For decades, the railway tracks slicing through Assam's Hollongapar Gibbon Sanctuary (HGS) in Jorhat district did more than just carry trains. They cut the forest canopy into two, turning treetop highways into dead ends and splitting families of India's only ape species, the Western Hoolock Gibbon, within their sole home in the country.
The Challenge of Fragmentation
Hoolock gibbons are almost entirely arboreal, moving through the upper canopy via brachiation—swinging by their arms in long, fluid arcs. Where the trees stop, their world stops. The railway corridor created a gap they could not safely cross, forcing groups on either side into isolation and leaving a small pocket of families stranded away from the larger population.
A Historic Crossing
But the story changed on Friday after a male gibbon was recorded crossing the newly installed canopy bridges with safety nets over the railway line, bringing hope not just for the apes but for many other canopy-dwelling animals. The Wildlife Institute of India (WII) stated: "This is the first confirmed use by a gibbon in the sanctuary—and the first documented use of a canopy bridge structure over a railway line anywhere in the world!"
WII noted that while this is a proud moment for all stakeholders and a ray of hope for mitigating the impacts of linear infrastructure on India's only ape species, careful infrastructure planning, eco-conscious settings, and creating reforested corridors to connect isolated populations remain critical for the long-term survival of exclusively arboreal and threatened species like gibbons.
Design and Deployment
The bridges were designed and deployed on the single-track Lumding–Dibrugarh railway line during February–March 2025, a stretch that has been slicing through Hollongapar since 1887. In a technical report released in May 2023, WII highlighted that a broad-gauge, electrified single track of about 1.65 route-km long fragmented the sanctuary into two unequal parts.
The Western Hoolock Gibbon, listed as endangered on the IUCN Red List, faces relentless threats—shrinking forests, tea cultivation, human encroachment, and illegal trade. "Over time, the sanctuary has become a 'forest island' having lost connectivity with surrounding forest patches. Since gibbons are exclusively arboreal animals inhabiting the forest upper canopy, they are particularly sensitive to canopy gaps. Gibbon families on both sides of the railway track have, thus, been effectively isolated from each other, thereby compromising their population genetic variability and further endangering their already threatened survival in the HGS," the report said.
Sanctuary and Population Details
The sanctuary spans an area of 20.98 sq km, characterized by tropical semi-evergreen forest, and is home to around 125 gibbons. Hoolock gibbons are primarily monogamous and live in small family groups of up to six closely related individuals. WII has highlighted just how fragmented the gibbon population at Hollongapar sanctuary has become. Around 26 family groups live within the sanctuary, but the railway line running through it has split the habitat into two unequal compartments. On one side, in the much smaller section, only four to five gibbon families survive. With no substantial canopy connectivity across the tracks, these families have remained effectively isolated for decades from the larger population on the other side.
The sanctuary also shelters arboreal mammals like the Bengal slow loris, the only nocturnal primate found in northeast, stump-tailed macaque, northern pig-tailed macaque, eastern Assamese macaque, rhesus macaque and capped langur, Malayan giant squirrel, and the particolored flying squirrel, all dependent on the dense canopy.
Previous Attempts
Attempts were made earlier to bridge the canopy gap. In 2015, the railways built an artificial canopy bridge made of iron at one location across the railway track, but the structure was not used by arboreal mammals, including gibbons, due to several design aspects not conforming to gibbons' specialized form of movement known as brachiation. Before that, in 2006, a natural canopy bridge was developed through long-term plantation activities on both sides of the railway track through joint efforts of Aaranyak (a Guwahati-based conservation NGO) and the state forest department. WII reported this natural canopy bridge was established and used by gibbons only by 2019, but this natural connectivity did not survive since railways regularly trims trees and branches as part of track maintenance activities.
Official Reactions
Union Environment Minister Bhupendra Yadav praised the effort, calling it "tech-led conservation" and proof that "science-led small-scale efforts can also be of great help in biodiversity conservation." Assam Chief Minister Himanta Biswa Sarma said in a post on X: "A heartening moment from Hollongapar. A year after installing the arboreal canopy bridge, a Hoolock Gibbon is now using it to safely cross the railway track. A small but significant example that shows how science-led interventions can make a real difference in conservation."
