THE COMMON CHIMPANZEE

 The common chimpanzee (Pan troglodytes), also known as the robust chimpanzee, is a species of great ape. Colloquially, the common chimpanzee is often called the chimpanzee (or "chimp"), though this term can be used to refer to both species in the genus Pan: the common chimpanzee and the closely related bonobo, formerly called the pygmy chimpanzee. Evidence from fossils and DNA sequencing show both species of chimpanzees are the sister group to the modern human lineage.

 

 

 

Name

The common chimpanzee was named Simia troglodytes by Johann Friedrich Blumenbach in 1776; Lorenz Oken moved it to the new genus Pan in 1816. The species name troglodytes is a reference to the Troglodytae (literally "cave-goers"), an African people described by Greco-Roman geographers. Blumenbach first used it in his De generis humani varietate nativa liber ("[Book] on the natural varieties of the human genus") in 1776,^[3][4] This book was based on his dissertation presented one year before (it had a date 16 Sep 1775 printed on its title page) to the University of Göttingen for internal use only,^[5] thus the dissertation did not meet the conditions for published work in the sense of zoological nomenclature.^[6]
The English name chimpanzee is first recorded in 1738. It is derived from a Tshiluba language term kivili-chimpenze, with a meaning of "mockman" or possibly just "ape". The colloquialism "chimp" was most likely coined some time in the late 1870s.^[7]

Evolutionary history

 


 Despite a large number of Homo fossil finds, chimpanzee fossils (genus Pan) 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. However, chimpanzee fossils have now been reported from Kenya. This would indicate that both humans and members of the Pan clade were present in the East African Rift Valley during the Middle Pleistocene.
According to John Gribbin and Jeremy Cherfas in their books The Monkey Puzzle: Reshaping the Evolutionary Tree and The First Chimpanzee: In Search of Human Origins, chimps and bonobos may be descended from Australopithecus. Chimpanzees were amongst the animals affected during the Toba eruption, hinting that Toba ash extended far west as Africa.

Taxonomy

DNA evidence suggests the bonobo and common chimpanzee species separated from each other less than one million years ago (similar in relation between Homo sapiens and Neanderthals).^[8][9] The chimpanzee line split from the last common ancestor of the human line approximately six million years ago. Because no species other than Homo sapiens has survived from the human line of that branching, both chimpanzee species are the closest living relatives of humans. The chimpanzee's lineage diverged from the gorilla's about seven million years ago.
Four subspecies of the common chimpanzee have been recognized,^[10][11] with the possibility of a fifth:^[12]

• Central chimpanzee or tschego, Pan troglodytes troglodytes, in Cameroon, the Central African Republic, Equatorial Guinea, Gabon, the Republic of the Congo, and the Democratic Republic of the Congo
• Western chimpanzee, Pan troglodytes verus, in Guinea, Guinea Bissau, Mali, Senegal, Sierra Leone, Liberia, Côte d'Ivoire, and Ghana
• Nigeria-Cameroon chimpanzee, Pan troglodytes ellioti (also known as Pan troglodytes vellerosus),^[10] in Nigeria and Cameroon
• Eastern chimpanzee, Pan troglodytes schweinfurthii, in the Central African Republic, South Sudan, the Democratic Republic of the Congo, Uganda, Rwanda, Burundi, Tanzania, and Zambia
• Southeastern chimpanzee, Pan troglodytes marungensis, in Burundi, Rwanda, Tanzania, and Uganda. Colin Groves argues that this subspecies is the result of enough variation between the northern and southern populations of P. t. schweinfurthii.^[12]

Physical description

The adult male common chimpanzee weighs between 40 and 60 kg (88 and 132 lb), the female weighs 32 to 47 kg (71 to 104 lb),.^[13] However, large wild males can weigh up to 70 kg (150 lb) and males in captivity, such as Travis the Chimp, have reached 91 kg (201 lb).^[14][15][16] Head-body length (from the nose to the rump while on all fours) ranges from 63 to 94 cm (25 to 37 in).^[16][17] Males can measure up to 1.6 m (5 ft 3 in) tall while standing and females up to 1.3 m (4 ft 3 in) tall. Their bodies are covered by coarse black hair, except for the face, fingers, toes, palms of the hands and soles of the feet. Both its thumbs and big toes are opposable, allowing a precise grip. The common chimpanzee is both arboreal and terrestrial and spends its nights in the trees, while most daylight hours are spent on the ground.^[18] Its habitual gait is quadrupedal, using the soles of its feet and resting on its knuckles, but it can walk upright for short distances. The common chimpanzee is a 'knuckle walker', like the gorilla and the bonobo,^[18] in contrast to the quadrupedal locomotion of the orangutan, a 'palm walker' that uses the outside edge of its palms.

