From Wikipedia:
The Siamang
(Symphalangus syndactylus) is an arboreal, black furred gibbon native to the forests of Malaysia, Thailand, and Sumatra. The largest of the lesser apes, the siamang can be twice the size of other gibbons, reaching 1 m in height, and weighing up to 23 kg. The Siamang is the only species in the genus Symphalangus.
The Siamang is distinctive for two reasons. The first is that two fingers on each hand are fused together (hence the name "syndactylus"). The second is the large "gular sac" (found in the male of the species), which is a throat pouch that can be inflated to the size of its head, allowing the Siamang to make loud resonating calls or songs.
