WESTERN LOWLAND GORILLA
Gorilla gorilla gorilla
Geographic Distribution and Habitat
The western lowland gorilla is one of two subspecies of western gorilla—the rare Cross River gorilla, Gorilla gorilla diehli, being the other. The western lowland gorilla is the most widespread of all gorillas, inhabiting the dense and remote tropical rainforests of Cameroon, the Central African Republic, the Democratic Republic of Congo, Equatorial Guinea, Gabon, Angola, and the isolated lowland swampy forests of the Republic of Congo on the African continent. The Congo is believed to harbor the largest number of these great apes.
Western lowland gorillas live in primary and secondary lowland tropical forests, swamp forests, and a variety of other forested areas. They tend to avoid areas near human settlements and villages. Their seclusion, in both habitat and behavior, makes it difficult for wildlife biologists to accurately determine how many of these great apes remain in the world. Estimates of their total population range from 95,000–100,000 individuals. Western lowland gorillas are more numerous than the other three gorilla subspecies including their Cross River gorilla “cousins” and the two subspecies of eastern gorilla: the mountain gorilla, Gorilla beringei beringei, and Grauer’s gorilla, Gorilla beringei graueri, formerly known as the eastern lowland gorilla.
Size, Weight, and Lifespan
Slightly smaller than the other gorilla subspecies, western lowland gorillas stand 4–6 feet (1.2–1.8 m) tall and weigh between 150 and 440 pounds (68 to 200 kg). Adult females are typically half the size of adult males. Lifespan in the wild is 35 to 50 years.
Besides being a bit smaller than other gorillas, western lowland gorillas have shorter hair, longer arms, and a more prominent brow ridge. They resemble their Cross River gorilla cousins but have a wider skull and smaller ears.
Like all gorillas, western lowland gorillas have flat, expressive faces with what appears to be a permanently furrowed brow, wide nostrils, and dark eyes that are deeply set. Their faces, hands, and feet are bare of fur. Their coats are typically brownish gray, and they have auburn-colored chests. A reddish crest adorns their cone-shaped heads. Adult males sport a silvery swath of hair down their backs, earning them the nickname “silverback.” Coarse black hair covers their entire body, save their face, ears, hands, and feet. They frequently stand upright but usually travel on all fours with their knuckles touching the ground, a mode of transportation referred to an “knuckle-walking.”
Western lowland gorillas are primarily vegetarian, relying on shoots, leaves, and pith (the white tissue lining the rinds of citrus fruits). Primatologists sometimes categorize their diet into three food categories: staple, seasonal, and fallback. Staple foods are eaten year-round, while seasonal and fallback foods are eaten during seasonal variation and times of scarcity, respectively. Staple foods include high-protein herbs, swamp herbs, and fruit. In times of fruit abundance, they especially rely on fruit. When fruit is scarce, they may incorporate bark and low-protein herbs as fallback foods. Their diet is remarkably diverse, incorporating hundreds of plant and fruit species throughout the year. Adults eat roughly 40 pounds (18 kg) of food per day.
Behavior and Lifestyle
Western lowland gorillas live in groups of four to eight individuals. Usually, groups comprise a dominant silverback, several adult females, and their young. Their group structure differs somewhat from other gorilla subspecies, which are larger and more regularly feature multiple male members. Occasionally, males form groups, referred to as “bachelor groups,” with no adult females. Western gorillas rely more heavily on fruit than other subspecies, which may explain their small group size; fruit occurs in clumped patches that cannot support large groups.
Gorillas nearly always walk on all fours and climb trees cautiously. For various physiological reasons, including their size, they cannot brachiate (swing from trees). They ford shallow streams, but cannot swim. Silverbacks adjust their stance to a bipedal position at the climax of their aggression displays, which follow a common sequence: a series of soft escalating hoots, rising to two legs, and (sometimes) running and tearing vegetation. Like other gorillas, silverbacks also beat their chests—recent studies indicate that their chest beats accurately signal their physical prowess and size.
Infanticide at the hands of a new alpha male may occur if the silverback is replaced by a new leader or the group disintegrates.
Western lowland gorillas are shy, live in low visibility areas, and range widely, all of which makes studying them in the wild challenging. Nonetheless, long-term field studies over the past thirty years provide a broad picture of their daily lives and the similarities and differences therein to other, more easily studied gorillas subspecies.
Western lowland gorillas vary between sleeping on the bare ground and constructing nests from vegetation. When choosing sleep sites, they prefer areas with easy proximity to their favorite fruits. Consequently, they often reuse sleep sites, though they rarely reuse the same bed/nest. For nest-building, western lowland gorillas favor herbs and liana—woody climbing plants that hang from trees—and especially prefer mattress-like herbs with large leaves and clumped stems. Folding the vegetation around and under their body, usually on level ground, takes one to three minutes.
