IV.

Behavior

Great apes have well-developed brains and are among the most intelligent of all animals. In the wild, chimpanzees and orangutans are known to make simple tools, such as sharpened sticks used to extract insects from holes in tree trunks. Toolmaking involves a preconceived image of what the tool will look like, a visualization ability that is only possible with an advanced brain. Orangutans have even been observed untying knots, working out for themselves the steps necessary to achieve this complex task. Some scientists believe that apes have the capability to learn and use language, and considerable success has been achieved in training chimps and gorillas to communicate with humans using symbols or sign language (see Animal Communication).

With the exception of orangutans, which spend most of their lives alone, apes live in social groups. These groups range from 3 or 4 animals in the case of some gibbons, an average of about 10 in gorillas, and often more than 50 in chimpanzees. In general, the larger the group size, the looser the social structure is. Gorilla groups are generally close-knit, and may be led by the same dominant male for many years. By contrast, chimpanzee social life is marked by constantly shifting alliances and sometimes by violent confrontations between neighboring groups. Gibbons are also strongly territorial, but they are not as aggressive. They lay claim to patches of forest with extraordinarily loud hooting calls that can be heard up to 3 km (2 mi) away.

Some of these behavioral differences are related to the way apes feed. Chimpanzees eat a wide variety of foods, including fruit, leaves, honey, and insects, and they sometimes hunt and eat other mammals. During the course of a day's foraging, they may travel nearly 18 km (11 mi). By banding together they improve their chances of finding food and also of spotting danger. Gorillas, on the other hand, are vegetarians, eating leaves, plant stems, and roots. This kind of food is comparatively easy to find, and although gorillas also move about, they rarely travel more than 2 km (1 mi) a day. Their great size also means that they are less vulnerable to attack. Consequently, gorillas would gain few advantages from living in larger groups.

Most apes breed throughout the year. Male and female orangutans come together only for a brief courtship and return to their solitary lifestyle immediately after mating. Other apes follow very different patterns: gibbons, for example, pair up for life. In these social species, long-term bonds develop between the adults, as well as between the adults and their young.

Female apes usually give birth to just a single young after a gestation period ranging from seven to nine months. Succeeding births normally occur only after the previous infant has been weaned. This process—like all aspects of ape development—takes a remarkable length of time. A gibbon is weaned by the age of about 2 years, while chimps take more than 4 years. Like young monkeys, young apes are carried by their mothers. They either cling to the mother's belly or, in the case of older chimps, ride on her back. In great apes, infant care is largely the job of the females, while in some gibbons the mother hands over responsibility to the father when the infant gibbon's first year is complete.