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Chimpanzee
Great ape native to the forest and savannah of tropical Africa

The chimpanzee, or common chimpanzee, is a great ape native to tropical Africa with four confirmed subspecies. It shares the genus Pan with the bonobo, and is humans' closest living relative, according to fossil and DNA evidence. Chimpanzees live in male-dominated groups and are known for using tools to hunt and gather food like honey and termites. With a gestation period of eight months and a long maternal bond, they face threats such as habitat loss and poaching, leading to their endangered status on the IUCN Red List. Efforts to teach them American Sign Language have met limited success.

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Etymology

The English word chimpanzee is first recorded in 1738.1 It is derived from Vili ci-mpenze2 or Tshiluba language chimpenze, with a meaning of "ape",3 or "mockman".4 The colloquialism "chimp" was most likely coined some time in the late 1870s.5 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.67

Taxonomy

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.8 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 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 chimpanzee by 1933.91011

Evolution

Further information: Chimpanzee–human last common ancestor

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.121314

According to studies published in 2017 by researchers at George Washington University, bonobos, along with chimpanzees, split from the human line about 8 million years ago; then bonobos split from the common chimpanzee line about 2 million years ago.1516 Another 2017 genetic study suggests ancient gene flow (introgression) between 200,000 and 550,000 years ago from the bonobo into the ancestors of central and eastern chimpanzees.17

Subspecies and population status

Four subspecies of the chimpanzee have been recognised,1819 with the possibility of a fifth:2021

Genome

Main article: Chimpanzee genome project

A draft version of the chimpanzee genome was published in 2005 and encodes 18,759 proteins,3132 (compared to 20,383 in the human proteome).33 The DNA sequences of humans and chimpanzees are very similar and the difference in protein number mostly arises from incomplete sequences in the chimpanzee genome. Both species differ by about 35 million single-nucleotide changes, five million insertion/deletion events and various chromosomal rearrangements.34 Typical human and chimpanzee protein homologs differ in an average of only two amino acids. About 30% of all human proteins are identical in sequence to the corresponding chimpanzee protein. Duplications of small parts of chromosomes have been the major source of differences between human and chimpanzee genetic material; about 2.7% of the corresponding modern genomes represent differences, produced by gene duplications or deletions, since humans and chimpanzees diverged from their common evolutionary ancestor.3536

Characteristics

Adult chimpanzees have an average standing height of 150 cm (4 ft 11 in).37 Wild adult males weigh between 40 and 70 kg (88 and 154 lb),383940 and females weigh between 27 and 50 kg (60 and 110 lb).41 In exceptional cases, certain individuals may considerably exceed these measurements, standing over 168 cm (5 ft 6 in) on two legs and weighing up to 136 kg (300 lb) in captivity.42

The chimpanzee is more robustly built than the bonobo but less than the gorilla. The arms of a chimpanzee 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, and the big toe is opposable. The pelvis is long with an extended ilium. A chimpanzee's head is rounded with a prominent and prognathous face and a pronounced brow ridge. It has forward-facing eyes, a small nose, rounded non-lobed ears and a long mobile upper lip. Additionally, adult males have sharp canine teeth. Like all great apes, it has a dental formula of 2.1.2.32.1.2.3, that is, two incisors, one canine, two premolars, and three molars on both halves of each jaw. Chimpanzees lack the prominent sagittal crest and associated head and neck musculature of gorillas.4344

Chimpanzee bodies are covered by coarse hair, except for the face, fingers, toes, palms of the hands, and soles of the feet. Chimpanzees lose more hair as they age and develop bald spots. The hair of a chimpanzee 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.4546 Chimpanzee skin that is covered with body hair is white, while exposed areas vary: white which ages into a dark muddy colour in eastern chimpanzees, freckled on white which ages to a heavily mottled muddy colour in central chimpanzees, and black with a butterfly-shaped white mask that darkens with age in western chimpanzees.4748 Facial pigmentation increases with age and exposure to ultraviolet light. Females develop swelling pink skin when in oestrus.4950 Like bonobos, male chimpanzees have a long filiform penis with a small baculum, but without a glans.51

