The western lowland gorilla (Gorilla gorilla gorilla) is one of two critically endangered subspecies of the western gorilla (Gorilla gorilla). It lives in mountain, primary, and secondary forests, as well as lowland swamps in central Africa. These areas include Angola (Cabinda Province), Cameroon, Central African Republic, Republic of the Congo, Democratic Republic of the Congo, Equatorial Guinea, and Gabon. It is the official name for the western gorilla and the smallest of the four gorilla subspecies.
The western lowland gorilla is the only subspecies found in zoos, except for Amahoro, a female eastern lowland gorilla at Antwerp Zoo, and a few mountain gorillas kept in captivity in the Democratic Republic of the Congo.
Description
The western lowland gorilla is the smallest type of gorilla but still has great size and strength. These gorillas show clear differences between males and females. They have no tails and black skin covered by coarse black hair on their bodies, except for their faces, ears, hands, and feet. The hair on the backs and rumps of males turns gray as they age and may disappear. This gray color is why older males are called "silverbacks." Their hands are large with nails on all fingers and thumbs, similar to humans. They have short faces, large brow ridges, big nostrils, and small eyes and ears. Their jaws have strong muscles and broad, strong teeth, including large canines and molars for chewing plants.
A male gorilla standing upright can be up to 1.83 meters (6 feet 0 inches) tall and weigh up to 227 kilograms (500 pounds). On average, males weigh about 140 kilograms (310 pounds), and females weigh about 70 kilograms (150 pounds). Males in captivity may reach up to 275 kilograms (606 pounds). Males stand about 1.67 meters (5 feet 6 inches) tall, and females stand about 1.5 meters (4 feet 11 inches) tall. Some people say a strong male gorilla has the strength of seven or eight Olympic weightlifters, but this claim has not been proven. Western gorillas often stand upright but walk on all fours with their hands curled and knuckles touching the ground. Their arms are longer than their height when standing.
The only known albino gorilla, named Snowflake, was a wild-born western lowland gorilla from Equatorial Guinea. He was captured as a baby and brought to the Barcelona Zoo in 1966. Snowflake had typical signs of albinism, such as white hair, pink skin, light-colored eyes, poor vision, and sensitivity to light. Scientists found that his albinism was caused by a specific genetic change in the SLC45A2 gene. This gene is linked to a type of albinism in humans. Since Snowflake’s parents were an uncle and niece who both carried the same genetic change, this discovery showed the first evidence of inbreeding in western lowland gorillas.
Behavior
Western lowland gorilla groups move within a home range that is usually between 8 and 45 km (3 and 17 sq mi) in size. Gorillas do not claim land as their own, and groups living near each other often share the same area. A group usually prefers a specific part of the home range, but their movement depends on the season and the availability of ripe fruits. At some places, they also move through large open areas like swamps and "bais." Gorillas typically travel 3–5 km (1.9–3.1 mi) each day. Gorillas that eat high-energy foods, which change with the season and location, travel farther than those that eat lower-quality but more constant food. Larger groups travel longer distances to find enough food.
Male gorillas can move alone more easily and switch groups because young males leave their birth group before reaching sexual maturity and live alone or with other nonbreeding males for several years. Both males and females leave their birth group, but females always stay in a group that includes breeding adults. Males often join other males from their family. A breeding group includes one silverback male, three adult females, and their young. The male’s job is to protect the group. Female gorillas form close bonds with other females from their birth group but also build strong relationships with males. Males compete aggressively for the chance to mate with females.
A group of gorillas is led by one or more adult males. If there is more than one silverback male, they are likely a father and son. Groups with only one male are considered the basic unit of a social group and grow larger as new members join. In a study at Lope, gorillas mostly eat food from trees but build fewer than half of their night nests in trees. They often stay on the ground, and groups can have up to 30 members. Western lowland gorillas live in the smallest family groups of all gorillas, with an average of four to eight members in each group. The leader, the silverback male, leads activities like eating, nesting, and traveling within the home range. Gorillas that challenge the alpha male are often scared by displays of strength, such as standing upright, throwing objects, charging, or pounding their chest while making loud noises. Gorillas are usually calm and not aggressive unless disturbed. Young gorillas, aged three to six, behave like children, spending much of their day playing, climbing, chasing, and swinging.
Female western lowland gorillas do not have many babies because they reach sexual maturity at 8 or 9 years old. A female gives birth to one baby after a pregnancy lasting nearly nine months. Female gorillas do not show signs of being pregnant. Newborns are very small, weighing about 4 lb (1.8 kg), but they can cling to their mothers’ fur. These babies ride on their mothers’ backs from four months old until they are two or three years old. They may depend on their mothers for up to five years.
