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Chimpanzees (sometimes called chimps) are one of two exclusively African species of extant great ape. Native to Sub Saharan Africa, both are currently found in the Congo jungle. Classified in the genus Pan, they were once considered to be one species. However, since 1928, they have been recognized as two distinct species: the common chimpanzee (P. troglodytes) live north of the Congo River and the bonobo (P. paniscus) who live south. In addition, P. troglodytes is divided into four subspecies, while P. paniscus has none. Based on genome sequencing, the two extant Pan species diverged around one million years ago. The most obvious differences are that chimpanzees are somewhat larger, more aggressive and male dominated, while the bonobos are more gracile, peaceful and female dominated.

Their hair is typically black or brown. Males and females differ in size and appearance. Both chimps and bonobos are some of the most social great apes, with social bonds occurring in large communities. Fruit is the most important component of an chimpanzee’s diet; however, they will also eat vegetation, bark, honey, insects and even other chimps or monkeys. They can live over 30 years in both the wild and captivity.

Chimpanzees and bonobos are equally humanity’s closest living relatives. As such, they are among the largest brained and most intelligent of primates; they use a variety of sophisticated tools and construct elaborate sleeping nests each night from branches and foliage. They have both been extensively studied for their learning abilities. There may even be distinctive cultures within populations. Field studies of Pan troglodytes were pioneered by primatologist Jane Goodall. Both Pan species are considered to be endangered as human activities have caused severe declines in the populations and ranges of both species. Threats to wild panina populations include poaching, habitat destruction, and the illegal pet trade. Several conservation and rehabilitation organizations are dedicated to the survival of Pan in the wild.

The chimpanzee fossil record has long been absent and thought to have been due to the preservation bias in relation to their environment. However, in 2005, chimpanzee fossils were discovered and described by Sally McBrearty and colleagues. Existing chimpanzee populations in West and Central Africa are separate from the major human fossil sites in East Africa; however, chimpanzee fossils have been reported from Kenya, indicating that both humans and members of the Pan clade were present in the East African Rift Valley during the Middle Pleistocene.

Anatomy and physiology

The chimpanzee’s brain on the left and the man’s brain on the right have been scaled to the same size to show the relative proportions of their parts. These drawings were in a book made in 1904 by Thomas Henry Huxley.

A chimpanzee’s arms are longer than its legs. The male common chimp stands up to 1.2 m (3.9 ft) high and weighs as much as 70 kg (150 lb); the female is somewhat smaller. When extended, the common chimp’s long arms span one and a half times the body’s height. The bonobo is slightly shorter and thinner than the common chimpanzee, but has longer limbs. In trees, both species climb with their long, powerful arms; on the ground, chimpanzees usually knuckle-walk, or walk on all fours, clenching their fists and supporting themselves on the knuckles. Chimpanzees are better suited for walking than orangutans, because the chimp’s feet have broader soles and shorter toes. The bonobo has proportionately longer upper limbs and walks upright more often than does the common chimpanzee. Both species can walk upright on two legs when carrying objects with their hands and arms.

The chimpanzee is tailless; its coat is dark; its face, fingers, palms of the hands, and soles of the feet are hairless. The exposed skin of the face, hands, and feet varies from pink to very dark in both species, but is generally lighter in younger individuals and darkens with maturity. A University of Chicago Medical Centre study has found significant genetic differences between chimpanzee populations. A bony shelf over the eyes gives the forehead a receding appearance, and the nose is flat. Although the jaws protrude, a chimp’s lips are thrust out only when it pouts.

The brain of a chimpanzee has been measured at a general range of 282–500 cc. The human brain, in contrast, is about three times larger, with a reported average volume of about 1330 cc.

Chimpanzees reach puberty between the age of eight and ten years. A chimpanzee’s testicles are unusually large for their body size, with a combined weight of about 4 oz (110 g) compared to a gorilla’s 1 oz (28 g) or a human’s 1.5 ounces (43 g). This relatively great size is generally attributed to sperm competition due to the polyandrous nature of chimpanzee mating behaviour.  One study estimates that chimps live about 33 years for males, 37 years for females, in the wild, but some have lived longer than 60 years in captivity.

Muscle strength

Chimpanzees are known for possessing great amount of muscle strength, especially in their arms. However, compared to humans the amount of strength reported in media and popular science is greatly exaggerated with numbers of four to eight times the muscle strength of a human. These numbers stem from two studies in 1923 and 1926 by a biologist named John Bauman. These studies were refuted in 1943 and an adult male chimp was found to pull about the same weight as an adult man. Corrected for their smaller body sizes, chimpanzees were found to be stronger than humans but not anywhere near four to eight times. In the 1960s these tests were repeated and chimpanzees were found to have twice the strength of a human when it came to pulling weights. The reason for the higher strength seen in chimpanzees compared to humans are thought to come from longer skeletal muscle fibers that can generate twice the work output over a wider range of motion compared to skeletal muscle muscle fibers in humans.

