Habitat 
More than half the population of our world’s mountain gorillas (604 individuals) resides high in the lush, dense cloud forests of the Virunga Mountains, a dormant volcanic range in central Africa that spans the borders of Rwanda, Uganda, and the Democratic Republic of the Congo. The remainder of this population (400 individuals) lives 15 mi (24 km) north in Bwindi Impenetrable National Park, which encompasses southern Uganda and adjoining areas of the Congo. Habitat destruction and fragmentation has confined both populations of mountain gorillas to isolated patches of forest.

With a home range consisting of several tropical vegetation zones, which include montane and bamboo forests, the aptly named mountain gorilla dwells at altitudes ranging from 7,200 to 14,100 ft (2,200–4,300 m) in Virunga and 4,921–7,550 ft (1,500–2,300 m) in Bwindi.

These great apes are one of two subspecies of the eastern gorilla, the other being Grauer’s gorilla, formerly known as the eastern lowland gorilla.

(Population numbers reported by Reuters news, taken from a census conducted in 2018.)

A NOTE ABOUT TAXONOMY

Some primatologists believe that the mountain gorillas of Bwindi Impenetrable National Park, because of their genetic differences from the Virunga mountain gorillas, might be a separate (and third) subspecies of the eastern gorilla. However, no new classification has yet been established (February 2016; June 2019).

Mountain gorilla geographic range in blue. Map credit: Chermundy and IUCN

Size, weight, and lifespan
As the second-largest primate species (Grauer’s gorilla is the largest), mountain gorilla males can weigh 430 pounds and stand 6 feet tall. Females are a bit more “petite,” weighing 220 pounds and standing 5 feet tall. The natural lifespan for these great apes in the wild is between 40 and 50 years.

Appearance
A long and thick fur coat, blue-black or brownish-gray in color, which provides insulation against colder temperatures, covers the mountain gorilla’s body. Young, reproductively immature male gorillas are known as “blackbacks.” Fully mature males sport a swath of silver-colored hair along their backs, leading them to be called “silverbacks.” This silvering begins at about age 13.

Both adult males and females exhibit a bony crest at the top of their skulls (the female’s crest is not as dramatic as that of her male counterpart), giving a conical shape to their large heads. Deeply set dark brown eyes are encircled by a black ring around the iris. The mountain gorilla’s countenance is completed by a flat nose with wide nostrils that, like the facial skin above its mouth, remains free from the great ape’s encroaching hirsuteness. Its broad chest, hands, and feet are also free of body hair.

What Does It Mean?

Harem:
A group of females guarded by a male, who prevents other males from mating with them; an animal group consisting of one or two males, a number of females, and their offspring. The dominant male drives off other males and maintains the unity of the group. If present, a second male is subservient to the dominant male.

Hirsuteness: 
Covered with hair; hairy.
 
Montane forest:
A forest that grows on the slope of a mountain, regardless of altitude or latitude, within a specific climate, just below the subalpine zone. Characterized by a low canopy cover and many species of epiphytes, plants that grow on other plants, such as ferns and orchids.

Slash and burn agriculture:
A method of unsustainable farming where farmers clear land by cutting and burning flora in order to create an empty field—a “swidden”—for cultivation. Such practices are detrimental to local ecosystems.

Visit the Glossary for more definitions

Diet
Mountain gorillas eat a mostly vegetarian diet that consists of leaves, stems, bark, shoots, roots, flowers, and fruit. Less than 1 percent of its diet comes from insects. Adult males consume up to 75 lb (34 kg) a day; adult females consume up to 40 lb (18 kg) a day. Elevation varies the type of vegetation that the two subspecies eat. For example, the diversity of fruit trees is greater in the lower altitude range of Bwindi gorillas, so these gorillas eat more fruit than the Virunga population. Weather is another influencer upon mountain gorillas’ diet. During the rainy season, Virunga gorillas frequent bamboo forests, where they are able to find young shoots to snack on. Gorillas get their daily water requirement through the plants they eat.

In studying mountain gorillas’ dental health, scientists have noted that while mountain gorillas rarely get teeth cavities (likely because fruits, which contain a lot of natural sugar, are not their primary food source), these great apes are prone to bad tartar, which can lead to periodontitis, ultimately causing the loss of teeth—and worse, deterioration of the gorillas’ jaw bones.