Chimpanzees are adapted for both arboreal and terrestrial locomotion. Arboreal locomotion consists of vertical climbing and brachiation.5253 On the ground, chimpanzees move both quadrupedally and bipedally. These movements appear to have similar energy costs.54 As with bonobos and gorillas, chimpanzees move quadrupedally by knuckle-walking, which probably evolved independently in Pan and Gorilla.55 Their muscles are 50% stronger per weight than those of humans due to higher content of fast twitch muscle fibres, one of the chimpanzee's adaptations for climbing and swinging.56 According to Japan's Asahiyama Zoo, the grip strength of an adult chimpanzee is estimated to be 200 kg (440 lb),57 while other sources claim figures of up to 330 kg (730 lb).58

Ecology

The 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.5960 In Gombe, the chimpanzee mostly uses semideciduous and evergreen forest as well as open woodland.61 At Bossou, the chimpanzee inhabits multistage secondary deciduous forest, which has grown after shifting cultivation, as well as primary forest and grassland.62 At Taï, it is found in the last remaining tropical rain forest in Ivory Coast.63 The chimpanzee has an advanced cognitive map of its home range and can repeatedly find food.64 The chimpanzee builds a sleeping nest in a tree in a different location each night, never using the same nest more than once. Chimpanzees sleep alone in separate nests except for infants or juvenile chimpanzees, which sleep with their mothers.65

Diet

The chimpanzee is an omnivorous frugivore. It prefers fruit above all other food, but it also eats leaves, leaf buds, seeds, blossoms, stems, pith, bark, and resin.6667 A study in Budongo Forest, Uganda found that 64.5% of their feeding time concentrated on fruits (84.6% of which being ripe), particularly those from two species of Ficus, Maesopsis eminii, and Celtis gomphophylla. In addition, 19% of feeding time was spent on arboreal leaves, mostly Broussonetia papyrifera and Celtis mildbraedii.68 While the chimpanzee is mostly herbivorous, it does eat honey, soil, insects, birds and their eggs, and small to medium-sized mammals, including other primates.6970 Insect species consumed include the weaver ant Oecophylla longinoda, Macrotermes termites, and honey bees.7172 The red colobus ranks at the top of preferred mammal prey. Other mammalian prey include red-tailed monkeys, infant and juvenile yellow baboons, bush babies, blue duikers, bushbucks, and common warthogs.73

Despite the fact that chimpanzees are known to hunt and to collect both insects and other invertebrates, such food actually makes up a very small 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.7475 Female chimpanzees appear to consume much less animal flesh than males, according to several studies.76 Jane Goodall documented many occasions within Gombe Stream National Park of chimpanzees and western red colobus monkeys ignoring each other despite close proximity.7778

Chimpanzees do not appear to directly compete with gorillas in areas where they overlap. When fruit is abundant, gorilla and chimpanzee diets converge, but when fruit is scarce gorillas resort to vegetation.79 The two apes may also feed on different species, whether fruit or insects.808182 Interactions between them can range from friendly and even stable social bonding,83 to avoidance,8485 to aggression and even predation of infants on the part of chimpanzees.86

Mortality and health

The average lifespan of a wild chimpanzee is relatively short. They usually live less than 15 years, although individuals that reach 12 years may live an additional 15 years. On rare occasions, wild chimpanzees may live nearly 60 years. Captive chimpanzees tend to live longer than most wild ones, with median lifespans of 31.7 years for males and 38.7 years for females.87 The oldest-known male captive chimpanzee to have been documented lived to 66 years,88 and the oldest female, Little Mama, was nearly 80 years old.89

