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Lifespan, longevity, and ageing

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"Maximum longevity: 59.4 years (captivity) Observations: Physiological observations indicate that chimpanzees develop age-related changes typical of humans at considerably earlier ages, and geriatric chimpanzees were originally defined as animals with 34 years of age and older (http://www.chimpanzoo.org/). Even in captivity, young adult chimpanzees have considerably higher mortality rates than humans (Hill et al. 2001). Therefore, while it is not known whether the pace of ageing is different between chimpanzees and humans, the onset of ageing appears to occur at earlier ages in chimpanzees. The MRDT and IMR were estimated based on mortality rates obtained from field sites (de Magalhaes 2006).+p Determining the maximum longevity of chimpanzees is problematic due to the many anecdotal reports. "Cheeta", a male chimpanzee who participated in some Tarzan movies, has been reported to be 75 years of age and still alive, though these claims have not been verified and might well be overestimated (http://cheetathechimp.org/). One wild-born female called "Little Mama" was estimated to be over 70 years of age at Lion Country Safari in Florida (http://www.lioncountrysafari.com/). Another specimen, called "Gregoire", reportedly died at the age of 66 in Congo after living most of his life in Brazzaville Zoo. In spite of these claims, chimpanzees in captivity have not been confirmed to live for more than 60 years (Richard Weigl 2005). Therefore, although the issue is controversial, the established maximum longevity of chimpanzees remains the 59.4 years that a female called "Gamma" was when she died in 1992 at Yerkes Regional Primate Research Center in Atlanta (Hakeem et al. 1996). Similarly, the maximum longevity in the wild belongs to one 55 year-old female, though there are unverified reports suggesting a longer lifespan (Hill et al. 2001)."
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Conservation Status

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Chimpanzee populations are jeopardized by human expansion into rainforests and mixed forest environments. Humans destroy habitats required by chimpanzees for survival and hunt them for bushmeat. They are listed as an Appendix I species by CITES, and are considered endangered by IUCN redlist. The United States Fish and Wildlife Service consideres the species endangered in the wild, and threatened in captivity outside of the natural range.

US Federal List: endangered

CITES: appendix i

IUCN Red List of Threatened Species: endangered

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Shefferly, N. 2005. "Pan troglodytes" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Pan_troglodytes.html
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Nancy Shefferly, Animal Diversity Web
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Tanya Dewey, Animal Diversity Web
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Associations

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Chimpanzees are hunted as food by humans in many parts of their range. There is no direct evidence of predation on chimps by other animals, although there are some sympatric predators that are likely candidates for taking an occasional chimpanzee--especially young ones. These are leopards (Panthera pardus), pythons (Phython sabae), and martial eagles (Poleamaetus bellicosus).

Known Predators:

  • humans (Homo sapiens)
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Shefferly, N. 2005. "Pan troglodytes" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Pan_troglodytes.html
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Nancy Shefferly, Animal Diversity Web
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Tanya Dewey, Animal Diversity Web
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Morphology

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Adult chimpanzees have a head and body length ranging between 635 and 925 mm. When standing erect, they are 1 to 1.7 m tall. In the wild, males weigh between 34 and 70 kg, whereas females are slightly smaller, weighing between 26 and 50 kg. In captivity individuals typically attain greater weights, with the top weight reaching 80 kg for males and 68 kg for females. Although data from individual subspecies are not available, it appears that P. t. schweinfurthi is smaller than P. t. verus, which is smaller than P. t. troglodytes. Some of the differences seen between captive chimps and wild chimps may be due to subspecific differences in size.

The arms are long, such that the spread of the arms is 1.5 times the height of an individual. Legs are shorter than are the arms, which allows these animals to walk on all fours with the anterior portion of the body higher than the posterior. Chimpanzees have very long hands and fingers, with short thumbs. This hand morphology allows chimpanzees to use their hands as “hooks” while climbing, without interference from the thumb. In trees, chimpanzees may move by swinging from their arms, in a form of brachiation. Although useful in locomotion, the shortness of the thumb relative to the fingers prevents precision grip between the index finger and thumb. Instead, fine manipulations require using the middle finger in opposition to the thumb.

The long hands of chimpanzees also function in quadrupedal locomotion. Fingertips are typically curled upward into the palm during locomotion, and the weight is borne along backs of the fingers. Much of the length of the hand thus contributes to the length of the forelimbs while walking. In combination with the short legs, this gives the back a downward slope from neck to rump, and orients the head into a forward facing position.

Chimpanzees have prominent ears, and a prominent superorbital crest. This gives the brows a somewhat rigid and bony appearance. A sagittal crest may be present on very large individuals, but is not common. There is no nuchal crest. Cranial capacity of these animals ranges from 320 to 480 cc. The face is slightly prognathic. The lips protrude and are very flexible, allowing an individual to accomplish many tasks through labial manipulation.

Dentition is typical of primates. The dental arch is square in shape, and there is a prominent diastema. Canines are large, as are molars. Molars decrease in size toward the back of the mouth, and lack the enamel wrinkling seen in orangutans.

The face of adults is typically black, or mottled with brown. Hair is black to brown, and there is no underfur present. There may be some white hairs around the face (looking a bit like a white beard in some individuals). Infant chimpanzees have a white tuft of hair on their rumps, which identifies their age quite clearly. This white tail tuft is lost as an individual ages.

Individuals of both sexes are prone to lose the hair on the head as they age, producing a bald patch behind the brow ridge. Graying of hairs in the lumbar region and on the back is common with age, also.

Range mass: 26 to 70 kg.

Range length: 635 to 925 mm.

Other Physical Features: endothermic ; homoiothermic; bilateral symmetry

Sexual Dimorphism: male larger

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Shefferly, N. 2005. "Pan troglodytes" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Pan_troglodytes.html
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Nancy Shefferly, Animal Diversity Web
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Tanya Dewey, Animal Diversity Web
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Life Expectancy

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Chimpanzees can live from 40 to 60 years.

A variety of ailments trouble chimpanzees in natural habitats, and affect survivorship and longevity. Respiratory diseases, such as colds and coughs, seem prevalent during the rainy season. Gastrointestinal problems, such as diarrhea, peritonitis, and enteritis have been seen and can be lethal, especially in young or very old chimps. Skin ulcers and osteoarthritis have affected some chimpanzees. One chimpanzee at Gombe had a goiter. Abcesses of various sorts have been seen, as have rashes, fungal diseases, and parasitic infections. Even human diseases may sometimes affect wild chimpanzees. A polio epidemic in local human populatons devastated the chimpanzees at Gombe Stream National Park in 1966, killing some and leaving many chimpanzees partially paralyzed.

In addition to disease, injuries are an important source of infections and can lead to mortality in chimpanzees. Injuries may be sustained during falls, or as a result of aggressive interactions within groups or among neighboring groups.

Range lifespan
Status: captivity:
59 (high) years.

Average lifespan
Status: wild:
51.0 years.

Average lifespan
Sex: male
Status: captivity:
56.0 years.

Average lifespan
Status: captivity:
44.5 years.

Average lifespan
Status: wild:
60.0 years.

Average lifespan
Status: captivity:
53.0 years.

Average lifespan
Status: wild:
45.0 years.

Average lifespan
Status: wild:
50.0 years.

Average lifespan
Status: captivity:
40.0 years.

Average lifespan
Sex: female
Status: captivity:
59.4 years.

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Shefferly, N. 2005. "Pan troglodytes" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Pan_troglodytes.html
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Nancy Shefferly, Animal Diversity Web
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Tanya Dewey, Animal Diversity Web
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Habitat

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Chimpanzees utilize a great diversity of habitat types. Although they are typically thought of as living in tropical rainforests, they are also found in forest-savanna mosaic environments, as well as in montain forests at elevations up to 2,750 m. Some populations are known to inhabit primarily savanna habitat.

Range elevation: 0 to 2,750 m.

