Wstern hoolock gibbon

The hoolock gibbons, are two  species of genus Hoolock in the gibbon family, native to eastern Bangladesh, North west india and south west china
Hoolocks are the second-largest of the gibbons, after the simang. 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, batching  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.

Western hoolock gibbon

The classification of this gibbon has changed several times in the past few years. Classically, all gibbons were classified in the genus Hylobates, with the exception of the simang. After some studies, the genus was divided into three subgenera (including the siamang's Symphalangus), and then into four (recognizing  as the hoolock subgenus distinct from other gibbon subgenera). These four subgenera were elevated to full genus status. Very recent investigations have shown the hoolocks are not closely related to B. sericus, so have been placed in their own genus, Hoolock. In the process, the two subspecies of hoolocks have been raised to species level. A new subspecies has been described recently from northeastern India, which has been named Mishmi Hills hoolock, Hoolock hoolock mishmiensis.