Leopards prey on chimpanzees in some areas.9091 It is possible that much of the mortality caused by leopards can be attributed to individuals that have specialised in killing chimpanzees.92 Chimpanzees may react to a leopard's presence with loud vocalising, branch shaking, and throwing objects.9394 There is at least one record of chimpanzees killing a leopard cub after mobbing it and its mother in their den.95 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, lions likely do kill chimpanzees occasionally, and the larger group sizes of savanna chimpanzees may have developed as a response to threats from these big cats. Chimpanzees may react to lions by fleeing up trees, vocalising, or hiding in silence.96

Chimpanzees 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.97 A 2017 study of gastrointestinal parasites of wild chimpanzees in degraded forest in Uganda found nine species of protozoa, five nematodes, one cestode, and one trematode. The most prevalent species was the protozoan Troglodytella abrassarti.98

Behaviour

Recent studies have suggested that human observers influence chimpanzee behaviour. One suggestion is that drones, camera traps, and remote microphones should be used to record and monitor chimpanzees rather than direct human observation.99

Group structure

Chimpanzees live in communities that typically range from around 15 to more than 150 members but spend most of their time traveling in small, temporary groups consisting of a few individuals. These groups may consist of any combination of age and sexes. Both males and females sometimes travel alone.100 This fission–fusion society may include groups of four types: all-male, adult females and offspring, adults of 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 organised to hunt for meat, while a group consisting of lactating females serves to act as a "nursery group" for the young.101

At the core of social structures are males, which patrol the territory, protect group members, and search for food. Males remain in their natal communities, while females generally emigrate at adolescence. 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.102 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,"103 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.104 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 with each other and congregate into parties in a particular area.105

Male chimpanzees exist in a linear dominance hierarchy. Top-ranking males tend to be aggressive even during dominance stability.106 This is probably due to the chimpanzee's fission-fusion society, with male chimpanzees leaving groups and returning after extended periods of time. With this, a dominant male is unsure if any "political maneuvering" has occurred in his absence and must re-establish his dominance. Thus, a large amount of aggression occurs within five to fifteen minutes after a reunion. During these encounters, displays of aggression are generally preferred over physical attacks.107108

Males maintain and improve their social ranks by forming coalitions, which have been characterised as "exploitative" and based on an individual's influence in agonistic interactions.109 Being in a coalition allows males to dominate a third individual when they could not by themselves, as politically apt chimpanzees can exert power over aggressive interactions regardless of their rank. Coalitions can also give an individual male the confidence to challenge a dominant or larger 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.110111 Chimpanzee alliances can be very fickle, and one member may suddenly turn on another if it is to his advantage.112

Low-ranking males frequently switch sides in disputes between more dominant individuals. Low-ranking males benefit from an unstable hierarchy and often find increased sexual opportunities if a dispute or conflict occurs.113114 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.115 Females in Taï have also been recorded to form alliances.116 While chimpanzee social structure is often referred to as patriarchal, it is not entirely unheard of for females to forge coalitions against males.117 There is also at least one recorded case of females securing a dominant position over males in their respective troop, albeit in a captive environment.118 Social grooming appears to be important in the formation and maintenance of coalitions. It is more common among adult males than either between adult females or between males and females.119

Chimpanzees have been described as highly territorial and will frequently kill other chimpanzees,120 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. Thus, the behaviour may not reflect innate characteristics of the species as a whole.121 In the years following her artificial feeding conditions at Gombe, Jane Goodall described groups of male chimpanzees patrolling the borders of their territory, brutally attacking chimpanzees that had split off from the Gombe group. A study published in 2010 found that the chimpanzees wage wars over territory, not mates.122 Patrols from smaller groups are more likely to avoid contact with their neighbours. Patrols from large groups even take over a smaller group's territory, gaining access to more resources, food, and females.123124 While it was traditionally accepted that only female chimpanzees immigrate and males remain in their natal troop for life, there are confirmed cases of adult males safely integrating themselves into new communities among West African chimpanzees, suggesting they are less territorial than other subspecies.125

