Guardians of Hollongapar

A visit to Hollongapar Gibbon Sanctuary in Assam is not considered complete until one hears the loud, echoing calls of the Western hoolock gibbons (Hoolock hoolock), perched atop the highest branches. Their distinctive “hoo-koo hooo-kooo” calls, an integral part of the rainforest’s soundscape, captivate visitors. Globally, Western hoolock gibbons are found across eastern Bangladesh, west of the Chindwin River in northeastern Myanmar, and Northeast India. In India, their range is limited to areas south of the Brahmaputra River and east of the Dibang River. Known for their ululating calls, they are the only ape species in India and one of humanity’s closest relatives on the evolutionary ladder. The sanctuary, one of the fragmented forest habitats where they persist, covers less than 21 sq km.

Western hoolock gibbons are dimorphic, meaning adult males and females differ in both colour and size. Males, slightly larger, have black coats, while females sport light brown or golden fur. They reach sexual maturity between six and eight years of age, at which point they leave their natal groups to find mates and establish their own territories. The inter-birth interval is around two years, and typical troop sizes range from two to five individuals. They are primarily monogamous, with males and females pairing for life. Western hoolock gibbons consume fruit as their main diet but also feed on young and mature leaves and insects. As frugivores, they play a critical role in seed dispersal. Exclusively arboreal, they rely on brachiation, a unique method of forest travel involving swinging from branch to branch using their long, specially adapted forelimbs. Canopy connectivity is essential for their survival, as they rarely descend to the forest floor. When forced to the ground due to gaps in the canopy, they walk bipedally, standing upright on their hind limbs with their forelimbs raised. This makes them vulnerable to predation by dogs and other animals, as they are clumsy on the ground. Habitat fragmentation caused by tree felling disrupts canopy connectivity, restricting their movement and reducing food availability. As a result, the population size of hoolock gibbons has declined precipitously throughout their range due to habitat loss, fragmentation, and hunting. They are listed as endangered on the IUCN Red List of Threatened Species and are protected under Schedule I of the Indian Wildlife (Protection) Act 1972.

Although the controversial oil exploration activities are set to take place 13 km away within the eco-sensitive zone of the Hollongapar Gibbon Sanctuary, they could still negatively impact local wildlife. Emissions from drilling can disperse pollutants over long distances, harming food sources. The tragic case of a gibbon family affected by the 2020 Baghjan blowout—a result of an oil leak at an Oil India plant—highlights the potential consequences. A pregnant female from nearby Barekuri village prematurely gave birth to a stillborn infant and died days later. The endangered Deohanh, or white-winged duck, also found in Hollongapar, depends on small forest pools that are at risk of contamination. Primates in the sanctuary already face threats from an existing railway track that divides the forest into two unequal halves. Oil exploration would exacerbate the degradation of this fragile environment.

(Neeharika Gogoi is a researcher and Narayan Sharma is an assistant professor at the Department of Environmental Biology and Wildlife Sciences, Cotton University in Guwahati, Assam)

This column was originally published as part of the special report Threat to Survival, published in the February 1-15, 2025 print edition of Down To Earth