Guwahati: In the forests of Kokrajhar district, golden langurs, a critically endangered species, face increasing threats from habitat loss and road accidents. These canopy-dwelling primates, native to the Indo-Bhutan border, are struggling with fragmented forests caused by deforestation and infrastructure development.
In an innovative effort to protect the endangered golden langurs in Assam, conservationists have turned to artificial canopy bridges to restore connectivity between fragmented forest habitats. These arboreal primates, which rely on tree canopies to travel, are vulnerable to habitat loss, deforestation, and linear infrastructure like roads, forcing them to descend to the ground. This behaviour makes them susceptible to road accidents and predators.
“We have been recording road kills, electric hazards, and high parasitic prevalence in golden langurs in Nadangiri RF, Nayekgaon PRF-Rubber Garden-Baxamara-Amguri area, and in Chakrashilla Wildlife Sanctuary since 2013,” says Dr. Jihosuo Biswas, a senior primatologist at the Primate Research Centre NE India, who led the innovative effort to build artificial canopy bridges.
“We conducted a comprehensive study on traffic behavior to understand the patterns and trends of vehicular movement along SH-14 road, alongside observing the locomotor behaviour and substrate use patterns of golden langurs during road crossings. Our observations revealed that on 71% of occasions, golden langurs opted to cross the road at ground level, disregarding the traffic. However, in 29% of cases, they utilized existing canopy connectivity, altering their route to avoid potential encounters with vehicles,” Biswas says.
“We continued our experiments and opted to use HDPE (high-density polyethylene) pipes with a 2″ diameter, typically used for water supply, and plastic rope of 1½” diameter instead of bamboo. This choice offers several advantages. HDPE pipe is resistant to various environmental factors and is known for long-term cost-effectiveness due to its durability and resistance to corrosion or melting, thanks to its lower thermal conductivity. Furthermore, the pipes are black, offering both a natural appearance and enhanced visibility, and are exceptionally lightweight, weighing about 3½ kilograms for an 11-meter-long pipe,” he added.
Biswas explained that they installed bamboo bridges, mixed bamboo-cum-rope bridges, and pipe bridges by fastening the rope to branches on each side of the anchor trees. “To ensure stability, we tied the remaining part of the ropes to adjacent trees next to the main anchor trees. We installed two such bridges and positioned a camera trap at each bridge to monitor their usage, supplementing our regular direct observation schedule. It was observed that the pipe bridge had a more appealing appearance, encouraging golden langurs to use it,” he said.
“We also constructed four ladder bridges at our field station in Siljan, Kokrajhar. For this purpose, we procured necessary materials such as 1″ diameter PVC hose pipe, 6 mm and 4 mm insulated steel cable typically used for gyms, tie cables of different sizes, both-side glue tape, and other instruments required for the construction of ladder bridges,” he added.
Biswas noted that despite their installation, it took six months for golden langurs to gain confidence in using the ladder bridges. “During the monsoon period, they typically utilized canopy gaps by jumping or avoiding certain areas,” he said.
However, in January 2024, the team observed that langur groups began using the ladder bridges regularly when crossing that part of the road.
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