Western hoolock gibbon  (Hoolock hoolock)

The western hoolock gibbon is classified as Endangered (EN) on the IUCN Red List (1), and listed on Appendix I of CITES (3).

One of the true ‘acrobats of the forest’, the western hoolock gibbon swings through the forest using its long arms, in a mode of locomotion known as brachiation (2) (4). The western hoolock gibbon is, like the other 15 species of gibbon currently recognised (5), a slender and graceful ape (2). Adult male and female western hoolock gibbons are similar in size (2), but in the colouration of their dense hair they are vastly different (2) (6). Adult males are black with a contrasting and distinctive white brow, giving them a highly expressive appearance. Females are copper-tan, with dark brown hair on the sides of the face and chest, and a distinct central parting in the head hair. Infants have grey-white hair with a yellow tinge at birth, which turns black with age. At puberty, the coat of females turns pale, whilst males remain unchanged (6). The gibbon is well known for its emotive call, which is mostly performed as a duet between a mated pair. Its call, both energetic and haunting (2) (4), can be heard over long distances, and has seeped into the folklore of Asia’s indigenous people (5).

The western hoolock gibbon occurs in eastern Bangladesh, north-eastern India and north-western Myanmar (1), between the Brahmaputra and Chindwin rivers (5). It is possible it may also occur in south-eastern Tibet (1).

The western hoolock gibbon is an inhabitant of forests, particularly dense evergreen and semi-evergreen forests (1), although it may also be seen moving through, or sleeping in, bamboo forest and plantations (1).

The forest habitat is one that the western hoolock gibbon is supremely adapted to. It swings effortlessly though the trees, reaching speeds of 56 kilometres per hour (4), and can also walk on two legs along branches that are too large to swing from, moving with impressive swiftness and balance (2). It feeds primarily on fruits, resulting in the western hoolock gibbon playing an important role as a seed disperser in the forest as the seeds are passed, undigested, in a new location (7). The gibbon’s simple stomach is well suited to this fruit-based diet which, unlike other primates, is limited in its ability to digest leaf material (5), although the western hoolock gibbon does consume some flowers, leaves and shoots (8).

Like other species of gibbon, male and female western hoolock gibbons form monogamous pairs that may remain together for many years. However, this faithfulness is not absolute, as both members of the pair may mate with other individuals (4). Each pair, along with any immature offspring they have, occupy a territory that is defended with both vocal and visual displays (4). The female gives birth every two and a half to three years to a single infant, which will remain in the family group for seven to ten years. Upon reaching maturity, the parent of the same sex will act aggressively towards the offspring until they leave the group (4).

Populations of western hoolock gibbon are reported to have declined by a shocking 90 percent over the past 30 years, as the result of numerous human activities (7), and it is now considered to be one of the most endangered 25 primate species in the world (8). As people expand their agricultural activities into forests, establish tea gardens and coffee estates, undertake logging and carry out other development projects, forest habitat is steadily fragmented and reduced. When forests are fragmented, the gibbon must spend more time on the ground, moving between forest patches; at these moments it becomes extremely vulnerable to both domestic and wild dogs (7). Adding to the threat of habitat loss is the pressure of hunting, with people hunting this charismatic primate for food, traditional medicine and the pet trade; unfortunately, the gibbon’s beautiful songs act as a signal that guides hunters directly to them (7).

Ironically, while humans have pushed the western hoolock gibbon into its precarious situation, this species is now entirely dependent on human action for its survival (8). The western hoolock gibbon occurs in numerous protected areas throughout its range (1), is listed on Schedule I of the Indian Wildlife (Protection) Act of 1972 (1) (9), and organisations are working together to carry out many other conservation actions (10). Hopefully enough will be done in time to save the western hoolock gibbon; a vital member of its habitat and a flagship species for Asia’s forests (7).

View information on this species at the UNEP World Conservation Monitoring Centre.

For further information on the western hoolock gibbon, see:

• The Gibbon Conservation Center: 
  http://www.gibboncenter.org/

To support conservation efforts in India visit:

• Wildlife Trust of India:
  www.wti.org.in