Chimpanzees belong to the order Primates, which includes monkeys, apes, and humans. Traditional classification schemes place chimpanzees in the family Pongidae, along with all of the other apes: gorillas, orangutans, and gibbons. Recent research on the relationships between apes and humans, especially research involving DNA and chromosomal analyses, has suggested a revised classification scheme that places the great apes and humans in the family Hominidae and the gibbons in the family Hylobatidae. Compared with monkeys, apes are larger, have no tail, and have longer arms and a broader chest. When apes stand upright, their long arms reach below their knees. They also have a great deal of upper body strength, needed for a life spent mostly in the forest canopy.

Chimpanzee species and habitat

There are two species of chimpanzees: the common chimpanzee (Pan troglodytes ) and the bonobo (Pan paniscus ), sometimes incorrectly called the pygmy chimpanzee. The common chimpanzee occurs in forested West and Central Africa, from Senegal to Tanzania. Their habitat includes rainforest and deciduous woodland, from sea level to above 6, 000 ft (1, 830m). Common chimpanzees are rarely found in open habitat, except temporarily when there is ready access to fruit-bearing trees. The bonobo is found in Central Africa, and is confined to the Democratic Republic of the Congo (formerly known as Zaire) between the Kasai and Zaire rivers. Its habitat is restricted to closed-canopy tropical rainforest below 5, 000 ft (1, 525 m).

Chimpanzees live within the borders of some 15 African countries. Both species are endangered by habitat destruction, a low rate of reproduction, and by hunting by humans as meat and for the live-animal trade.

Physical characteristics

The height of chimpanzees varies from about 39 in (1 m) in males to about 36 in (90 cm) in females. Adult wild males weigh about 132 lb (60 kg), but in captivity they can grow up to 220 lb (100 kg). The weight of females ranges from about 66 lb (30 kg) in the wild to as much as 190 lb (87 kg) in captivity.

Chimpanzee pelage is primarily black, turning gray on the back after about 20 years of age. Both sexes have a short white beard, and baldness frequently develops in later years. The skin on the hands and feet is black, and that of the face ranges from pink to brown or black. The ears are large and he nostrils are small. Chimpanzees have heavy eyebrows, a flattened forehead, large protruding ears, and a short neck. The jaw is heavy and protruding, and the canine teeth are large. Male chimpanzees have larger canines than females, and these are used in battle with other males and during predation. Although chimpanzees have long powerful fingers, they have small weak thumbs. Their big toes function like thumbs, and their feet as well as their hands are used for grasping. Bonobos can be distinguished from chimpanzees by darker facial skin, by longer hair on the head with a distinct center part, and by a slighter, less robust and muscular physical appearance.

The genitalia of both sexes are prominent. Areas of the female’s genital skin become pink during estrus, a period that lasts two to three weeks and occurs every four to six weeks.

The characteristic gait on the ground is the “knuckle-walk,” which involves the use of the knuckles for support. Chimps spend much of their time climbing in trees, and they sleep alone every night in nests that they make with branches and leaves.

A mother sleeps with her baby until her next infant is born. Chimpanzees live up to 40-45 years.

Behavior

Life in the wild presents many challenges for chimpanzees. They have complex social systems and use their considerable mental skills daily for their survival. They are presented with a multitude of choices in their natural habitat, and exercise highly developed social skills. For instance, males aspire to attain a high position of dominance within the hierarchy of chimpanzee society; consequently, low-ranking individuals must learn the art of deception, doing things in secret to satisfy their own needs.

Researchers have discovered that chimpanzees experience a full range of emotions, from joy to grief, fear, anger, and curiosity. Chimpanzees also have a the ability to learn and understand concepts and the elements of language.

Chimpanzees have a sophisticated social organization. They band together in groups, which vary in size and the age of its members. Between 15 and 120 individuals will form a community, with generally twice as many adult females as adult males in the group. The range and territory of a particular group depends on the number of sexually mature males.

Chimpanzees generally do not travel as an entire social unit. Instead, they move around in smaller groups of three to six individuals. While traveling about, individuals may separate and join other chimpanzees. Temporary bonds with other chimps are created by an abundant food source, or by a female in estrus. The strongest social bond is between a mother and her young. Offspring that are under eight years of age are always found with their mother.

Parenting

A female’s estrus cycle averages 38 days, which includes 2-4 days of menstruation. When a female begins her estrus cycle, her genital area swells for approximately ten days, and this is when she is sexually attractive and receptive. The last three or four days of estrus are when the likelihood of conception is highest. Mating is seemingly random and varied, and receptive females are often mounted by most of the mature males in the community. A high-ranking male may, however, claim “possession” and prevent other males from mating with a female. Regardless of rank and social status in the community, all males and females have a chance to pass on their genes.