Ecology

Chimpanzee in a tree with a kill.

The common chimpanzee is a highly adaptable species. It lives in a variety of habitats, including dry savanna, evergreen rainforest, montane forest, swamp forest, and dry woodland-savanna mosaic.^[19][20] In Gombe, the chimpanzee lives in subalpine moorland, open woodland, semideciduous forest, evergreen forest, and grassland with scattered trees.^[20] At Bossou, the chimpanzee inhabits multistage secondary deciduous forests, which have grown after shifting cultivation, as well as primary forests and grasslands.^[21] At Taï, it can be found in the last remaining tropical rainforest in Côte d'Ivoire.^[22]
The chimpanzee has an advanced cognitive map of its home range and can repeatedly find food.^[20] The chimpanzee makes a night nest in a tree in a new location every night, with every chimpanzee in a separate nest other than infants or small chimpanzees, which sleep with their mothers.^[23] When confronted by a predator, the chimpanzee will react with loud screams and use any object it can against the threat. Leopard predation is apparently a significant cause of mortality in chimpanzees at Taï and Lopé National Parks.^[22][24]
In some cases, the common chimpanzee has been documented killing leopard cubs,^[25] an act which primarily seems to be a protective effort. Lions may have also preyed on the chimpanzees at Mahale Mountains National Park, where at least four chimpanzees could have fallen prey to them.^[26] Although no other instances of lion predation on chimpanzees have been recorded, the larger group sizes of savanna chimps may have developed as a response to threats from these big cats.^[26] Isolated cases of cannibalism have also been documented.^[27]

Diet and foraging

Chimpanzee eating fruit.

The chimpanzee is an omnivorous frugivore. It prefers fruit above all other food items and will even seek out and eat them when they are not abundant. It will also eat leaves and leaf buds. Seeds, blossoms, stems, pith, bark and resin, insects and meat make up the rest of its diet.^[20][28] While the common chimpanzee is mostly herbivorous, it does eat honey, soil, insects, birds and their eggs, and small to medium-sized mammals, including other primates.^[20][29] The western red colobus ranks at the top of preferred mammal prey. Other mammalian prey include red-tailed monkeys, yellow baboons, blue duikers, bushbucks and common warthogs.^[30]
When hunting small monkeys such as the red colobus, the chimpanzee hunts where the forest canopy is interrupted or irregular.^[31] This allows it to easily corner the monkeys when chasing them in the appropriate direction. Chimps may also hunt as a coordinated team, so that they can corner their prey even in a continuous canopy.^[31] During an arboreal hunt, each chimp in the hunting groups has a role. "Drivers" serve to keep the prey running in a certain direction and follow them without attempting to make a catch. "Blockers" are stationed at the bottom of the trees and will climb up to block prey that take off in a different direction. "Chasers" move quickly and try to make a catch. Finally, "ambushers" hide and rush out when a monkey nears.^[31] While both adults and infants are taken, adult male black-and-white colobus monkeys will attack the hunting chimps. In Gombe, the chimpanzee also fears adult female colobus monkeys, and prefers to snatch infants from their mother's bellies without harming the mothers.^[20] Male chimps hunt more than females. When caught and killed, the meal is distributed to all hunting party members and even bystanders.^[31]
Despite the fact that common chimpanzees are known to hunt, and to collect insects and other invertebrates, such food actually makes up a small portion of their diet; from as little as two percent yearly to as much as 65 grams of meat 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.^[32][33]

Behavior

Group structure

Chimpanzees grooming one another.