A study of more than 300 births among captive female gorillas found that older females are more likely to have male babies than younger females under 8 years old. This pattern may happen because older females can better support their sons, who may have more success in reproduction than females.
Female western lowland gorillas in groups led by a single male have been seen showing sexual behavior during all stages of their reproductive cycle and even when not fertile. Three out of four females have been seen showing sexual behavior while pregnant, and two out of three have done so while nursing. Females are more likely to engage in sexual behavior when another female is also sexually active. This behavior may help a female increase her own chances of having babies while reducing the chances of other females.
Adult male gorillas have sometimes been seen killing babies that are not related to them. A male does this to have the chance to mate with the baby’s mother, who would otherwise be too busy caring for her young to mate.
Intelligence
Chimpanzees show their intelligence by making tools from natural materials to help them gather food more easily. Using and making tools to get ants and termites is a well-known behavior in wild chimpanzees. This behavior has never been seen in other great apes in the wild or in other primates in zoos.
Western lowland gorillas can change tools to fit a specific purpose by choosing branches, removing parts like leaves and bark, and adjusting the branch’s length to match the depth of a hole. It seems they plan how to use the tool, starting with the largest stick available and gradually shaping it until it fits perfectly into a hole with food. This shows the gorillas have developed advanced abilities to use their senses and motor skills, similar to those of young human children.
A gorilla was seen using a stick to check the depth of water. In 2009, a western lowland gorilla at Buffalo Zoological Gardens used a bucket to collect water. In an experiment, one adult male gorilla and three adult female gorillas were given five-gallon buckets near a pool. Two of the younger females filled the buckets with water. This was the first time gorillas were recorded using tools to drink in zoos without being taught.
Another example of gorillas’ intelligence is their ability to understand simple sign language. In the mid-1970s, researchers began teaching gorillas to communicate using signs. A gorilla named Koko was born on July 4, 1971, at San Francisco Zoo. Francine Patterson started teaching Koko sign language on July 12, 1972. At first, Patterson taught Koko three basic signs: "food," "drink," and "more." Koko learned by watching and by having her hands shaped into the correct signs. After a formal teaching routine began, Koko quickly started using the sign "food" when asked. In three months, Koko made 16 different sign combinations and began forming simple questions using eye contact and body positioning. Koko eventually learned over 1,000 signs and could connect up to eight words to express needs, thoughts, or simple ideas.
A study looked at whether western lowland gorillas could exchange items with humans. In the experiment, humans held objects like fruit, leaves, or peanuts in one hand. When gorillas gave twigs to humans, they received one of these objects. If gorillas did not give a twig, they did not get the object they wanted. The gorillas quickly learned how to get rewards, as their mistakes during the experiment decreased over time.
Ecology
Western lowland gorillas live in many types of forests, including rainforests, swamp forests, brush, areas with new plant growth, forest edges, old farmland, and river forests. They are found in lowland tropical forests, both primary and secondary, at elevations from sea level up to 1,300 meters (4,300 feet). These areas receive about 1,500 millimeters (60 inches) of rain each year, with the most rain falling between August and November. Western lowland gorillas are usually found far from human settlements, villages, roads, and farms. They prefer places where there are many edible plants. Swamp forests are now considered an important source of food and shelter for these gorillas. These areas support them during both wet and dry seasons. The Republic of the Congo is home to most of the western lowland gorilla population. The large, isolated swamp forests help protect the gorillas.
Western lowland gorillas are mostly plant-eaters. They eat roots, young plant parts, fruit, wild celery, tree bark, and pulp found in the forests of Central and West Africa. During the wet season, they often eat fruit. In the dry season, they eat fewer soft fruits but still consume other types of fruit. When their favorite fruit is not available, they eat a wider variety of fruits. They may also eat insects sometimes. Herbaceous stems, which are stems of plants that grow from the ground, are a common source of fiber in their diet.
Important food sources are grouped into three categories: staple foods, which are eaten daily or weekly throughout the year; seasonal foods, which are available when they grow; and fallback foods, which are always present but eaten mainly when fruit is scarce. An adult gorilla eats about 18 kilograms (40 pounds) of food each day. Gorillas climb trees up to 15 meters (50 feet) tall to find food. They do not remove all plants from one area because the vegetation grows back quickly, allowing them to stay in a small area for long periods.