On average, female chimpanzees give birth every five to six years. Gestation is 230-240 days. Newborn chimps have only a weak grasping reflex, and initially require full support from their mother as she moves about. After a few days, however, the infant is able to cling securely to its mother’s underside. At about the age of five to seven months, the youngster is able to ride on its mother’s back. At the age of four years, a young chimp can travel well by walking. Weaning occurs before its third year, but the youngster will stay with its mother until it is five to eight years old.

When an infant chimp is born, its older sibling will start to become more independent of its mother. It will look for food and will build its own sleeping nest, but a close relationship remains with the mother and develops between the siblings. Young males stay close to their family unit until about the age of nine. At this time, they find an adult male to follow and watch his behavior. Thus begins the long process by which the male develops his place in the community.

Young females stay with their mothers until about ten years of age. After her first estrus, a young female typically withdraws from her natal group and moves to a neighboring one, mating with its males. At this time a female may transfer out of her initial group to form a family of her own. This exchange helps to prevent inbreeding and maintains the diversity of the gene pool.

Eating habits

Chimpanzees are omnivorous, eating both meat and plant material. Their diet includes fruits, leaves, buds, seeds, pith, bark, insects, bird eggs, and smaller mammals. Chimps eat some 200-300 species of plants, depending on local availability. Animal prey is eaten less regularly than fruits and leaves. Chimpanzees (usually males) have been observed to kill and eat baboons, antelopes, other monkeys, and young bush pigs, and they sometimes practice cannibalism.

Chimpanzees seem to know the medicinal value of certain plants. In the Gombe National Forest in Tanzania, chimps have been seen to eat the plant Apilia mossambicensis to help rid themselves of parasites in their digestive system. A branch of science, zoopharmacognosy, has recently developed to study the medicinal use of plants by wild animals.

Fruit is the main component of the chimpanzee diet, and they spend at least four hours a day finding and eating varieties of this food. In the afternoon chimps also spend another hour or two feeding on young leaves. They also eat quantities of insects that they collect by hand.

In some cases, chimpanzees may use simple tools; for example, they may collect termites with sticks or break open the hard shells of nuts with sticks or smash them between two rocks. Chimpanzees are able to devise simple tools to assist for other activities as well. They peel leaves from bamboo shoots for use as washcloths to wipe off dirt or blood and to collect rainwater from tree-cavities. The use of tools by chimpanzees varies from region to region, which indicates that it is a learned behavior. Young chimps have been observed to imitate their elders in the use of tools and to fumble with the activity until they eventually become proficient.

Communication

Chimpanzees use a multitude of calls to communicate. After being separated, chimpanzees often embrace, kiss, touch, stroke, or hold hands with each other. When fighting, the opponents may strike with a flat hand, kick, bite, or stomp, or drag the other along the ground. Scratching and hair pulling are favorite tactics of females. When the fighting is over, the loser will approach the winner and weep, crouch humbly, or hold out its hand. The victor usually responds by gently touching, stroking, embracing, or grooming the defeated chimp.

Body contact is of utmost importance in maintaining social harmony in a chimpanzee community. Chimpanzees will often groom each other for hours. Grooming a way to maintain calmness and tranquility, while preserving close relationships.

Chimpanzees also communicate through a combination of postures, gestures, and noises. While avoiding direct contact, a male chimpanzee will charge over the ground and through the trees, swinging and pulling down branches. He will drag branches, throw sticks and stones, and stomp on the ground. By doing this, he gives the impression that he is a dangerous and large opponent. The more impressive this display, the better the position achieved in the male ranking order.

Confrontations between members of different communities can, however, be extremely violent. Fighting is ferocious and conducted without restraint, often resulting in serious injury and sometimes death. These encounters usually occur in places where communities overlap. Chimpanzees behave cautiously in such places, often climbing trees to survey the area for members of the neighboring community.

When two community groups of balanced strength meet, they may show aggression by performing wild dances, throwing rocks, beating tree trunks, and making fierce noises. This display is usually followed by retreat into their territory. However, when one or a small number of strangers, whether male or female, is met by a larger group, there is a danger of being viciously attacked. Chimpanzees have been seen to twist the limbs, tear the flesh, and drink the blood of strangers they have murdered in such aggressive encounters.

This hostile activity often occurs when male chimpanzees are routinely involved in “border patrols.” Males may patrol for several hours, keenly watching and listening for signs of nearby activity. It is not known if the purpose of the patrols is to protect the local food source of the community; if the males are engaged in competition for females; or if they are engaging in predatory cannibalism.

Jane Goodall’s observations

In 1960, Jane Goodall (1934–), a young Englishwoman, first set up camp in Gombe, Tanzania, to conduct a long-term study of chimpanzees. Louis Leakey (1903–1972), a famous anthropologist, helped Goodall by providing the initial funding for her research and by serving as her mentor. Leakey is best known for his discovery of hominid fossils in eastern Africa and his contributions to the understanding of human evolution.