Common chimpanzees live in communities that typically range from 20 to more than 150 members, but spend most of their time travelling in small, temporary groups consisting of a few individuals, "which may consist of any combination of age and sex classes."^[23] Both males and females will sometimes travel alone.^[23] The common chimpanzee lives in a fission-fusion society and may be found in groups of the following types: all-male, adult females and offspring, consisting of both sexes, or one female and her offspring. Chimpanzees have complex social relationships and spend a large amount of time grooming each other.^[34]
At the core of social structures are males, which roam around, protect group members, and search for food. Males remain in their natal communities, while females generally emigrate at adolescence. As such, males in a community are more likely to be related to one another than females are to each other. Among males, there is generally a dominance hierarchy and males are dominant over females.^[35] 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,"^[20] is highly variable in terms of which particular individual chimpanzees congregate at a given time. This is mainly due to chimpanzees having a high level of individual autonomy within their fission-fusion social groups. Also, communities have large ranges that overlap with those of other groups.
As a result, individual chimpanzees often forage for food alone, or in smaller groups (as opposed to the much larger "parent" group, which encompasses all the chimpanzees which regularly come into contact and congregate into parties in a particular area). As stated, these smaller groups also emerge in a variety of types, for a variety of purposes. For example, an all-male troop may be organized to hunt for meat, while a group consisting of lactating females serves to act as a "nursery group" for the young.^[36] An individual may encounter certain individuals quite frequently, but have run-ins with others almost never or only in large-scale gatherings. Due to the varying frequency at which chimpanzees associate, the structure of their societies is highly complicated.

Male chimpanzees in Mahale National Park, Tanzania.

Male chimpanzees exist in a linear dominance hierarchy. Top-ranking males tend to be aggressive even during dominance stability.^[37] This is likely due to the chimp’s fission-fusion society, with male chimps leaving groups and returning after extended periods of time. With this, a dominant male is unsure if there has been any "political maneuvering" and must re-establish his dominance. Thus, a large amount of aggression occurs 5–15 minutes after a reunion.^[37][38] During aggressive encounters, displays are preferred over attacks.^[37][38]
Males maintain and improve their social ranks by forming coalitions. These coalitions have been characterized as "exploitive" and are based on an individual’s influence in agonistic interactions.^[39] Being in a coalition allows males to dominate a third individual when they could not by themselves, as politically apt chimps can exert power over aggressive interactions regardless of their rank. Coalitions can also give an individual male the confidence to challenge a dominant male. The more allies a male has, the better his chance of becoming dominant. However, most changes in hierarchical rank are caused by dyadic interactions.^[37] Chimpanzee alliances can be very fickle and one member may turn on another if it serves him.^[40]
Low ranking males commonly switch sides in disputes between more dominant individuals. Low ranking males benefit from an unstable hierarchy and have increased sexual opportunities.^[39][40] 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.^[41] Females in Taï have also been recorded to form alliances.^[42] Social grooming appears to be important in the formation and maintenance of coalitions.^[43] It is more common among adult males than adult females.
Chimpanzees have been described as highly territorial and are known to kill other chimps,^[44] although Margaret Power wrote in her 1991 book The Egalitarians that the field studies from which the aggressive data came, Gombe and Mahale, use artificial feeding systems that increased aggression in the chimpanzee populations studied, so might not reflect innate characteristics of the species as a whole.^[45] In the years following her artificial feeding conditions at Gombe, Jane Goodall described groups of male chimps patrolling the borders of their territory brutally attacking chimps which had split off from the Gombe group. A study published in 2010 found that the chimpanzees wage wars over land, not mates.^[46] Patrol parties from smaller groups are more likely to avoid contact with their neighbors. Patrol parties from large groups will even take over a smaller group's territory, gaining access to more resources, food and females.^[20][40]
Goodall documented many occasions within Gombe Stream National Park of chimpanzees and western red colobus monkeys ignoring each other within close proximity.^[47][48]

