Assam: Wildlife passages and canopy bridges reduce Golden Langur road accidents

Increased deaths and injuries of endangered Golden langur (Trachypithecus geei) due to road accidents have led the primate research centre to construct wildlife passages as linear corridors and artificial canopy bridges.

The animal is a canopy-dwelling primate species endemic to the Indo-Bhutan border, primarily found in four districts of western Assam, India, and six districts of south-central Bhutan.

'The species is threatened by habitat loss and population decline. In India, the Golden langur has experienced substantial habitat loss, with over half of its habitat disappearing over recent decades, particularly in Assam's Kokrajhar and Bongaigaon districts', Senior Primatologist of Primate Research Centre NE India Dr Jihosuo Biswas said on Wednesday.

Habitat loss is mainly due to deforestation, exacerbated by conversion of forests into farmland and human settlements, which has threatened the survival of the Golden langur population in these regions.

As an arboreal primate, golden langurs may need to engage in terrestrial behaviours to cross disconnected areas within their home ranges, with lack of connectedness due to recent infrastructure development has led to high mortality due to road accidents.

'We have been recording road kills, electric hazards, and high parasitic prevalence in golden langurs in Nadangiri Reserve Forest, Nayekgaon, Rubber Gardens in the Baxamara-Amguri area, and in Chakrashilla Wildlife sanctuary', Biswas said.

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A comprehensive study on traffic behaviour was conducted to understand the patterns and trends of vehicular movement along State Highway 14, alongside observing the locomotor behaviour and the pattern adopted by golden langurs during road crossings, he said.

According to the study, Golden langurs for 71 per cent of the time opted to cross the road at ground level, disregarding the traffic while in 29 per cent of cases, they opted to utilize existing canopy connectivity, altering their route to avoid vehicles.

'We have successfully developed some alternatives to mitigate mortality and restore landscape connectivity in some areas, such as the construction of wildlife passages as linear corridors and artificial canopy bridges are a recent innovation that aims to provide functional connectivity', he said.

Initially, the project involved constructing artificial canopy bridges (ACB) at the Nayekgaon-Rubber Garden-Baxamara-Amguri forest complex to minimize accidents under the Nayekgaon Range of Kokrajhar district.

ACB's were designed and constructed while necessary measurements and data were collected for bridge construction, including bridge length, anchor trees, canopy height, angle of the bridges, owner of the area, bridge type, and bridge viability along the State Highway -14 (SH-14), where the majority of golden langur deaths were caused by road accidents.

'We 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', he said.

After installing two such bridges, a camera trap was positioned at each bridge to monitor its utilization, supplementing the regular direct observation schedule.

It was observed that this pipe bridge had a more appealing appearance, which encouraged golden langurs to use it, Biswas pointed out.

Four ladder bridges were also constructed at Siljan in Kokrajhar but it took six months for Golden langurs to gain confidence in using these bridges.

During the monsoon period, the Golden Langurs typically utilized canopy gaps by jumping or avoiding certain areas. However, in January 2024, we observed that langur groups began using the ladder bridges regularly when crossing that part of the road.

It was observed while monitoring their movement that golden langur groups used bamboo bridges, mixed bamboo-cum-rope bridges, and pipe bridges more frequently than rope bridges (insulated cable rope) and ladder bridges. he said.

During dry seasons, however, most of the langur groups were observed crossing the road utilizing the ladder bridges.

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 langurs died immediately, five received major injuries, including broken legs or the amputation of palms or tails, but they survived and in six instances, the animals sustained minor injuries due to the collisions.