Mating and parenting

Chimpanzees mate throughout the year, although the number of females in oestrus varies seasonally in a group.126 Female chimpanzees are more likely to come into oestrus when food is readily available. Oestrous females exhibit sexual swellings. Chimpanzees are promiscuous: during oestrus, females mate with several males in their community, while males have large testicles for sperm competition. Other forms of mating also exist. A community's dominant males sometimes restrict reproductive access to females. A male and female can form a consortship and mate outside their community. In addition, females sometimes leave their community and mate with males from neighboring communities.127128 These alternative mating strategies give females more mating opportunities without losing the support of the males in their community.129 Infanticide has been recorded in chimpanzee communities in some areas, and the victims are often consumed. Male chimpanzees practice infanticide on unrelated young to shorten the interbirth intervals in the females.130131 Females sometimes practice infanticide. This may be related to the dominance hierarchy in females or may simply be pathological.132

Inbreeding was studied in a relatively undisturbed eastern chimpanzee community that displayed substantial bisexual philopatry.133 Despite an increased inbreeding risk incurred by females who do not disperse before reaching reproductive age, these females were still able to avoid producing inbred offspring.134

Copulation is brief, lasting approximately seven seconds.135 The gestation period is eight months.136 Care for the young is provided mostly by their mothers. The survival and emotional health of the young is dependent on maternal care. Mothers provide their young with food, warmth, and protection, and teach them certain skills. In addition, a chimpanzee's future rank may be dependent on its mother's status.137138 Male chimpanzees continue to associate with the females they impregnated and interact with and support their offspring.139 Newborn chimpanzees are helpless. For example, their grasping reflex is not strong enough to support them for more than a few seconds. For their first 30 days, infants cling to their mother's bellies. Infants are unable to support their own weight for their first two months and need their mothers' support.140

When they reach five to six months, infants ride on their mothers' backs. They remain in continual contact for the rest of their first year. When they reach two years of age, they are able to move and sit independently and start moving beyond the arms' reach of their mothers. By four to six years, chimpanzees are weaned and infancy ends. The juvenile period for chimpanzees lasts from their sixth to ninth years. Juveniles remain close to their mothers, but interact an increasing amount with other members of their community. Adolescent females move between groups and are supported by their mothers in agonistic encounters. Adolescent males spend time with adult males in social activities like hunting and boundary patrolling.141 A captive study suggests males can safely immigrate to a new group if accompanied by immigrant females who have an existing relationship with this male. This gives the resident males reproductive advantages with these females, as they are more inclined to remain in the group if their male friend is also accepted.142

Communication

Chimpanzees use facial expressions, postures, and sounds to communicate with each other. Chimpanzees have expressive faces that are important in close-up communications. When frightened, a "full closed grin" causes nearby individuals to be fearful, as well. Playful chimpanzees display an open-mouthed grin. Chimpanzees may also express themselves with the "pout", which is made in distress, the "sneer", which is made when threatening or fearful, and "compressed-lips face", which is a type of display. When submitting to a dominant individual, a chimpanzee crunches, bobs, and extends a hand. When in an aggressive mode, a chimpanzee swaggers bipedally, hunched over and arms waving, in an attempt to exaggerate its size.143 While travelling, chimpanzees keep in contact by beating their hands and feet against the trunks of large trees, an act that is known as "drumming". They also do this when encountering individuals from other communities.144

Vocalisations are also important in chimpanzee communication. The most common call in adults is the "pant-hoot", which may signal social rank and bond along with keeping groups together. Pant-hoots are made of four parts, starting with soft "hoos", the introduction; that gets louder and louder, the build-up; and climax into screams and sometimes barks; these die down back to soft "hoos" during the letdown phase as the call ends.145146 Grunting is made in situations like feeding and greeting.147 Submissive individuals make "pant-grunts" towards their superiors.148149 Whimpering is made by young chimpanzees as a form of begging or when lost from the group.150 Chimpanzees use distance calls to draw attention to danger, food sources, or other community members.151 "Barks" may be made as "short barks" when hunting and "tonal barks" when sighting large snakes.152

