CHIMPANZEE

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The chimpanzee (Pan troglodytes), also known as the common chimpanzee, robust chimpanzee, or simply “chimp”, is a species of great ape native to the forests and savannahs of tropical Africa. It has four confirmed subspecies and a fifth proposed subspecies. The chimpanzee and the closely related bonobo (sometimes called the “pygmy chimpanzee”) are classified in the genus Pan. Evidence from fossils and DNA sequencing shows that Pan is a sister taxon to the human lineage and are humans’ closest living relatives.

The chimpanzee is covered in coarse black hair, but has a bare face, fingers, toes, palms of the hands, and soles of the feet. It is larger and more robust than the bonobo, weighing 40–60 kg (88–132 lb) for males and 27–50 kg (60–110 lb) for females and standing 100 to 170 cm (3.3 to 5.6 ft). Its gestation period is eight months. The infant is weaned at about three years old, but usually maintains a close relationship with its mother for several years more. It lives in groups which range in size from 15 to 150 members, although individuals travel and forage in much smaller groups during the day. The species lives in a strict male-dominated hierarchy, where disputes are generally settled without the need for violence. Nearly all chimpanzee populations have been recorded using tools, modifying sticks, rocks, grass and leaves and using them for acquiring honey, termites, ants, nuts and water. The species has also been found creating sharpened sticks to spear small mammals.

The chimpanzee is listed on the IUCN Red List as an endangered species. Between 170,000 and 300,000 individuals are estimated across its range. The biggest threats to the chimpanzee are habitat loss, poaching and disease. Chimpanzees appear in Western popular culture as stereotyped clown-figures, and have featured in entertainments such as chimpanzees’ tea parties, circus acts and stage shows. They are sometimes kept as pets, though their strength and aggressiveness makes them dangerous in this role. Some hundreds have been kept in laboratories for research, especially in America. Many attempts have been made to teach languages such as American Sign Language to chimpanzees, with limited success; for example, sentences do not grow in length with more training.

The English name “chimpanzee” is first recorded in 1738. It is derived from Vili ci-mpenze or Tshiluba language chimpenze, with a meaning of “ape”. The colloquialism “chimp” was most likely coined some time in the late 1870s. The genus name Pan derives from the Greek god, while the specific name troglodytes was taken from the Troglodytae, a mythical race of cave-dwellers.

The first great ape known to Western science in the 17th century was the “orang-outang” (genus Pongo), the local Malay name being recorded in Java by the Dutch physician Jacobus Bontius. In 1641, the Dutch anatomist Nicolaes Tulp applied the name to a chimpanzee or bonobo brought to the Netherlands from Angola. Another Dutch anatomist, Peter Camper, dissected specimens from Central Africa and Southeast Asia in the 1770s, noting the differences between the African and Asian apes. The German naturalist Johann Friedrich Blumenbach classified the common chimpanzee as Simia troglodytes by 1775. Another German naturalist, Lorenz Oken, coined the genus Pan in 1816. The bonobo was recognised as distinct from the common chimpanzee by 1933.

Despite a large number of Homo fossil finds, Pan fossils were not described until 2005. Existing chimpanzee populations in West and Central Africa do not overlap with the major human fossil sites in East Africa, but chimpanzee fossils have now been reported from Kenya. This indicates that both humans and members of the Pan clade were present in the East African Rift Valley during the Middle Pleistocene.

DNA evidence suggests the bonobo and common chimpanzee species separated from each other less than one million years ago (similar in relation between Homo sapiens and Neanderthals). A 2017 genetic study suggests ancient gene flow (introgression) between 200 and 550 thousand years ago from the bonobo into the ancestors of central and eastern chimpanzees. The chimpanzee line split from the last common ancestor of the human line around six million years ago. Because no species other than Homo sapiens has survived from the human line of that branching, both chimpanzee species are the closest living relatives of humans; the lineage of humans and chimpanzees diverged from gorillas (genus Gorilla) about seven million years ago. A 2003 study argues the common chimpanzee should be included in the human branch as Homo troglodytes, and notes “experts say many scientists are likely to resist the reclassification, especially in the emotionally-charged and often disputed field of anthropology”.