Behaviour

Anatomical differences between the common chimpanzee and the bonobo are slight, but sexual and social behaviours are markedly different. The common chimpanzee has an omnivorous diet, a troop hunting culture based on beta males led by an alpha male, and highly complex social relationships. The bonobo, on the other hand, has a mostly frugivorous diet and an egalitarian, nonviolent, matriarchal, sexually receptive behavior. Bonobos frequently have sex, sometimes to help prevent and resolve conflicts. Different groups of chimpanzees also have different cultural behavior with preferences for types of tools. The common chimpanzee tends to display greater aggression than does the bonobo. The average, captive chimpanzee sleeps 9.7 hours per day.

Social structure

Chimpanzees live in large multi-male and multi-female social groups, which are called communities. Within a community, the position of an individual and the influence the individual has on others dictates a definite social hierarchy. Chimpanzees live in a leaner hierarchy wherein more than one individual may be dominant enough to dominate other members of lower rank. Typically, a dominant male is referred to as the alpha male. The alpha male is the highest-ranking male that controls the group and maintains order during disputes. In chimpanzee society, the ‘dominant male’ sometimes is not the largest or strongest male but rather the most manipulative and political male that can influence the goings on within a group. Male chimpanzees typically attain dominance by cultivating allies who will support that individual during future ambitions for power. The alpha male regularly displays by puffing his normally slim coat up to increase view size and charge to seem as threatening and as powerful as possible; this behavior serves to intimidate other members and thereby maintain power and authority, and it may be fundamental to the alpha male’s holding on to his status. Lower-ranking chimpanzees will show respect by submissively gesturing in body language or reaching out their hands while grunting. Female chimpanzees will show deference to the alpha male by presenting their hindquarters.

Female chimpanzees also have a hierarchy, which is influenced by the position of a female individual within a group. In some chimpanzee communities, the young females may inherit high status from a high-ranking mother. Dominant females will also ally to dominate lower-ranking females: whereas males mainly seek dominant status for its associated mating privileges and sometimes violent domination of subordinates, females seek dominant status to acquire resources such as food, as high-ranking females often have first access to them. Both genders acquire dominant status to improve social standing within a group.

Community female acceptance is necessary for alpha male status; females must ensure that their group visits places that supply them with enough food. A group of dominant females will sometimes oust an alpha male which is not to their preference and back another male, in whom they see potential for leading the group as a successful alpha male.

Intelligence

Chimpanzees make tools and use them to acquire foods and for social displays; they have sophisticated hunting strategies requiring cooperation, influence and rank; they are status conscious, manipulative and capable of deception; they can learn to use symbols and understand aspects of human language including some relational syntax, concepts of number and numerical sequence; and they are capable of spontaneous planning for a future state or event.

Tool use

In October 1960, Jane Goodall observed the use of tools among chimpanzees. Recent research indicates chimpanzee stone tool use dates to at least 4,300 years ago. Chimpanzee tool usage includes digging into termite mounds with a large stick tool, and then using a small stick that has been altered to “fish” the termites out. There have been occasional unsubstantiated or controversial reports of chimpanzees using rocks or sticks as weapons. A recent study claimed to reveal the use of spears, which common chimpanzees in Senegal sharpen with their teeth and use to stab and pry Senegal bush babies out of small holes in trees. Chimpanzees are also known to use stones as anvils and hammers in order to break open nuts. Before the discovery of tool use in chimps, humans were believed to be the only species to make and use tools, but several other tool-using species are now known.

Nest-building

Nest-building, sometimes considered as tool use, is seen in chimpanzees which construct arboreal night nests by lacing together branches from one or more trees. It forms an important part of their behaviour, especially in the case of mothers who teach this trait to infants. Nests consist of a mattress, supported on a strong foundation, and lined above with soft leaves and twigs, and are built in trees with a minimum diameter of 5 metres (16 ft) and may be located at a height of 3 to 45 metres (10 to 150 ft). Both day and night nests are built; they may be located in groups. A study in 2014 found that the Muhimbi tree is favoured for nest building by chimpanzees in Uganda due to its physical properties, such as bending strength, inter-node distance, and leaf surface area.

Altruism and emotivity

Studies have shown chimpanzees engage in apparently altruistic behavior within groups. Some researchers have said that chimpanzees are indifferent to the welfare of unrelated group members, but a more recent study of wild chimpanzees found that both male and female adults would adopt orphaned young of their group. Also, different groups sometimes share food, form coalitions, and cooperate in hunting and border patrolling. Sometimes, chimpanzees have adopted young that come from unrelated groups. And in some rare cases, even male chimps have been shown to take care of abandoned infant chimps of an unrelated group, though in most cases they would kill the infant.