Goodall was not initially a trained scientist, but Leaky felt that this could be an advantage in her work, because she would not bring preconceived scientific bias into her research. One of the most difficult hurdles that Goodall had to overcome when presenting the results of her work to the scientific community was to avoid making references to the fact that chimps have feelings. Projecting human emotions onto animals is thought to signal anthropomorphic bias and is often regarded as a scientific flaw. However, as Goodall demonstrated, chimps do experience a wide range of emotions, and they perceive themselves to be individuals. These are some of the compelling similarities that they share with humans.

Goodall made several particularly significant discoveries early in her research. Her first chimpanzee friend was a male individual she named David Greybeard. One day she was observing him when he walked over to a termite mound, picked up a stiff blade of grass, carefully trimmed it, and poked it into a hole in the mound. When he pulled the grass out of the mound, termites were clinging to it, and he ate them. This remarkable discovery showed that chimps are toolmakers.

Goodall’s second discovery also involved David Greybeard; she observed him eating the carcass of an infant bush pig (a medium-sized forest mammal). David Greybeard shared the meat with some companions, although he kept the best parts for himself. The use of tools and the hunting of meat had never before been observed in apes. Numerous other observations have since been made of chimps making and using simple tools and engaging in sometimes well-organized group hunts of monkeys and other prey.

The bonobo

Several projects begun in the early 1970s were the first to study bonobos in the wild. Their alternative name, pygmy chimp, is inaccurate because these animals are only slightly smaller than common chimpanzees. The reference to “pygmy” has more to do with so-called pedomorphic traits of bonobos, meaning they exhibit certain aspects of adolescence in early adulthood, such as a rounded shape of their head.

Another characteristic of the bonobo that differs from the common chimp is the joining of two digits of their foot. Additionally, the bonobo’s body frame is thinner, its head is smaller, its shoulders narrower, and legs are longer and stretch while it is walking. Furthermore, the eyebrow ridges of the bonobo are slimmer, its lips are reddish with a black edge, its ears are smaller, and its nostrils are widely spaced. Bonobos also have a flatter and broader face with a higher forehead than do common chimpanzees, and their hair is blacker and finer.

Bonobos also have a somewhat more complex social structure than common chimpanzees. Like common chimpanzees, bonobos belong to large communities and form smaller groups of 6-15 that will travel and forage together. Groups of bonobos have an equal sex ratio, unlike those of the common chimpanzee. Among bonobos the strongest bonds are created between adult females and between the sexes. Bonds between adult males are weak (whereas in common chimps they may be strong). Bonobo females take a more central position in the social hierarchy of the group.

Sex is an important pastime among bonobo chimpanzees. Female bonobos are almost always receptive and are willing to mate during most of their monthly cycle. The ongoing sexual exchanges within their communities help to maintain peace and ease friction.

KEY TERMS

Anthropomorphic —Ascribing human feelings or traits to other species of animals.

Bipedal —The ability to walk on two legs.

Border patrol —Routine visits that common chimpanzees make to the edges of their communal areas to observe neighboring territories.

Estrus —A condition marking ovulation and sexual receptiveness in female mammals.

Pedomorphic —Having juvenile traits in adulthood.

Zoopharmacognosy —A field of research that studies the medicinal values of plants that animals eat.

Bonobos try to avoid social conflict, especially when it relates to food.

Bonobos are extremely acrobatic and enjoy spending time in the trees. They do not fear water and have been observed to become playful on rainy days, unlike common chimpanzees, who seem to hate the rain. It is believed that there are fewer than 100, 000 bonobos in the wild; they are threatened by hunting as food and for sale in foreign trade, and by the destruction of their natural forest habitat.

Language

Sign language has been used successfully for communication between humans and captive chimpanzees. Sign language research has shown that some chimpanzees are able to create their own symbols for communication when none has been given for a specific object. Other studies of the use of language by chimps suggest that they understand the syntax of language, that is, the relationship of words to action and to the actor. Chimpanzees also have pre-mathematical skills, and are able to differentiate and categorize. For example, they can learn the difference between fruits and vegetables, and to divide sundry things into piles of similar objects.

Use in research

The chimpanzee is the closest living relative of humans. In fact, the DNA of humans and chimpanzees differs by less than 4%. Because of this genetic closeness, and anatomical and biochemical similarities, chimps have been widely used for testing new vaccines and drugs in biomedical research.

Chimpanzees can also become infected by certain diseases that humans are susceptible to, such as colds, flu, AIDS, and hepatitis B. Gorillas, gibbons, and orangutans are the only other animals that show a similar susceptibility to these diseases. Consequently, these species are used in biomedical research seeking cures for these ailments, including work that would be considered ethically wrong if undertaken on human subjects. However, many people are beginning to object to using chimpanzees and other apes in certain kinds of invasive biomedical research. This is because of the recent understanding that chimpanzees, other apes, and humans are so closely related, and that all are capable of experiencing complex emotions, including pain and suffering. Some people are even demanding that apes should be given legal rights and protection from irresponsible use in research.