Habitat Regions: tropical ; terrestrial

Terrestrial Biomes: savanna or grassland ; chaparral ; forest ; rainforest ; scrub forest

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Shefferly, N. 2005. "Pan troglodytes" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Pan_troglodytes.html
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Nancy Shefferly, Animal Diversity Web
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Tanya Dewey, Animal Diversity Web
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Distribution

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Chimpanzees (Pan troglodytes) inhabit the tropical forests of central Africa. They are distributed from about 10 degrees N to 8 degrees S, and from 15 degrees W to 32 degrees E. They are found from Gambia in the west to Uganda in the east, excluding the region bordered by the Congo and Lualaba rivers in central Zaire (Congo) where their sister species, bonobos (Pan paniscus), are found.

There are three recognized subspecies of common chimpanzee. Pan troglodytes verus occurs in the western portions of the range, from Gambia to the Niger river. From the Niger river to Congo, in the central portion of the range, P. troglodytes troglodytes inhabits forested regions. In the far eastern portion of the range, from the northwestern corner of Zaire into western Uganda and Tanzania, P. troglodytes schweinfurthi is found.

Biogeographic Regions: ethiopian (Native )

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Shefferly, N. 2005. "Pan troglodytes" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Pan_troglodytes.html
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Nancy Shefferly, Animal Diversity Web
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Trophic Strategy

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Chimpanzees are broadly omnivorous. They rely heavily on ripe fruits and young leaves, with additional consumption of stems, buds, bark, pith, seeds, and resins. This diet is supplemented by a variety of insects, small vertebrates, and eggs. Soil is sometimes consumed, especially that associated with termite mounds, presumably for the minerals it contains. Diets vary seasonally, as different foods are available at different times of year. Diets also vary geographically. Some foods eaten by chimpanzees in one location are not eaten by chimpanzees in another location, even when the food in question is present at both locations, making it possible that geographical differences in diet are cultural.

Chimpanzees spend the bulk their time feeding or moving from one food source to another. Although foods may be eaten at any time of the day or night, there are typically two major peaks in feeding activities. The first occurs in the morning between 7 and 9 AM. The other is in the afternoon, between 3:30 and 7:30 PM.

Chimps may use a food source until the food is gone, or they may leave before having consumed all of the food. This may depend upon how many chimps are feeding at the site. Variety in the diet seems to be important, and after consuming enough of a particular food, chimps may move on in search of something else to eat.

Chimpanzees are known to hunt other large vertebrates on occasion. The largest animals hunted are bush pigs (Potamochoerus larvatus), colobus monkeys (Colobinae) and baboons (Papio). Although adults are sometimes taken, it is more common for chimps to take young animals.

The predatory behaviors of chimpanzees vary between sexes, individuals, and locations. Males typically consume more meat than females, who seem to specialize more on insect foods than do males. Chimps in the Ivory Coast are known to use more cooperative hunting techniques than the chimpanzees in Tanzania and Uganda. This may be related to differences in the habitat and the behavior of prey. In the Ivory Coast, there is a well developed canopy to the forest, and monkeys may escape chimp predators by climbing high into the trees. In this situation, only cooperative hunting tactics work well for capturing prey. However, at both Gombe and Mahale in Tanzania, the forest is not as dense, and the upper portions of the canopy are not as well developed. As a result, individuals have high success at hunting without enlisting the aid of other chimps.

Another consequence of habitat differences between western and eastern populations of chimpanzees is that in the east, the colobus monkeys preyed cannot take refuge in areas inacessible to chimpanzees. Under these conditions, colobus monkeys are more aggressive toward the chimpanzees. Coupled with the smaller size of the subspecies of chimp found in this area (P. t. schweinfurthi), a different dynamic is established between predator and prey. Chimpanzees in this area are sometimes fearful of adult male monkeys, and are most likely to attack females with young, in the hope of snatching a baby monkey to eat.

Cannibalism has been reported in chimpanzees. The circumstances under which this behavior has been observed vary, although typically chimps do not kill and eat members of their own communities. Most commonly, infants killed during intercommunity aggression may be eaten by the males of the neighboring community. However, in a famous case at Gombe, an adult female and her adolescent daughter were responsible for killing several infants of other females in their community. These infants were eaten, often in front of the mother. This behavior ended when the adult female died. The daughter has shown no inclination toward cannibalism since her mother's death.

Captive chimps commonly exhibit coprophagy and repetitive regurgitation and reingestion. These behaviors appear to be an aberration seen in captivity, as they are not found in wild chimpanzees.

Finally, sick chimpanzees are known to consume a variety of plants with potentially medicinal value. For a more comprehensive discussion of this behavior, please refer to the behavior section.

Animal Foods: birds; mammals; reptiles; eggs; insects

Plant Foods: leaves; wood, bark, or stems; seeds, grains, and nuts; fruit; flowers; sap or other plant fluids

Primary Diet: omnivore

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Shefferly, N. 2005. "Pan troglodytes" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Pan_troglodytes.html
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Nancy Shefferly, Animal Diversity Web
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Tanya Dewey, Animal Diversity Web
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Associations

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As predators, chimpanzees may be a factor in structuring populations of their prey species. Certainly, they have a strong impact on red colobus monkeys (Colobus mitis) at Gombe, and they are likely to have effects on other species as well. As frugivores, chimps may help to disperse seeds of certain plants, either through transportation, or by processing the fruit. There are competitive interactions with other primates, and so chimpanzees may have an additional negative effect on other primate species.

Various parsites, such as intestinal helminths, trematodes, and schistosomes, have been reported in these animals.

Ecosystem Impact: disperses seeds

Commensal/Parasitic Species:

  • intestinal helminths
  • trematodes (Trematoda)
  • schistosomes (Schistosomatidae)
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Shefferly, N. 2005. "Pan troglodytes" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Pan_troglodytes.html
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Nancy Shefferly, Animal Diversity Web
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Tanya Dewey, Animal Diversity Web
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Benefits

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Chimpanzees, being among our closest living relatives, are of tremendous importance in medical research. They are also heavily used in studies of behavior, both in captivity and in the wild. They are the focus of valuable ecotourism enterprises and are popular in zoos. Finally, there is some illegal pet trade in chimpanzees and they are hunted for bushmeat.

Positive Impacts: pet trade ; food ; ecotourism ; research and education

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Shefferly, N. 2005. "Pan troglodytes" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Pan_troglodytes.html
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Nancy Shefferly, Animal Diversity Web
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Benefits

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Chimpanzees have been known to prey upon young humans when the opportunity arises, although the propensity for this behavior is closely related to the presence of waste from human beer-making facilities. Chimpanzees eat these attractive, fermented leavings and become intoxicated, making them more likely to become aggressive. When frightened or aggressive chimpanzees can be dangerous, even to adult humans. In addition, because of their biological similarity to humans, they may serve as a reservoir or host for diseases that affect humans.

Negative Impacts: injures humans (bites or stings, carries human disease)

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Shefferly, N. 2005. "Pan troglodytes" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Pan_troglodytes.html
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Nancy Shefferly, Animal Diversity Web
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Behavior

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Communication in this highly social species is an area of great interest to human researchers. Chimpanzees in captivity have been involved in a number of experiments designed to show how their minds work with regard to signs, signals, and speech. In this account, communication in wild chimpanzees will be discussed first, followed by a discussion of what language studies in captivity have helped us to understand about these animals.

Visual Communication

Chimpanzees communicate with a wide variety of gestures, postures, and facial expressions. In addition, body language and physical cues are used in communication.

Gestures such as arm raising, slapping the ground, or a direct stare are threatening signals used between individuals. Male courtship signals, like branch shaking or foot stamping, may be directed at particular female with whom he wishes to mate. Some facial expressions and vocalizations may also be directed at particular individuals. Loud arm scratching while looking at another individual may be interpreted as a request for grooming.

When excited or fearful, chimps may show low closed grins, full closed grins, or open grins. Snears may also be shown in a fearful context. When the distress is less severe, communicative facial expressions include pouts and horizontal pouts. Compressed lips are often used in threatening displays, and play is generally accompanied by a “play face”, in which the chimp has an open smile with top teeth covered.