Hunting

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. Chimpanzees may also hunt as a coordinated team, so that they can corner their prey even in a continuous canopy. During an arboreal hunt, each chimpanzee 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 takes off in a different direction. "Chasers" move quickly and try to make a catch. Finally, "ambushers" hide and rush out when a monkey nears.153 While both adults and infants are taken, adult male colobus monkeys will attack the hunting chimps.154 When caught and killed, the meal is distributed to all hunting party members and even bystanders.155

Male chimpanzees hunt in groups more than females. Female chimpanzees tend to hunt solitarily. If a female chimpanzee were to participate in the hunting group and catch a Red Colobus, it would likely immediately be taken by an adult male. Female chimpanzees are estimated to hunt ≈ 10-15% of a community's vertebrates.156

Wound care

A 2022 study reported that chimpanzees crushed and applied insects to their own wounds and the wounds of other chimpanzees.157

Intelligence

Further information: Primate cognition

Chimpanzees display numerous signs of intelligence, from the ability to remember symbols158 to cooperation,159 tool use,160 and varied language capabilities.161 They are among species that have passed the mirror test, suggesting self-awareness.162 In one study, two young chimpanzees showed retention of mirror self-recognition after one year without access to mirrors.163 Chimpanzees have been observed to use insects to treat their own wounds and those of others. They catch them and apply them directly to the injury.164 Chimpanzees also display signs of culture among groups, with the learning and transmission of variations in grooming, tool use and foraging techniques leading to localized traditions.165

A 30-year study at Kyoto University's Primate Research Institute has shown that chimpanzees are able to learn to recognise the numbers 1 to 9 and their values. The chimpanzees further show an aptitude for eidetic memory, demonstrated in experiments in which the jumbled digits are flashed onto a computer screen for less than a quarter of a second. One chimpanzee, 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.166

In controlled experiments on cooperation, chimpanzees show a basic understanding of cooperation, and recruit the best collaborators.167 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 behaviours.168

Great apes show laughter-like vocalisations in response to physical contact, such as wrestling, play chasing, or tickling. This is documented in wild and captive chimpanzees. 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.169

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. In one case chimpanzees cared for a dying elder, then attended and cleaned the corpse. Afterward, they avoided the spot where the elder died and behaved in a more subdued manner.170 Mothers have been reported to carry around and groom their dead infants for several days.171

Experimenters now and then witness behaviour that cannot be readily reconciled with chimpanzee intelligence or theory of mind. Wolfgang Köhler, for instance, reported insightful behaviour in chimpanzees, but he likewise often observed that they experienced "special difficulty" in solving simple problems.172 Researchers also reported that, when faced with a choice between two persons, chimpanzees were just as likely to beg food from a person who could see the begging gesture as from a person who could not, thereby raising the possibility that chimpanzees lack theory of mind.173

Tool use

Further information: Tool use by animals

Nearly all chimpanzee populations have been recorded using tools. They modify sticks, rocks, grass, and leaves and use them when foraging for termites and ants,174 nuts,175176177178 honey,179 algae180 or water. Despite the lack of complexity, forethought and skill are apparent in making these tools.181 Chimpanzees have used stone tools since at least 4,300 years ago.182

A chimpanzee from the Kasakela chimpanzee community was the first nonhuman animal reported making a tool, by modifying a twig to use as an instrument for extracting termites from their mound.183184 At Taï, chimpanzees simply use their hands to extract termites.185 When foraging for honey, chimpanzees use modified short sticks to scoop the honey out of the hive if the bees are stingless. For hives of the dangerous African honeybees, chimpanzees use longer and thinner sticks to extract the honey.186

