Gibbons are small apes traditionally grouped in the genus Hylobates, though at least some authorities now divide them into four genera Hylobates, Bunopithecus, Nomascus, and Symphalangus. They are also called lesser apes, and differ from great apes in being smaller, generally monogamous, in not making nests, and in certain anatomical details in which they more closely resemble monkeys than the great apes do. They occur in tropical and subtropical rainforests from northeast India to Indonesia and north to southern China. Gibbons are masters of their primary mode of locomotion, brachiation, allowing them to swing from branch to branch distances of up to 50 feet, at speeds as much as 35 mph. Strongly territorial, they defend their boundaries with vigorous visual and vocal displays. The vocal element, which can often be heard for long distances, consists of a duet between the mated pair, the young animals sometimes joining in. This eerie song can make them an easy find for poachers who engage in the illegal wildlife trade and sales of body parts for use in traditional medicine. Most species are threatened or endangered, and the most important reason is degradation or loss of their forest habitat.
The species include the siamang, the lar, and the hoolock. The siamang, which is the largest, is distinguished by having two digits on each side stuck together, hence the species name syndactylus.