Hominoids

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 [Page 1183]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 forelimb-dominated vertical climbing and suspension.

Source: © Robert Jurmain.

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 sub-species, 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.

A white-handed gibbon (Hylobates lar)in the forests of Thailand

An infant orangutan (Pongo pygmaeus pygmaeus) from Borneo

A young male gorilla (Gorilla

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