Guwahati: A new study has revealed an alarming decline in the population of Capped Langurs (Trachypithecus pileatus) in Assam’s Upper Brahmaputra Valley. Once widely distributed across the region’s lush lowland rainforests, these arboreal primates now survive in only 11 of the 40 forest fragments surveyed.
The findings, published in the Journal of Asia-Pacific Biodiversity, point to forest fragmentation and the loss of food tree diversity as the primary drivers of local extinction. Forests once rich in native tree species have been carved up by tea plantations, agriculture, and infrastructure development, leaving isolated green patches unable to sustain viable primate populations.
The study was authored by Neeharika Gogoi from the Department of Environmental Biology and Wildlife Sciences, Cotton University; Samrat Sengupta from the Department of Zoology, Debraj Roy College, Golaghat; Bijay Basfore from the Department of Zoology, Pandu College; and Narayan Sharma from the Wildlife Biology and Conservation Group, Cotton University, and the School of Natural Sciences and Engineering, National Institute of Advanced Studies (NIAS), Bangalore.
Among the most striking findings is the disappearance of langur groups from seven sites where they had previously been recorded. In some cases, small remnants of troops were found living outside the forest, navigating human-dominated landscapes such as tea gardens — a sign of increasing ecological stress.
Researchers assessed the presence of Capped Langurs across six districts, combining direct observations with interviews of local forest staff and residents. Over 423 kilometers of forest trails were walked, and 540 vegetation plots were studied to evaluate factors such as tree diversity, canopy cover, and human disturbance.
Food tree diversity emerged as the strongest predictor of langur presence. Even relatively large forest patches were unable to support langur populations if they lacked sufficient food tree species. In contrast, fragments with higher food tree richness tended to retain their langur groups, despite other pressures.
Sites like Hollongapar Gibbon Sanctuary, Upper Dehing, and Joypur still host multiple langur troops, but others, including Borajan and Chala, have seen drastic population declines. In Borajan, a troop that once had six individuals was reduced to just three females by 2022 — an isolated and unsustainable group.
The Upper Brahmaputra Valley is home to seven primate species, including the charismatic Western Hoolock Gibbon. But while the gibbon often draws conservation attention, the Capped Langur has long been overlooked — until now.
“The disappearance of Capped Langurs from many forest fragments indicates chronic habitat degradation, which must be urgently addressed. Prioritizing the restoration of these habitats is essential to secure the future of Capped Langurs and other arboreal species,” says Narayan Sharma, the corresponding author of the study.
Neeharika Gogoi, a co-author of the paper, adds that the vulnerable Capped Langurs are the only colobines found in the Upper Brahmaputra Valley of Assam.
“Their distinctive black faces, long tails, and the tufted fur framing their features capture our attention and leave a lasting impression. Yet, their disappearance from fragmented forests paints a bleak picture for their future. Circumventing this decline calls for human intervention through the restoration of their extant but degraded habitats and the prevention of food tree sapling extraction.”
The study warns that many forest patches are approaching ecological thresholds beyond which species like the Capped Langur may not recover. Ongoing threats include illegal logging, non-timber forest product collection, and competition for food resources. Even low-intensity activities, when persistent, can erode habitat quality and connectivity over time.
As a species listed as Vulnerable on the IUCN Red List and protected under Indian wildlife law, the Capped Langur now faces increasing risk in Assam. Conservation efforts, the study suggests, must focus on restoring native food tree species, improving forest connectivity, and managing land use in surrounding areas such as tea estates and agroforestry systems.
Without targeted intervention, these charismatic but often overlooked primates may continue to disappear from Assam’s shrinking rainforests — silently, and one fragment at a time.
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