The hoolock gibbons are two primate species of genus Hoolock in the gibbon family, Hylobatidae, native to eastern Bangladesh, Northeast India and Southwest China.
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 sexes 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 their eyes and mouths give their faces a mask-like appearance.
In northeast India, the hoolock is found south of Brahmaputra and east of the Dibang Rivers. Its range extends into seven states covering Arunachal Pradesh, Assam, Manipur, Meghalaya, Mizoram, Nagaland, and Tripura (The seven northeastern states of India).
Like the other gibbons, they are diurnal and arboreal, brachiating through the trees with their long arms. They live together in monogamous pairs, which stake out a territory. Their calls serve to locate family members and ward off other gibbons from their territory. Their diet consists mainly of fruits, insects and leaves.
Young hoolocks are born after a seven-month gestation, with milky white or buff-colored hair. After about six months, the hair of males will darken and turn black, while the female hair remains buff-colored throughout her life. After eight to 9 years, they are fully mature and their fur reaches its final coloration. Their life expectancy in the wild is about 25 years.