Erection of body hair (piloerection) is an important signal communicating excitement. It occurs in most chimps when a strange or frightening stimulus is encountered, during times of aggression, and in other contexts of social excitment. This bristling of the hair is an autonomic response, so it is not under the conscious control of an individual animal. It is a reliable signal of excitement in this species, just as blushing is a reliable signal of embarrassment in humans.

In times of fear induced by the behavior or presence of a dominant animal, chimpanzees never show piloerection. Instead, they have incredibly sleek hair, making them appear smaller. Also, the alpha male chimpanzee in a community, although not frightened or excited, almost always has bristled hair--making him appear even larger than he is.

The swelling of the anogenital skin of females clearly communicates their sexual state to other members of the community. Because the bright pink swelling is highly visible, even at a distance, and can be seen by all, it is considered a non-directed signal.

Auditory Communication

All chimpanzee vocalizations are closely tied to their emotions. Their vocalizations are usually spontaneous, signalling the excitment of arriving at a food source, greeting of old friends, or moments of acute fear or distress. However, producing a particular vocalizations without experiencing the underlying emotion seems to be a task that surpasses a chimpanzee's abilities. Conversely, chimps can learn through experience to suppress a particular call in contexts where the vocalizations may lead to an unwelcome result.

Chimpanzees can be quite vocal. They use a variety of grunts, barks, squeaks, whimpers, and screams. Each call is typically tied to a particular emotional context, such as fear, excitment, bewilderment, or annoyance, so that vocalizations provide information to other chimps about what is happening to other members of their community, even if they cannot see them directly. Subordinate animals direct pant-grunts at more dominant animals. During grooming, chimpanzees often lip-smack or tooth-clack. Play is often accompanied by laughter which, although very raspy-sounding to humans, is similar enough to our own laughter to be easily recognized. Some vocalizations (food grunts) attract other party members to an plentiful food source. Some louder vocalizations (food aaa calls) may attract other chimpanzees in the community from a greater distance. The famous “pant hoot” call of chimps seems to serve as a means of individual identification, and allows friends and family to locate one another even though they may not be within visual range. A detailed listing of calls made by the chimps of Gombe is available in Goodall (1986), and should be consulted by those wishing to know more about specific calls.

That chimpanzees understand the meaning of their vocalizations is clear from contexts in which they purposefully supress vocalizations. Although typically vocal--especially when traveling in groups-- male chimpanzees are almost entirely silent when they are performing a border patrol, or when raiding into the home range of a neighboring group. It is as if they understand that the success of their mission depends upon remaining covert, and that vocalizations will assuredly attract the notice of neighboring animals whom they would prefer to surprise. Similarly, during the course of a consortship, both male and female remain almost entirely silent. This silence may serve two different functions. First, it may prevent the pair from being discovered by other males in the community, disrupting the temporarily monogamous union. Second, because most consortships take place on the outskirts of the community’s range, silence helps the consorting pair to avoid attracting the attention of neighboring males, who may themselves be out patrolling their borders.

Tactile Communication

Various forms of tactile communication occur between pairs of chimps. Physical contact helps to reassure distressed individuals, to placate aggressive individuals, and to appease stress. Embracing, patting, kissing, mounting, and touching all occur in a variety of contexts, including greetings, reconciliations, and reunions. As mentioned in the section on behavior, relaxed physical contact is provided by frequent bouts of social grooming. Such friendly contact helps to cement social bonds. Playful contact, such as finger wrestling or tickling may also occur.

Although the bulk of physical contact seen in chimpanzees is friendly, there is also physical contact associated with aggression. Hitting, slapping, kicking, and biting also occur, as do pounding, dragging, and stamping. Although such aggressive physical contact usually occurs between two individuals as the result of a specific conflict, it may also sometimes be incidental, as when a chimpanzee is in the wrong place at the wrong time, and becomes incorporated into the display of a dominant or irritated individual.

Chemical Communication

Chimpanzees are very interested in smells, and seem to be using them in a variety of contexts. However, the degree to which they use smells, or the specific information they obtain from smells, is not known. Chimpanzees sniff and smell at the anogenital swellings of females. They smell the ground after a mother with a new infant has moved away, apparently trying to catch the scent of the newborn. Individual chimps may have unique odors, recognized by their fellows, but research on this point is lacking. Wild chimpanzees sometimes appear to use scent cues in tracking missing family members. Olfactory cues may be used in helping males to identifiy the approach of ovulation in females, although the specific mechanism or chemicals used for this have not been described.

Communication Studies in Captivity

Although wild chimpanzees have complex communication, they do not possess what we would call language. They do not use specific calls to identify specific objects or individuals. Indeed, they seem unable to produce vocalizations at will, instead uttering cries and calls as a result of impulsive emotions. However, in spite of having no true language, the mental function of chimpanzees is well developed and they possess many of the cognitive abilities necessary for language to develop, as studies of their acquisition of lexigrams (keyboard symbols) and sign language have shown.

Chimpanzees can be taught large numbers of signs or symbols, which they can use to respond to questions reliably and repeatably. They can identify sizes, shapes, colors, and can distinguish what attributes of objects make them different (e.g., two circles, one blue, one red, differ in color). They can use abstract concepts and generalize. For example, they can know that a wrench is a tool and a banana is a food. They are able to spontaneously mix and use symbols they know to describe novel objects. For example, one chimpanzee described a cucumber as a “banana which is green”. Further, research has demonstrated that chimpanzees can understand spoken language, responding appropriately to requests, even though they are, themselves, unable to speak.

Communication Channels: visual ; tactile ; acoustic ; chemical

Perception Channels: visual ; tactile ; acoustic ; chemical

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Shefferly, N. 2005. "Pan troglodytes" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Pan_troglodytes.html
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Nancy Shefferly, Animal Diversity Web
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Tanya Dewey, Animal Diversity Web
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Untitled

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The scientific name for this species is somewhat misleading. Pan refers to the greek god of forests, and is not entirely inappropriate for these animals. However the term troglodytes means one who crawls into holes or caves. As demonstrated in the report above, these animals do not typically use caves.

Relationships among the subspecies are of interest to many, especially those seeking to understand human diversity by looking at diversity in our closest relatives. Research on the DNA of these subspecies indicates that P. troglodytes troglodytes and P. troglodytes schweinfurthi are most closely related. Their lineages likely separated around 440,000 years before present. In contrast, the lineage of P. t. verus separated from the other common chimpanzees around 1.58 million years before present. Because of this relatively distant relationship, some researchers believe that P. t. verus may warrant elevation to species status.

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Shefferly, N. 2005. "Pan troglodytes" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Pan_troglodytes.html
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Nancy Shefferly, Animal Diversity Web
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Tanya Dewey, Animal Diversity Web
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Reproduction

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Chimpanzee reproduction is very complex, and many misconceptions arose early in the study of these animals about the nature of their mating system. Both males and females are known to mate with multiple partners, so they can be considered polygynandrous. However, at times a male may control sexual access to a female, preventing other males from mating with her. A male may do this either through force and dominance in a group mating situation, or by taking the female on a consortship away from other males and thereby securing exclusive sexual access to her. Each of these situations will be discussed at length below.

It is important to note that copulation may serve a number of social functions in this species. Females and males mate more often than would be necessary to ensure impregnation. Copulation may help to develop bonds between males and females. It may funtion in establishing and maintaining group unity.

Females have an estrus cycle which lasts approximately 36 days. During the course of this cycle, as her hormone levels change, a female experiences changes is the size, shape, and color of her genital skin. As circulating estrogens increase during the follicular phase of the cycle, the size of the swelling increases. When the anogenital skin is fully engorged, it is typically bright pink, and can measure from 938 to 1400 cc. The state of maximal tumescence is of variable length in different individuals and at different stages of maturity, but lasts an average of 6.5 days. It is during this time that females are sexually receptive and that the bulk of copulations with mature males occur.