Behavior and Lifestyle
Mountain gorillas are highly social animals who live in family groups, referred to as troops, led and protected by a dominant silverback. Three to four sexually mature females, one or two younger male gorillas, and three to six juveniles and infants form this cohesive family. Subordinate silverbacks might also be part of a troop; they are most often the younger brothers or adult sons of the dominant silverback. A large mountain gorilla troop can exceed 20 individuals.

Females, more frequently than males, leave their birth groups when they become reproductively mature. Their exit is a deliberate act to avoid inbreeding. In groups with a lone silverback male, females are inclined to leave their birth group following the silverback’s death. Without the silverback’s protection, a mother’s infants may be vulnerable to infanticide by outsider males. (In groups with multiple male members, the deceased silverback’s son is likely to assume the role as leader.)

Emigrating females may try out several groups before settling into one where they feel comfortable and protected by the group’s silverback leader. Some silverbacks are known to kill the babies of female newcomers. Because infanticide shortens interbirth intervals, killing a female’s infant is a strategic act on the part of the silverback, who is intent on passing on his genes. He gets to mate with the soon-to-be ovulating bereaved mother.

Males who remain in their birth group are subordinate to the dominant silverback, who has exclusive mating privileges with a troop’s females. Subordinate silverbacks might get the chance to mate with new female members of the troop, or if the dominant silverback dies.

Those males not satisfied with a chance of copulating with a female member of their troop leave. Some choose a solitary life; these lone males hope to attract emigrating females so they can start a new social group and instill themselves as leader. Younger males who disperse from their birth group might live in all-male groups, known as bachelor groups. These groups tend to dissolve once the males reach full reproductive maturity.

Fun Facts

Individual mountain gorillas can be identified and distinguished from one another by their unique nose prints.

Mountain gorillas appear to possess an innate phobia toward certain reptiles.

Fearful of water, mountain gorillas walk atop fallen logs to avoid getting wet when crossing a stream. When surprised by a rain shower, they are known to sit motionless until the rain has stopped. If a cave is nearby, they will seek shelter.

Humans are more closely related to gorillas than gorillas are to chimpanzees! We share 98 percent of our genetic sequence (DNA) with gorillas. Scientists believe humans and our gorilla brethren diverged from a common ancestor around 10 million years ago.

Certain shared genes in our DNA cause diseases like dementia and heart disease in humans, but not in gorillas.

Daily Life and Group Dynamics 
Mountain gorillas are primarily terrestrial animals; that is, they spend most of their time on the ground, where they move quadrupedally (on all four limbs). With arms longer than their legs, mountain gorillas propel themselves forward on the forest floor by “knuckle walking,” curving their fingers inward to support their massive weight. They are capable, however, of walking upright, or “bipedally,” for short distances. This type of locomotion often accompanies a threat display.

Active during the daylight hours, mountain gorillas are “diurnal.” They rise with the sun, unless the day is cold or overcast—then the gorillas are inclined to sleep in! Days are spent foraging and eating. Members forage an average of 0.31–0.62 mi (0.5–1 km) each day, but they will travel greater distances for a favorite food. They forage on open grassland when unable to find food sources in the forest. Although mountain gorillas are more comfortable on the ground, they are known to climb a tree to retrieve a delectable fruit, so long as the tree’s branches can support their weight.

During late morning and midday, the gorillas indulge in rest periods. Youngsters play with one another by chasing, wrestling, and climbing trees—not unlike human youngsters—while the silverback and the devoted females of his harem watch and occasionally play with their children. Playing helps to instill family bonds. Social grooming that occurs between individuals during these rest periods is another activity that helps strengthen family bonds.

When it’s time to turn in for the night, each gorilla constructs a nest from surrounding vegetation that serves as his or her bed. A new nest is constructed every night. Only infants sleep in the same nest with their mothers. These nighttime nests are most often situated on the ground, but occasionally, for safety reasons, they are constructed in trees—again, so long as the branches can support the gorillas’ weight. Females and young infants are more likely to have their nest in trees, whereas silverbacks (likely because of their impressive girth) rarely sleep in trees.

Communication
Mountain gorillas are highly vocal animals, and they can be loud! They are most vocal during feeding times, but they are also known to converse during rest periods. (Some scientists posit that gorillas are able to recognize one another’s voice within their family group.)

