- Capped langurs have vanished from seven of their previously known habitats in Assam’s Upper Brahmaputra Valley due to forest fragmentation and habitat degradation.
- A new study across 40 forest fragments found the primates in only 11 sites, with food tree diversity emerging as the strongest factor influencing their presence.
- Despite being listed as ‘Vulnerable’ by IUCN, the capped langur populations continue to decline, with some troops now forced into human-dominated areas.
The capped langur (Trachypithecus pileatus), a leaf eating primate with a distinctive ‘cap-like’ hairstyle, is the second most commonly seen primate in Assam after the rhesus macaque. Once thriving in the tropical rainforests of the Upper Brahmaputra Valley, the only colobine primate (characterised by their leaf-based diet) in the region, is now in a precarious situation.
The Valley was once covered by a vast lowland tropical rainforest. However, over the years, agricultural expansion, deforestation and development of tea gardens have turned this forest into scattered fragments.
A new study published in the Journal of Asia-Pacific Biodiversity surveyed 40 rainforest fragments across six districts of the Upper Brahmaputra Valley between February 2019 and January 2020. Researchers found capped langurs in only 11 sites, with evidence that they had disappeared from at least seven forests where they were once recorded, a decade ago.
“By comparing the surveys of the same fragments, it became clear that capped langurs have now disappeared from several of these areas,” said Narayan Sharma, an author of the study, and assistant professor at the Department of Environmental Biology and Wildlife Sciences in Cotton University, Guwahati. “This kind of longitudinal study helps us track not just numbers but also their overall occurrence and distribution over time.”
The capped langur has reported a 30% decline in its population globally over the past two decades, making it to the vulnerable category of International Union for Conservation of Nature (IUCN), and in Appendix I of CITES. In India, it is included in Schedule I of the Wildlife (Protection) Act, 1972.
During the year-long survey, researchers recorded 82 langurs across eight sites, the largest troop comprising 15 langurs in Hollongapar Gibbon Sanctuary. Even protected areas such as Borajan-Bherjan-Padumoni Wildlife Sanctuary, previously home to the species, reported no sightings during this study.
At least one-third of the capped langur’s habitat in Assam has been lost since the 1980s due to tree felling and encroachment.
Pradipta Baruah, Indian Forest Service (IFS), and field director of the Orang Tiger Reserve, said, “We see rhesus macaque populations multiplying in the same environment where capped langurs are struggling. This indicates that something specific is affecting the langurs, and it calls for long-term investigation to understand the issue better.”
A fragmented rainforest
The Upper Brahmaputra Valley is among the last remaining lowland tropical rainforest zones in Northeast India, spread across six districts: Dibrugarh, Tinsukia, Sibsagar, Charaideo, Jorhat and Golaghat. These rainforest fragments were once part of a vast, unbroken stretch of forests.
However, deforestation, due to agricultural expansion and the development of tea gardens, has broken up these continuous forests into smaller, isolated fragments. “Historical deforestation here is not a recent phenomenon; it began nearly a century ago,” said Sharma. “For instance, the railway line that cuts through Hollongapar Gibbon Sanctuary was laid during the British era, around 90 to 100 years ago.”
But these fragmented forests still host seven primate species, including the capped langur, but loss of canopy connectivity has left populations isolated in pockets.
For the study, the researchers analysed three factors: habitat, landscape, and anthropogenic. “Our hypothesis was that the larger the habitat, the greater the chances of capped langur presence; the smaller the habitat, the higher the risk of their disappearance,” Sharma added. “In smaller fragments, food sources are limited, and this ecological constraint plays a major role in their decline.
A drop in numbers
In 1989, when naturalist Anwaruddin Choudhury was studying capped langurs for his PhD, he estimated their population in Assam at around 39,000. By 2014, that number had dropped to 18,600. Since then, no fresh estimate has been made.
“What makes this worrying is that the capped langur is a forest-dwelling species,” said Choudhury, now a retired civil servant who has studied the species for decades. “With forests becoming fragmented, these langurs end up isolated in separate patches, making breeding difficult and ultimately causing their numbers to decline.”
