Members of the genus Gorilla are the largest of the non-human family members, weighing in at up to 450 pounds for adult males and up to 225 pounds for adult females. Gorillas live in the central African tropical rainforest and savannah. Over two million years ago their ancestors divided, becoming two separate species on either side of the vast Congo River basin.
Eastern and western gorillas share several physical features: large heads, broad shoulders and chests, and hairless, shiny faces. Maturing adult males have a distinctive silvering of the hair on their backs and along a prominent boney ridge in the midline at the top of the skull—hence the name “silverbacks.” Western gorillas live at or near sea level in present-day Cameroon, Central African Republic, Congo Republic (Congo-Brazzaville), and Nigeria. They sport a sleek coat, often with a reddish-bronze tinge, that is shorter than the longer, shaggy black coat of the eastern gorilla. Eastern gorillas live in Rwanda, Democratic Republic of the Congo (Congo-Kinshasa), and Uganda, with some in the Virunga Massif, the eastern end of their range, at an altitude of up to ten thousand feet.
Gorillas’ large body size allows them to eat a relatively poor-quality diet compared to that of the other great apes. Their stomachs are simple and non-fermenting. Their diet varies with the season, but consists primarily of herbaceous leaves, bamboo shoots, seeds, and fruits. They will eat ants, termites, and other insects, as well as bark and twigs. The poor digestibility of their diets dictates that they spend most of their daylight hours feeding and resting. Gorilla groups settle into a small area for a day or two, then move on, seldom returning to any spot for several months to allow the trampled vegetation to recover.
Gorillas live in social groups of between seven and sixteen individuals, including a dominant male, three or four females, and four or five of their offspring. Their social bonds are not particularly strong, except between mothers and their infants, who are weaned at three years of age. Female gorillas deliver their first offspring when they are approximately eight or nine years old, and will then reproduce every four years. Gorillas sleep at night in nests they construct each evening, males on the ground and females and their offspring on the ground or in trees.
Chimpanzees and bonobos, the two species of the genus Pan, descend from a common ancestor who likely split off some 1.3 to 3 million years ago. The chimpanzee lineage occupies a wide arc, from Senegal and Guinea in West Africa through central Africa north of the Congo River to Tanzania in East Africa. Chimpanzees are found in twenty-two countries in Africa. Bonobo ancestors appear to have become isolated south of the Congo River in the heartland of the Congo basin—the modern-day Democratic Republic of the Congo.
Chimps and bonobos are of a similar size, with adult males ranging from 65 to 130 pounds and females being approximately 35 percent smaller. Both have black faces as adults, black fur, and arms as long as their legs. Bonobos have a more slender build, with longer limbs, and sport a central part in the hair on the top of their head. Bonobos have red lips; those of chimpanzees are brown or black. Bonobos are born with black faces, chimpanzees with pink ones. The skull of a bonobo is shorter and more rounded; chimpanzees have a lower forehead, with prominent brow ridges.
Their widespread distribution suggests that chimpanzees are opportunistic and highly adaptable to a variety of habitats. Unable to digest large quantities of leaves, they consume primarily fruits, algae, mushrooms, flowers, and seeds. They also eat some young leaves, small mammals, and invertebrates. Bonobos live in lowland forest and, like chimpanzees, their diet is primarily fruit, supplemented with leaves, pith, sprouts, mushrooms, seeds and nuts, honey, earthworms, invertebrates, and fish. They will occasionally consume small mammals such as infant duikers, flying squirrels, and bats.
Chimpanzees live in variably sized communities consisting of from twenty to one hundred members. Group sizes fluctuate as individuals merge with or split from a given community, sometimes for mere hours or days. They form both strategic political alliances and long-lasting friendships. They exhibit compassion and concern for others who may or may not be related to them, and chimpanzees have been known to grieve upon the death of an individual significant to them. Bonobo community size is slightly larger, at between 50 and 120 individuals. Bonobos forage in larger groups than chimpanzees, and those groups can contain members of different communities.
The most obvious difference between chimpanzees and bonobos is observed in the relationship between and roles of males and females. Chimpanzee society is male-dominated, with groups of males cooperating in hunting expeditions, in patrolling the borders of their territories, and in mating with receptive females. In contrast, bonobo society is dominated by female coalitions. Female bonobos form alliances among themselves that facilitate food allocation, dictate mating strategies, and offset the greater physical strength of males. Males are much less territorial, with significantly less competition for females and less aggression with males of other groups.
Chimpanzee and bonobo males reach sexual maturity when they are thirteen years old and females when they are eleven. Their gestation period is approximately eight months. They have been known to live up to sixty years in the wild. They all sleep in tree nests at night.
Classified in the genus Pongo, orangutans are three species of great apes native to Indonesia and Malaysia. They inhabit the tropical rainforests and peat swamp forests of the islands of Borneo and Sumatra. Sumatran and Bornean orangutans are believed to have diverged from their common orangutan ancestor some 1.1 to 2.3 million years ago, with the newly recognized Tapanuli orangutan representing the earliest split, approximately 3.4 million years ago.
Adult female and male orangutans are significantly different in size, weighing up to 90 and 165 pounds respectively. Their diet consists primarily of ripe, sugary fruit, which is supplemented by “famine” foods that include leaf shoots, insects, flowers, bark, honey, pith, termites, bird eggs, and small mammals. With big toes able to grasp like hands, powerful long arms, and highly flexible hip joints, they are masters of swinging within the forest canopy. These features enable them to access edible seeds and fruits. They spend most of their time in the forest canopy of their large home range.
Prominent cheek pads are seen in sexually mature males that have reached a certain social status. The status that these so-called “flanged” males are thought to have provides an advantage in establishing their home range, accessing food, and choosing females with whom to breed. The hair coat of the shorter and stockier Bornean orangutan is a darker red-brown compared to the lighter cinnamon of the slimmer Sumatran orangutan. Sumatran orangutans have a fuller beard and moustache, whereas their Bornean counterparts have little fur around the face. Male Bornean orangutans have a distinctive figure eight-shaped face, with forward-facing cheek flanges, in contrast to the Sumatran males, with their flat cheek flanges covered in downy hair.
The home range size, population density, and sociability of orangutans are largely a function of the abundance and continuous availability of fruit. There are few territorial disputes among orangutans that live within the areas they habitually share. They live in loose social communities consisting of one or more clusters of related females and the adult male with which they prefer to mate. Adult males are intolerant of each other and exercise caution when entering each other’s territory. The mother-infant bond is very close in orangutans, with suckling continuing for five or six years. Orangutans sleep in nests throughout the day and night; they build these nests daily, high in the forest canopy.
Male orangutans reach sexual maturity at around fourteen years of age; female orangutans mature approximately five years earlier. Orangutan pregnancy lasts approximately 8.5 months, with an interval between births of between eight and nine years—the longest in any primate species. Lifespan estimates for orangutans in the wild range from forty-five to sixty years.
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