Each of mountain gorillas’ 25 or so vocalizations conveys a specific message or emotion and may accompany specific activities. Their verbal repertoire includes whines and whimpers, emitted by infants; play chuckles, most often emitted by young gorillas engaged in play (though even adult gorillas enjoy play time); intense and mild cough grunts, emitted during mild threat displays; copulatory grunts and whimpers, emitted by a male or female during copulation; a pig grunt or dog grunt, emitted by gorilla who is telling another gorilla to keep his or her hands off a plant food item; a dog whine, a high-pitched vocalization most often emitted while eating, when a gorilla is feeling content (this call has been described as “singing”); deep, rumbling belches often emitted during rest periods—not unlike humans who might let out a large belch after an enjoyable meal; barks, alarm calls used to alert troop members of a potential predator approaching; roars and screams, warning calls most often sounded by a troop’s silverback, intended to intimidate or threaten (these are expressions of fear that by sounding, a gorilla hopes to scare away a potential predator); hoots, emitted in series, used in long-distance threat displays when another gorilla group is detected. Hoots are often accompanied by chest beating by a troop’s silverback.

Despite their great strength, mountain gorillas are usually gentle and shy in disposition. Aggression is rare. To wit, a silverback resorts to posturing and intimidation—including beating his chest and thumping the ground with his hands—to frighten away potential attackers. If, however, his troop is attacked, a silverback will risk his life to protect his family. When younger males are part of the troop, they will help drive members away from the source of danger. Silverbacks have been known to remove poachers’ snares from the hands or feet of family members.

Chest beating is, no doubt, the dramatic physical activity that humans most often associate with their gorilla cousins. But other less-dramatic body postures are important in gorilla communication, too. A direct gaze, expressions of concern, social grooming, and touch (which can be used to convey affection) are all a part of gorilla “language.” So is scent.

Like their human cousins, gorillas sweat. Armpit sweat from silverback males is especially pungent. A silverback releases this powerful scent when he senses a predator approaching; the scent emanates throughout his troop, putting members on alert. Ovulating females release a pheromonal scent, signaling to adult males that they are ready to mate.

Reproduction and Family
Much of what is known about the reproductive behavior of gorillas is based on research of wild mountain gorillas and captive western lowland gorillas. Through extensive research, scientists conclude that all gorilla subspecies share similarities to each other, and to humans.

Gorillas have similar life stages as their human cousins, albeit at an accelerated pace. Infanthood quickly passes into childhood, which typically lasts until a gorilla is 5 months old; followed by a juvenile stage that lasts from age 3 to 6; leading to an adolescent stage from age 6 to 8; then to an early adult stage; and, finally, to a mature adult who is able to reproduce (reproductive age varies between male and female gorillas).

Although female gorillas begin ovulating at age 8, they are between the ages of 10 and 12 when they first give birth. Males lag behind, not reaching sexual maturity until they are between the ages of 11 and 13, when the hair on their back begins to develops its distinctive silver swath and their large canine teeth have come in. They may not successfully breed (sire offspring) until the age of 15, when they have reached their full adult size.

Like their human cousins, gorillas do not have a specific mating season and births occur throughout the year. Gestation period for gorillas is about 8.5 months. A female gorilla’s menstrual cycle is also similar to humans, 28 days with about 3 fertile days (the period known as estrus).

Gorillas are polygamous, meaning they mate with multiple individuals. In groups with a lone silverback, he mates with all the females. However, in multi-male groups, a female may have multiple partners as well—but the lead silverback always has first privilege.  

Females, more often than males, initiate copulation. To get a male’s attention, a female locks eyes with him and languidly approaches while pursing her lips. If her less-than-subtle art of seduction fails, and he does not respond to her pheromonal scent, she may reach out and touch the male. If he still doesn’t respond, she may slap the ground to arouse his interest. When a male initiates copulation, he skips the foreplay and boldly reaches out and touches the female, while grunting. Unlike female chimpanzees, whose genitals become swollen when they are in estrus, external reproductive clues are not evident in female gorillas. No worries; male gorillas are able to sense which females are ready to mate, so they focus their attention on these prospective breeding partners. The couple copulates on the ground, often facing one another. Scientists previously believed that only humans and bonobos had face-to-face positioning while having sex. But apparently, gorillas like to gaze into one another’s face, too.

