Golden Langurs in Assam face population decline amid loss of habitat

Guwahati (Assam) [India], September 10 : The golden langur species are facing serious habitat loss and population decline due to deforestation in Assam.
Dr Jihosuo Biswas, a senior primatologist at Primate Research Centre Northeast India (NGO), said that the main driver of this habitat loss is deforestation, exacerbated by the conversion of forests into farmland and human settlements.
The golden langur (Trachypithecus geei) is an obligate canopy-dwelling primate species endemic to the Indo-Bhutan border, primarily found in four districts of western Assam and six districts of south-central Bhutan.
In India, the species has experienced substantial habitat loss, with over half of its habitat reportedly disappearing over recent decades, particularly in the southern part of its distribution range in the Kokrajhar and Bongaigaon districts of Assam.
"This transformation has led to the fragmentation of once extensive forested areas into smaller, isolated fragments, further threatening the survival of the Golden Langur population in these regions," said Dr. Jihosuo Biswas.
He elaborated that lack of connectedness due to linear infrastructure development has led to high mortality from road accidents or predators, altered home ranges, and higher physiological stress.
"As an arboreal primate, golden langurs, may need to engage in terrestrial behaviours to cross disconnected areas within their home ranges. Lack of connectedness due to recent linear infrastructure development within fragments comes at a price, including high mortality from road accidents or predators, food alterations, altered home ranges, and higher physiological stress and parasite burdens," he added.
He further highlighted that there have been recorded instances of increased road kills, electric hazards, and parasitic prevalence within the primate population.
"Nowadays, road kills are considered the major direct human cause of mortality of terrestrial animals worldwide, and wildlife electrocutions have been documented as having an important impact on some animal populations in urban landscapes. 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 WLS since 2013," the researcher said.
Biswas further mentioned a study conducted to understand road crossing behaviours "We conducted a comprehensive study on traffic behaviour 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 opted to utilise existing canopy connectivity, altering their route to avoid potential encounters with vehicles," Dr. Jihosuo Biswas said.
Initially, the project involved constructing artificial canopy bridges (ACB) at the Nayekgaon-Rubber Garden-Baxamara-Amguri forest complex to minimise accidents under the Nayekgaon Range of Kokrajhar district.
"We began designing and constructing artificial canopy bridges. We collected all of the necessary measurements and data for bridge construction, including bridge length, anchor trees, canopy height, angle of the bridges, owner of the area, bridge type, and bridge viability along Charikhola-Bahalpur State Highway -14 (SH-14), where the majority of golden langur deaths were caused by road accidents. We continued our experiments and opted to use HDPE (high-density polyethylene) pipes with 2 inches of diameter typically used for water supply and plastic rope of 1 1/2 inches of diameter instead of bamboo. HDPE pipe is resistant to various environmental factors and is known for long-term cost-effectiveness due to its durability and resistance to corrosion, breakdown, or melting, due to its lower thermal conductivity. Furthermore, the pipes are black in colour, offering both a natural appearance and enhanced visibility," Dr. Jihosuo Biswas said.
He further said that they installed bamboo bridges, mixed bamboo-cum-rope bridges, and pipe bridges up on the anchor trees by fastening the rope to the branches on each side of the 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 its utilisation, supplementing our regular direct observation schedule. It was observed that this pipe bridge had a more appealing appearance, which encouraged golden langurs to use it. 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. Despite their installation, it took six months for Golden Langurs to gain confidence in using the ladder bridges," Dr. Jihosuo Biswas said.
Between June 2022 and February 2024, seventeen road collision incidents were documented along the stretch from Nayekgaon to Charaibari area of the SH-14 road. Of these incidents, six tragically resulted in fatalities, with the animals succumbing to their injuries immediately.
Five incidents reportedly caused major injuries to the animals, including broken legs or the amputation of palms or tails, though the animals managed to survive. In six other instances, the animals sustained minor injuries but ultimately survived the collisions.
To sensitise the local communities and to earn their support, the people were involved in planting trees in village backyards to connect fragments by restoring natural corridors to facilitate future population exchange of golden langurs in the Nadangiri-Baxamara-Amguri-Nayekgaon-Chakrashilla WLS forest complex in Kokrajhar district matrix.