The Siamang is distinctive for two reasons. The first is that two fingers on each hand are fused together. The second is the large "gular sac" (found in both male and female 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. Male and female couples will often call in duet.
While the illegal pet trade takes a toll on wild populations, the principal threat to the Siamang is habitat loss in both Malaysia and Sumatra. Palm oil production is clearing large swathes of forest, reducing the habitat of the Siamang, along with that of other species such as the Orang utan and Sumatran Tiger.
Click here to listen to my interview with Fleur Butcher of Melbourne Zoo about the Siamang Gibbon and the distruction of their habitat.
For more information on Melbourne Zoo, please vist http://www.zoo.org.au/