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Gorillas move around by knuckle-walking, although they sometimes walk bipedally for short distances while carrying food or in defensive situations.[11] Wild male gorillas weigh 135 to 180 kg (298 to 400 lb) while adult females usually weigh half as much as adult males at 68–113 kg (150–249 lb). Adult males are 1.7 to 1.8 m (5.6 to 5.9 ft) tall, with an arm span that stretches from 2.3 to 2.6 m (7.5 to 8.5 ft). Female gorillas are shorter with smaller arm spans.[12] Adult male gorillas are known as silverbacks due to characteristic silver hair on their backs reaching to the hips. 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 can weigh as much as 270 kg (600 lb).[13] Gorilla facial structure is described as mandibular prognathism, that is, the 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 gorillas. The eastern gorilla also has a longer face and broader chest than the western gorilla.[14]

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.[15] Like humans, gorillas have individual finger prints.[16][17] 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.[18]

Behavior and ecology

Young gorilla in tree Range and habitat

Gorillas have a patchy distribution. The range of the two species is separated by the Congo River and its tributaries. The western gorilla lives in west central Africa, while the eastern gorilla lives in east central Africa. Between the species and even within the species, gorillas live in a variety of habitats and elevations. Gorilla habitat ranges from montane forests to swamps. Eastern gorillas inhabit montane and submontane forests between 650 and 4000 m (2132-13,123 ft) above sea level.[19] Mountain gorillas live in the montane forests at the higher ends of the elevation range, while eastern lowland gorillas live in submontane forests at the lower ends of the elevation range. In addition, eastern lowland gorillas live in montane bamboo forests, as well as lowland forests ranging from 600–3308 m (1969-10,853 ft) in elevation.[20] Western gorillas live in both lowland swamp forests and montane forests, and elevations ranging from sea level to 1600 m (5249 ft).[19] Western lowland gorillas live in swamp and lowland forests ranging up to 1600 m (5249 ft), and Cross River gorillas live in low-lying and submontane forests ranging from 150–1600 m (492–5249 ft).

Food and foraging

Gorillas moving in habitat

Gorilla foraging A gorilla's day is synchronized, divided between rest periods and travel or feeding periods. Diets differ between and within species. Mountain gorillas mostly eat foliage, such as leaves, stems, pith, and shoots, while fruit makes up a very small part of their diets.[21] Mountain gorilla food is widely distributed and both individuals and groups do not have to compete with each other. Their home ranges average 3–15 km2 (1.16–5.79 mi2), and their movements range around 500 m (0.311 mi) or less on an average day.[21] Despite eating a few species in each habitat, mountain gorillas have flexible diets and can live in a variety of habitats.[21]

Eastern lowland gorillas have more diverse diets, which vary seasonally. Leaves and pith are commonly eaten, but fruits can make up as much as 25% of their diets. Since fruit is less available, lowland gorillas must travel farther each day, and their home ranges 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.[22] Western lowland gorillas depend on fruits more than the others and they are more dispersed across their range.[23] 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).[23] Western lowland gorillas have less access to terrestrial herbs, although they can access aquatic herbs in some areas. Termites and ants are also eaten.

Gorillas rarely drink water "because they consume succulent vegetation that is comprised of almost half water as well as morning dew",[24] 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 this may be the result of scavenging.[25] 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.[26] George Schaller reported, "[a] silverback gorilla and a leopard were both found dead from mutually inflicted wounds".[27]

Social structure

Silverback gorilla with one of his females Gorillas live in groups called troops. Troops tend to be made of one adult male or silverback, multiple adult females and their offspring.[28][29][30] However, multiple-male troops also exist.[29] A silverback is typically more than 12 years of age, and is named for the distinctive patch of silver hair on its back, which comes with maturity. Silverbacks also have large canine teeth which also come with maturity. Both males and females tend to emigrate from their natal groups. For mountain gorillas, females disperse from their natal troops more than males.[28][31] Mountain gorillas and western lowland gorillas also commonly transfer to second new groups.[28] Mature males tend to also leave their groups and establish their own troops by attracting emigrating females. However, male mountain gorillas sometimes stay in their natal troops and become subordinate to the silverback. If the silverback dies, these males may be able to become dominant or mate with the females. This behavior has not been observed in eastern lowland gorillas. In a single male group, when the silverback dies, the females and their offspring disperse and find a new troop.[31][32] Without a silverback to protect them, the infants will likely fall victim to infanticide. Joining a new group is likely to be a tactic against this.[31][33] However, while gorilla troops usually disband after the silverback dies, female eastern lowlands gorillas and their offspring have been recorded staying together until a new silverback transfers into the group. This likely serves as protection from leopards.[32] All-male troops have also been recorded.