The anogenital swelling of females is very important in the sexual behavior of these animals. Most copulations involving mature males and females (96.2%) seen at the Gombe Stream National Park in Tanzania were observed with females who were maximally tumescent. Of the few copulations observed when females were not maximally swollen, almost 75% were performed by one of two adult males, indicating that the propensity to copulate with females who are not at their peak swelling may be something of an individual idiosynchracy.

The role of the anogenital swelling is many-fold. First, it serves as a signal, visible to males from a great distance, that a female is sexually receptive. Since females tend to be relatively solitary, advertizement of their sexual state to potential mates is essential for reproduction to occur. Males are very interested in the condition of the genitals of each female they encounter. Second, the anogenital swelling may aid females in obtaining food resources including meat. Females who are maximally swollen are often able to supplant more dominant animals at a food source, and are more successful at begging food from males than are unswollen females. Finally, because the males find sexual swellings so attractive, having a maximally engorged genital region may help stranger females to interact peacefully with unfamiliar males as they disperse into new areas.

There are several possible mating scenarios that males and females may encounter. Each of these is based in part on the phase of the female’s cycle. A female may experience one or more of these scenarios during a particular cycle. The types of situations she encounters depend upon a female’s popularity as a sexual partner, how many other females are in estrus at the same time, how popular those females are, and how attractive the female is to the dominant male.

First, during early tumescence, females are mated by infants, juveniles and early adolescents. Infants and juveniles are probably gaining experience through the copulation, they are unlikely to sire offspring. Mature males do not typically copulate with females until they are maximally tumescent, although exceptions to this rule have been observed.

In the second sexual scenario, a female who has achieved maximal tumescence becomes the nucleus of a multi-male party. Other estrus females may travel in the same sexual party. These parties can include some or all of community males. During this phase of a female’s cycle, mating can be promiscuous. The males are typically not comepetitive in this situation, and different males may mate with the female in rapid succession.

The third situation a female might encounter occurs during the second half of maximal tumescence. As the timing of ovulation approaches, dominant males may become possessive and prevent subordinates from copulating with the female. This may involve outright conflicts or, because the dominance relationship between males is well established, may be as simple as the dominant male maintaining close proximity to the female, thereby communicating to his subordinates that the female is no longer up for grabs. Inhibition of the copulations of other males may also occur through threats or attacks. Interestingly, these attacks and threats are often directed at the female, should she express sexual interest in another male. Directing aggression toward the female benefits the male in several ways: 1) it prevents potentially costly fights with other large males, 2) it teaches the female not to copulate with anybody else, and 3) it prevents a third male from mating with the female while the possessive male is punishing another sexual rival. If the possesive male is the highest ranking male in a party, he can inhibit copulations between the female and all other males.

The result of this restriction of mates late in the course of maximal tumescence has the effect of reducing the number of potential sires for any offspring conceived during that estrus cycle. Since sperm remain viable in the fallopian tubes for 48 hours, only males copulating with a female during the last four days of her swelling could fertilize an egg. Even though a female may mate with many males during any particular cycle, not all of these matings have the potential of resulting in impregnation.

The fourth sexual mating situation is the consortship. During a consortship, the female may be led away from the group by a particular male. When consorting, male/female pairs often move to the periphery of the community range. Pairs can stay together up to 3 months. During consortship, both members of the pair maintain relative vocal silence, helping to avoid the attention of other community members, as well as attention from the males of neighboring communities who might behave with hostility toward the pair. Consortships inherently involve the cooperation of female.

Whether males engage in any of these sexual scenarios is highly variable among individuals. A male's preference for, or success in, group mating versus consortships may change depending upon the rise and fall of his fortunes in the constant struggle for dominance between community males. Males who are actively moving up the dominance heirarchy may not spend time away from other males frequently, as in doing so, they may sacrifice social status. Such males, who are in their prime, are more likely to be able to monopolize sexual access to a female in a group situation. High ranking males, especially the alpha male, may take females on consortships, but because of the need to maintain their social standing, these consortships tend to be short in duration.

It may benefit lower-ranking males to initiate consortships when possible, as in consortships there is no mating competition from other males. This may represent the most likely chance the male has to sire offspring. However, it is harder to entice a female to come on a consortship if she is close to ovulation because of competition/possessiveness of other males. A female may benefit from consortships by being able to choose the male with whom she mates. There may also be better access to food and reduced aggression during a consortship as compared to a group mating situation. However, these benefits must be weighed against the potential cost of encountering hostile neighboring chimpanzee groups when spending time in the periphery of the range.

Most consortships (40%) at Gombe Stream National Park were initiated when a female was maximally swollen. Only 16% of consortships were intiated when the female was at variable tumescence, and even fewer consortships began when females were flabby (12%) or pregnant (12%).

To initiate a consortship, a male may gaze toward the female he desires to consort with. This is often accompanied by piloerection (fluffed-out hair), branch shaking, arm stretching, and rocking. If the male succeeds in getting the female to follow him away from the group, he will often walk while looking over his shoulder to make sure she’s still tagging along. This sequence of behaviors may be repeated until the female follows him. If the female does not comply with the male’s wishes, he may become hostile, using aggression to force her to follow him.

During just over half of fertile cycles, females are confined to multi-male groups. About 21% of fertile cycles occur on consortships. The remaining 15% of cycles occur when young females visit males in other communities. In spite of the numbers of fertile cycles which fall under each mating situation, females are disproportionately likely to conceive during consortships. The exact mechanism of this is not understood.

Male mate choice

Because males become possesive of females only late in the course of maximal tumescence, it appears that they have some ability to discern the fertile period of females. The ability of male chimpanzees to gauge the potential fertility of a given female can unquestionably be inferred from patterns of copulations. The increase in the copulation frequency of dominant or older males as ovulation approaches demonstrates that males do not respond the same to females throughout the duration of maximal tumescence. Copulations increase as fertilization and impregnation become more likely. In addition, females who were presumed to be undergoing nonfertile cycles (such as during pregnancy and early in the postpartum period) are typically not sexually popular with mature males.

Aside from potential fertility, one characteristic involved in male mate choice is the age of the female. When presented with two receptive females, males typically show a preference for copulating with the older of the two. Personality traits of individual females may also contribute to males favoring them. A female who is relaxed in the presense of males may be prefered over a more skittish female. Novelty can also play a role in attracting males, since they seem to prefer unfamiliar females over those with whom they have longstanding relationships.

Female mate choice

Females have some ability to choose the males with whom they mate. They may choose to accept or decline a male’s invitation to consortship. This may allow a female to ensure that a particular male who is low in dominance standing, and therefore is less successful in group mating competitions, sires her offspring. The characteristics of males with whom females consort may vary. It seems that the overall “friendliness” of a female’s relationship with a male may play some role in her choice of him as a consortship partner. Whether the male has played with her, groomed her, or engaged in other friendly behaviors with her as she matured or when she is not maximally swollen, may play some role.

Although consenting to a consortship clearly demonstrates choice on the part of the female, it should not be assumed that by staying in a multi-male mating party as ovulation approaches, a female is relinquishing her mate choice. She may be choosing to mate with particular dominant individuals. Or, she may be enhancing her social status and familiarity to all the community males by remaining in the group.

That females discriminate between various males in mating situations is clear. Females avoid copulations with their mature sons and their brothers. There is also some evidence that young females avoid copulations with the older males in their communities (who may potentially have sired them). Although matings do occur between siblings, and occasionally between mothers and their mature sons, the frequency of such matings is much less than would be expected by random pairings of adults within the community.

Initiating a copulation

Copulations are typically initiated by males. The male sits in what is called the “male invite” posture, with his legs flexed and slightly splayed. This displays his erect penis to a potential mate. A male chimpanzee’s penis is bright pink, thin, and tapered to a point. It is very visible against the black hair and pale skin on the male’s lower abdomen and thighs. The value of the erect penis as a signal may be enhanced as the male “flicks” it-- causing the penis to make a rapid “tapping” movement.

