Western hoolock gibbons (Hoolock hoolock) are large primates from the gibbon family, Hylobatidae. They live in family groups and form long-term pair bonds. Their primary mode of locomotion is swinging from branch to branch. They can also leap and are the fastest of all tree-dwelling, nonflying mammals.
The male and the female of this species are about the same size, but they differ considerably in coloration. Males are black-colored with remarkable white brows, while females have 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.
Western hoolock gibbons are found in Assam, Mizoram, and Meghalaya in India, Bangladesh, and Myanmar west of the Chindwin River. In India and Bangladesh, they prefer to live in areas where the canopy is contiguous, broad-leaved, wet evergreen. They inhabit mixed evergreen forests, including dipterocarp forests, and often in mountainous terrain. The can also visit rural gardens and plantations.
Western hoolock gibbons are diurnal primates. They live in trees and move swiftly by brachiating with their long arms. Hoolock gibbons live in family groups which consist of a mated pair and their young and they fiercely defend their territory. Like other gibbons, hoolock gibbon pairs produce a loud, elaborate song, usually as a duet from the forest canopy, in which younger individuals of the family group may join. The song includes an introductory sequence, an organizing sequence, and a great call sequence, with the male also contributing to the latter (unlike in some other gibbon species).
Western hoolock gibbons are herbivores (frugivores, folivores). Their diet includes mostly ripe fruits, with some flowers, leaves, and shoots. However, they also supplement their diet with silkworms, ants, and various other insect species. During the monsoon season of northeast Inda, insects are their second preferred food item after a fruit-bearing tree in the same genus of jackfruit.
Western hoolock gibbons are monogamous and form stable pair bonds. Females typically reproduce every 2-3 years. After the gestation period of around 7 months, they give birth to 1 or 2 infants. Young hoolocks are born with milky white or buff-colored hair. After about 6 months, the hair of males darkens and turns black, while the females' hair remains buff-colored throughout their lives. At the age of 8-9 years, young hoolock gibbons become fully mature and their fur reaches its final coloration.
Western hoolock gibbons face many threats in the wild, and they are now entirely dependent on human action for their survival. Threats include habitat encroachment by humans, forest clearance for tea cultivation, the practice of jhuming (slash-and-burn cultivation), hunting for food and "medicine", capture for trade, and forest degradation.
According to the IUCN Red List, the total population size of the Western hoolock gibbon is unknown but there are estimated populations in the following areas: 200-280 individuals in Bangladesh; over 12,000 individuals in northeastern India; over 15,000 individuals in Namdapha National Park in the Changlang District of Arunachal Pradesh. Currently, this species is classified as Endangered (EN) on the IUCN Red List, and its numbers today are decreasing.
Western hoolock gibbons are important seed dispersers within their native ecosystem. They eat a wide range of fruits and thus help spread the seeds of these fruits throughout the forests.
