Gorillas' natural habitats cover tropical or subtropical forests in Africa. Although their range covers a small percentage of 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 2,200–4,300 metres (7,200–14,100 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.
The American physician and missionary Thomas Staughton Savage and naturalist Jeffries Wyman first described the western gorilla (they called it Troglodytes gorilla) in 1847 from specimens obtained in Liberia. The name was derived from Greek Γόριλλαι (Gorillai), meaning "tribe of hairy women", described by Hanno the Navigator, a Carthaginian navigator and possible visitor (circa 480 BC) to the area that later became Sierra Leone.
Evolution and classification
The closest relatives of gorillas are chimpanzees and humans, all of the Hominidae having diverged from a common ancestor about 7 million years ago. Human genes differ only 1.6% on average from their corresponding gorilla genes in their sequence, but there is further difference in how many copies each gene has. Until recently there was considered to be a single gorilla species, with three subspecies: the western lowland gorilla, the eastern lowland gorilla and the mountain gorilla. There is now agreement that there are two species with two subspecies each. More recently it has been claimed that a third subspecies exists 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.
Primatologists continue to explore the relationships between various gorilla populations. The species and subspecies listed here are the ones upon which most scientists agree.
|Taxonomy of genus Gorilla||Phylogeny of superfamily Hominoidea|
Some variations that distinguish the classifications of gorilla include varying density, size, hair color, length, culture, and facial widths. There are now thought to be over 100,000 western lowland gorillas in the wild, with 4,000 in zoos; eastern lowland gorillas have a population of 4,000 in the wild and 24 in zoos. Mountain gorillas are the most severely endangered, with an estimated population of about 620 left in the wild and none in zoos.
Gorillas move around by knuckle-walking, although they sometimes walk bipedally for short distances while carrying food or in defensive situations. Adult males, also called silverbacks, range in height 1.65–1.75 metres (5 ft 5 in–5 ft 9 in), and in weight 140–200 kg (310–440 lb). Adult females are often half the size of a silverback, averaging about 1.4 metres (4 ft 7 in) tall and 100 kg (220 lb). Occasionally, a silverback of over 1.8 metres (5 ft 11 in) and 230 kg (510 lb) has been recorded in the wild. Obese gorillas in captivity have reached a weight of 270 kg (600 lb). Gorillas have a facial structure which is described as mandibular prognathism, that is, their mandible protrudes farther out than the maxilla. Adult males also have a prominent sagittal crest.
The eastern gorilla is more darkly colored than the western gorilla, with the mountain gorilla being the darkest of all. The mountain gorilla also has the thickest hair. The western lowland gorilla can be brown or grayish with a reddish forehead. In addition, gorillas that live in lowland forests are more slender and agile than the more bulky mountain gorilla. The eastern gorilla also has a longer face and broader chest than the western gorilla.
Studies have shown that gorilla blood is non-reactive to anti-A and anti-B monoclonal antibodies which would, in humans, indicate type O blood. However, due to novel sequences it is different enough to not conform with the human ABO blood group system, which the other great apes fit into. Like humans, gorillas have individual finger prints. Their eye color is dark brown, framed by a black ring around the iris. Similar to humans, the leading cause of death in gorillas is cardiovascular disease.
Behavior and ecology
Range and habitat
Food and foraging
Eastern lowland gorillas have a more diverse diet which varies seasonally. Leaves and pith are commonly eaten but fruits can make up as much as 25% of their diet. Since fruit is less available, lowland gorillas must travel farther each day and have home ranges that vary from 2.7–6.5 km2 (1.04 to 2.51 mi2) with day ranges 154–2280 m (0.096–1.42 mi). Eastern lowland gorillas will also eat insects, preferably ants. Western lowland gorillas depend on fruits more than the others and they are more dispersed across their range. They travel even further than the other gorilla subspecies, at 1105 m (0.687 mi) per day on average, and have larger home ranges of 7–14 km2 (2.70–5.41 mi2). Western lowland gorillas have less access to terrestrial herbs, although they can access aquatic herbs in some areas. Termites and ants also are also eaten.
Gorillas rarely drink water "because they consume succulent vegetation that is comprised of almost half water as well as morning dew", although both mountain and lowland gorillas have been observed drinking.
One possible predator of gorillas is the leopard. Gorilla remains have been found in leopard scat but it is possible that this may be the result of scavenging. 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. George Schaller reported that a "silverback gorilla and a leopard were both found dead from mutually inflicted wounds".
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. Younger males subordinate to the silverback, known as blackbacks, may serve as backup protection. Blackbacks are aged between 8 and 12 years of age and lack the silver back hair. The bond a silverback has with his females forms the core of gorilla social life. Bonds between them are maintained by grooming and stay close together. Females form strong relationships with males to gain mating opportunities and protection from predators and infanticidal outside males. However aggressive behaviors between males and females do occur but rarely lead to serious injury. Relationships between females may vary. Maternally related females in a troop tend to be friendly towards each other and associate closely. Otherwise, females have few friendly encounters and commonly act aggressively towards each other. Females may fight for social access to males and a male may intervene. Male gorillas have weak social bonds, particularly in multi-male groups with apparent dominance hierarchies and strong competition for mates. However, males in all-male groups tend to have friendly interactions and socialize through play, grooming and staying together, and occasionally they even engage in homosexual interactions.