In addition to displaying his penis, a courting male may show piloerection (fluffed-out hair). A male may gaze directly at a female. Such a gaze directed at a male rival is an unambiguous threat, but in a sexual context appears to serve as an invitation. He may place his raised hand on a branch overhead, and he may shake the branch. This is all a low-key invitation to the female to present her hindquarters to the male for copulation. If he fails to attract the notice of the female, he may incorporate one or more of the following behaviors into his display: arms outstretched toward the female, a bipedal swagger, a sitting hunch, side to side rocking, swaying of vegetation, or stamping with the foot or knuckes.

Copulation usually occurs in a squatting position after the female crouches and presents her rump to the male. Often, there is no contact between the participants in the mating except at their genitals, although sometimes the male may hold the female.

Ejaculation is usually achieved within 8.8 thrusts. The copulation is ended as the male scoots back or the female darts forward. Males and females have been seen to clean themselves with leaves after copulating. Females not infrequently consume the vaginal plug (congealed semen) after mating.

Mating System: polygynandrous (promiscuous) ; cooperative breeder

There is no clear seasonality in the reproduction of chimpanzees. Females cycle throughout the year, and males copulate with them when they are receptive. Females may have infertile cycles, such as are seen during the period of adolescent sterility, during pregnancy, and early in the postpartum period. Males copulate with females during infertile cycles as well as during fertile cycles. This indicates that there are social functions other than reproduction related to sexual behavior in these animals.

The female reproductive cycle lasts an average of 36 days. As hormone levels change during their cycle, so does the size of the female’s anogenital swelling. There are four main phases to the cycle, including inflation, as the size of the swelling increases, maximal tumescence. when the sexual skin is fully distended, detumescence, when the previously swollen skin looses all turgidity, and flat, when there is no sign of swelling in the anogenital area. Menstruation occurs about nine days after detumescence begins, and lasts for about three days. Ovulation typically occurs on the last day of maximal tumescence.

Females do not reproduce frequently. There is a prolonged period of juvenile dependence during which the offspring relies on the mother for milk, protection, and education. Becuse of the care required by a single offpspring, females cannot produce offspring frequently. The duration of the interbirth interval varies from population to population. Some of the variability may be due to ecological factors (highly productive habitats may allow females to wean their young sooner, or may result in higher rates of infant survival, both of which would affect interbirth intervals). Because different populations may also represent different subspecies, genetic differences in the timing of reproduction may also be involved.

Average interbirth intervals range between 3 and 6 years. Gestation lasts from 202 to 260 days, with a mean of 230 days. Typically, a single young is born, weighing about 2 kg. Twinning is rare, but may be more common than in humans. The infant is carried ventrally by the mother for about 3 to 6 months, after which infants may either ride on their mother’s ventrum or on her back. As the young chimp grows, it increasingly rides on its mother’s back during travel. Although young chimps sometimes walk on their own, they regularly ride on mom until the time of weaning at 3.5 to 4.5 years.

The age of independece is somewhat difficult to judge in this very social species. Young chimps can survive without their mother after they are weaned. However, orphaned chimps are often “adopted” by an older sibling or another close relative, who provides the young chimp with care similar to that which the mother would provide. Young typically travel all the time with their mother until they reach puberty. At puberty, females may become the focus of sexual parties, and males become very interested in establishing themselves in the dominance hierarchy. The activities of maturing chimps around the age of 10 years lead to the parting of ways between a mother and her son.

Females and males enter puberty around the age of 7 years. Females experience a period of adolescent sterility of about three years, during which they cycle, but do not ovulate. During this period, females may transfer into a neighboring chimpanzee community.

As in humans, there is a great deal of variability between different populations and between individuals within populations in the timing of first birth. Female chimpanzees in the wild give birth to their first offspring between the ages of 11 and 23 years. In Tanzania, the average age at which a female first gives birth is between 14.5 and 15 years. Captive chimpanzees reach sexual maturity at younger ages, and have been known to have babies at ages as young a 7.5 years. However, even for well-fed captive animals, the average age at which a female has her first offspring is between 10.5 and 11.15 years.

Like females, males enter puberty around the age of 7 years. Males of any age, including infants, may mate with females, but these copulations are unlikely to result in impregnation of the female. It is not until males attain social maturity that they can effectively compete for access to females who are fertile. In the wild, males are first seen to ejaculate around the age of 9 years. They do not reach adult weight and social maturity until they are about 15 years of age.

In general, chimpanzees can be classified into age categories that represent developmental stages. Until the age of 5 years, chimpanzees are infants. From 5 to 7 years of age, chimpanzees are called juveniles. From 7 to 10 years of age, females are called adolescents. Similarly aged males, from 7 to 12 years are also called adolescents. Females aged 10 to 13 years are considered subadults, as are males aged 12 to 15 years. Females are considered fully adult around the age of 13 years, whereas males reach maturity later, around 15 years.

Breeding interval: The breeding interval varies. Female cycles last about 36 days, and females mate during each cycle. However, if pregnancy ensues, a female may not begin cycling again for 2.5 to 5.5 years.

Breeding season: Chimpanzees may breed throughout the year.

Range number of offspring: 1 to 2.

Average number of offspring: 1.

Range gestation period: 202 to 260 days.

Average gestation period: 230 days.

Range weaning age: 30 to 54 months.

Average time to independence: 6 years.

Range age at sexual or reproductive maturity (female): 10 to 13 years.

Range age at sexual or reproductive maturity (male): 12 to 15 years.

Key Reproductive Features: iteroparous ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; viviparous

Average birth mass: 1821 g.

Average number of offspring: 1.

As is true for most mammals, females provide the bulk of parental care. They carry their offspring, groom them, nurse them, and provide them with opportunites to learn all of the complex behavioral patterns of the species. Young are completely dependent upon their mother until weaning at 3 to 4 years of age, but continue to travel with her and rely heavily upon her for support until they reach adulthood. Bonds with the mother extend throughout an individual’s life. In spite of having achieved independence, both males and females may maintain social bonds with their mother for the remainder of their lives. Although females sometimes emmigrate into a new community of chimpanzees, thereby severing ties with their mother, females may also stay in their natal community as adults. In this case, they may occasionally travel with their mother. Males often use their mother for emotional support when establishing themselves in the male dominance hierarchy. When things are not going particularly well for them, some males may seek the comfort, stability, and quiet that only their natal family can provide.

Because multiple young of different ages may be traveling with their mother at any time, bonds between siblings are also strong. These bonds may remain strong during adulthood, and brothers are frequent allies in intragroup intrigues. Older siblings frequently help to carry infants and play with infants. If the mother should die, older siblings will often assume the care of their immature, weaned siblings.

Males do not provide any direct parental care for young, although they can be quite gentle and playful with young members of their community, especially those still possessing a white tail tuft. Males may indirectly provide protection for their young. Adult males in the community engage in border patrols, which may help to protect the young from potentially dangerous stanger males.

The relationship between a mother and her offspring can have many repercussions during the life of the offspring. Although rank is not technically inherited from the mother, the rank of a female does affect her offspring. A mother who has a high rank, who is confident and relaxed in dealing with other chimpanzees, is likely to have offspring who behave in a similar fashion. Nervous mothers may produce offspring who are fearful of other chimpanzees, and who may not do well in dominance competition.

Because young males do not emmigrate at maturity, they inherit the home range of dominant males. There is no certainty of paternity in this polygynandrous species, so transmission is not directly from father to son and it is unlikely that such relatives would recognize one another as such. Females may remain in their natal community also, although they may transfer to a different chimpanzee social unit upon reaching maturity.