Common chimpanzees have a standing height of 100–170 cm (3.3–5.6 ft). Adult males weigh between 40–60 kg (88–132 lb) with females weighing between 27–50 kg (60–110 lb). The build is more robust than the bonobo’s but less than the gorilla’s. The arms of a chimp are longer than its legs, and can reach below the knees. The hands have long fingers with short thumbs and flat fingernails. The feet are adapted for grasping, the big toe being opposable. A chimp’s head is rounded with a prominent and prognathous face. It has forward-facing eyes, a small nose, rounded non-lobed ears, a long mobile upper lip and, in adult males, sharp canine teeth. Chimps lack the prominent sagittal crest and associated head and neck musculature of gorillas.

Chimpanzee bodies are covered by coarse hair, except for the face, fingers, toes, palms of the hands, and soles of the feet. Chimps lose more hair as they age, and develop bald spots. The hair of a chimp is typically black but can be brown or ginger. As they get older, white or grey patches may appear, particularly on the chin and lower region. The skin may range from pale to dark, females develop swelling pink skin when in oestrus.

Chimpanzees are adapted for both arboreal and terrestrial locomotion. Arboreal locomotion consists of vertical climbing and brachiation. On the ground, chimps move both quadrupedally and bipedally, which appear to have similar energy costs. As with bonobos and gorillas, chimps move quadrupedally by knuckle-walking, which probably evolved independently in Pan and Gorilla. The physical strength of chimps is around 1.5 times greater than humans, due to higher content of fast twitch muscle fibres, one of the chimpanzee’s adaptations for climbing and swinging.

The common chimpanzee is a highly adaptable species. It lives in a variety of habitats, including dry savanna, evergreen rainforest, montane forest, swamp forest and dry woodland-savanna mosaic. In Gombe, the chimpanzee mostly uses semideciduous and evergreen forest as well as open woodland. At Bossou, the chimpanzee inhabits multistage secondary deciduous forests, which have grown after shifting cultivation, as well as primary forests and grasslands. At Taï, it can be found in the last remaining tropical rain forest in Ivory Coast. The chimpanzee has an advanced cognitive map of its home range and can repeatedly find food. The chimpanzee makes a night nest in a tree in a new location every night, with every chimpanzee in a separate nest other than infants or juvenile chimpanzees, which sleep with their mothers.

The chimpanzee is an omnivorous frugivore. It prefers fruit above all other food items but also eats leaves and leaf buds, seeds, blossoms, stems, pith, bark and resin. A study in Budongo Forest, Uganda found that 64.5% of their chimp feeding time concentrated on fruits (84.6% of which being ripe), particularly those from two species of FicusMaesopsis eminii and Celtis durandii. In addition, 19% of feeding time was spent on arboreal leaves, mostly Broussonetia papyrifera and Celtis mildbraedii. While the common chimpanzee is mostly herbivorous, it does eat honey, soil, insects, birds and their eggs, and small to medium-sized mammals, including other primates. Insect species consumed include the weaver ant Oecophylla longinodaMacrotermes termites and honey bees. The western red colobus ranks at the top of preferred mammal prey. Other mammalian prey include red-tailed monkeys, yellow baboons, bush babies, blue duikers, bushbucks, and common warthogs.

Despite the fact that common chimpanzees are known to hunt, and to collect insects and other invertebrates, such food actually makes up a tiny portion of their diet, from as little as 2% yearly to as much as 65 grams of animal flesh per day for each adult chimpanzee in peak hunting seasons. This also varies from troop to troop and year to year. However, in all cases, the majority of their diet consists of fruits, leaves, roots, and other plant matter. Female chimpanzees appear to consume much less animal flesh than males, according to several studies. Jane Goodall documented many occasions within Gombe Stream National Park of chimpanzees and western red colobus monkeys ignoring each other within close proximity.

Chimpanzees do not appear to directly compete with gorillas in areas where they overlap. When fruit is abundant, gorilla and chimp diets converge but diverge when fruit is scarce, gorillas resorting to vegetation. The two apes may also feed on different species, whether fruit or insects. Chimps and gorillas ignore or avoid each other when feeding on the same tree, although one hostile encounter has been documented.

