The siamang (/ˈsiːəmæŋ/, also UK: /ˈsaɪə-/;[5][6] Symphalangus syndactylus) is an endangered arboreal, black-furred gibbon native to the forests of Indonesia, Malaysia, and Thailand.

First, two digits on each foot—the second and third toes—are partially joined by a membrane, hence the specific name syndactylus, from the Ancient Greek σύν, sun-, "with" + δάκτυλος, daktulos, "finger".

Second, a large gular sac (throat pouch), found in both males and females of the species, can be inflated to the size of the siamang's head, allowing it to make resonating calls.

[8] While the illegal pet trade takes a toll on wild populations, the principal threat to the siamang is habitat loss in both Indonesia and Malaysia.

The palm oil production industry is clearing large swaths of forest, reducing the habitat of the siamang, along with those of other species, such as the Sumatran tiger.

The siamang inhabits the forest remnants of Sumatra Island and the Malay Peninsula, and is widely distributed from lowland forest to mountain forest—even rainforest—and can be found at altitudes up to 3800 m.[9] It lives in groups of up to six individuals (four individuals on average) with an average home range of 23 hectares.

The siamangs in Sumatra and the Malay Peninsula are similar in appearance, but some behaviors differ between the two populations.

[14] Siamang males tend to offer more paternal care than do other members of the family Hylobatidae, taking up a major role in carrying an infant after it is about 8 months old.

[15] Siamangs are generally known to have monogamous mating pairs, which have been documented to spend more time in close proximity to each other, in comparison to other gibbon species.

[17] This figure may be less in the 21st century: Bukit Barisan Selatan National Park is the third-largest protected area (3,568 km2 or 1,378 sq mi) in Sumatra, of which roughly 2,570 km2 (990 sq mi) remain under forest cover inhabited by 22,390 siamangs (in 2002 censuses).

In Sumatra, the siamang prefers to inhabit lowland forest between 500 and 1,000 m (1,600 and 3,300 ft) above sea level.

[10] The siamang tends to rest for more than half of its waking period from dawn to dusk, followed by feeding, moving, foraging, and social activities.

A siamang may spend an entire day in a single fruiting tree, moving out when it wants to rest and returning to feed.

Calls in the late morning typically happen when it meets or sees another siamang group.

The siamang prefers calling in the living, tall, and big trees, possibly where another group is easy to see.

[21] Unlike other gibbons in which vocalization is added by laterally expanded laryngeal sacs, those in siamangs fuse with each other and extend into the ventral area of the neck.

[23] As an arboreal primate whose survival absolutely depends on the forest, the siamang faces population pressure due to habitat loss,[10] poaching, and hunting.

[17][24] A major threat to the siamang is habitat fragmentation due to plantations, forest fire, illegal logging, encroachment, and human development.

Firstly, palm-oil plantations have removed large areas of the siamang's habitat in recent decades.

Since 2002, 107,000 km2 of oil palm have been planted,[25] which has replaced much rainforest in Indonesia and Malaysia, where the siamang originally lived.

Secondly, in the second decade of the 21st century, forests in the Malay Peninsula have been destroyed due to illegal logging.

Sixteen out of the 37 permanent forest reserves in Kelantan, in the Malay Peninsula, where most of the siamangs live, have been encroached upon by illegal loggers.