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Hominoids are the superfamily to which apes and humans belong. Hominoids are distinguished from cercopithecoids (Old World monkeys), the catarrhine group to which they are most closely related, in having primitive nonbilophodont molars, larger brains, longer arms than legs (except in humans), a broader chest, a shorter and less flexible lower back, and no tail. Many of these specializations relate to a more upright posture in apes, associated with a greater emphasis on forelimbdominated vertical climbing and suspension.
Hominoids can be classified into two families: the Hylobatidae, which includes gibbons and siamangs, and the Hominidae, which includes the great apes and humans. The gibbons and siamang (Hylobates) are the smallest of the hominoids (with average body weights ranging from 5–11 kg), and for this reason, they are sometimes referred to as the “lesser apes.” The 12 or so species are common throughout the tropical forests of Asia, ranging from southern China to the Indonesian islands. They are remarkable in having the longest arms of any primates, being 30% to 50% longer than their legs. This is related to their highly specialized mode of locomotion, called brachiation, in which they progress below branches using only their forelimbs to swing from support to support. Gibbons are specialist fruit eaters, while the larger siamang incorporates a higher proportion of young leaves in its diet. Hylobatids live in monogamous family groups, in which males and females are similar in size (unlike most catarrhines). Both sexes defend a territory, mainly through complex vocalizations or songs (with male and female pairs of most species producing a duet), which serve to advertise their presence to neighboring pairs. Siamangs have large specialized throat sacs that they inflate during calling. As in Old World monkeys, hylobatids sit upright on branches to sleep, resting on their ischial callosities (i.e., sitting pads on the buttocks), rather than sleeping in nests like great apes.
The great apes include the orangutan (Pongo) from Asia and the gorilla (Gorilla) and chimpanzees (Pan) from Africa. These were formerly included together in their own family, the Pongidae, to distinguish them from humans, who were placed in the Hominidae. However, recent anatomical, molecular, and behavioral evidence has confirmed that humans are more closely related to the great apes, especially the African apes, and for this reason most scientists now classify them together in a single family, the Hominidae. The orangutan is restricted to the tropical rain forests of Borneo and Sumatra. Traditionally, the Bornean and Sumatran populations, which are morphologically and behaviorally quite distinct, have been separated as subspecies, but there is growing support to recognize them as two separate species, Pongo pygmaeus and P. abelii. They are large (35–80 kg) arboreal primates that climb cautiously through the trees using all four limbs for support. When they descend to the ground, they move quadrupedally, with clenched fists, or bipedally, with the arms raised for better balance. Orangutans subsist mainly on ripe fruits, but they do eat a variety of leaves and small animal prey. Modified sticks are used as simple tools by Sumatran orangutans to extract honey, insects, and seeds as food sources. As is typical of other great apes, they lack ischial callosities and build simple nests to sleep in at night. Adult male orangutans are about twice as heavy as females, and the sides of their faces are adorned with huge fatty flaps. Orangutans are the least sociable of the hominoids and tend to lead solitary lives. Males aggressively defend their territory from other males, which encompasses the ranges of several female consorts. The male long call, a series of reverberating grunts, which can be heard at a distance of 1 km, serves to attract receptive females.
The gorilla, the largest of the hominoids (70–175 kg), has a disjunct distribution in tropical Africa. Because of their great size, gorillas are almost entirely terrestrial, although females and young individuals frequently climb trees. Nests are often built on the ground. They move quadrupedally, and like chimpanzees, the hand is specialized for knuckle-walking, in which the weight of the animal is borne on the dorsal surface of the middle joints of the fingers. Three subspecies of gorillas are generally recognized: mountain gorillas (G. gorilla beringei), from the volcanic highlands of Uganda, Rwanda, and Democratic Republic of Congo; western lowland gorillas (G. gorilla gorilla), from West Africa; and eastern lowland gorillas (G. gorilla graueri), from the Democratic Republic of Congo.Mountain gorillas eat a variety of leaves, stem, and roots, while lowland gorillas eat a greater proportion of fruits and insects. Groups consist of a dominant male, called a “silverback” (because of the silvery white hair on their backs and hips), as well as several adult females, subadults, and infants. The silverback male defends the troop from neighboring groups, which have overlapping home ranges.
There are two species of chimpanzees, the common chimpanzee (Pan troglodytes) and the bonobo or pygmy chimpanzee (Pan paniscus). The common chimpanzee is widely distributed in the forests and woodlands stretching across equatorial Africa, while the bonobo is restricted to the tropical rain forests of the Democratic Republic of Congo. Bonobos differ from the more familiar common chimpanzee in having a more slender build, with longer limbs (despite their name, they are not smaller than common chimpanzees); the face is black with pink lips, the hair on top of the head is parted in the middle, breasts are more protuberant in adult females, and they are more adept at walking bipedally. Both species feed and build their nests in trees, but they mostly travel on the ground. Chimpanzees have an eclectic diet of fruits, leaves, insects, and small vertebrates. Common chimpanzees also include meat in their diet, which they obtain by hunting small to medium-sized mammals, especially colobus monkeys. Tool-using behaviors are common, and more than a dozen simple tool types have been identified. Differences in tool use and other behaviors between populations of common chimpanzees have led some researchers to interpret these as cultural traditions. Examples include termite fishing in Tanzania and the use of rocks to crack nuts in West Africa. Chimpanzees are gregarious and sociable, and they live in large multimale-multifemale communities (20–200 individuals), which divide into smaller subgroups (3–15 individuals) for foraging. Common chimpanzees have a hierarchical social structure, in which males maintain their dominance through intimidating displays and overt aggression; they have been known to kill members of other communities. Bonobos are much less aggressive and are able to reduce tension within the group through more intensive use of sexual behavior.
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