A female gives birth to a single infant and won’t ovulate again for another 3 to 5 years (a biological phenomenon known as lactational amenorrhea, the absence or suppression of menstruation [ovulation] during nursing). This suppression of ovulation is indicative of the enormous stress nursing places upon the female’s body and explains the species’ lengthy birth interval of 4 to 8 years. Over the course of her life, she may give birth to 3 to 6 infants. A male with 3 or 4 female partners may sire between 10 and 20 offspring over his lifespan.

Baby gorillas are tiny! At birth, they weigh only 4 lb (1.8 kg). By 8 weeks old, the little ones are smiling, playing, and bouncing on their mother’s lap while she grooms them. At about 9 weeks they begin to crawl. At 3 months old, they become interested in exploring their immediate environment, and they will reach out and grab nearby objects.

Mothers are the primary caregivers. However, a troop’s silverback acts as his family’s protector, and he squelches any intragroup conflict that might stress nursing mothers. By playing with his children, he teaches them socialization skills. Other members of the troop also take interest in their newest members and will attempt to socialize with these babies as soon as the mothers allow.

For their first five months his life, a baby mountain gorilla is in constant contact with his mother, who initially carries her child before transferring him to her back, where he gets a free ride (while clinging tightly to her coat!) until he begins walking on his own, at about 5 to 8 months old. During this period, mothers seek protection for their infants by remaining in close proximity to the troop’s silverback. At 1 year old, a young gorilla begins to venture short distances, up to 16.4 ft (5 m), from his mother. By 2.5 years old, he is spending only half his time in direct contact with his mother, returning periodically to nurse.

Young gorillas begin eating some vegetation at about 2.5 months old; by the time they are 8 months old, their bodies are able to fully digest their plant-based diet. However, young gorillas are not considered fully weaned until age 3 or 4. Three or four years of nursing at their mother’s breast instills strong mother-child bonds. Sadly, nearly 40 percent of infants die during their first three years of life. But those who survive will remain with their mother for the next few years.

Ecological Role
As with other primates who have a frugivorous diet, mountain gorillas are important seed dispersers, playing a critical role in forest regeneration in their fragile ecosystem. Since they travel long distances, they disperse seeds (via their feces) far from the mother tree, adding to the diversity of the forest flora. Their large-scale grazing helps to naturally keep the diversity of vegetation in balance.

Conservation Status and Threats
Mountain gorillas are classified as Endangered by the International Union for Conservation of Nature, appearing on the IUCN Red List of Threatened Species (IUCN, 2018). Endangered means that a species faces “a very high risk of extinction in the wild.” While dire, this status is a slight improvement from the gorillas’ former classification as Critically Endangered (IUCN, 2008), “an extremely high risk of extinction in the wild.” A modest increase in the overall mountain gorilla population led the IUCN to “downlist” the species’ threat level.

Today’s total mountain gorilla population count of 1,004 individuals represents an increase in the Virunga population, which has risen incrementally from 380 individuals in 2003, to 480 individuals living within 36 social groups in 2010, to 604 individuals living within 41 social groups in 2018, taken from a census conducted in 2018. The remaining 400 mountain gorillas live within Bwindi Impenetrable National Park in Uganda, according to a 2012 census.

The IUCN’s downlisting offers a glimmer of optimism, proof that educational programs and serious conservation actions work. But unless conservation efforts continue in earnest, this incremental improvement from Critically Endangered to Endangered could be reduced to an exercise in semantics. The IUCN warns that the threat level against mountain gorillas could easily revert to Critically Endangered, a perilous backward step toward extinction.

Their most dangerous predator continues to be another great ape: humankind. Except for the occasional leopard or crocodile attack, humans pose the greatest threat to the lives of mountain gorillas.

Poaching tops this ignominious human threat list. Gorillas are maimed or killed by traps set for other animals; their heads, hands, and feet are deliberately severed to be sold on the black market as trophies; and infants are kidnapped and sold to zoos.

Habitat loss through “slash and burn agriculture” and encroaching human settlements (refugees fleeing civil war) have led to the deforestation of the mountain gorilla’s home. The illegal production of charcoal—a multimillion-dollar black market industry—within Virunga National Park has also contributed to the destruction of the mountain gorilla’s habitat. Without forests, the gorillas cannot survive.

