The largest of the lesser apes, the Siamang is twice as heavy as the other hylobatid species. The fur is long and soft and shining, but less dense than the true Gibbons. It is always black. Only the eyebrows are reddish brown. Extremely long hair covers the scrotum, which looks like a tail from certain angles.
Water is obtained mainly from the fruit and moisture adhering to leaves.
They will dip hands into tree holes and pour water into the mouth from above. When running on ground or on thick branches, they move in a short gallop with arms extended laterally or lifted up. Calls of the Siamangs by far excel those of other gibbons, because of the sound-amplifying throat sac. Siamangs, like other brachiators, use muscular arms to swing from branch to branch. Good swimmers, they are very adept at the breast stroke.