OLL LEWIS: The Disappearing Western Hoolock

As you will have seen from some of the previous articles I've written for this blog the Garo Hills is home to a large number of rare species. One of those species, and indeed a species that some people have postulated may be responsible for some sightings of the Indian Yeti, is the western hoolock gibbon (Hoolock hoolock). The western hoolock is an endangered species of tail-less gibbon and can grow to a maximum size of around 90cm, making it, in terms of size of individuals, the second largest known species of gibbon in the world.

The western hoolock is under considerable pressure from several different sources, including deforestation and hunting, and as a result the population has declined by 50% in the last 40 years. Unless conservation efforts pay off this trend is likely to continue. As the population becomes more fragmented the speed of decline will only accelerate as groups of the gibbons become more and more isolated from each other. This isolation of groups will lead to smaller local gene-pools and the inevitable loss of potential genetic combinations that could reduce resistance to disease and other environmental pressures, and if this isolation is caused by deforestation and the fragmentation of habitat then, if one local group is wiped out it is less likely that the area will be recolonised naturally. There are estimated to be less than 5000 individual western hoolock gibbons across their entire range in Bangladesh, North Eastern India and North Western Myanmar. It is also thought to occur in parts of the Tibet area of China, and it has been listed among the worlds 25 most at risk primates by the IUCN.

Another potential threat to the western hoolock's survival is its close relative the eastern hoolock gibbon (Hoolock leuconedys), which is also a threatened species but classed as vulnerable rather than the more threatened endangered western hoolock. Typically the eastern hoolock is only found to the east of the Chindwin River but the two species are not reproductively isolated and hybridisation between the two species occurs towards the river's source. It is known that there is a population of the eastern hoolock, within the range of the western hoolock, in Arunachal Pradesh and it is certainly a possibility that hybridisation will have occurred around the edges of this population as well. Isolated hybridisation is one thing but if the eastern hoolock encroaches further into the western hoolocks territory there is a danger that hybridisation could spread, which would further impact the population of western hoolocks.

It is certainly not too late to save the western hoolock and along with a CITES listing there are several habitat conservation projects in India from which the western hoolock benefits, but in order to halt the decline of the species and even reverse that trend a lot more will have to be done.