Hoolocks are the second largest of the gibbons, after the Siamang.
They reach a size of 60 to 90 cm and weigh 6 to 9 kg. The genders are about the same size, but they differ considerably in coloration: males are black colored with remarkable white brows, while females have a grey-brown fur, which is darker at the chest and neck.
White rings around the eyes and around the mouth give their face a mask-like appearance. Young hoolocks are born after a seven month gestation, with a milky white fur.
After about six months their fur turns black. After 8 to 9 years they are fully mature and their fur reaches its final coloration. Their life expectancy in the wild is about 25 years.
The range of the hoolocks is the most northwestern of all the gibbons, extending from Assam in North-East India, to Myanmar.
Small populations (in each case few hundred animals) live also in the eastern Bangladesh and in southwest China.