Hoolock Gibbon (ulluk) a smaller arboreal ape, Bunopithecus hoolock, of the family Hylobatidae, order Primates.
It is also known as White-browed Gibbon. Its body is 45-63 cm long and weighs 6-7 kg. Hoolock lives in the natural and semi-natural forests of Chittagong, Cox's Bazar, Chittagong Hill Tracts, Sylhet and Mymensingh (Karnojhora area of Balijhuri Forest Range adjacent to Tura Hills of India). It is tailless and its arms are longer than legs.
The Gibbon’s body has a dense and shaggy fur while it has beard-like tufts in the groin. Adults are sexually dichromatic. The male is jet black while the female is buff/blonde to various shades of greyish brown. The young are dark brown to black. Gibbons are diurnal and sleep at night on high tree branches; they awake before dawn and retire from day activities before dusk. They seldom come to cross open patches.
Group size is 2-5. Gibbons give long and elaborate call like hoo-u, hoo-yu, hoo-koo-u, etc, for which they are is called Hoolock Gibbon; their calls are mostly performed in the morning and last for 1-46 min. They are frugivorous ie they feed mostly on ripe fruits. New birth occurs in between mid-September and January; their inter-birth interval is at least 3 years; and the gestation period is 4-8 months. [M Farid Ahsan]