Mating and parenting

Common chimpanzee infant and mother

Chimpanzees mate throughout the year, although the number of females in estrus varies seasonally in a group.^[22][49] Female chimps are more likely to come into estrus when food is readily available. Estrous females exhibit sexual swellings. Chimp mating tends to be promiscuous, with females mating with multiple males in her community during estrus.^[20] As such, males have large testicles for sperm competition. However, other forms of mating also exist. A community's dominant males sometimes restrict reproductive access to females. A male and female can form consortship and mate outside their community. In addition, females sometimes leave their communities and mate with males from neighboring communities.^[20][50]
These alternative mating strategies give females more mating opportunities without losing the support of the males in their community.^[50] Infanticide has been recorded in chimp communities in Gombe, Mahale, and Kibale National Parks. Male chimps practice infanticide on unrelated young to shorten the interbirth intervals in the females. There are also accounts of infanticide by females. There are questions whether cases of female infanticide are related to the dominance hierarchy in females or are simply isolated pathological behaviors.^[20][41]
Care for the young is provided mostly by their mothers. The survival and emotional health of the young is dependent on maternal care.^[20] Mothers provide their young with food, warmth, and protection, and teach them certain skills. In addition, a chimp’s future rank may be dependent on its mother’s status.^[20][22] For their first 30 days, infants cling to their mother's bellies. Newborn chimps are helpless; their grasping reflex is not strong enough to support them for more than a few seconds. Infants are unable to support their own weight for their first two months and need their mothers' support.^[51]
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.^[51] By three years, infants will move farther away from their mothers. By four to six years, chimps are weaned and infancy ends.^[51]
The juvenile period for chimps lasts from their sixth to ninth years. Juveniles remain close to their mothers, but they also have more interactions 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.^[51]

Communication

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Chimpanzees use a variety of facial expressions, postures and sounds to communicate with each other.^[20] Chimps have expressive faces which are important in close-up communications. When frightened, a "full closed grin" causes nearby individuals to be fearful, as well. Other facial expressions include the "lip flip", "pout", "sneer", and "compressed-lips face".^[20] When submitting to a conspecific, a chimp will crunch, bob and extend a hand. When in an aggressive mode, a chimp will swagger bipedally, hunched over and arms waving, in an attempt to exaggerate its size.^[20] Chimps will beat their hands and feet against the trunks of large tree, an act known as "drumming".^[52]
Vocalizations are also important in chimp communication. The most common and important call in adults is the "pant-hoot". These calls are made when individuals are excited.^[20] Pant-hoots are made of four parts, starting with soft "hoos" that get louder and louder and climax into screams and sometimes barks; the former die down to soft "hoos" again as the call ends.^[52] Submissive individuals will make "pant-grunts" towards their superiors.^[20][41] Chimps use distance calls to draw attention to danger, food sources or other community members.^[20] "Barks" may be made as "short barks" when hunting and "tonal barks" when sighting large snakes.^[52]

Tool use

Further information: Tool use by animals

Nearly all chimpanzee populations have been recorded using tools. They will modify sticks, rocks, grass, and leaves and use them when foraging for honey, termites, ants, nuts, and water. Despite the lack of complexity, there does seem to be forethought and skill in making these tools and should be considered such.^[53] While it has been known since Jane Goodall's 1960s discovery that modern chimpanzees use tools, research published in 2007 indicates that chimpanzee stone tool use dates to at least 4,300 years ago.^[54]
A common chimpanzee from the Kasakela chimpanzee community was the first non-human animal reported making a tool, by modifying a twig to use as an instrument for extracting termites from their mound.^[55][56][57] At Taï, chimps simply use their hands to extract termites.^[53] When foraging for honey, chimps will use modified short sticks to scoop the honey out of the hive, that is, if the bees are stingless. For hives of the dangerous African honeybees, chimps use longer and thinner sticks to extract the honey.^[58] Chimps will also fish for ants using the same tactic.^[20][53]
Ant dipping is difficult and some chimps never master it. West African chimps will crack open hard nuts with stones or branches.^[22][53] There seems to be some forethought in his activity as these items are not found together or near a source of nuts. Nut cracking is also difficult and must be learned.^[22] Chimps will also use leaves as sponges or spoons to drink water.^[59]
A recent study revealed the use of such advanced tools as spears, which West African chimpanzees in Senegal sharpen with their teeth, being used to spear Senegal bushbabies out of small holes in trees.^[60] An eastern chimpanzee has been observed using a modified branch as a tool to capture a squirrel.^[61]

Chimpanzees and humans

Field study

Jane Goodall undertook the first long-term field study of the common chimpanzee, begun in Tanzania at Gombe Stream National Park in 1960. Other long-term study sites begun in 1960 include A. Kortlandt in eastern Democratic Republic of the Congo and Junichiro Itani in Mahale Mountains National Park in Tanzania.^[62] Current understanding of the species' typical behaviors and social organization are formed largely from Goodall's ongoing 50-year Gombe research study.^[45]