Severe habitat loss has led to another grim impact: As mountain gorillas become ever more geographically isolated from others in their species, their gene pool becomes less diverse, leading to inbreeding and resulting in birth defects and abnormalities, including webbed hands and feet in younger gorillas.

Infectious Diseases, largely respiratory in nature, are a growing threat to the mountain gorilla. Ironically, ecotourism plays an unintentional role in transmitting human diseases to the gorillas. Ebola, tuberculosis, and even the common cold, which can be fatal to mountain gorillas, endanger the population. Additionally, pathogens from livestock and domesticated animals can transmit disease to mountain gorillas through contaminated water sources.

War and civil unrest, specifically the 1994 Rwanda Genocide and continued civil unrest and violent crime in the Democratic Republic of the Congo, have dramatically shrunken and transformed the mountain gorilla’s once bucolic habitat into a marred and volatile landscape occupied by both refugees and government rebels.

According to the World Wildlife Fund, 15 of Virunga’s mountain gorillas have been killed since the outbreak of civil war in 1990; between 1990 and 1994, four silverbacks and members of their troop were killed. Land mines, in addition to poaching, have led to the deaths of mountain gorillas.

And in 2007, seven mountain gorillas were slaughtered—execution style—in Virunga National Park. A senior park official was arrested as being complicit in their murders, which have been linked to the illegal charcoal trade.

Conservation Efforts
The mountain gorilla is listed in Appendix I of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival. However, even though the entire population of mountain gorillas lives within national parks or reserves, laws to protect them have been ignored.

But today there is reason for hope. Local and international conservation efforts are leading a gradual increase in the mountain gorilla population. Education, habitat protection, revised guidelines for ecotourism (including safety protocols that limit the number of visitors and the duration of visits, and mandating that face masks be worn when within less than 33 ft [10 m] from gorillas), and community-based conservation management are some of the efforts under way.

A treaty signed in October 2015 by the governments of the Democratic Republic of Congo, Republic of Rwanda, and Republic of Uganda may have reversed the mountain gorilla’s downward trajectory. Known as the “Treaty on the Greater Virunga Transboundary Collaboration on Wildlife Conservation and Tourism Development,” this inter-governmental treaty covers the greater Virunga landscape inclusive of 13 national parks and reserves at the borders of the three countries, inclusive of all protected areas in the mountain gorilla range. Associated with the treaty is collaborative conservation effort known as the “Transboundary Strategic Plan.”

The Dian Fossey Gorilla Fund, “dedicated to the conservation and protection of gorillas and their habitats in Africa” may be anointed with the most recognized name in gorilla conservation. Eponymously named for the famed primatologist, the fund takes a multifaceted approach to helping people and wildlife thrive together.

A key program of the Fossey Fund is the Karisoke Research Center in Rwanda’s Volcanoes National Park. Founded by Dian Fossey in 1967 to study endangered mountain gorillas, the center remains today the “world’s centerpiece for gorilla conservation and science.” The center employs more than 100 expert gorilla trackers, hired from the local community, who protect about half of Rwanda’s mountain gorillas. Local university students visit the center each year to learn about conservation methods and the importance of the region’s biodiversity, and to participate in field studies. The center also hosts regional conservation and scientific meetings.

A modern facility, presently under construction and scheduled to open in 2021, is slated to become the new home of the Karisoke Research Center. Largely made possible through a generous gift from the Ellen DeGeneres Wildlife Fund (another eponymously named fund, this one with Hollywood recognition!), the 50,000-sq-ft building will house state-of-the-art laboratories, interactive exhibits, indoor and outdoor classrooms . . . all on 11 acres of land, adjacent to Volcanoes National Park. The Ellen fund, named for American talk show host/comedian/animal protection advocate Ellen DeGeneres, was founded in 2018 with the specific purpose of continuing the work of Dr. Fossey (an environmental hero to Ms. DeGeneres) in helping to save wild mountain gorillas from extinction. (The Diane Fossey Fund was the first beneficiary of the Ellen Fund.) Officially, the new Karisoke Research Center facility will be known as The Ellen DeGeneres Campus of The Dian Fossey Fund. According to the Ellen Fund’s website, “The Campus aims to inspire all who visit toward a lifetime of gorilla conservation: tourists, scientists, government partners, students and community members.”

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Written by Kathleen Downey, February 2016; updated by Kathleen Downey, July 2019.