Parental Investment: altricial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Male, Female); pre-weaning/fledging (Provisioning: Female, Protecting: Male, Female); pre-independence (Provisioning: Female, Protecting: Male, Female); post-independence association with parents; extended period of juvenile learning; inherits maternal/paternal territory; maternal position in the dominance hierarchy affects status of young

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Shefferly, N. 2005. "Pan troglodytes" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Pan_troglodytes.html
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Biology

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Chimpanzees are highly intelligent, social animals with a startlingly complex culture. They live in stable communities which range in size from 15 to 150 members, although individuals tend to travel and forage in much smaller groups (4). Males stay in their natal community for life and male relationships are ordered in a strict linear hierarchy, which allows many disputes to be settled without the need for violence (4); males are also dominant over all of the females in the group (5). Chimpanzees are long-lived and do not reach sexual maturity until they are around 10 to 11 years old, with inter-birth intervals of around five years (5). Young chimpanzees develop slowly and are weaned at around four years old (4), although they retain strong ties with their mother after this. Chimps are active throughout the day and feed mainly on fruit, supplementing their diet with leaves, flowers, seeds and insects when fruit is scarce (4). Meat is a favourite food for chimps and groups will cooperate together to hunt and kill monkeys (5). Chimpanzees are remarkably dextrous and are one of the few species to exhibit tool use; from fly wisps and nutcrackers to rods used to probe for ants and termites (4). Chimps exhibit complex communication in the form of expressions, postures and calls (5); social grooming is vital for maintaining bonds between individuals (4).
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Conservation

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Chimpanzees are protected by law throughout their range although this is often poorly enforced. The precise impact of the bushmeat trade is currently being investigated by the Bushmeat Working Group, part of the Convention on International Trade in Endangered Species (CITES) (9). The United Nations Environmental Program (UNEP) has recently recognised the urgent need to protect our closest relatives and has established a Great Ape Survival Project (GRASP) aimed at identifying the conservation initiatives required to secure the future of the apes and obtaining political support and funding to allow these to be achieved (10). The Jane Goodall Institute, amongst other organisations, is involving local people in chimp conservation in the form of sanctuaries and education programmes (5).
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Description

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Along with the pygmy chimp or bonobo (Pan paniscus), the chimpanzee is the closest living relative (4) to humans and is estimated to share 98 percent of our genes (6). There are currently four recognised subspecies of chimpanzee, showing differences in appearance and geographic range: the western or masked chimpanzee (Pan troglodytes verus), central or black-faced chimpanzee (P. t. troglodytes), eastern or long-haired chimpanzee (P. t. schweinfurthii) and the eastern Nigeria chimpanzee (P. t. vellerosus) (3). They all have the characteristic chimpanzee body shape with longer arms than legs, together with opposable thumbs and big toes (5). The bare skin on the face, ears, palms, and soles of the feet is pinkish to black (5), whilst the rest of the body is covered with brown to black hairs (6). Chimpanzees have very expressive features with their bulging eyebrows and protrusive lips (6). The long arms and fingers and mobile shoulder joints allow chimps to move easily in the trees where they forage and rest (4). The majority of their locomotion however, takes place on the ground in the form of 'knuckle-walking' (4).
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Habitat

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Inhabits a variety of woodlands from humid evergreen forests to deciduous forest and dry savanna woodlands (7).
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Range

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Once found throughout the equatorial forest belt of Africa from the west to east coast of the continent (5), between 13°N and 7°S (2). Populations are today found where tracts of these forests remain and the largest populations are located in Gabon, Zaire and Cameroon (2).
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Status

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Classified as Endangered (EN) on the IUCN Red List 2007 (1), and listed on Appendix I of CITES (3). Subspecies: western or masked chimpanzee (P. t. verus); central or black-faced chimpanzee (P. t. troglodytes); eastern or long-haired chimpanzee (P. t. schweinfurthii); eastern Nigeria chimpanzee (P. t. vellerosus) are all classified as Endangered (EN) on the IUCN Red List 2007 (1).
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Threats

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Habitat loss from mining, forestry and agriculture is one of the major threats to wildlife in equatorial Africa at present (8). Deforestation in west Africa is most severe and remaining pockets of undisturbed forest are home to highly fragmented populations of chimpanzees (8). The demand for bushmeat has recently exploded due in part to the growing human population, more accessible forests and the increased availability of firearms (4); some conservationists believe this trade is currently the biggest threat to species' survival in the area. Exposure to human diseases presents a further threat to the chimpanzee (8).
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Niger Delta Terrestrial Vertebrate Associates

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The Niger Delta is an enormous classic distributary system located in West Africa, which stretches more than 300 kilometres wide and serves to capture most of the heavy silt load carried by the Niger River. The peak discharge at the mouth is around 21,800 cubic metres per second in mid-October. The Niger Delta coastal region is arguably the wettest place in Africa with an annual rainfall of over 4000 millimetres. Vertebrate species richness is relatively high in the Niger Delta, although vertebrate endemism is quite low. The Niger Delta swamp forests occupy the entire upper coastal delta. Historically the most important timber species of the inner delta was the Abura (Fleroya ledermannii), a Vulnerable swamp-loving West African tree, which has been reduced below populations viable for timber harvesting in the Niger Delta due to recent over-harvesting of this species as well as general habitat destruction of the delta due to the expanding human population here. Other plants prominent in the inner delta flood forest are: the Azobe tree (Lophira alata), the Okhuen tree (Ricinodendron heudelotii ), the Bitter Bark Tree (Sacoglottis gabonensis), the Rough-barked Flat-top Tree (Albizia adianthifolia), and Pycnanthus angolensis. Also present in its native range is the African Oil Palm (Elaeis guineensis)

There are a number of notable mammals present in the upper (or inner) coastal delta in addition to the The Endangered Chimpanzee (Pan troglodytes). The near-endemic White-cheeked Guenon (Cercopithecus erythrogaster, VU) is found in the inner delta. The Critically Endangered Niger Delta Red Colubus (Procolobus pennantii ssp. epieni), which primate is endemic to the Niger Delta is also found in the inner delta. The limited range Black Duiker (Cephalophus niger) is fournd in the inner delta and is a near-endemic to the Niger River Basin. The restricted distribution Mona Monkey (Cercopithecus mona), a primate often associated with rivers, is found here in the Niger Delta. The Near Threatened Olive Colobus (Procolobus verus) is restricted to coastal forests of West Africa and is found here in the upper delta.

Some of the reptiles found in the upper coastal Niger Delta are the African Banded Snake (Chamaelycus fasciatus); the West African Dwarf Crocodile (Osteolaemus tetraspis, VU); the African Slender-snouted Crocodile (Mecistops cataphractus); the Benin Agama (Agama gracilimembris); the Owen's Chameleon (Chamaeleo oweni); the limited range Marsh Snake (Natriciteres fuliginoides); the rather widely distributed Black-line Green Snake (Hapsidophrys lineatus); Cross's Beaked Snake (Rhinotyphlops crossii), an endemic to the Niger Basin as a whole; Morquard's File Snake (Mehelya guirali); the Dull Purple-glossed Snake (Amblyodipsas unicolor); the Rhinoceros Viper (Bitis nasicornis). In addition several of the reptiles found in the outer delta are found within this inner delta area.

Five threatened marine turtle species are found in the mangroves of the lower coastal delta: Leatherback Sea Turtle (Dermochelys coricea, EN), Loggerhead Sea Turtle (Caretta caretta, EN), Olive Ridley Turtle (Lepidochelys olivacea, EN), Hawksbill Sea Turtle (Eretomychelys imbricata, CR), and Green Turtle (Chelonia mydas, EN).

Other reptiles found in the outer NIger Delta are the Nile Crocodile (Crocodylus niloticus), African Softshell Turtle (Trionyx triunguis), African Rock Python (Python sebae), Boomslang Snake (Dispholidus typus), Cabinda Lidless Skink (Panaspis cabindae), Neon Blue Tailed Tree Lizard (Holaspis guentheri), Fischer's Dwarf Gecko (Lygodactylus fischeri), Richardson's Leaf-Toed Gecko (Hemidactylus richardsonii), Spotted Night Adder (Causus maculatus), Tholloni's African Water Snake (Grayia tholloni), Smith's African Water Snake (Grayia smythii), Small-eyed File Snake (Mehelya stenophthalmus), Western Forest File Snake (Mehelya poensis), Western Crowned Snake (Meizodon coronatus), Western Green Snake (Philothamnus irregularis), Variable Green Snake (Philothamnus heterodermus), Slender Burrowing Asp (Atractaspis aterrima), Forest Cobra (Naja melanoleuca), Rough-scaled Bush Viper (Atheris squamigera), and Nile Monitor (Varanus niloticus).