Gorillas eat a mix of fruit and leaves, depending on the season. When ripe fruit is available, they eat more fruit than leaves. When ripe fruit is not available, they eat more leaves, herbs, and bark. Fruit is often ripe in July and August during the rainy season, but it is less common in the dry season. Gorillas choose fruit that has high sugar and fiber content for energy and nutrition.
Predation
In Mbeli Bai, western lowland gorillas may view West African slender-snouted crocodiles as a serious threat. This is because they avoid crossing water where crocodiles are present and show strong reactions when they see crocodiles.
Relationship with humans
The presence of western lowland gorillas has helped scientists study how gorillas compare to humans in terms of diseases, behavior, and how they think and communicate. These gorillas are hunted illegally in Africa for their skin and meat, and some are captured to be sold to zoos. Although some people argue that hunting is helpful for local economies, it has greatly contributed to the endangered status of western lowland gorillas. In western Africa, they are also considered pests because they destroy crops by eating them from plantations.
In tropical forests, gorillas are hunted for the bushmeat trade, which sells their meat for food. Logging also harms their habitats, though it may create more plant growth in some areas. Destroying gorilla habitats can damage the entire forest ecosystem. Western lowland gorillas help spread seeds, which benefits many forest animals. If they become extinct, other animals could suffer, and the ecosystem might eventually be harmed.
The western lowland gorilla population in the wild faces many threats. These include cutting down forests, farming, grazing, and growing human settlements that take over forest areas. Human activity is linked to habitat loss and increased hunting for bushmeat. Another threat is low reproduction rates. Female gorillas usually start having babies at 7–8 years old, but most wait until 10–12 years. Males take longer to become strong and dominant enough to reproduce, often not until 15–20 years old. Female gorillas’ ability to have many young decreases by age 18. In zoos, about 30% of female gorillas of childbearing age have only one baby. Gorillas that do not reproduce can still help maintain genetic diversity in zoo populations through artificial reproduction methods.
In the 1980s, researchers estimated there were about 100,000 western lowland gorillas in equatorial Africa. Later studies reduced this number because of hunting and disease. Surveys in 2006 and 2007 found about 125,000 gorillas living in swamp forests near Lake Télé Community Reserve and neighboring dryland forests in the Republic of the Congo. However, gorillas remain at risk from diseases like Ebola, deforestation, and hunting.
In 2002 and 2003, an Ebola outbreak in the Lossi sanctuary population and another in 2004 in the Lokoué forest clearing in Odzala-Kokoua National Park, both in the Republic of the Congo, caused significant population declines. The Lokoué outbreak hurt adult females and group-living gorillas more than solitary males, leading to fewer group-living individuals. The population dropped from 377 to 38 individuals two years after the outbreak and to 40 six years later. It is slowly recovering, with hopes of returning to a healthy population structure through new births and breeding groups. The outbreak also reduced the Maya Nord population from 400 to fewer than that. Because of these events, the International Union for Conservation of Nature (IUCN) changed the status of western lowland gorillas from "endangered" to "critically endangered."
In the northeastern part of the Republic of the Congo, western lowland gorillas are still hunted for bushmeat and for their young to be sold as pets. About 5% of the subspecies is killed each year for this reason. Deforestation in the area has made bushmeat hunting and poaching easier. Increased commercial logging and infrastructure, such as roads, have allowed hunters to go deeper into forests, increasing poaching. The Republic of the Congo has taken steps to protect species like chimpanzees, forest elephants, and western gorillas from poaching and deforestation. These efforts aim to help these species access food and other important resources.
Hunting for bushmeat and cutting down trees in western lowland gorilla habitats have made it harder for them to survive. The IUCN lists them as critically endangered. Like other gorillas, western lowland gorillas help maintain rainforests by spreading seeds. Many organizations have made protecting them a priority. The Wildlife Conservation Society (WCS) works with local communities in the Congo Basin to create wildlife management programs. The WCS also helps reduce the bushmeat trade by enforcing hunting laws and helping local people find alternative sources of protein.
Zoos around the world have 550 western lowland gorillas. The Cincinnati Zoo has the most births of this species in the United States.
In captivity
Stress can lead to long-term physical and behavioral problems in animals living in captivity. These issues include changes in reproductive cycles and behavior, weaker immune systems, imbalances in hormones and growth, lower body weight, increased unusual activities and aggression, and less exploration with more hiding. These stress reactions may be caused by loud noises, bright lights, strong smells, uncomfortable temperatures or humidity, the materials used in enclosures, small living spaces, lack of hiding spots, being too close to humans, regular care and feeding routines, or improper social groups. Using both inside and outside privacy screens on exhibit windows has been shown to reduce stress from seeing too many people, which helps lower repetitive behaviors in gorillas. Playing natural sounds, such as those found in the wild, instead of classical music, rock music, or no sound (which allows visitors' noise and machines to be heard) has also been found to reduce stress behaviors. Changing feeding methods, like adding clover-hay to the floor of an exhibit, can decrease repetitive behaviors while encouraging positive food-related actions.
