The Siamang is different form other gibbons. In Siamangs, two fingers on each hand are fused together, that's where the name "syndactylus" is from. Siamangs have large "gular sacs". These can be found in both males and females. The gular sac is a throat pouch that can be inflated to the size of its head. This allows the Siamang to make loud resonating calls or songs.