There are a limited number of amphibians in the inner coastal delta including the Marble-legged Frog (Hylarana galamensis). At the extreme eastern edge of the upper delta is a part of the lower Niger and Cross River watersheds that drains the Cross-Sanaka Bioko coastal forests, where the near endemic anuran Cameroon Slippery Frog (Conraua robusta) occurs.

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Chimpanzee

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The chimpanzee (/ɪmpænˈzi/; Pan troglodytes), also known as simply the chimp, is a species of great ape native to the forest and savannah of tropical Africa. It has four confirmed subspecies and a fifth proposed one. When its close relative the bonobo was more commonly known as the pygmy chimpanzee, this species was often called the common chimpanzee or the robust chimpanzee. The chimpanzee and the bonobo are the only species in the genus Pan. Evidence from fossils and DNA sequencing shows that Pan is a sister taxon to the human lineage and is humans' closest living relative. 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 40–70 kg (88–154 lb) for males and 27–50 kg (60–110 lb) for females and standing 150 cm (4 ft 11 in).

The chimpanzee lives in groups that 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 hunting and acquiring honey, termites, ants, nuts and water. The species has also been found creating sharpened sticks to spear small mammals. 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.

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. Although many chimpanzees have been kept as pets, their strength, aggressiveness, and unpredictability makes them dangerous in this role. Some hundreds have been kept in laboratories for research, especially in the United States. Many attempts have been made to teach languages such as American Sign Language to chimpanzees, with limited success.

Etymology

The English word chimpanzee is first recorded in 1738.[5] It is derived from Vili ci-mpenze[6] or Tshiluba language chimpenze, with a meaning of "ape".[7] The colloquialism "chimp" was most likely coined some time in the late 1870s.[8] 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.[9][10]

Taxonomy and genetics

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.[11] 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.[9][10][12]

Evolution

Relationships among apes. The branch lengths are a measure of evolutionary distinctness. Based on genome sequencing by The Chimpanzee Sequencing and Analysis Consortium. Figure from Yousaf et al. 2021,[13] adapted from Prado-Martinez et al. 2013.[14]

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.[15]

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; moreover, bonobos split from the common chimpanzee line about 2 million years ago.[16][17] Another 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.[18]

Subspecies and population status

Four subspecies of the chimpanzee have been recognised,[19][20] with the possibility of a fifth:[18][21]

Genome

A draft version of the chimpanzee genome was published in 2005 and encodes 18,759 proteins,[28][29] (compared to 20,383 in the human proteome).[30] The DNA sequences of humans and chimpanzees are very similar and the difference in protein number mostly arises from incomplete sequences in the chimp genome. Both species differ by about 35 million single-nucleotide changes, five million insertion/deletion events and various chromosomal rearrangements.[31] 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.[28][31]

Characteristics

Skeleton

Adult chimpanzees have an average standing height of 150 cm (4 ft 11 in).[32] Wild adult males weigh between 40 and 70 kg (88 and 154 lb)[33][34][35] with females weighing between 27 and 50 kg (60 and 110 lb).[36] 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.[a]

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. Chimpanzees lack the prominent sagittal crest and associated head and neck musculature of gorillas.[39][12]

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. The skin may range from pale to dark, though females develop swelling pink skin when in oestrus.[39][12]

Chimpanzees are adapted for both arboreal and terrestrial locomotion. Arboreal locomotion consists of vertical climbing and brachiation.[40][41] On the ground, chimpanzees move both quadrupedally and bipedally. These movements appear to have similar energy costs.[42] As with bonobos and gorillas, chimpanzees move quadrupedally by knuckle-walking, which probably evolved independently in Pan and Gorilla.[43] Their physical strength 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.[44] According to Japan's Asahiyama Zoo, the grip strength of an adult chimpanzee is estimated to be 200 kg (440 lb),[45] while other sources claim figures of up to 330 kg (730 lb).[b]

Ecology

Overnight nest in a tree

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.[48][49] In Gombe, the chimpanzee mostly uses semideciduous and evergreen forest as well as open woodland.[50] At Bossou, the chimpanzee inhabits multistage secondary deciduous forest, which has grown after shifting cultivation, as well as primary forest and grassland.[51] At Taï, it is found in the last remaining tropical rain forest in Ivory Coast.[52] The chimpanzee has an advanced cognitive map of its home range and can repeatedly find food.[53] 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.[54]

Diet

A mother with young eating Ficus fruit in Kibale National Park, Uganda

The chimpanzee is an omnivorous frugivore. It prefers fruit above all other food items but also eats leaves, leaf buds, seeds, blossoms, stems, pith, bark, and resin.[55][56] 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.[57] 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.[55][58] Insect species consumed include the weaver ant Oecophylla longinoda, Macrotermes termites, and honey bees.[59][60] 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.[61]

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.[56][62] Female chimpanzees appear to consume much less animal flesh than males, according to several studies.[63] Jane Goodall documented many occasions within Gombe Stream National Park of chimpanzees and western red colobus monkeys ignoring each other despite close proximity.[64][54]

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.[65] The two apes may also feed on different species, whether fruit or insects.[66][59][60] Interactions between them can range from friendly and even stable social bonding,[67] to avoidance,[65][68] to aggression and predation on part of chimpanzees.[69]

Mortality and health

Chimpanzee named "Gregoire" on 9 December 2006, born in 1944 (Jane Goodall sanctuary of Tchimpounga, Republic of the Congo)

The average lifespan of a chimpanzee in the wild is relatively short, usually 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.[70] The oldest known male captive chimpanzee to have been documented lived to 66 years,[71] and the oldest female, Little Mama, was over 70 years old.[72]

Leopards prey on chimpanzees in some areas.[73][74] It is possible that much of the mortality caused by leopards can be attributed to individuals that have specialised in chimp-killing.[73] Chimpanzees may react to a leopard's presence with loud vocalising, branch shaking, and throwing objects.[75][73] There is at least one record of chimpanzees killing a leopard cub after mobbing it and its mother in their den.[76] 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.[77]

The chimpanzee louse Pediculus schaeffi is closely related to the human body louse P. humanus.

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.[78]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.[79]

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.[80]

Group structure

Group in Uganda

Chimpanzees live in communities that typically range from around 20 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.[54] 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.[81]

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.[82] 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,"[83] 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.[39] 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.[81]

Mutual grooming, removing lice

Male chimpanzees exist in a linear dominance hierarchy. Top-ranking males tend to be aggressive even during dominance stability.[84] 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.[84][85]

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.[86] 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.[84][87] Chimpanzee alliances can be very fickle, and one member may suddenly turn on another if it is to his advantage.[88]

Males in Mahale National Park, Tanzania

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.[86][88] 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.[89] Females in Taï have also been recorded to form alliances.[90] While chimpanzee social structure is often referred to as patriarchal, it is not entirely unheard of for females to forge coalitions against males.[91] 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.[92] 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.[87]

Chimpanzees have been described as highly territorial and will frequently kill other chimpanzees,[93] 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.[94] 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.[95] 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.[96][88] 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.[97]

Mating and parenting

Infant and mother

Chimpanzees mate throughout the year, although the number of females in oestrus varies seasonally in a group.[98] 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.[99][100]

These alternative mating strategies give females more mating opportunities without losing the support of the males in their community.[100] 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.[101][102] Females sometimes practice infanticide. This may be related to the dominance hierarchy in females or may simply be pathological.[89]

Copulation is brief, lasting approximately seven seconds.[103] The gestation period is eight months.[39] 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.[104][105] Male chimpanzees continue to associate with the females they impregnated and interact with and support their offsping.[106] 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.[107]

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.[107] 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.[108]

Communication

Pant-hoot call made by an adult male, demonstrating the introduction, build-up, climax, and let-down phases.[109]

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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.[110] 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.[111]

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.[111][109] Grunting is made in situations like feeding and greeting.[111] Submissive individuals make "pant-grunts" towards their superiors.[112][89] Whimpering is made by young chimpanzees as a form of begging or when lost from the group.[111] Chimpanzees use distance calls to draw attention to danger, food sources, or other community members.[113] "Barks" may be made as "short barks" when hunting and "tonal barks" when sighting large snakes.[111]

Adult male eastern chimpanzee snatches a dead bushbuck antelope from a baboon in Gombe Stream National Park.