Gorillas construct nests for daytime and night use. Nests tend to be simple aggregations of branches and leaves about 2 to 5 feet (0.61 to 1.5 m) in diameter and are constructed by individuals. Gorillas, unlike chimpanzees or orangutans, tend to sleep in nests on the ground. The young nest with the mother but construct nests after three years of age, initially close to that of their mother. Gorilla nests are distributed arbitrarily and use of tree species for site and construction appears to be opportunistic. Nest building by great apes is now considered to be not just animal architecture but as an important instance of tool use.
Reproduction and parenting
Females mature at 10–12 years (earlier in captivity); males at 11–13 years. A female’s first ovulatory cycle occurs when she is six years of age and is followed by a two year long period of adolescent infertility. The estrous cycle last 30–33 days with outward ovulation signs subtle compared to that of chimpanzees. The gestation period lasts 8.5 months. Female mountain gorillas first give birth at 10 years of age and have four year interbirth intervals. Males can be fertile before reaching adulthood. Gorillas mate year round.
Females will purse their lips and slowly approach a male while and make eye contact. This serves to urge the male to mount her. If the male does not respond, then she will try to attract his attention by reaching towards him or slapping the ground. In multi-male groups, solicitation indicates female preference. However females can be forced to mate with multiple males. Males incite copulation by approaching a female and displaying at her or touching her and giving a "train grunt". Recently, gorillas have been observed engaging in face-to-face sex, a trait that was once considered unique to humans and the bonobo.
Gorilla infants are vulnerable and dependant and thus mothers, their primary caregivers, are important to their survival. Male gorillas are not active in caring for the young. However they do play a role in socializing them to other youngsters. The silverback has a largely supportive relationship with the infants in his troop and shields them from aggression within the group. Infants remain in contact with their mothers for the first five months and mothers stay near the silverback for protection. Infants will suckle at least once per hour and will sleep with their mothers in the same nest.
Infants begin to break contact with their mothers after five months but only for brief period each time. By 12 months, infants move up to five meters (16.4 ft) from their mothers. At around 18–21 months, the distance between mother and offspring increases and they regularly spend time away from each other. In addition, nursing decreases to once every two hours. Infants spend only half of their time with their mothers by 30 months. They enter their juvenile period at their third year and this lasts until their sixth year. At this time, gorillas are weaned and they sleep in a separate nest from their mothers. After their offspring are weaned, females begin to ovulate and soon become pregnant again. The presence of play partners, including the silverback, minimize conflicts in weaning between mother and offspring.
Gorillas are considered highly intelligent. A few individuals in captivity, such as Koko, have been taught a subset of sign language. Like the other great apes, gorillas can laugh, grieve, have "rich emotional lives," develop strong family bonds, can make and use tools, and can think about the past and future. Some researchers believe that gorillas have spiritual feelings or religious sentiments. Gorillas have been shown to have cultures in different areas revolving around different methods of food preparation, and gorillas will show individual color preferences.
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 that all of the great apes are now known to use tools.
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. 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 a semi-precision grip, and have been able to use both simple tools and even weapons, by improvising a club from a convenient fallen branch.
Interactions with humans
American physician and missionary Thomas Staughton Savage obtained the first specimens (the skull and other bones) during his time in Liberia in Africa. The first scientific description of gorillas dates back to an article by Savage and the naturalist Jeffries Wyman in 1847 in Proceedings of the Boston Society of Natural History, where Troglodytes gorilla is described, now known as the Western Gorilla. Other species of gorilla are described in the next couple of years.
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.
The first systematic study was not conducted until the 1920s, when Carl Akeley of the American Museum of Natural History traveled to Africa to hunt for an animal to be shot and stuffed. On his first trip he was accompanied by his friends Mary Bradley, a mystery writer, and her husband. After their trip, Mary Bradley wrote On the Gorilla Trail. She later became an advocate for the conservation of gorillas and wrote several more books (mainly for children). In the late 1920s and early 1930s, Robert Yerkes and his wife Ava helped further the study of gorillas when they sent Harold Bigham to Africa. Yerkes also wrote a book in 1929 about the great apes.
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. It was not until she published her work that many misconceptions and myths about gorillas were finally disproved, including the myth that gorillas are violent.
Since they came to the attention of western society in the 1860s, gorillas have been a recurring element of many aspects of popular culture and media. For example, gorillas have featured prominently in monstrous fantasy films such as King Kong, and pulp fiction such as the stories of Tarzan and Conan the Barbarian have featured gorillas as physical opponents to the titular protagonists.