Genome Project

Main article: Chimpanzee Genome Project

Genomic information

NCBI genome ID	202
Ploidy	diploid
Genome size	3,323.27 Mb
Number of chromosomes	24 pairs

Relationships among apes. The numbers in this diagram are branch lengths, a measure of evolutionary distinctness. Based on protein electrophoresis data of Goldman et al.^[63]

Human and common chimpanzee DNA are very similar. After the completion of the Human Genome Project, a Chimpanzee Genome Project was initiated. In December 2003, a preliminary analysis of 7600 genes shared between the two genomes confirmed that certain genes, such as the forkhead-box P2 transcription factor which is involved in speech development, have undergone rapid evolution in the human lineage. A draft version of the chimpanzee genome was published on September 1, 2005, in an article produced by the Chimpanzee Sequencing and Analysis Consortium.^[64]
The DNA sequence differences between humans and chimpanzees is about thirty-five million single-nucleotide changes, five million insertion/deletion events, and various chromosomal rearrangements. Typical human and chimp protein homologs differ in only an average of two amino acids. About 30% of all human proteins are identical in sequence to the corresponding chimp protein. Duplications of small parts of chromosomes have been the major source of differences between human and chimp genetic material; about 2.7% of the corresponding modern genomes represent differences, produced by gene duplications or deletions, during the approximately four to six million years since humans and chimps diverged from their common evolutionary ancestor. Results from human and chimp genome analyses, currently being conducted by geneticists including David Reich, should help in understanding the genetic basis of some human diseases.

Attacks

Common chimpanzees have been known to attack humans.^[65][66]

On infant humans

In Uganda, there have been several attacks on children, some of them fatal. It has been suggested some of these attacks are due to the chimpanzees being intoxicated (from alcohol obtained from rural brewing operations) and mistaking human children^[67] for the Western red colobus, one of their favorite meals.^[68] Human interactions with chimpanzees may be especially dangerous if the chimpanzees perceive humans as potential rivals.^[69] There are at least six documented cases of chimpanzees snatching and eating human babies.^[70]

On adult humans

A chimpanzee's great strength and sharp teeth mean that attacks, even on full-grown men, 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 chimps before they were killed.^[71][72] Another example of chimpanzees being aggressive toward humans occurred in 2009 in Stamford, Connecticut, when a 200-pound (91 kg), 14-year old pet chimp named Travis attacked his owner's friend, who lost her hands, eyelids, nose and part of her maxilla from the attack.^[73][74]

Link with human immunodeficiency virus type 1

This Cameroonian chimpanzee was brought to a rescue centre after its mother was killed by poachers.

Two types 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.^[75] Both types originated in west and central Africa, jumping from primates to humans. HIV-1 has evolved from a simian immunodeficiency virus (SIVcpz) found in the common chimpanzee subspecies, Pan troglodytes troglodytes, native to southern Cameroon.^[76][77] 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 SIV, found in the sooty mangabey monkeys in Guinea-Bissau.^[75]

Status and conservation

Chimpanzee are a legally protected species in most of their range and can be found both in and outside national parks.^[2] There are thought to be between 172,700 to 299,700 individuals living in the wild.^[2]
The biggest threats to the common chimpanzee are habitat destruction, poaching and disease.^[2] 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 have not been seriously impacted by humans.^[2] While deforestation rates are low in western Central Africa, selective logging may be done outside national parks.^[2]
Chimpanzees are a common target for poachers. In Côte d’Ivoire, chimpanzees make up 1–3% of bushmeat sold in urban markets.^[2] They are also taken in pet trades despite it being illegal in many countries where they live.^[2] Chimpanzees are also hunted for medicinal purposes in some areas.^[2] Capturing chimpanzees for scientific research is still allowed in some countries, such as Guinea.^[2] People will kill any chimpanzee that threatens their crops.^[2] Chimps may also be unintentionally maimed or killed by snares meant for other animals.
Infectious diseases are a main cause of death for chimpanzees. Chimpanzees succumb to many diseases that afflict humans since the two species are so similar.^[2] As human populations grow, so does the risk of disease transmission between humans and chimpanzees.^[2]