Silverback gorilla 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[30] 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 staying close together.[34] Females form strong relationships with males to gain mating opportunities and protection from predators and infanticidal outside males.[35] 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.[28] Females may fight for social access to males and a male may intervene.[34] Male gorillas have weak social bonds, particularly in multiple-male groups with apparent dominance hierarchies and strong competition for mates. Males in all-male groups, though, tend to have friendly interactions and socialize through play, grooming and staying together,[30] and occasionally they even engage in homosexual interactions.[36]

Nesting

Gorilla night nest constructed in a tree. Gorillas construct nests for daytime and night use. Nests tend to be simple aggregations of branches and leaves about 2 to 5 ft (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 their mothers, but construct nests after three years of age, initially close to those of their mothers.[37] Gorilla nests are distributed arbitrarily and use of tree species for site and construction appears to be opportunistic.[38] Nest-building by great apes is now considered to be not just animal architecture, but as an important instance of tool use.[38]

Reproduction and parenting

Young gorilla riding on mother 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 period of adolescent infertility.[39] The estrous cycle last 30–33 days, with outward ovulation signs subtle compared to those 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.[39] Males can be fertile before reaching adulthood. Gorillas mate year round.[40]

Females will purse their lips and slowly approach a male while making 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.[41] In multiple-male groups, solicitation indicates female preference, but females can be forced to mate with multiple males.[41] Males incite copulation by approaching a female and displaying at her or touching her and giving a "train grunt".[40] Recently, gorillas have been observed engaging in face-to-face sex, a trait once considered unique to humans and bonobos.[42]

Mother gorilla with 10-day-old infant Gorilla infants are vulnerable and dependent, thus mothers, their primary caregivers, are important to their survival.[33] Male gorillas are not active in caring for the young, but they do play a role in socializing them to other youngsters.[43] The silverback has a largely supportive relationship with the infants in his troop and shields them from aggression within the group.[43] Infants remain in contact with their mothers for the first five months and mothers stay near the silverback for protection.[43] Infants will suckle at least once per hour and will sleep with their mothers in the same nest.[44]

Infants begin to break contact with their mothers after five months, but only for a brief period each time. By 12 months old, 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.[45] In addition, nursing decreases to once every two hours.[44] 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.[43] After their offspring are weaned, females begin to ovulate and soon become pregnant again.[43][44] The presence of play partners, including the silverback, minimize conflicts in weaning between mother and offspring.[45]

Lifespan

A gorilla's lifespan is between 35 and 40 years, although zoo gorillas may live for 50 years and more. Dallas Zoo's Jenny was the oldest living gorilla in captivity[46][47][48] until 2008, when she died at the age of 55. Now it is Colo - on 22 December 2011 she celebrated her 55th birthday at the Columbus Zoo and Aquarium.[49]

Communication

"Gorilla communication" redirects here. It is not to be confused with Guerrilla communication.

Twenty-five distinct vocalizations are recognized, 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.[50] 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.[51] Severe aggression is rare in stable groups, but when two mountain gorilla groups meet, the two silverbacks can sometimes engage in a fight to the death, using their canines to cause deep, gaping injuries.[51]

For this reason, conflicts are most often resolved by displays and other threat behaviors that are intended to intimidate without becoming physical. The ritualized charge display is unique to gorillas. The entire sequence has nine steps: (1) progressively quickening hooting, (2) symbolic feeding, (3) rising bipedally, (4) throwing vegetation, (5) chest-beating with cupped hands, (6) one leg kick, (7) sideways running, two-legged to four-legged, (8) slapping and tearing vegetation, and (9) thumping the ground with palms to end display.[52]

Intelligence

See also: Animal language

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, make and use tools, and think about the past and future.[53] Some researchers believe gorillas have spiritual feelings or religious sentiments.[2] They have been shown to have cultures in different areas revolving around different methods of food preparation, and will show individual color preferences.[2]

Tool use

A female gorilla exhibiting tool use by using a tree trunk as a support whilst fishing herbs 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.[54]

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.[55] 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, by improvising a club from a convenient fallen branch, for example.