Wild Chimpanzees

The Chimpanzee in Scientific Perspective

1. Chimpanzees and bonobos form the two species of great apes that are most closely related to humans. Chimpanzees inhabit tropical forests of equatorial Africa. Individuals vary considerably in size and appearance, but most chimpanzees stand from 3–5 feet tall and weigh from 70–130 pounds. Males tend to be larger and more robust than females. Chimpanzees are covered by a coat of brown or black hair, but their faces are bare except for a short white beard. Skin color is generally white except for the face, hands, and feet, which are black. The faces of younger animals may be pinkish or whitish. Among older males and females, the forehead often becomes bald and the back becomes gray.

2. Chimpanzees awaken at dawn, and their day is spent both in the trees and on the ground. After a lengthy midday rest, late afternoon is usually the most intensive feeding period. In the trees, where most feeding takes place, chimps use their hands and feet to move about. They also leap and swing by their arms (brachiate) skillfully from branch to branch. Movement over any significant distance usually takes place on the ground. Though able to walk upright, chimpanzees more often move about on all fours, leaning forward on the knuckles of their hands (knuckle walking). At night they usually sleep in the trees in nests they build of branches and leaves.

3. The chimpanzee diet is primarily vegetarian and consists of more than 300 different items, mostly fruits, berries, leaves, blossoms, and seeds but also bird eggs and chicks, many insects. Chimpanzees also hunt, both alone and in groups, stalking and killing various mammals such as monkeys, small deer, and wild pigs.

4. The female chimpanzee bears a single young at any time of year after a gestation period of about eight months. The newborn weighs about four pounds, is almost helpless, and clings to the fur of the mother’s belly as she moves. From about 6 months to 2 years, the youngster rides on the mother’s back. Weaning from the mother’s breast takes place at about 5 years. Males are considered adults at 16 years of age, and females usually begin to reproduce at about 13 years. The longevity of chimps is about 45 years in the wild and 58 in captivity; however, older individuals have been documented.

5. Chimpanzees are an endangered species; their population in the wild has been reduced by hunting (primarily for meat), destruction of habitat from logging or farming, and commercial exportation for use in zoos and research laboratories. Lions and leopards also prey upon chimpanzees.

6. Chimpanzees are lively animals with more extraverted dispositions than either gorillas or orangutans. They are highly social and live in loose and flexible groups known as communities, or unit groups, that are based on associations between adult males within a home range, or territory. Home ranges of forest-dwelling communities can be as small as a few square miles, but home ranges covering fone hundred square miles are known among some communities. A community can number from 20 to a 100 members. Each consists of several subgroups of varying size and unstable composition.

7. Social dominance exists, with adult males being dominant over adult females and adolescent males. Within a community, there are twice or three times as many adult females as adult males; the number of adults is about equal to the number of immature individuals. Commun-ities usually divide into subgroups called parties, which vary widely in size. The dominance hierarchy among male chimpanzees very fluid; individuals associate with each other and join and leave different subgroups with complete freedom. The dominant (alpha) male of a group can monopolize females – and thereby spread his genes – through possessive behavior. On the other hand, gang attack by subordinate males can expel an alpha male. Males spend all of their lives in the community in which they are born, but occasionally a juvenile male may transfer to another community with his mother.

8. In contrast to males, most females leave their group of birth to join a neighboring group when they mature - at around age 11. Female chimpanzees spend most of their time with their young or with other females. Those with dependent offspring are more likely to range alone or in small parties within narrow “core areas.” Females have been known to form coalitions against a bullying adult male or newly immigrated female.

9. Relations between different chimp communities tend to be hostile. Intruders on a group’s home range may be attacked, and adult males engage in boundary patrol. On some occasions, a group may invade a neighboring territory that is much smaller in size, and fatalities among the smaller group result. Infanticide and cannibalism by adult males, and to a lesser extent by adult females, have been observed. Victimized infants are not only those of neighboring groups but also those born to newly immigrated females.

10. Sometimes a male and female will form a consortship, engaging in exclusive mating relationships by leaving other members of the group and staying in the periphery of the group range. This strategy, however, brings increased risk of attack by neighboring groups.

11. Chimpanzees exhibit complex social strategies such as cooperation in combat and the cultivation of coalitions and alliances via ranging together, reciprocal grooming, and the sharing of meat (sometimes in exchange for mating opportunities). An alpha male, for instance, may interfere with his rival in grooming with a third party because such a coalition might jeopardize the alpha’s status. On the other hand, the third party might show strategic opportunism in such a situation, since his assistance to either side could determine which of his superiors prevails. Chimpanzees, therefore, appear to have some concept of “trade.” They console, reconcile, and retaliate during fighting and so share emotions and aspects of psychology similar to those found in humans: self-recognition, curiosity, sympathy, grief, and attribution. Although chimps take care of orphaned infants, they also tease handicapped individuals, conceal information that would bring disadvantage to themselves, and manipulate others for their own advantage by expressing deceptive postures, gestures, and facial expressions.

12. Chimpanzees are highly intelligent and are able to solve many kinds of problems posed to them by human trainers and experimenters. A number of researchers claim to have taught chimpanzees sign language or languages based on the display of tokens or pictorial symbols, but not all scientists agree with these claims. Communication between chimps in the wild takes the form of facial expressions, gestures, and a large array of vocalizations, including screams, hoots, grunts, and roars. Males display excitement by standing up on two legs, stamping or swaying, and letting out a chorus of screams. Chimps use louder calls and gestures for long-distance communication (such as drumming on tree buttresses) and quieter calls and facial expressions for short-distance communication. Similarities to human laughter and smiling might be seen in their “play panting” and grinning.

13. Chimpanzees use various tools in several ways. Chimpanzees “fish” for termites and ants in mounds of soil formed by insect nests. They collect insects with probes made of grass stalks, vines, branches, peeled bark, and midribs of leaves. They crack hard nuts open by using stones, roots, and wood as hammers or anvils, and they use leafy sponges to drink water. In threat displays, chimps throw rocks and drag and throw branches. Sticks are used to inspect dead animals or other unfamiliar objects that might be dangerous. Leaves are used for cleanliness in wiping the mouth or other soiled body parts.

14. Chimps learn tool use as an aspect of maturation, with younger animals acquiring tool-using behaviors from their elders. Many scientists claim that this education demonstrates that chimpanzees have a primitive culture – a set of ideas – about behavior, tools, or communication – transmitted from individual to individual.

15. Such cultural influences are seen in food items consumed and in gestural communication. Chimpanzees indeed possess culture when it is defined as the transmission of information from generation to generation via social learning shared by most members of a single age or sex class in a given group.

16. Chimpanzees’ appearance, intelligence, and behavior make it clear that they are related to humans. The genetic features of chimpanzees resemble those of humans more closely than any other animal. Genetic analysis suggests that humans and chimps diverged four million to eight million years ago and that at least 98 percent of the human and chimpanzee genomes are identical.