The average lifespan of a chimpanzee is usually less than 15 years, although individuals that reach 12 years may live an additional 15. Wild individuals may live over 27 years and occasionally over 60. Captive chimps live longer with median lifespans of 31.7 years for males and 38.7 years for females. Captive chimps have been recorded to live up to 63 years.

Leopards are recorded to have preyed on chimpanzees in some areas. It is possible that much of the mortality caused by leopards can be attributed to individuals that have specialised in chimp-killing. Chimps may react to a leopard’s presence with loud vocalising, branch shaking and throwing objects. There is at least one record of chimps killing a leopard cub, after mobbing it and its mother in their den. Four chimpanzees could have fallen prey to lions at Mahale Mountains National Park. Although no other instances of lion predation on chimpanzees have been recorded, the larger group sizes of savanna chimps may have developed as a response to threats from these big cats. Chimps may react to lions by fleeing up trees, vocalising or silence.

Chimps and humans share only 50% of their parasite and microbe species. This is due to the differences in environmental and dietary adaptations; human internal parasite species overlap more with omnivorous, savanna-dwelling baboons. The chimpanzee is host to the louse species Pediculus schaeffi, a close relative of P. humanus which infests human head and body hair. By contrast, the human pubic louse Pthirus pubis is closely related to Pthirus gorillae which infests gorillas. A 2017 study of gastrointestinal parasites of wild chimps in degraded forests in Uganda found nine species of protozoa, five nematodes, one cestode, and one trematode. The most prevalent species was the protozoan Troglodytella abrassarti.

Common chimpanzees live in communities that typically range from 20 to more than 150 members, but spend most of their time travelling in small, temporary groups consisting of a few individuals, which may consist of any combination of age and sex classes. Both males and females sometimes travel alone. This fission-fusion society may include groups of four types: all-male, adult females and offspring, both sexes, or one female and her offspring. These smaller groups emerge in a variety of types, for a variety of purposes. For example, an all-male troop may be organized to hunt for meat, while a group consisting of lactating females serves to act as a “nursery group” for the young.

At the core of social structures are males, which roam around, protect group members, and search for food. Males remain in their natal communities, while females generally emigrate at adolescence. As such, males in a community are more likely to be related to one another than females are to each other. Among males there is generally a dominance hierarchy, and males are dominant over females. However, this unusual fission-fusion social structure, “in which portions of the parent group may on a regular basis separate from and then rejoin the rest,” is highly variable in terms of which particular individual chimpanzees congregate at a given time. This is caused mainly by the large measure of individual autonomy that individuals have within their fission-fusion social groups. As a result, individual chimpanzees often forage for food alone, or in smaller groups as opposed to the much larger “parent” group, which encompasses all the chimpanzees which regularly come into contact and congregate into parties in a particular area.

Male chimpanzees exist in a linear dominance hierarchy. Top-ranking males tend to be aggressive even during dominance stability. This is probably due to the chimp’s fission-fusion society, with male chimps leaving groups and returning after extended periods of time. With this, a dominant male is unsure if any “political manoeuvring” has occurred and must re-establish his dominance. Thus, a large amount of aggression occurs 5–15 minutes after a reunion. During aggressive encounters, displays are preferred to attacks.

Males maintain and improve their social ranks by forming coalitions, which have been characterized as “exploitative” and are based on an individual’s influence in agonistic interactions. Being in a coalition allows males to dominate a third individual when they could not by themselves, as politically apt chimps can exert power over aggressive interactions regardless of their rank. Coalitions can also give an individual male the confidence to challenge a dominant male. The more allies a male has, the better his chance of becoming dominant. However, most changes in hierarchical rank are caused by dyadic interactions. Chimpanzee alliances can be very fickle and one member may turn on another if it serves him.

Low-ranking males commonly switch sides in disputes between more dominant individuals. Low-ranking males benefit from an unstable hierarchy and have increased sexual opportunities. In addition, conflicts between dominant males cause them to focus on each other rather than the lower-ranking males. Social hierarchies among adult females tend to be weaker. Nevertheless, the status of an adult female may be important for her offspring. Females in Taï have also been recorded to form alliances. Social grooming appears to be important in the formation and maintenance of coalitions. It is more common among adult males than adult females and between males and females.

