Gorillas' natural habitats cover tropical or subtropical forests in Sub-Saharan Africa. Although their range covers a small percentage of Sub-Saharan Africa, gorillas cover a wide range of elevations. The mountain gorilla inhabits the Albertine Rift montane cloud forests of the Virunga Volcanoes, ranging in altitude from 2,200 to 4,300 metres (7,200 to 14,100 ft). Lowland gorillas live in dense forests and lowland swamps and marshes as low as sea level, with western lowland gorillas living in Central West African countries and eastern lowland gorillas living in the Democratic Republic of the Congo near its border with Rwanda.^[2]

Etymology

The word "gorilla" comes from the history of Hanno the Navigator, (c. 500 BC) a Carthaginian explorer on an expedition on the west African coast to the area that later became Sierra Leone.^[3][4] Members of the expedition encountered "savage people, the greater part of whom were women, whose bodies were hairy, and whom our interpreters called Gorillae".^[5][6] The word was then later used as the species name, though it is unknown whether what these ancient Carthaginians encountered were truly gorillas, another species of ape or monkeys, or humans.^[7]

Evolution and classification

The closest relatives of gorillas are the other two Homininae genera, chimpanzees and humans, all of them having diverged from a common ancestor about 7 million years ago.^[10] Human gene sequences differ only 1.6% on average from the sequences of corresponding gorilla genes, but there is further difference in how many copies each gene has.^[11] Until recently, gorillas were considered to be a single species, with three subspecies: the western lowland gorilla, the eastern lowland gorilla and the mountain gorilla.^[7][12] There is now agreement that there are two species, each with two subspecies. More recently, a third subspecies has been claimed to exist in one of the species. The separate species and subspecies developed from a single type of gorilla during the Ice Age, when their forest habitats shrank and became isolated from each other.^[2]

Primatologists continue to explore the relationships between various gorilla populations.^[7] The species and subspecies listed here are the ones upon which most scientists agree.

The proposed third subspecies of Gorilla beringei, which has not yet received a trinomen, is the Bwindi population of the mountain gorilla, sometimes called the Bwindi gorilla.

Some variations that distinguish the classifications of gorilla include varying density, size, hair colour, length, culture, and facial widths.^[2] Population genetics of the lowland gorillas suggest that the western and eastern lowland populations diverged ~261 thousand years ago.^[14]

Physical characteristics

Gorillas move around by knuckle-walking, although they sometimes walk bipedally for short distances while carrying food or in defensive situations,^[2] and some Mountain Gorillas use other parts of their hand to aid locomotion (studies of 77 Mountain Gorillas published in 2018 showed 61% only used knuckle walking, but the remainder used knuckle walking plus other parts of their hand—fist walking in ways that do not use the knuckles, using the backs of their hand, and using their palms).^[16] Wild male gorillas weigh 136 to 195 kg (300 to 430 lb), while adult females usually weigh about half as much as adult males at 68–113 kg (150–250 lb).

Adult males are 1.4 to 1.8 m (4 ft 7 in to 5 ft 11 in) tall, with an arm span that stretches from 2.3 to 2.6 m (7 ft 7 in to 8 ft 6 in).

The eastern gorilla is more darkly coloured than the western gorilla, with the mountain gorilla being the darkest of all.

Studies have shown gorilla blood is not reactive to anti-A and anti-B monoclonal antibodies, which would, in humans, indicate type O blood. Due to novel sequences, though, it is different enough to not conform with the human ABO blood group system, into which the other great apes fit.^[24] Like humans, gorillas have individual fingerprints.^[25][26] Their eye colour is dark brown, framed by a black ring around the iris.

Distribution and habitat

Gorillas have a patchy distribution.

Gorillas construct nests for daytime and night use.

Food and foraging

A gorilla's day is divided between rest periods and travel or feeding periods.

Eastern lowland gorillas have more diverse diets, which vary seasonally.

Gorillas rarely drink water "because they consume succulent vegetation that is comprised of almost half water as well as morning dew",^[34] although both mountain and lowland gorillas have been observed drinking.

Behaviour

Gorillas live in groups called troops.

Mature males also tend to leave their groups and establish their own troops by attracting emigrating females.

The silverback is the center of the troop's attention, making all the decisions, mediating conflicts, determining the movements of the group, leading the others to feeding sites, and taking responsibility for the safety and well-being of the troop.

Females may fight for social access to males and a male may intervene.^[41] Male gorillas have weak social bonds, particularly in multiple-male groups with apparent dominance hierarchies and strong competition for mates.

One possible predator of gorillas is the leopard. Gorilla remains have been found in leopard scat, but this may be the result of scavenging.^[45] When the group is attacked by humans, leopards, or other gorillas, an individual silverback will protect the group, even at the cost of his own life.^[46]

Females mature at 10–12 years (earlier in captivity), and males at 11–13 years.

Females will purse their lips and slowly approach a male while making eye contact.

Gorilla infants are vulnerable and dependent, thus mothers, their primary caregivers, are important to their survival.^[40] Male gorillas are not active in caring for the young, but they do play a role in socialising them to other youngsters.^[51] The silverback has a largely supportive relationship with the infants in his troop and shields them from aggression within the group.^[51] Infants remain in contact with their mothers for the first five months and mothers stay near the silverback for protection.^[51] Infants suckle at least once per hour and sleep with their mothers in the same nest.^[52]

Infants begin to break contact with their mothers after five months, but only for a brief period each time.

Twenty-five distinct vocalisations are recognised, many of which are used primarily for group communication within dense vegetation. Sounds classified as grunts and barks are heard most frequently while traveling, and indicate the whereabouts of individual group members.^[54] They may also be used during social interactions when discipline is required. Screams and roars signal alarm or warning, and are produced most often by silverbacks. Deep, rumbling belches suggest contentment and are heard frequently during feeding and resting periods. They are the most common form of intragroup communication.^[44]

For this reason, conflicts are most often resolved by displays and other threat behaviours that are intended to intimidate without becoming physical.

Lifespan

A gorilla's lifespan is normally between 35 and 40 years, although zoo gorillas may live for 50 years or more.

Intelligence

Gorillas are considered highly intelligent.

The following observations were made by a team led by Thomas Breuer of the Wildlife Conservation Society in September 2005. Gorillas are now known to use tools in the wild. A female gorilla in the Nouabalé-Ndoki National Park in the Republic of Congo was recorded using a stick as if to gauge the depth of water whilst crossing a swamp. A second female was seen using a tree stump as a bridge and also as a support whilst fishing in the swamp. This means all of the great apes are now known to use tools.^[58]

In September 2005, a two-and-a-half-year-old gorilla in the Republic of Congo was discovered using rocks to smash open palm nuts inside a game sanctuary.^[59] While this was the first such observation for a gorilla, over 40 years previously, chimpanzees had been seen using tools in the wild 'fishing' for termites. Great apes are endowed with semiprecision grips, and have been able to use both simple tools and even weapons, such as improvising a club from a convenient fallen branch.

Scientific study

The explorer Paul Du Chaillu was the first westerner to see a live gorilla during his travel through western equatorial Africa from 1856 to 1859. He brought dead specimens to the UK in 1861.^[62][63][64]

After World War II, George Schaller was one of the first researchers to go into the field and study primates. In 1959, he conducted a systematic study of the mountain gorilla in the wild and published his work. Years later, at the behest of Louis Leakey and the National Geographic, Dian Fossey conducted a much longer and more comprehensive study of the mountain gorilla. When she published her work, many misconceptions and myths about gorillas were finally disproved, including the myth that gorillas are violent.