Gibbon

Gibbons are apes in the family Hylobatidae. The family historically contained one genus, but now is split into four genera. Gibbons occur in tropical and subtropical rainforests from northeast India to Indonesia and north to southern China, including the islands of Sumatra, Borneo, and Java.

Also called the lesser apes, gibbons differ from great apes (chimpanzees, gorillas, orangutans and humans) in being smaller, exhibiting low sexual dimorphism, in not making nests, and in certain anatomical details in which they superficially more closely resemble monkeys than great apes do. But like all apes, gibbons evolved to become tailless. Gibbons also display pair-bonding, unlike most of the great apes. Gibbons are masters of their primary mode of locomotion, brachiation, swinging from branch to branch for distances of up to 15 m, at speeds as high as 55 km/h. They can also make leaps of up to 8 m, and walk bipedally with their arms raised for balance. They are the fastest and most agile of all tree-dwelling, non-flying mammals.

Depending on species and gender, gibbons' fur coloration varies from dark to light brown shades, and anywhere between black and white. It is rare to see a completely white gibbon.

Gibbon species include the siamang, the white-handed or lar gibbon, and the hoolock gibbons.

Evolutionary history
The dating of the evolution of these genera has been difficult. The best current estimates place Nomascus diverging from the other genera about 8 million years ago (Mya), and Symphalangus and Hylobates diverging at 7 Mya. At the species level, Hylobates pileatus diverged from H. lar and H. agilis at 3.9 Mya, and H. lar and H. agilis separated at 3.3 Mya. The extinct Bunopithecus sericus is a gibbon or gibbon-like ape which, until recently, was thought to be closely related to the hoolock gibbons.

Taxonomy
The family is divided into four genera based on their diploid chromosome number: Hylobates (44), Hoolock (38), Nomascus (52), and Symphalangus (50).
 * Family Hylobatidae: gibbons
 * Genus Hylobates: dwarf gibbons
 * Lar gibbon or white-handed gibbon, H. lar
 * Malaysian lar gibbon, H. l. lar
 * Carpenter's lar gibbon, H. l. carpenteri
 * Central lar gibbon, H. l. entelloides
 * Sumatran lar gibbon, H. l. vestitus
 * Yunnan lar gibbon, H. l. yunnanensis
 * Bornean white-bearded gibbon, H. albibarbis
 * Agile gibbon or black-handed gibbon, H. agilis
 * Müller's Bornean gibbon, H. muelleri
 * Müller's gray gibbon, H. m. muelleri
 * Abbott's gray gibbon, H. m. abbotti
 * Northern gray gibbon, H. m. funereus
 * Silvery gibbon, H. moloch
 * Western silvery gibbon or western Javan gibbon, H. m. moloch
 * Eastern silvery gibbon or central Javan gibbon, H. m. pongoalsoni
 * Pileated gibbon or capped gibbon, H. pileatus
 * Kloss's gibbon, Mentawai gibbon or bilou, H. klossii
 * Genus Hoolock
 * Western hoolock gibbon, Hoolock hoolock
 * Eastern hoolock gibbon, Hoolock leuconedys
 * Genus Symphalangus
 * Siamang, S. syndactylus
 * Genus Nomascus: crested gibbons
 * Northern buffed-cheeked gibbon, N. annamensis
 * Concolor or black crested gibbon, N. concolor
 * N. c. concolor
 * N. c. lu
 * N. c. jingdongensis
 * N. c. furvogaster
 * Eastern black crested gibbon or Cao Vit black crested gibbon, N. nasutus
 * Hainan black crested gibbon, N. hainanus
 * Northern white-cheeked gibbon, N. leucogenys
 * Southern white-cheeked gibbon, N. siki
 * Yellow-cheeked gibbon, N. gabriellae

Hybrids
Many gibbons are hard to identify based on fur coloration, so are identified either by song or genetics. These morphological ambiguities have led to hybrids in zoos. Zoos often receive gibbons of unknown origin and therefore rely on morphological variation or labels that are impossible to verify to assign species and subspecies names, so it is common for separate species of gibbons to be misidentified and housed together. Interspecific hybrids, hybrids within a genus, also occur in wild gibbons where the ranges overlap.

Physical description
One unique aspect of gibbon anatomy is that the wrist is composed of a ball and socket joint, allowing for biaxial movement. This greatly reduces the amount of energy needed in the upper arm and torso, while also reducing stress on the shoulder joint. Sometimes when a gibbon is swinging, its wrist will naturally dislocate until the gibbon finishes its swing. Gibbons also have long hands and feet, with a deep cleft between the first and second digits of their hands. Their fur is usually black, gray, or brownish, often with white markings on hands, feet, and face. The male gibbon will sometimes end up with some dark patches in the white to show it is a suitable choice for mating. Some species have an enlarged throat sac, which inflates and serves as a resonating chamber when the animals call. This structure is enormous in a few species, equaling the size of the animal's head.

Gibbon skulls and teeth resemble those of the great apes, and their noses are similar to those of all catarrhine primates. The dental formula is The siamang, which is the largest of the 17 species, is distinguished by having two fingers on each foot stuck together, hence the generic and species names Symphalangus and syndactylus.

Behavior
Gibbons are social animals. They are strongly territorial, and defend their boundaries with vigorous visual and vocal displays. The vocal element, which can often be heard for distances of up to 1 km (0.6 mi), consists of a duet between a mated pair, with their young sometimes joining in. In most species, males, and in some also females, sing solos to attract mates, as well as advertise their territories. The song can be used to identify not only which species of gibbon is singing, but also the area from which it comes.

Gibbons are masters of their primary mode of locomotion, brachiation, swinging from branch to branch for distances of up to 15 m, at speeds as high as 55 km/h. The gibbons' ball-and-socket joints allow them unmatched speed and accuracy when swinging through trees. Nonetheless, their mode of transportation can lead to hazards when a branch breaks or a hand slips, and researchers estimate that the majority of gibbons suffer bone fractures one or more times during their lifetimes. They are the fastest and most agile of all tree-dwelling, non-flying mammals.

Conservation status
Most species are endangered, primarily due to degradation or loss of their forest habitats.

In traditional Chinese culture
The sinologist Robert van Gulik concluded gibbons were widespread in Central and Southern China until at least the Song Dynasty, and furthermore, based on an analysis of references to primates in Chinese literature and their portrayal in Chinese paintings, the Chinese word yuán (猿) referred specifically to gibbons until they were extirpated throughout most of the country due to habitat destruction (circa 14th century). In modern usage, however, yuán is a generic word for ape. Early Chinese writers viewed the "noble" gibbons, gracefully moving high in the treetops, as the "gentlemen" (jūnzǐ, 君子) of the forests, in contrast to the greedy macaques, attracted by human food. The Taoists ascribed occult properties to gibbons, believing them to be able to live a several hundred years and to turn into humans.

Gibbon figurines as old as from the fourth to third centuries BCE (the Zhou Dynasty) have been found in China. Later on, gibbons became a popular object for Chinese painters, especially during the Song Dynasty and early Yuan Dynasty, when Yì Yuánjí and Mùqī Fǎcháng excelled in painting these apes. From Chinese cultural influence, the Zen motif of the "gibbon grasping at the reflection of the moon in the water" became popular in Japanese art, as well, though gibbons have never occurred naturally in Japan.