Chimpanzees also fish for ants using the same tactic.187 Ant dipping is difficult and some chimpanzees never master it. West African chimpanzees crack open hard nuts with stones or branches.188189 Some forethought in this activity is apparent, as these tools are not found together or where the nuts are collected. Nut cracking is also difficult and must be learned.190 Chimpanzees also use leaves as sponges or spoons to drink water.191

West African chimpanzees in Senegal were found to sharpen sticks with their teeth, which were then used to spear Senegal bushbabies out of small holes in trees.192 An eastern chimpanzee has been observed using a modified branch as a tool to capture a squirrel.193 Chimpanzees living in Tanzania were found to deliberately choose plants that provide materials that produce more flexible tools for termite fishing.194

Whilst experimental studies on captive chimpanzees have found that many of their species-typical tool-use behaviours can be individually learnt by each chimpanzees,195 a 2021 study on their abilities to make and use stone flakes, in a similar way as hypothesised for early hominins, did not find this behaviour across two populations of chimpanzees—suggesting that this behaviour is outside the chimpanzee species-typical range.196

Language

Main article: Great ape language

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, including her adopted son, Loulis.197 Over a longer period of time, Washoe was reported to have learned over 350 signs.198

Debate is ongoing among scientists such as David Premack about chimpanzees' ability to learn language. Since the early reports on Washoe, numerous other studies have been conducted, with varying levels of success.199 One involved a chimpanzee jokingly named Nim Chimpsky (in allusion to the theorist of language Noam Chomsky), trained by Herbert Terrace of Columbia University. Although his initial reports were quite positive, in November 1979, Terrace and his team, including psycholinguist Thomas Bever, re-evaluated the videotapes of Nim with his trainers, analyzing them frame by frame for signs, as well as for exact context (what was happening both before and after Nim's signs). In the reanalysis, Terrace and Bever concluded that Nim's utterances could be explained merely as prompting on the part of the experimenters, as well as mistakes in reporting the data. "Much of the apes' behaviour is pure drill", he said. "Language still stands as an important definition of the human species." In this reversal, Terrace now argued Nim's use of ASL was not like human language acquisition. Nim never initiated conversations himself, rarely introduced new words, and mostly imitated what the humans did. More importantly, Nim's word strings varied in their ordering, suggesting that he was incapable of syntax. Nim's sentences also did not grow in length, unlike human children whose vocabulary and sentence length show a strong positive correlation.200

Human relations

In culture

Chimpanzees are rarely represented in African culture, as people find them "too close for comfort". The Gio people of Liberia and the Hemba people of the Congo make chimpanzee masks. Gio masks are crude and blocky, and worn when teaching young people how not to behave. The Hemba masks have a smile that suggests drunken anger, insanity or horror and are worn during rituals at funerals, representing the "awful reality of death". The masks may also serve to guard households and protect both human and plant fertility. Stories have been told of chimpanzees kidnapping and raping women.201

In Western popular culture, chimpanzees have occasionally been stereotyped as childlike companions, sidekicks or clowns. They are especially suited for the latter role on account of their prominent facial features, long limbs and fast movements, which humans often find amusing. Accordingly, entertainment acts featuring chimpanzees dressed up as humans with lip-synchronised human voices have been traditional staples of circuses, stage shows and TV shows like Lancelot Link, Secret Chimp (1970–1972) and The Chimp Channel (1999).202 From 1926 until 1972, London Zoo, followed by several other zoos around the world, held a chimpanzees' tea party daily, inspiring a long-running series of advertisements for PG Tips tea featuring such a party.203204 Animal rights groups have urged a stop to such acts, considering them abusive.205