Gorilla groups range roughly 2 miles (3 km) per day, covering more or less distance depending on the availability of fruit. Groups that sleep in the same sites may split into distinct foraging parties throughout the day to prevent intra-group competition. However, the parties remain in vocal contact with one another and reconvene at nightfall.
The composition of foraging parties varies widely—lone males split off occasionally, as do groups of females. Females and males both forage in the trees, but males spend more time on the ground than females when fruit is difficult to access. The apes forage intensely in the morning, followed by a long midday rest. Afternoon feeding is longer but less rigorous. Around one hour before dusk, the silverback initiates nest-building.
Mutual grooming is less common in western lowland gorillas than in other great apes and gorillas subspecies. Mother-child and female-silverback grooming occurs; however, grooming between adult females is very rare. Aggression among group members is minimal. Groups do not defend territory from one another, and their ranging areas overlap extensively. Compared to other subspecies, western lowland gorillas spend more time feeding and traveling but less time resting and socializing—perhaps because fruit (their dietary staple) is widely dispersed.
Recent studies on gorilla social networks have revealed a more complex range of inter-group interactions than previously observed, including social play between inter-group juveniles and member exchanges based on kinship (i.e., allowing adult males to transfer back into their natal group after a period with another group). Their extended sociality contrasts with the more aggressive patterns of interaction between other subspecies.
Western lowland gorillas use body language and gestures, a variety of vocalizations, and olfactory signals to communicate. The character of their vocalizations depends on several variables, including the distance, age, group membership, and relative status of the individual or individuals with whom they are communicating. For instance, when gorillas of differing statuses vocalize to one another, the more dominant individual vocalizes more than the subordinate individual.
Western lowland gorillas also avoid overlapping their calls with one another when communicating, which has led primatologists to liken their vocal interactions to a primitive form of rule-based conversation. Their vocal repertoire contains 11 main call types associated with socializing, foraging, and other activities. Grunts are especially important. They occur in many situations—most crucially when gorillas want to maintain contact with one another—but often coincide with disputes over personal space or feeding sites. In tense or aggressive situations, gorillas sometimes emit a loud roar or high-pitched scream.
The gorilla chest beat constitutes both vocal and bodily communication. The sound is famously loud and expansive, while its length and frequency indicate a gorilla’s competitive ability. (This study has been conducted with other gorilla subspecies. We can assume similar results for western lowland gorillas.)
Before or after initiating a display—in which the gorilla screams and carries out a bluff charge—western lowland gorillas often assume a “strut” stance: back arched, head up, lips tight and stiff.
Silverbacks also use odor to communicate. Like vocalizations, their emissions vary with context and relationship. Primatologists have theorized that olfactory signals are especially useful when visibility is limited, as is often the case in Central African forests. Researchers also report several different facial expressions in western lowland gorillas. Some are displayed alongside other forms of communication, like baring teeth while screaming, for example. Others are displayed apart from any clear sequence or behavior.
Koko, the legendarily communicative gorilla, was a western lowland gorilla who was taught American Sign Language (ASL) in a study that lasted from the time she was a juvenile until her death at 46 year of age in 2018. According to popular accounts, she had a working vocabulary of 1,000–2,000 signs, some of which she strung into sentences. Scientists remain divided on the true extent of her communicative ability via sign language. Some argue that she did not understand the meaning of all of her signs and was sometimes, instead, responding to unconscious cues from her handlers.
Western lowland gorillas display sexual behavior year-round and at all points of their reproductive cycle, including after conception and while non-fertile. In one study of a captive population, females were more likely to engage in sexual behavior while other females were fertile, regardless of their own fertility. Primatologists theorize that this is one strategy (among others) used by females as they compete for access to the sole silverback male in their breeding group.
Female gorillas reach sexual maturity after eight or nine years. Their gestation period lasts about nine months. Newly born infants weigh roughly four pounds (1.8 kg). When infants are strong enough to grasp onto their mothers’ hair, they ride on their backs. Adult females give birth every four or five years. Infant mortality is high; only half of western lowland gorilla infants make it to weaning age. Interestingly, in captivity, older females tend to give birth to more males than younger females.
Like other great apes, juvenile gorillas are playful and highly social, though dependent on their mothers for up to five years. Infants are very interested in their mothers’ activities. They often imitate their behaviors and encourage their mothers to share food, play, or follow them. This relationship, at least as studied in captivity, is a bit one-sided; mothers show little interest in soliciting particular behaviors from their children.