Chimpanzees have been described as highly territorial and are known to kill other chimps, although Margaret Power wrote in her 1991 book The Egalitarians that the field studies from which the aggressive data came, Gombe and Mahale, used artificial feeding systems that increased aggression in the chimpanzee populations studied, and might not reflect innate characteristics of the species as a whole as such. In the years following her artificial feeding conditions at Gombe, Jane Goodall described groups of male chimps patrolling the borders of their territory, brutally attacking chimps which had split off from the Gombe group. A study published in 2010 found that the chimpanzees wage wars over land, not mates. Patrol parties from smaller groups are more likely to avoid contact with their neighbours. Patrol parties from large groups even take over a smaller group’s territory, gaining access to more resources, food, and females.

When hunting small monkeys such as the red colobus, chimpanzees hunt where the forest canopy is interrupted or irregular. This allows them to easily corner the monkeys when chasing them in the appropriate direction. Chimps may also hunt as a coordinated team, so that they can corner their prey even in a continuous canopy. During an arboreal hunt, each chimp in the hunting groups has a role. “Drivers” serve to keep the prey running in a certain direction and follow them without attempting to make a catch. “Blockers” are stationed at the bottom of the trees and climb up to block prey that take off in a different direction. “Chasers” move quickly and try to make a catch. Finally, “ambushers” hide and rush out when a monkey nears. While both adults and infants are taken, adult male colobus monkeys will attack the hunting chimps. Male chimps hunt more than females. When caught and killed, the meal is distributed to all hunting party members and even bystanders.

Chimpanzees display numerous signs of intelligence, from the ability to remember symbols to cooperation, tool use, and perhaps language. They are among species that have passed the mirror test, indicating self-awareness. In one study, two young chimpanzees showed retention of mirror self-recognition after one year without access to mirrors. Chimps also display signs of culture among groups, with the learning and transmission of variations in grooming, tool use and foraging techniques leading to localised traditions.

A 30-year study at Kyoto University’s Primate Research Institute has shown that chimps are able to learn to recognise the numbers 1 to 9 and their values. The chimps further show an aptitude for photographic memory, demonstrated in experiments in which the jumbled digits are flashed onto a computer screen for less than a quarter of a second. One chimp, Ayumu, was able to correctly and quickly point to the positions where they appeared in ascending order. Ayumu performed better than human adults who were given the same test.

In controlled experiments on cooperation, chimpanzees show a basic understanding of cooperation, and recruit the best collaborators. In a group setting with a device that delivered food rewards only to cooperating chimpanzees, cooperation first increased, then, due to competitive behaviour, decreased, before finally increasing to the highest level through punishment and other arbitrage behaviour.

Chimpanzees, bonobos, gorillas, and orangutans show laughter-like vocalizations in response to physical contact, such as wrestling, play-chasing, or tickling. This is documented in wild and captive chimpanzees. Common chimpanzee laughter is not readily recognisable to humans as such, because it is generated by alternating inhalations and exhalations that sound more like breathing and panting. Instances in which nonhuman primates have expressed joy have been reported. Humans and chimpanzees share similar ticklish areas of the body, such as the armpits and belly. The enjoyment of tickling in chimpanzees does not diminish with age.

Chimpanzees have displayed different behaviours in response to a dying or dead group member. When witnessing a sudden death, the other group members act in frenzy, with vocalisations, aggressive displays and touching of the corpse. A dying elder elicited care in the others and upon death, the attendance and cleaning of the corpse and avoidance of the spot where it died. The chimps also behaved in a more subdued manner following death. Mothers have been reported to carry around and groom their dead infants.

Scientists have attempted to teach human language to several species of great ape. One early attempt by Allen and Beatrix Gardner in the 1960s involved spending 51 months teaching American Sign Language to a chimpanzee named Washoe. The Gardners reported that Washoe learned 151 signs, and had spontaneously taught them to other chimpanzees. Over a longer period of time, Washoe was reported to have learned over 800 signs.

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