The hoolock gibbon is an extremely endangered primate. Measuring less than a metre and weighing about 68kg, the animal is Indias only ape. This tailless primate is known more by its distinctive, haunting song, than by sight. Gibbon songs resound in the Namdapha forests at dawn. Vocalised as a loud hookoo-hookoo, the songs are sung by one group at a time, with other groups taking over in turns. The chorus continues till the sun is high up in the sky.

The male gibbon is black with white, silvery brows and the young females are similarly coloured. Adult females, however, turn a golden blonde. Both are tailless and have long arms, almost double the length of their bodies, helping them to swing from tree to tree using brachiation (hanging by the arms and swinging at high speed from branch to branch). Gibbons can also stand upright and run along branches. Their diet consists primarily of fruits and flowers. These fascinating apes live in pairs and are said to be monogamous for life.