Western Hoolock Gibbons

The endangered western hoolock gibbon (Hoolock hoolock), India’s only non-human ape species, is facing severe threats in the Hollongapar Gibbon Sanctuary in Jorhat, Assam, due to habitat fragmentation, railway intrusion, and other anthropogenic pressures.

Railway Line Splits Habitat
A British-era Northeast Frontier Railway line cuts through the Hollongapar reserve forest, dividing it into two unequal parts. The constant movement of passenger trains disrupts the forest ecosystem, with the gibbons’ calls competing against the roar of trains passing through their habitat.

Sharp Decline in Population

• The western hoolock gibbon population has declined drastically from an estimated over 100,000 individuals to less than 5,000.
• The species is now listed among the most endangered primates in the world.
• In Assam, the gibbon’s distribution has become highly restricted due to shrinking and fragmented forest cover.

Major Threats

• Habitat loss and fragmentation caused by deforestation and infrastructure development
• Expansion of settled agriculture and jhoom (shifting) cultivation
• Hunting and human interference
• Changing land-use patterns across Northeast India

These threats affect gibbon populations across Assam, Meghalaya, Arunachal Pradesh, Manipur, Nagaland and Mizoram.

Habitat and Distribution

• Western hoolock gibbons prefer dense forests with continuous canopy
• Found at altitudes up to 8,202 feet
• The Chindwin River separates them from the eastern hoolock gibbon (Hoolock leuconedys), though a hybrid zone exists near the river’s northern headwaters

Biological Characteristics

• Belong to the family Hylobatidae (lesser apes)
• Characterised by very long arms, long legs, and a dense, plush coat
• Exhibit sexual dichromatism (different coat colours in males and females)
• No tail, with hook-shaped hands and opposable thumbs

Behaviour and Diet

• Arboreal and diurnal in nature
• Primarily frugivorous, feeding largely on fruits
• Follow a monogamous social structure, with a bonded male–female pair forming the family unit

Sources: TH & Other