The black-furred gibbon with a big vocal sac on its throat is called Siamang, it's native to the mountain and lowland forests of Indonesia, Malaysia and Thailand.

Siamang's most striking feature – the throat sac – can expand to the size of its head in order to amplify booming calls, which can be heard three kilometers away.

Generally, they call from the top of tall trees every morning.

Male and female duets can often be heard as a way to reinforce bonds as well as to announce their territory.

Listen to their singing:

Another feature is the webbing between their second and third toes, the purpose of which is not known.

Their body structure enables them to swing acrobatically between tree branches at high speed – an arm span that could be twice the body length.

Although they are the largest gibbon member, their territory is rather limited, roughly about 0.2 square kilometers.

They eat mainly fruits, especially figs.

Generally speaking, siamangs are monogamous, but cases of polygamy have also been found in the wild.

(All images via VCG)

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