Siamang

The siamang is an arboreal, black-furred gibbon native to the forests of Indonesia, Malaysia, and Thailand. The largest of the gibbons, the siamang can be twice the size of other gibbons, reaching 1 m in height, and weighing up to 14 kg. It is the only species in the genus Symphalangus.

Two features distinguish the siamang from other gibbons. 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, found in both males and females of the species, can be inflated to the size of the siamang's head, allowing it to make loud, resonating calls or songs.