Western hoolock gibbon (Hoolock hoolock)

Male western hoolock gibbon leaping
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Western hoolock gibbon fact file

Western hoolock gibbon description

GenusHoolock (1)

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).

Also known as
hoolock gibbon, western hoolock.
Bunopithecus hoolock hoolock, Hylobates hoolock.
Gibbon Hoolock Occidental, Hoolock.
Gibón Hulock.
Head-body length: 45 – 64 cm (2)
5.5 – 5.6 kg (2)

Western hoolock gibbon biology

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).


Western hoolock gibbon range

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).


Western hoolock gibbon habitat

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).


Western hoolock gibbon status

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

IUCN Red List species status – Endangered


Western hoolock gibbon threats

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).


Western hoolock gibbon conservation

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.

Find out more

For further information on the western hoolock gibbon, see:

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  1. IUCN Red List (September, 2009)
  2. Macdonald, D.W. (2006) The Encyclopedia of Mammals. Oxford University Press, Oxford.
  3. CITES (September, 2009)
  4. Cunningham, C. and Mootnick, A. (2009) Gibbons. Current Biology, 19(14): 543 - 544.
  5. Kakati, K., Raghavan, R., Chellam, R., Qureshi, Q. and Chivers, D.J. (2009) Status of western hoolock gibbon (Hoolock hoolock) populations in fragmented forests of eastern Assam. Primate Conservation, 24: 1 - 11.
  6. Das, J., Biswas, J., Bhattacharjee, P.C. and Mohnot, S.M. (2006) First distribution records of the eastern hoolock gibbon Hoolock hoolock leuconedys from India. Zoos’ Print Journal, 21(7): 2316 - 2320.
  7. Kumar, A., Mary, P.P. and Bagchie, P. (2009) Present distribution, population status, and conservation of western hoolock gibbons Hoolock hoolock (Primates: Hylobatidae) in Namdapha National Park, India. Journal of Threatened Taxa, 1(4): 203 - 210.
  8. Mittermeier, R.A., Ratsimbazafy, J., Rylands, A.B., Williamson, L., Oates, J.F., Mbora, D., Ganzhorn, J.U., Rodríguez-Luna, E., Palacios, E., Heymann, E.W., Kierulff, M.C., Yongcheng, L., Supriatna, J., Roos, C., Walker, S. and Aguiar, J.M. (2007) Primates in Peril: The World’s 25 Most Endangered Primates, 2006 – 2008. Primate Conservation, 2007(22): 1 - 40.
  9. Government of India, Ministry of Environment and Forests (September, 2009)
  10. Molur, S., Walker, S., Islam, A., Miller, P., Srinivasulu, C., Nameer, P.O., Daniel, B.A. and Ravikumar, L. (2005) Conservation of the western hoolock gibbon (Hoolock hoolock hoolock) in India and Banglasdesh. Zoo Outreach Organisation and CBSG-South Asia, Coimbatore, India.

Image credit

Male western hoolock gibbon leaping  
Male western hoolock gibbon leaping

© Bernard Castelein / naturepl.com

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