The recent study also points to an “extinction debt”, where the consequences of habitat loss are not immediate, but are felt after several generations. Sharma noted that capped langurs are highly adaptive. “These landscapes were degraded long ago, yet it took decades for their populations to decline. This is what we are witnessing today.”
Choudhury also stressed on the need for more research on the species. “We need more studies on capped langurs to better understand their status especially in smaller forest pockets.”
He meanwhile clarified that capped langurs still have a healthy population in the state. “They are found in Garbhanga Reserve Forest and Amchang Reserve Forest near Guwahati, in Dhansiri Reserve Forest and East Karbi Anglong Wildlife Sanctuary, as well as in Dihing Patkai National Park, Borail National Park, and Hollongapar, which supports a sizeable population,” he noted.
Human pressures on the forests
While hunting is not a major threat in this region, indirect human pressures are. These include illicit logging, extraction of non-timber forest products (NTFP), and loss of food and roosting trees.
A large number of indigenous communities in Northeast India depend heavily on forest resources and products for their livelihoods, but more extensively on NTFP, which play a significant role in their subsistence and food security, states a 2020 study. With limited income sources, locals widely collect non-timber forest products for personal use or trade.
An overlooked threat is the removal of saplings of food and roosting trees from forest fragments, which prevents young trees from maturing and reduces both food availability and canopy connectivity, both essential components of capped langur ecology and survival.
“Locals often cut saplings for firewood,” said Neeharika Gogoi, a co-author of the study and PhD scholar at the Department of Environmental Biology and Wildlife Sciences in Cotton University, Guwahati. “Over time, this prevents food trees from maturing and reduces diversity in the forest. One of our key findings was that capped langurs survive best in fragments with high food tree density.”
Logging is another major stressor. “When the canopy is broken, langurs are forced to the ground, exposing them to predators and other threats,” Gogoi added. “Logging also removes fruit-bearing trees essential to their diet.”
As primarily leaf-eaters, capped langurs require a steady supply of young leaves, fruits and seeds. In degraded forests, some troops have been forced into tea estates and villages to survive.
“We found that food tree density emerged as the most important predictor of capped langur presence,” Sharma said. “Even in small forest fragments, if the density of food trees is high, langurs can survive there for a long time. While many factors influence their occurrence, the availability of food trees is the strongest determinant.”
Capped langurs in human-inhabited areas
The study documented capped langurs in Borajan and Chala Reserve Forest, where they now forage, rest and even establish home ranges in human-dominated areas.
In Borajan, Gogoi initially observed a troop of six langurs in 2019. By 2022, only three adult females remained. In Chala, just two females were spotted, moving alongside a troop of rhesus macaques.
“Without males, they cannot reproduce, and that group will likely become locally extinct, unless males join them,” Gogoi said, while citing examples of human-related disturbances such as an instance where a langur was killed by a speeding train, according to local accounts.
“In Borajan, there is a presence of leopards, and pythons,” Choudhary said. “Due to reduced tree cover, langurs are forced to move to lower levels, and when they roost on small or degraded trees, they become more vulnerable to predation. Other primates too have declined, though this trend is unique to Borajan.”
The way forward
Experts stress that the key to survival lies in habitat restoration. Protecting and replanting native food and roosting trees such as Gmelina arborea, Dalbergia sissoo, and Albizia lebbeck could help capped langurs sustain themselves.
“Although degraded, many of these areas are still under the Forest Department,” Sharma said. “With collaboration between the [forest] department and NGOs, restoration can help capped langur populations recover.”
Researchers also recommend easing pressure on forests by providing local communities with alternatives to firewood, such as subsidised LPG and biogas.
Sharma also emphasised the role of the “matrix” or the areas surrounding fragments, such as tea gardens, settlements, orchards, and paddy fields. “If the quality of this matrix is good, langurs will use it,” he added. “But it shouldn’t become more resource-rich than the forest itself, or the animals may abandon forests for human habitats.”
Read more: Species no bar: Langurs forge unusual alliances in the wild
Editor’s note: A quote from Anwaruddin Choudhury has been updated on September 29, 2025, for clarity.
Banner image: A capped langur at Bordubi. Image by Neeharika Gogoi.
Credits