According to James W. Harrod, evidence for chimpanzee emotivity includes display of mourning, “incipient romantic love”, rain dances, appreciation of natural beauty (such as a sunset over a lake), curiosity and respect towards wildlife (such as the python, which is neither a threat nor a food source to chimpanzees), altruism toward other species (such as feeding turtles) and animism, or “pretend play”, in chimps cradling and grooming rocks or sticks.

Communication

Chimps communicate in a manner similar to human nonverbal communication, using vocalizations, hand gestures, and facial expressions. There is even some evidence that they can recreate human speech. Research into the chimpanzee brain has revealed that chimp communication activates an area of the chimp brain in the same position as Broca’s area, a language center in the human brain.

There is some debate as to whether chimpanzees have the ability to express hierarchical ideas in language. Studies have found that chimps are capable of learning a limited set of sign language symbols, which they can use to communicate with human trainers. However, it is clear that there are distinct limits to the complexity of knowledge structures with which chimps are capable of dealing. The sentences that they can express are limited to specific simple noun-verb sequences and are not capable of the indefinite expansion of complexity characteristic of humans.

Aggression

Adult common chimpanzees, particularly males, can be very aggressive. They are highly territorial and are known to kill other chimps.

Hunting

Chimpanzees also engage in targeted hunting of lower-order primates, such as the red Colobus and bush babies, and use the meat from these kills as a “social tool” within their community.

History

Chimps, as well as other apes, had also been purported to have been known to ancient writers, but mainly as myths and legends on the edge of European and Near Eastern societal consciousness. Apes are mentioned variously by Aristotle. The English word ape translates Hebrew qőf in English translations of the Bible (1 Kings 10:22), but the word may refer to a monkey rather than an ape proper.

The diary of Portuguese explorer Duarte Pacheco Pereira (1506), preserved in the Portuguese National Archive (Torre do Tombo), is probably the first written document to acknowledge that chimpanzees built their own rudimentary tools. The first of these early transcontinental chimpanzees came from Angola and were presented as a gift to Frederick Henry, Prince of Orange in 1640, and were followed by a few of its brethren over the next several years. Scientists described these first chimpanzees as “pygmies”, and noted the animals’ distinct similarities to humans. The next two decades, a number of the creatures were imported into Europe, mainly acquired by various zoological gardens as entertainment for visitors.

Darwin’s theory of natural selection (published in 1859) spurred scientific interest in chimpanzees, as in much of life science, leading eventually to numerous studies of the animals in the wild and captivity. The observers of chimpanzees at the time were mainly interested in behavior as it related to that of humans. This was less strictly and disinterestedly scientific than it might sound, with much attention being focused on whether or not the animals had traits that could be considered ‘good’; the intelligence of chimpanzees was often significantly exaggerated, as immortalized in Hugo Rheinhold’s Affe mit Schädel (see image, left). By the end of the 19th century, chimpanzees remained very much a mystery to humans, with very little factual scientific information available.

In the 20th century, a new age of scientific research into chimpanzee behavior began. Before 1960, almost nothing was known about chimpanzee behavior in their natural habitats. In July of that year, Jane Goodall set out to Tanzania’s Gombe forest to live among the chimpanzees, where she primarily studied the members of the Kasakela chimpanzee community. Her discovery that chimpanzees made and used tools was groundbreaking, as humans were previously believed to be the only species to do so. The most progressive early studies on chimpanzees were spearheaded primarily by Wolfgang Köhler and Robert Yerkes, both of whom were renowned psychologists. Both men and their colleagues established laboratory studies of chimpanzees focused specifically on learning about the intellectual abilities of chimpanzees, particularly problem-solving. This typically involved basic, practical tests on laboratory chimpanzees, which required a fairly high intellectual capacity (such as how to solve the problem of acquiring an out-of-reach banana). Notably, Yerkes also made extensive observations of chimpanzees in the wild which added tremendously to the scientific understanding of chimpanzees and their behavior. Yerkes studied chimpanzees until World War II, while Köhler concluded five years of study and published his famous Mentality of Apes in 1925 (which is coincidentally when Yerkes began his analyses), eventually concluding, “Chimpanzees manifest intelligent behavior of the general kind familiar in human beings … a type of behavior which counts as specifically human”

The August 2008 issue of the American Journal of Primatology reported results of a year-long study of chimpanzees in Tanzania’s Mahale Mountains National Park, which produced evidence of chimpanzees becoming sick from viral infectious diseases they had likely contracted from humans. Molecular, microscopic and epidemiological investigations demonstrated the chimpanzees living at Mahale Mountains National Park have been suffering from a respiratory disease that is likely caused by a variant of a human paramyxovirus.