Chimpanzees in media include Judy on the television series Daktari in the 1960s and Darwin on The Wild Thornberrys in the 1990s. In contrast to the fictional depictions of other animals, such as dogs (as in Lassie), dolphins (Flipper), horses (The Black Stallion) or even other great apes (King Kong), chimpanzee characters and actions are rarely relevant to the plot. Depictions of chimpanzees as individuals rather than stock characters, and as central rather than incidental to the plot can be found in science fiction. Robert A. Heinlein's 1947 short story "Jerry Was a Man" concerns a genetically enhanced chimpanzee suing for better treatment. The 1972 film Conquest of the Planet of the Apes, the third sequel of the 1968 film Planet of the Apes, portrays a futuristic revolt of enslaved apes led by the only talking chimpanzee, Caesar, against their human masters.206

As pets

Chimpanzees have traditionally been kept as pets in a few African villages, especially in the Democratic Republic of Congo. In Virunga National Park in the east of the country, the park authorities regularly seize chimpanzees from people keeping them as pets.207 Outside their range, chimpanzees are popular as exotic pets despite their strength and aggression. Even in places where keeping non-human primates as pets is illegal, the exotic pet trade continues to prosper, leading to injuries from attacks.208

Use in research

See also: Countries banning non-human ape experimentation and Animal testing on non-human primates § Chimpanzees in the U.S.

Hundreds of chimpanzees have been kept in laboratories for research. Most such laboratories either conduct or make the animals available for invasive research,209 defined as "inoculation with an infectious agent, surgery or biopsy conducted for the sake of research and not for the sake of the chimpanzee, and/or drug testing".210 Research chimpanzees tend to be used repeatedly over decades for up to 40 years, unlike the pattern of use of most laboratory animals.211 Two federally funded American laboratories use chimpanzees: the Yerkes National Primate Research Center at Emory University in Atlanta, Georgia, and the Southwest National Primate Center in San Antonio, Texas.212 Five hundred chimpanzees have been retired from laboratory use in the US and live in animal sanctuaries in the US or Canada.213

A five-year moratorium was imposed by the US National Institutes of Health in 1996, because too many chimpanzees had been bred for HIV research, and it has been extended annually since 2001.214 With the publication of the chimpanzee genome, plans to increase the use of chimpanzees in America were reportedly increasing in 2006, some scientists arguing that the federal moratorium on breeding chimpanzees for research should be lifted.215216 However, in 2007, the NIH made the moratorium permanent.217

Other researchers argue that chimpanzees either should not be used in research, or should be treated differently, for instance with legal status as persons.218 Pascal Gagneux, an evolutionary biologist and primate expert at the University of California, San Diego, argues, given chimpanzees' sense of self, tool use, and genetic similarity to human beings, studies using chimpanzees should follow the ethical guidelines used for human subjects unable to give consent.219 A recent study suggests chimpanzees which are retired from labs exhibit a form of post-traumatic stress disorder.220 Stuart Zola, director of the Yerkes laboratory, disagrees. He told National Geographic: "I don't think we should make a distinction between our obligation to treat humanely any species, whether it's a rat or a monkey or a chimpanzee. No matter how much we may wish it, chimps are not human."221

Only one European laboratory, the Biomedical Primate Research Centre in Rijswijk, the Netherlands, used chimpanzees in research. It formerly held 108 chimpanzees among 1,300 non-human primates. The Dutch ministry of science decided to phase out research at the centre from 2001.222 Trials already under way were however allowed to run their course.223 Chimpanzees including the female Ai have been studied at the Primate Research Institute of Kyoto University, Japan, formerly directed by Tetsuro Matsuzawa, since 1978. 12 chimpanzees are currently[when?] held at the facility.224

Two chimpanzees have been sent into outer space as NASA research subjects. Ham, the first great ape in space, was launched in the Mercury-Redstone 2 capsule on 31 January 1961, and survived the suborbital flight. Enos, the third primate to orbit Earth after Soviet cosmonauts Yuri Gagarin and Gherman Titov, flew on Mercury-Atlas 5 on 29 November of the same year.225226

