About 250 species of primates live in the world today. However, there are only five main types of apes. Scientists divide these five into two categories, based chiefly on size. Great apes include gorillas, chimpanzees, bonobos, and orangutans. Three of the great apes—the bonobo, chimpanzee, and gorilla—live in the tropical equatorial forests of Africa, while the fourth—the orangutan—lives in Indonesia on the islands of Sumatra and Borneo. The gibbon, which lives in Southeast Asia, is the only lesser ape.

Although apes and monkeys appear similar, apes share certain characteristics that set them apart. Apes lack tails, which most monkeys possess. Apes have rotating shoulder joints that allow them to hang or swing by their arms. (You have the same kind of rotating shoulder, which allows you to throw a football or swing from a gymnastics high bar.) Apes also have a specialized way of walking, placing the weight of their forearms on their knuckles.

The most important trait shared by all great apes, however, is a large brain. Great apes top the list of brainy creatures, exceeded only by human beings. For this reason, ape intelligence has become an important focus for scientists seeking to understand the evolution of human intelligence. Some scientists study apes' intellectual abilities in laboratories, giving them tests similar to the intelligence tests given in school. My approach, however, is to try to understand ape intelligence in nature, by studying apes' strategies for using tools, finding mates, or hunting. Some primatologists also try to understand whether the emotions apes display—such as guilt, shame, fear, and anger—are the same as human emotions.

Among the great apes, the chimpanzee is probably the best known. An estimated 150,000 to 200,000 chimpanzees live in the wild—more than all the other great apes combined. Chimpanzees live in a variety of habitats, from lowland tropical rain forests to open grasslands with only patches of trees. These habitats range across equatorial Africa from Gambia in the west to Lake Victoria in the east.

In some ways, chimpanzees strongly resemble people in their behavior, anatomy, and ways of life. For example, infant chimpanzees spend many years with their mother. Chimpanzees reach sexual maturity at about age 11. Females usually give birth for the first time in their midteen-age years and have a baby every four years for the rest of their life. Chimpanzees may live to about age 45 in the wild and may reach 60 in captivity.

Chimpanzees live in complex social groupings called communities. Males spend their entire life in the community in which they were born, while females typically immigrate at maturity to a neighboring group. Males of a community have strong social bonds, and they act together to defend their community boundaries against all outsiders. They attack and even kill chimpanzees from neighboring communities that wander into their area. Male chimpanzees spend a great deal of energy intimidating and fighting other males in an attempt to move up the chain of command, so to speak, in their community.

Males and females form temporary subgroups called parties that come together when trees are laden with ripe fruit or when females are ready to mate. Chimpanzees seem to relish the meat of other mammals and spend much time and energy hunting. However, they often get less energy from the meat they catch than the amount of energy they spend hunting. Most of the chimpanzee diet consists of ripe fruit.

Bonobos, also sometimes called pygmy chimpanzees, live in the rain forest south of the Congo River in the Democratic Republic of the Congo, often called Congo (Kinshasa). Bonobos look like chimpanzees, but their social behavior differs greatly. Although males dominate chimpanzee social life, female bonobos form alliances with other females to intimidate males and so try to prevent the males from dominating them. In addition, like human beings, bonobos do not use sexual contact solely for reproduction. Bonobos engage in sexual activity with members of their own gender as well as with members of the opposite sex as a way of reducing tensions, settling conflicts, and strengthening the bonds among members of a group.

The gorilla is the world's largest primate. Like chimpanzees and bonobos, gorillas live in tropical forests near the equator. Because of their size, gorillas spend more time on the ground and less time climbing trees than chimpanzees or bonobos do.

The name gorilla actually refers to two similar species. Western gorillas live in western and central Africa, primarily in lowland rain forests. Eastern gorillas live higher in mountain ranges. One race (subspecies) of eastern gorilla, the mountain gorilla, lives at elevations up to 4,000 meters (13,000 feet) in two small patches of forest near the border between Uganda, Rwanda, and Congo (Kinshasa). As males reach their midteen-age years, the hair on their backs becomes silvery, earning them the common name of silverback. At about the same age, females tend to leave their home groups and join another group.

The orangutan is the only Asian great ape, and is, perhaps, still the least understood of the four great apes. These large red apes, which live in the tropical rain forests of the Indonesian islands of Sumatra and Borneo, are mostly solitary. As a result, scientific studies of orangutans have been able to produce only limited amounts of information. Adult male orangutans spend their lives alone, while adult females live in the company of their infants. Considering that all other great apes are highly social, this is strange behavior, which scientists have not been able to explain.

Like all the other great apes, orangutans grow slowly and reproduce infrequently; a female may go more than five years between the births of offspring. Orangutans spend most of their life high in trees, where they feed on ripe fruits. These apes live in complex societies that, when mapped out, resemble overlapping rings. Several females live alone or with their offspring on overlapping areas. One resident male, meanwhile, tries to control a territory that encompasses the areas of all his females. He constantly patrols his area in an attempt to keep other males away. At the same time, younger, “bachelor” males wander the forest hoping to find females when the resident male is not nearby.