Western Hoolock Gibbon Hoolock hoolock (Harlan, 1831) Bangladesh, India, Myanmar (2006)

The hoolock gibbon was formerly in the genus Bunopithecus with just one species and two subspecies: B. hoolock hoolock, the western hoolock gibbon, and B. hoolock leuconedys Groves, 1967, the eastern hoolock gibbon from Myanmar and China. Mootnick and Groves (2005) informed that the name Bunopithecus was not valid, and placed it in a new genus, Hoolock, and at the same time argued that the two forms were distinct species (but see Mootnick 2006). The western hoolock gibbon (Hoolock hoolock) occurs in Bangladesh, northeastern India and western Myanmar, west of the Chindwin River. Its range in Myanmar, known from just a few field studies and mostly informal sightings, is restricted to the western parts, delineated from the populations of Hoolock leuconedys by the Chindwin River as far as the head waters in the north. In India and Bangladesh its range is strongly associated with the occurrence of contiguous canopy, broad-leaved, wet evergreen and semi-evergreen forests. The species is an important seed disperser; its diet includes mostly ripe fruits, with some flowers, leaves and shoots. Western hoolock gibbons face numerous threats in the wild, and are now entirely dependent on human action for their survival. The debilitating threats include habitat encroachment to accommodate ever-growing human populations and immigration, forest clearance for tea cultivation, the practice of jhuming (slash-and-burn cultivation), hunting for food and “medicine”, capture for trade, and the degradation and decline in quality of their forests that impacts fruiting trees, canopy cover and the viability of their home ranges. Isolated populations face the additional threats arising from the intrinsic effects of small populations. Some populations surviving in just a few remaining trees are subjected to harassment by locals and to lack of food, and are attacked by dogs while attempting to cross clearings between forest patches. Based on habitat loss over the last 30-40 years, western hoolock gibbons are estimated to have declined from more than 100,000 (Assam state alone was estimated to have around 80,000 in the early 1970s) to less than 5,000 individuals (a decline of more than 90%). The species was known to occur in good numbers in contiguous forests, which have borne the brunt of persistent human impacts. Isolated forest fragments hold just some few families—numbers insufficient for survival in the mid- to long-term. Apart from some border forests between India and Myanmar, the remaining habitat is fragmented, holding minimal populations of this sort. We have documented the extirpation of western hoolock gibbons from 18 locations over the last 3–5 years; eight in Bangladesh and 10 in India. Bangladesh has about 200 western hoolock gibbons in 22 separate locations, twenty of which have less than 20 individuals each: 17 of these have less than 15 individuals, and 14 have less than 10 individuals. About 100 locations with hoolock gibbons have been recorded in India; 77 have less than 20 individuals, and 47 of these have less than 10 individuals. The Population Viability Analysis (PVA) predicts a 95% decline in the population in Bangladesh and a 75% decline in the population in India over the next two decades based on the current effects of human impacts and the intrinsic factors acting on very small and isolated populations. The population of the western hoolock gibbon in Myanmar has not been surveyed. West of the Ayeyarwaddy-Chindwin River, there is about 50,000 km² of forest in the Rakhine Yoma region, but much of it is degraded and hunted. The area includes the Rakhine Yoma Elephant Range (about 175,500 ha), managed by the Nature and Wildlife Conservation Division of the Forest Department of Myanmar, in Rakhine State, in the lower part of the country (about 17ºN). There are other forested areas farther to the north, including the Chin Hills Complex and the Naga Hills area, but they are considered unsafe for travelers. No published information is available on the current range and status of the western hoolock in Myanmar. Warren Brockelman has been carrying out surveys of the eastern hoolock, Hoolock leuconedys Groves, 1967, in accessible protected areas east of the Chindwin River in Myanmar since 2005, and preliminary results indicate that the situation there is considerably more encouraging, with relatively large populations still surviving. The population trends for the western hoolock observed over recent years in Bangladesh and northeast India indicate a very rapid decline in numbers and immediate measures are required by their governments, forest departments, local communities and NGOs. Sally Walker, Sanjay Molur & Warren Y. Brockelman

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