Chimpanzee

I

Introduction

Chimpanzee, ape of equatorial Africa that, physically and genetically, is the animal most closely related to humans. Two species of chimpanzee, or chimp, exist: the common chimpanzee and the bonobo, also known as the pygmy chimpanzee. The common chimp is found in dense jungle and more open wooded savanna from Sierra Leone and Guinea on the Atlantic Ocean to the lakes Tanganyika and Victoria in east central Africa. The bonobo is found only in a small region of thick jungle in the Democratic Republic of the Congo (DRC, formerly Zaire) in central Africa. Both species of chimpanzee are listed as endangered species in the wild by the World Conservation Union (also known as IUCN) and the Convention on International Trade in Endangered Species (CITES).

II

Characteristics

The male common chimp is up to 1.7 m (up to 5.6 ft) high when standing, and weighs as much as 70 kg (154 lb); the female is somewhat smaller. The common chimp’s long arms, when extended, have a span half again as long as the body’s height and are longer than its legs. The bonobo is a little shorter and thinner than the common chimpanzee but has longer limbs. Both species use their long, powerful arms for climbing in trees. On the ground, chimpanzees usually walk on all fours using their knuckles for support with their hands clenched, a form of locomotion called knuckle-walking. Chimpanzee feet are better suited for walking than are those of the orangutan because the chimp’s soles are broader and the toes shorter. Both the common chimpanzee and bonobo can walk upright on two legs when carrying objects with their hands and arms. The coat is dark; the face, fingers, palms of the hands, and soles of the feet are hairless; and the chimp has no tail. A bony shelf over the eyes gives the forehead a receding appearance, and the nose is flat. Although the jaws protrude, the lips are thrust out only when a chimp pouts. The brain of a chimpanzee is about half the size of the human brain.

The chimpanzee is diurnal (active during daytime) and omnivorous, eating about 200 kinds of leaves and fruit; termites, ants, honey, and birds’ eggs; and birds and small mammals. It lives in or near trees, avoiding direct sunlight. An adult builds a sleeping nest each night in a tree. The female has a 35-day menstrual cycle, is receptive to mating for 6.5 days out of each cycle, and can breed at any time of the year. The female may mate with different partners. The gestation period is more than seven months long, and a single offspring or rarely, twins, is produced. Immediately after birth the helpless young clutches its mother’s hair, riding on the mother’s back when she travels. Common chimps are weaned at about four years, and bonobos are weaned at about five years. In both species, the young may continue to travel with the mother until the age of ten. Offspring sometimes maintain a bond with the mother throughout life. Chimps in the wild live about 40 years and many in captivity live more than 50 years.

III

Social Behavior

Chimpanzees form loosely organized bands of 2 to 80 individuals on fairly large home ranges, where the animals remain for years. Within a community, smaller groups may form, break up, and reform; sometimes a female migrates to another community. Males never migrate. Except between mother and young, little permanency exists in individual relationships. Members of a community cooperate in hunting and sharing food. On finding a food source, they hoot, scream, and slap logs to attract others. A constant interplay occurs between adults, and all members of the group groom one another.

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Major differences exist between the social structure of groups of common chimpanzees and bonobos. A dominant male typically heads a group of chimpanzees, using size and strength, or even aggression to assert authority. Bonobo groups are dominated by females and males rarely show aggression. Male common chimpanzees form gangs to hunt prey or patrol their territories. In a behavior that has been likened to warfare, the gangs of males will sometimes attack and kill chimpanzees from neighboring troops that are found near their territories. Bonobos are much less aggressive toward neighboring troops and have been seen intermingling peacefully with other groups of bonobos for short periods.

IV

Communication and Intelligence

Chimpanzees communicate through vocalizations, facial expressions, posture, touch, and movement. Studies show that a young chimp is able to make as many as 34 different calls, and the facial musculature can express a wide range of emotions. The animals show great intelligence in problem solving. A number of experiments have shown that chimps can even learn to use sign language or other languages based on pictures or symbols. However, critics maintain that this does not constitute an understanding or use of words.

Researchers have documented a range of simple tools made or used by chimpanzees in the wild. Many chimpanzees have been recorded using stripped twigs or grass stems to draw termites out of termite mounds, or using sticks to enlarge holes or openings in trees enough to reach in for honey or small birds. Other tools include sponges made from chewed leaves or bark to sop up water. Males sometimes toss or brandish rocks and sticks in their charging displays to assert authority or drive off predators. In certain areas groups of chimpanzees have been observed using stones to crack open nuts for food. More controversially, scientists studying chimpanzees in Senegal witnessed females apparently fashioning small sticks as spears to stab and kill galagos (small prosimians also called bush babies) found hiding in holes in trees—in some cases, the chimpanzees sharpened the tip of the sticks with their teeth.

The discovery of distinct kinds of tool use and toolmaking among different groups of chimpanzees has led to claims that chimpanzees have local cultures. According to some researchers, certain skills may be passed on from one generation to another within a particular group of chimpanzees. Other scientists, however, think that some tool use by chimpanzees such as hammering nuts with stones may have come from imitating humans living nearby. Recent archaeological finds in Africa include stones that chimpanzees possibly used as tools thousands of years ago. A number of researchers see the ancient stones as evidence that chimpanzees developed the tool-using skills themselves or that such tool use may have existed in the common primate ancestor shared by both chimpanzees and humans.

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