All siamangs can be differentiated from other gibbons in two major ways: A naked throat sac (also called a gular sac) that can expand to the size of a siamang’s head when inflated prior to making calls; webbing between their second and third toes, the purpose of which is not yet known. This webbing provides the siamang with its scientific name, Symphalangus syndactylus, which translates from Ancient Greek to “together phalanx” + “united finger.”

The siamang’s body is perfectly adapted to a life of swinging from branch to branch (also known as brachiation). As with other gibbons, their arms are incredibly long with an arm span sometimes twice as long as body length. These proportions make walking along the forest floor awkward, but it allows them to move through the canopy with ease and grace. To help grip onto branches, siamang thumbs have evolved to be shorter and lower on the hand so that they do not get in the way. This trait is also observed in the hands of orangutans, spider monkeys, and, of course, other gibbons. Not only does the siamang’s expertise in the trees give them access to more food, but it also takes much less energy to swing from branch to branch than it does to walk on the ground.

Siamangs have dark black fur with gray or reddish hairs scattered around their mostly naked face. Their fur is relatively shaggy compared to other gibbons.