The boundary between the two species of Hoolock is the Chindwin River, which flows into the Ayerawady (Irrawady) River. At the headwaters in the north there is a hybrid zone or cline between the two species (since they are almost certainly not reproductively isolated). Das et al. (2006) reported the discovery of a population of H. leuconedys in Arunachal Pradesh, northeast India, which has traditionally been considered to be part of the range of H. hoolock.
Habitat and Ecology
Home ranges in most populations range from 8-63 ha (Ahsan 2001; Alfred 1992; Alfred and Sati 1986, 1990; Feeroz and Islam 1992; Gittins and Tilson 1984; Islam and Feeroz 1992; Kakati 2004; Mukherjee 1986; Tilson 1979), but unusually large home ranges of 200-400 ha were reported from Tripura and Arunachal Pradesh, northeast India (Mukherjee 1982; Mukherjee et al. 1988). The western hoolock is a frugivorous species, with ripe fruits composing a majority of its diet (Ahsan 2001; Alfred and Sati 1986, 1994; Feeroz and Islam 1992; Islam and Feeroz 1992; Kakati 1997; Tilson 1979). A dominantly folivorous diet was reported during studies carried out in Assam’s Borajan Reserved Forest, and in Tripura (Kakati 1997; Mukherjee 1986), and gibbons living in small forest fragments were observed to experience a period of almost total lack of fruit in their diet at the end of the dry season (Kakati, 2004). Low fruit availability may contribute to the relatively large home range sizes of some populations. In northeast India, the hoolock gibbon is recognized as being an important disperser of undigested seeds from large and small fruit-bearing trees (Das 2003).
Life History and Behavior
Lifespan, longevity, and ageing
IUCN Red List Assessment
Red List Category
Red List Criteria
This species occurs in several of India’s northeastern states, but populations there tend to be isolated. It is common in certain areas of occurrence, but rare in others due to intense hunting by local tribes (Choudhury 1991), and is considered rare throughout its range (Choudhury 2001). The species was found in all forested patches in northeastern India about 30 years ago, but they are reduced to a few forest fragments now. The total population in northeastern India was estimated to be about 2,600, of which the majority—about 2,000—occurs in the state of Assam (Molur et al. 2005). A population of about 170 gibbons has more recently been identified as H. leuconedys (Das et al. 2006) and should be subtracted from the population estimate if this identity is proven. Moreover, there are surveys needed in Mehao region, where there is uncertainty as to which species the gibbons there represent (Das et al. 2006; Das pers. comm.). Namdapha National Park in the Changlang District of Arunachal Pradesh is a relative stronghold for this species in India, offering the population there its largest contiguous stretch of protected habitat (Chetry et al. 2003).
There are no population estimates available for Myanmar. It is possible that the largest and most viable populations of western hoolock are to be found in this country, where at present almost no attention is paid to it (W. Brockelman pers. comm.). There are several thousand square kilometers of unsurveyed forest habitat in the central-west and north-west of this country, with a particular need to survey the western areas west of Chindwin/Ayerawady River. There are reports of gibbons in Rakhine Yoma Elephant Range, but there is no knowledge of the actual population level there (W. Brockelman pers. comm. 2006). The western part of Hukuang Tiger Reserve with a large area of forest (>1,000 km2) has not been surveyed, but is likely to have this species. The northern limit is just south of Hkakaraborazi National Park.
In some Indian locales, these animals are rare due to large scale hunting for food and because some ethnic groups believe the gibbons have medicinal properties (Gupta 2005; J. Das pers. comm.). Additionally, jhoom cultivation threatens the habitats of Indian populations, some of which are relatively isolated already (Choudhury 1991). Affecting all northeastern Indian primate populations are harvesting of bamboo for paper mills, oil mining and exploration, and coal mining, which deplete habitat and cause pollution and disturbance (Choudhury 2001). Habitat fragmentation and loss are major threats in India (Molur et al. 2005). Small and restricted groups may not be viable because of genetic and demographic instabilities and because they are more affected by hunting pressure and habitat loss. Many small forest fragments are reported to have only one or a few gibbon groups. These have limited chances of surviving more than a few generations without translocation.
