Great apes are large when compared to other primates, although a range of sizes and body forms exist within the
family. Without question, gorillas are the largest of the great apes. Adult males may exceed 350 lb (158 kg), while adult females are generally about half that size. Bonobos are the most slender great apes, having very long limbs and a gracile appearance. Adult males average less than 100 lb (45 kg) and adult females are usually about 20% smaller.
All of the great apes share an overall similarity in body type, having arms that are longer than their legs. When standing with both hands and both feet on the ground, their limb proportions slope their back downwards, and their head is positioned as the highest point of their body. While this is their most common posture, all are also able to stand bipedally and walk upright for short distances. Whether on the ground or in the trees, great apes are able to move their arms with maximum rotation at the shoulder joint, unlike most other species of primates that have a more limited degree of flexibility. This superior range of motion allows great apes to suspend themselves with their arms, and move using a style of locomotion referred to as semi-brachiation. The gibbons, genus Hylobates, are the only true brachiators.
The African species (gorillas, chimpanzees, and bonobos) are primarily terrestrial, although they do spend considerable amounts of time in the trees. When moving on the ground, they "knuckle-walk," meaning that they support their body weight on the knuckles of their hands, and the soles of their feet. Orangutans, the largest arboreal species on Earth, have a number of physical adaptations that make them uniquely suited for life in the trees. Spending most of their lives in the forest canopy, these apes have exceptionally long arms, and the majority of their strength is located in the upper body. Their fingers and toes are also elongated, allowing them to secure themselves with a vise-like grip. When moving high above the ground, orangutans use their hands and feet interchangeably in a style that is referred to as quadrumanous, literally translated as "four-hands." Unlike the African apes, orangutans descend from the trees infrequently. When moving on the ground, they do not knuckle-walk, but either hold their hands and feet open and flat as they move, or close them completely and support themselves on their fists.
The general appearance of the great apes varies by species. All of the African apes have dark hair, and orangutans are aptly called the "red ape" due to their rusty orange appearance. Gorillas and orangutans show extreme sexual dimorphism, with males reaching an adult size that is usually twice that of most females. In addition to their size, adult male gorillas are most easily recognized by their prominent sagittal crest, which becomes dome-shaped as they mature. Male orangutans develop very long hair, a beard, cheek pads, and an obvious throat sac, as they reach adulthood. These physi cal indications of sexual maturity all serve to intimidate rival males, as well as to attract potential mates. Male and female chimpanzees and bonobos lack these extreme differences in size and appearance, with males being approximately 20% larger than females. However, bonobos and chimpanzees are distinct in their appearance. The skin coloration on the faces of adult chimpanzees may be lighter than the very dark complexion of bonobos, who also have pinkish lips. Bonobos have longer hair on their head, with an obvious part in the middle. In terms of general appearance, chimpanzees are robust and muscular, while bonobos appear much slighter by comparison.
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