gibbon, small ape, genus Hyloblates, found in the forests of SE Asia. The gibbons, including the siamang, are known as the small, or lesser, apes; they are the most highly adapted of the apes to arboreal life. Gibbons are about 3 ft (90 cm) tall and weigh about 15 lb (6.4 kg). Their arms are extremely long in proportion to their body length, and they swing through the trees with great speed and agility, clearing gaps up to 20 ft (6 m) wide. On the ground they walk on two feet, holding their arms up awkwardly; they can also run on all fours. Members of most gibbon species have black faces surrounded by a white ruff; their fur ranges in color from black to buff. Some species, e.g., white-handed gibbon, have sexual dimorphism in coloration. Like Old World monkeys and unlike other apes, gibbons have callosities on their buttocks. Gibbons live in permanent families consisting of a male, a female, and their young; families occupy definite territories. They feed on fruits and other plant matter as well as insects and other small animals. Gibbons have powerful voices and at times engage in loud howling, which is answered by other gibbons in the vicinity. The largest gibbon is the siamang, sometimes classified in a separate genus, Symphalangus. Deep black, with a reddish brown face, the siamang may weigh up to 25 lb (11.3 kg). Siamangs are further distinguished by the presence in both sexes of a large vocal sac on the throat; this sac is inflated before the animal howls and probably functions to magnify the sound. Such a sac is also found in the male concolor gibbon ( Hyloblates concolor ). Siamangs are found in the high mountain forests of Sumatra and the Malay Peninsula. The gibbons are highly endangered because of habitat destruction. Gibbons are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Primates, family Pongidae.