In Myanmar, shifting cultivation is a major threat, and so is hunting. Although logging is restricted on the western side of the Chindwin, it is still considered a threat for this species. Political and ethnic conflicts have prevented the Myanmar government to promote development and conservation activities effectively in areas of northwestern and central western Myanmar along the borders with India and Bangladesh. Thus, most conservation efforts have been concentrated within the range of H. leuconedys.
Western hoolock gibbon
Mootnick and Groves stated that hoolock gibbons do not belong in the genus Bunopithecus, and placed them in a new genus, Hoolock. This genus was argued to contain two distinct species which were previously thought to be subspecies: Hoolock hoolock and Hoolock leuconedys which were later have found that the 2 species have a bigger difference to that of white handed gibbons than bonobos to chimpanzees.
In India and Bangladesh it is found where there is contiguous canopy, broad-leaved, wet evergreen and semi-evergreen forests dipterocarpus forest often mountainous. The species is an important seed disperser; its diet includes mostly ripe fruits, with some flowers, leaves and shoots.
There are numerous threats to western hoolock gibbons in the wild, and 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.
Over the last 30–40 years, western hoolock gibbon numbers are estimated to have dropped from more than 100,000 (Assam alone was estimated to have around 80,000 in the early 1970s) to less than 5,000 individuals (a decline of more than 90%). In 2009 it was considered to be one of the 25 most endangered primates, though it has been dropped from the later editions of the list. 
Hoolock gibbons feed submissively on leaves and flower buds from the forest canopy, but fresh fruits are mainly part of their diet as well.
- Brockelman, W., Molur, S. & Geissmann, T. (2008). Hoolock hoolock. In: IUCN 2008. IUCN Red List of Threatened Species. Retrieved 4 January 2009.
- Groves, C. P. (1967). "Geographic variation in the hoolock or white-browed gibbon (Hylobates hoolock harlan 1834).". Folia Primatologica 7: 276–283. doi:10.1159/000155125. PMID 5626313.
- Mootnick, A. R. and C. P. Groves (2005). "A new generic name for the hoolock gibbon (Hylobatidae)". Int. J. Primatol 26: 971–976. doi:10.1007/s10764-005-5332-4.
- Mootnick, A. R. (2006). "Gibbon (Hylobatidae) species identification recommended for rescue or breeding centers". Primate Conserv 21: 103–138. doi:10.1896/0898-6188.8.131.52.
- "Western Hoolock Gibbon, Hoolock hoolock". Retrieved 2008-03-10.
- Mittermeier, R.A.; Wallis, J.; Rylands, A.B.; Ganzhorn, J.U.; Oates, J.F.; Williamson, E.A.; Palacios, E.; Heymann, E.W.; Kierulff, M.C.M.; Long, Yongcheng; Supriatna, J.; Roos, C.; Walker, S.; Cortés-Ortiz, L.; Schwitzer, C., eds. (2009). Primates in Peril: The World's 25 Most Endangered Primates 2008–2010. Illustrated by S.D. Nash. Arlington, VA: IUCN/SSC Primate Specialist Group (PSG), International Primatological Society (IPS), and Conservation International (CI). pp. 1–92. ISBN 978-1-934151-34-1.
- Mittermeier, R.A.; Schwitzer, C.; Rylands, A.B.; Taylor, L.A.; Chiozza, F.; Williamson, E.A.; Wallis, J., eds. (2012). Primates in Peril: The World's 25 Most Endangered Primates 2012–2014. Illustrated by S.D. Nash. IUCN/SSC Primate Specialist Group (PSG), International Primatological Society (IPS), Conservation International (CI), and Bristol Conservation and Science Foundation. pp. 1–40.
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