Male infants play more than female infants, and infants of both sexes prefer to play with male infants—again, this pattern was noted in captivity, not the wild. Play consists of both lone activities, like running, climbing, slapping leaves, and shredding vegetation, and social activities, like wrestling, and mock biting.
Young males reach the “blackback” stage at eight or ten years, when they’re as large as adult females. Males reach their full size at 18 and begin growing their distinct “silverback” fur between ages 12 and 15. Upon becoming blackbacks, male gorillas leave their groups and roam widely, either alone or with male kin, before establishing their own breeding groups (with varying degrees of success). Female gorillas also leave their natal groups, but they settle quickly into a new group. Consequently, young males cover a wider geographic range than females, because females (unlike males) are bound to the movement patterns of new breeding groups. New breeding groups form when migrating females join a solitary male.
The home ranges of gorillas groups tend to overlap, so inter-group meetings are common. When these occur, silverback males will display and (sometimes) attack one another to retain their own females and/or persuade females from the opposing group to join them. Young females nearly always transfer, but older females that have recently given birth may sometimes transfer. These are known as “parous” females; usually, they transfer after their most recent infant is weaned or dies. Females are more likely to leave large groups for small groups and groups led by older males for groups led by males near the beginning of their “tenure.”
Western lowland gorillas are effective seed dispersers due to their largely fruit-based diet, high body mass (and thus high amount of consumption), and pattern of seed disposition. They tend to defecate in their nests, which are well-suited for seed germination and growth. One study found that seeds grow 2–10 times more effectively in nests than closed-canopy forest. Crucially, they also eat the pulp from fruits, which can otherwise inhibit seed growth. In 2018, fecal analysis in Gabon indicated that western lowland gorillas distributed seeds from 58 species and that gut passage contributed positively to seed growth. Further evidence of their “gardening” abilities comes from their fruit selection—they preferentially select and eat high-quality fruits, which are more likely to have viable seeds.
The International Union for Conservation of Nature lists western lowland gorillas as Critically Endangered (IUCN, 2016), appearing on the IUCN Red List of Threatened Species. In the last 20 to 25 years, the western lowland gorilla population has declined over 60% due to disease, poaching, and habitat loss. At the current rate of annual population reduction—conservatively, about 2.7%—the population will decline by 80% over the next three generations.
The Ebola virus has been especially harmful. Researchers estimate that the disease has killed roughly one-third of wild gorillas, most of whom were western gorillas. During the worst disease outbreaks, the mortality rate of Ebola in wild gorillas reached 95%. Poaching is similarly destructive, and continues despite legal restrictions against it across the entirety of the apes’ geographic range. In Northeast Congo, for instance, about 5% of western gorillas are killed each year. Poachers hunt gorillas for meat, pets, and trinkets made from their body parts.
Timber extraction and clearance for subsistence farming have decimated their native habitats, and both efforts have further intensified the threat of poaching via access roads built for logging and agriculture. Road projects (for resource extraction and otherwise) also lead to habitat fragmentation and, consequently, inbreeding and genetic homogeneity, since young gorillas are less able to move to and reproduce with other groups.
The governance may turn a blind eye to poaching, which makes enforcing restrictions against poaching and habitat destruction difficult. However, many of the main threats to gorilla habitats are unregulated. Throughout Central Africa, for example, mining permits are being issued over an increasingly large area. Economic imperatives also encourage local governments to allow farmers, loggers, miners, and other groups to encroach in gorilla territory. One notable case is the palm oil trade. As oil palm plantations reach capacity in Asia, Africa has become the new frontier, and most of the forests in which gorillas live are suitable for the crop.
Western lowland gorillas are 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.
Recent assessments by the IUCN and other groups recommend several courses of action to preserve and support the recovery of the Western lowland gorilla population:
- An increase in effective law enforcement throughout both protected and unprotected areas, supported by updated regulations and sanctions and efforts to encourage community support of local law enforcement. Organizations assisting with this effort include the Zoological Society of London, the Wildlife Conservation Society, WildAid, and the Bushmeat Conservation Track Force.
- Effective land-use planning to coordinate economic and agricultural expansion in ways that avoid the clearing of gorilla habitats, and creating profitable alternatives (i.e., ecotourism) to subsistence farming, logging, poaching, and other destructive activities for communities in key areas. Organizations assisting with this effort include the World Wildlife Fund, the World Land Trust, and the African Wildlife Foundation.
- Further research into and prevention of human disease transmission, including the possibility of large-scale vaccine administration to portions of the Western gorilla population. One key organization assisting with this effort is Gorilla Doctors (based at UC Davis).
Written by Eli Elster, July 2023