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.[114] While both adults and infants are taken, adult male colobus monkeys will attack the hunting chimps.[115] Male chimpanzees hunt more than females. When caught and killed, the meal is distributed to all hunting party members and even bystanders.[114]

Intelligence and cognition

Drawing of human and chimpanzee skull and brain
Human and chimpanzee skull and brain. Diagram by Paul Gervais from Histoire naturelle des mammifères (1854).

Chimpanzees display numerous signs of intelligence, from the ability to remember symbols[116] to cooperation,[117] tool use,[118] and perhaps language.[119] They are among species that have passed the mirror test, suggesting self-awareness.[120] In one study, two young chimpanzees showed retention of mirror self-recognition after one year without access to mirrors.[121] 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.[122] 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.[123]

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.[116]

In controlled experiments on cooperation, chimpanzees show a basic understanding of cooperation, and recruit the best collaborators.[117] 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.[124]

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.[125]

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.[126] Mothers have been reported to carry around and groom their dead infants for several days.[127]

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.[128] 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.[129]

Tool use

Chimpanzees using twigs to dip for ants

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,[130] nuts,[131][132][130][133] honey,[134] algae[135] or water. Despite the lack of complexity, forethought and skill are apparent in making these tools.[118] Chimpanzees have used stone tools since at least 4,300 years ago.[136]

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.[137][138] At Taï, chimpanzees simply use their hands to extract termites.[118] 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.[139]

Chimpanzees also fish for ants using the same tactic.[140] Ant dipping is difficult and some chimpanzees never master it. West African chimpanzees crack open hard nuts with stones or branches.[140][118] 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.[140] Chimpanzees also use leaves as sponges or spoons to drink water.[141]

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.[142] An eastern chimpanzee has been observed using a modified branch as a tool to capture a squirrel.[143]

Whilst experimental studies on captive chimpanzees have found that many of their species-typical tool-use behaviours can be individually learnt by each chimpanzees,[144] 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.[145]

Language

Hugo Rheinhold's Affe mit Schädel ("Ape with skull"), c. 1893

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.[146] Over a longer period of time, Washoe was reported to have learned over 350 signs.[147]

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.[119] 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.[148]

Relations with humans

In culture

Chimpanzee mask, Gio tribe, Liberia

Chimpanzees are rarely represented in African culture, as people find their resemblance to humans discomforting. The Gio people of Liberia and the Hemba people of the Congo have created masks of the animals. 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.[149]

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).[150] 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.[151][152] Animal rights groups have urged a stop to such acts, considering them abusive.[153]

Poster for the 1931 film Aping Hollywood. Media like this relied on the novelty of performing apes to carry their gags.[150]

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.[150]

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 confiscate chimpanzees from people keeping them as pets.[154] Outside their range, chimpanzees are popular as exotic pets despite their strength and aggression. Even where keeping non-human primates as pets is illegal, the exotic pet trade continues to prosper, leading to injuries from attacks.[155]

Use in research

Hundreds of chimpanzees have been kept in laboratories for research. Most such laboratories either conduct or make the animals available for invasive research,[156] 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".[157] Research chimpanzees tend to be used repeatedly over decades for up to 40 years, unlike the pattern of use of most laboratory animals.[158] 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.[159] Five hundred chimpanzees have been retired from laboratory use in the U.S. and live in animal sanctuaries in the U.S. or Canada.[156]

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.[159] 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.[159][160] However, in 2007, the NIH made the moratorium permanent.[161]

Ham, the first great ape in space, before being inserted into his Mercury-Redstone 2 capsule on 31 January 1961

Other researchers argue that chimpanzees either should not be used in research, or should be treated differently, for instance with legal status as persons.[162] 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.[159] A recent study suggests chimpanzees which are retired from labs exhibit a form of post-traumatic stress disorder.[163] 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."[159]

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.[164] Trials already under way were however allowed to run their course.[165] Chimpanzees including the female Ai have been studied at the Primate Research Institute of Kyoto University, Japan, formerly directed by Tetsuro Matsuzawa, since 1978. Some 12 chimpanzees are currently held at the facility.[166]

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.[167][168]

Field study

Feeding station at Gombe, where Jane Goodall used to feed and observe the chimpanzees

Jane Goodall undertook the first long-term field study of the chimpanzee, begun in Tanzania at Gombe Stream National Park in 1960.[169] Other long-term studies begun in the 1960s include A. Kortlandt's in the eastern Democratic Republic of the Congo and Toshisada Nishida's in Mahale Mountains National Park in Tanzania.[170][171] Current understanding of the species' typical behaviours and social organisation has been formed largely from Goodall's ongoing 60-year Gombe research study.[172][173][94]

Attacks

Chimpanzees have attacked humans.[174][175] 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.[176] Human interactions with chimpanzees may be especially dangerous if the chimpanzees perceive humans as potential rivals.[177] At least six cases of chimpanzees snatching and eating human babies are documented.[178]

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 (in the St. James Davis Chimpanzee Attack) while he and his wife were celebrating the birthday of their former pet chimpanzee.[179][180] 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.[181][182]

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 is largely confined to west Africa.[183] 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.[184][185] 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.[183]

Status and conservation

Cameroonian chimpanzee at a rescue centre after its mother was killed by poachers

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,[3] a decrease from about a million chimpanzees in the early 1900s.[186] 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.[4]

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.[3]

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.[3]

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 human populations grow, so does the risk of disease transmission between humans and chimpanzees.[3]

See also

Notes

  1. ^ One captive male, "Kermit", attained a height of 168 cm (5 ft 6 in) and a body weight of 82 kg (181 lb) when he was 11 years old.[37] As a fully grown adult, he weighed almost 136 kg (300 lb).[38]
  2. ^ According to A. S. Vanesyan's "Anthropology" (2015), a study by "Vorden" (probably 'Worden' or 'Warden') reported that a 54 kg (119 lb) male chimpanzee squeezed 330 kg (730 lb) on a dynamometer, while an angry female squeezed 504 kg (1,111 lb) with both hands. Of the hundreds of human students who also participated in the experiment, only one could squeeze more than 200 kg (440 lb) with both hands.[46] The source is said to be "Jan Dembowskiy, The Psychology of Monkeys."[47] This study is listed in: Dembowski, J. (1946). "Psychology of Monkeys". The Chimpanzee: A Topical Bibliography (PDF) (2nd ed.). Warsaw: Ksrazka. p. 359. Archived from the original (PDF) on 20 July 2021. Retrieved 19 March 2021.

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Chimpanzee: Brief Summary

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The chimpanzee (/tʃɪmpænˈzi/; Pan troglodytes), also known as simply the chimp, is a species of great ape native to the forest and savannah of tropical Africa. It has four confirmed subspecies and a fifth proposed one. When its close relative the bonobo was more commonly known as the pygmy chimpanzee, this species was often called the common chimpanzee or the robust chimpanzee. The chimpanzee and the bonobo are the only species in the genus Pan. Evidence from fossils and DNA sequencing shows that Pan is a sister taxon to the human lineage and is humans' closest living relative. 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 40–70 kg (88–154 lb) for males and 27–50 kg (60–110 lb) for females and standing 150 cm (4 ft 11 in).

The chimpanzee lives in groups that 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 hunting and acquiring honey, termites, ants, nuts and water. The species has also been found creating sharpened sticks to spear small mammals. 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.

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. Although many chimpanzees have been kept as pets, their strength, aggressiveness, and unpredictability makes them dangerous in this role. Some hundreds have been kept in laboratories for research, especially in the United States. Many attempts have been made to teach languages such as American Sign Language to chimpanzees, with limited success.

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