Field study

Jane Goodall undertook the first long-term field study of the chimpanzee, begun in Tanzania at Gombe Stream National Park in 1960.227 Other long-term studies begun in the 1960s include Adriaan Kortlandt's in the eastern Democratic Republic of the Congo and Toshisada Nishida's in Mahale Mountains National Park in Tanzania.228229 Current understanding of the species' typical behaviours and social organisation has been formed largely from Goodall's ongoing 60-year Gombe research study.230231232

Attacks

Chimpanzees have attacked humans.233234 In Uganda, several attacks on children have happened, some of them fatal. Some of these attacks may have been due to the chimpanzees being intoxicated (from alcohol obtained from rural brewing operations) and becoming aggressive towards humans.235 Human interactions with chimpanzees may be especially dangerous if the chimpanzees perceive humans as potential rivals.236 At least six cases of chimpanzees snatching and eating human babies are documented.237

A chimpanzee's strength and sharp teeth mean that attacks, even on adult humans, can cause severe injuries. This was evident after the attack and near death of former NASCAR driver St. James Davis, who was mauled by two escaped chimpanzees while he and his wife were celebrating the birthday of their former pet chimpanzee.238239 Another example of chimpanzees being aggressive toward humans occurred in 2009 in Stamford, Connecticut, when a 90-kilogram (200 lb), 13-year-old pet chimpanzee named Travis attacked his owner's friend, who lost her hands, eyes, nose, and part of her maxilla from the attack.240241

Human immunodeficiency virus

Two primary classes of human immunodeficiency virus (HIV) infect humans: HIV-1 and HIV-2. HIV-1 is the more virulent and easily transmitted, and is the source of the majority of HIV infections throughout the world; HIV-2 occurs mostly in west Africa.242 Both types originated in west and central Africa, jumping from other primates to humans. HIV-1 has evolved from a simian immunodeficiency virus (SIVcpz) found in the subspecies P. t. troglodytes of southern Cameroon.243244 Kinshasa, in the Democratic Republic of Congo, has the greatest genetic diversity of HIV-1 so far discovered, suggesting the virus has been there longer than anywhere else. HIV-2 crossed species from a different strain of HIV, found in the sooty mangabey monkeys in Guinea-Bissau.245

Conservation

The chimpanzee is on the IUCN Red List as an endangered species. Chimpanzees are legally protected in most of their range and are found both in and outside national parks. Between 172,700 and 299,700 individuals are thought to be living in the wild,246 a decrease from about a million chimpanzees in the early 1900s.247 Chimpanzees are listed in Appendix I of the Convention on International Trade in Endangered Species (CITES), meaning that commercial international trade in wild-sourced specimens is prohibited and all other international trade (including in parts and derivatives) is regulated by the CITES permitting system.248

The biggest threats to the chimpanzee are habitat destruction, poaching, and disease. Chimpanzee habitats have been limited by deforestation in both West and Central Africa. Road building has caused habitat degradation and fragmentation of chimpanzee populations and may allow poachers more access to areas that had not been seriously affected by humans. Although deforestation rates are low in western Central Africa, selective logging may take place outside national parks.249

Chimpanzees are a common target for poachers. In Ivory Coast, chimpanzees make up 1–3% of bushmeat sold in urban markets. They are also taken, often illegally, for the pet trade and are hunted for medicinal purposes in some areas. Farmers sometimes kill chimpanzees that threaten their crops; others are unintentionally maimed or killed by snares meant for other animals.250

Infectious diseases are a main cause of death for chimpanzees. They succumb to many diseases that afflict humans because the two species are so similar. As the human population grows, so does the risk of disease transmission between humans and chimpanzees.251

See also

  • Animals portal
  • Mammals portal
  • Primates portal
  • Africa portal

Notes

Literature cited

  • Goodall, J. (1986). The Chimpanzees of Gombe: Patterns of Behavior. The Belknap Press of Harvard University Press. ISBN 978-0-674-11649-8.
Wikimedia Commons has media related to Pan troglodytes. Wikispecies has information related to Pan troglodytes.

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