Repetitive or unusual behaviors are abnormal actions that some animals in captivity may show. These behaviors often differ from how they act in the wild. Captive gorillas may display behaviors such as eating problems, like regurgitating food, eating feces, or re-swallowing food; self-harm or aggression toward other gorillas; pacing, rocking, sucking fingers, lip-smacking, or over-grooming. They may also become overly alert to visitors, showing actions like staring, posturing, or charging. Gorilla groups with young silverbacks (mature males) that are not mixed with other ages or sexes tend to have more aggression and injuries.
One unusual behavior is hair-plucking, which happens in many mammals and birds. Studies show that 15% of western lowland gorillas in zoos and aquariums display hair-plucking behavior, and 62% of zoos and aquariums that house gorillas have at least one gorilla that plucks its own hair. Gorillas that are more solitary or were exposed to another gorilla plucking its hair as a young, immature animal are more likely to develop this behavior themselves.
Recent research on gorilla well-being highlights the importance of looking at each gorilla individually instead of treating all gorillas the same. Factors like age, sex, personality, and past experiences help explain how different stressors affect each gorilla's well-being in unique ways.
Genetics
The gorilla was the second-to-last great ape group to have its complete set of genes mapped out. This happened in 2012. This mapping helped scientists learn more about how humans evolved and where they came from. Even though chimpanzees are the closest living relatives to humans, 15% of human DNA is more similar to that of gorillas. Also, 30% of the gorilla DNA is more similar to human or chimpanzee DNA than chimpanzee DNA is to human DNA. This is less common in parts of the DNA that control how genes work, showing that natural selection has played a big role in the evolution of great apes. This has affected how genes are used in the body. Studying the gorilla genome made scientists question whether the fast changes in hearing-related genes caused the development of human language, because similar changes were also found in gorillas.
In 2013, a study was done to learn more about genetic differences in gorillas by looking at parts of their DNA. The study used samples from 12 western lowland gorillas and 2 eastern lowland gorillas, all living in captivity. The study found that western lowland gorillas are more likely to have two different versions of a gene (heterozygous) than two identical versions (homozygous). Most western lowland gorillas that are not closely related to others (not inbred) have a ratio of homozygous to heterozygous genes between 0.5 and 0.7. Because of these differences, scientists concluded that western lowland gorillas show some variety in their population. The study also looked at how often different gene versions appear (allele frequency spectrum) in western lowland gorillas. This information helps scientists understand population history and how evolution has shaped these animals. The study found that western lowland gorillas have fewer rare gene versions than expected.
Disease
Western lowland gorillas are thought to be one of the animal sources of the HIV/AIDS virus. A virus called SIV, which infects these gorillas, is similar to a type of HIV-1. HIV-1 groups into clusters based on location, often separated by large rivers. These clusters help scientists identify where two human virus groups likely began. In southern Cameroon, scientists tested feces from 2,934 gorillas. Of these, 70 samples reacted to at least one HIV-1 protein. These samples came from four areas in southern Cameroon.
The origin of AIDS is connected to a virus that infects over 40 types of nonhuman primates in Africa. HIV-1 has four groups, each formed when SIV spread from animals to humans. SIV infects African primates, including gorillas and chimpanzees.
Disease has affected the survival of western lowland gorillas. The Ebola outbreak in western and central Africa caused more than 90% of these gorillas to die. Between 2003 and 2004, two Ebola outbreaks reduced their population by two-thirds. Scientists, including Magdalena Bermejo, studied the outbreak in the Republic of Congo. The disease spread to humans through contact with bushmeat. Because of this, the World Conservation Union listed western lowland gorillas as critically endangered. Malaria is another problem for these gorillas. In a study of 51 fecal samples from gorillas, 25 showed signs of a parasite called Plasmodium. A group of these parasites, called Laverania, was found in the samples. Differences in exposure to mosquitoes that carry Plasmodium explain how malaria spreads among gorillas.
Wild western lowland gorillas eat seeds from a plant called "grains of paradise," which may improve their heart health. Gorillas in zoos sometimes have poor heart health, possibly because they do not have access to these seeds. Adult male gorillas are at risk for fibrosing cardiomyopathy, a heart disease that weakens the heart muscle over time.