Endangered Species Spotlight: Delacour’s Langur

Delacour’s langur, named after French-American ornithologist Jean Théodore Delacour, is a species of Old World monkey endemic to the limestone karst forests of north-central Vietnam. It is one of the world’s most endangered primates.

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Until recently, the rugged and generally inaccessible nature of the species’ habitat, coupled with the fact that populations tend to be small and highly fragmented, had rendered behavioural and ecological studies on these elusive monkeys incredibly challenging.

Now regarded as one of the most endangered primates in the world, conservationists have undertaken significant efforts to study, monitor, and implement protection measures for remaining Delacour’s langur populations in an attempt to save the species from the brink of extinction. 

Appearance 

Locally known as voọc quần đùi trắng, which translates to “white-legged langur” or “langur with white pants”, the Delacour’s langur is easily recognizable by its colouration: a predominantly black torso with a clear demarcation to white fur between its hips and its knees. A long, black, bushy tail extends from its rear, utilized primarily for maintaining balance whilst climbing challenging terrain.

Categorized as a crested langur (genus Trachypithecus), Delacour’s langurs have long, upright tufts of hair at the top of their heads, which extend down their necks and behind their ears, as well as long white whiskers on their cheeks. Due to the rough, jagged texture of the limestone karst cliffs they traverse, Delacour’s langurs have specialized pads on their hands and feet – an adaptation that allows these remarkable primates to run, climb and jump on the karst, and serves as a form of protection against predators.  

Male Delacour’s langurs are slightly heavier than females, weighing between 7.5 and 10.5 kilograms. Females weigh between 6.2 to 9.2 kilograms, and both genders have a body length of approximately 60 centimeters, with a 90 centimeter long tail. As with most Old World monkeys, Delacour’s langurs have ischial callosities, also known as sitting pads, on their buttocks. These thickened, hardened areas of skin develop over the ischial tuberosities (sitting bones) and act as specialized protection when sitting on hard surfaces. Female Delacour’s langurs are further distinguishable in the wild by a distinctive pubic patch of white fur and pale skin, found anterior to the ischial callosities. Infant Delacour’s langurs are born with orange fur, which turns black at the age of four months. Juveniles then develop their full coat pattern at approximately three years of age.

The Delacour’s langur was once regarded as a subspecies of the Francois’ leaf monkey (Trachypithecus francoisi) due to their significant morphological similarities, shared habitat preferences, and as a result of early taxonomic practices. However, Delacour’s langur was later classified as a separate species in 1984; a conclusion which was supported in 2001 by molecular genetic studies and parasitological evidence.

Diet 

The Cercopithecidae family (Old World monkeys) is divided into two main subfamilies based on general diet and digestive anatomy: the Cercopithecinae are omnivorous monkeys, possessing cheek pouches for food storage and simple stomachs; meanwhile, the Colobinae are folivorous monkeys, with complex sacculated stomachs for digesting leaves and no cheek pouches.

The Delacour’s langur belongs to the Colobinae subfamily, with leaves comprising up to 80% of the species’ diet (60% young leaves and 20% mature leaves), and the rest consisting of fruit, flowers, shoots, and bark. With a preference for leaves that contain a high protein to fibre ratio, Delacour’s langurs only consume a small variety of plant species within their home range, opting for leaves with fewer structural or chemical defensive compounds. Key food species include dragon plum (Dracaena marginata), wild rhododendrons (Rhododendron irroratum), powdered giant bamboo (Dendrocalamus barbatus), and red-fox orchids (Bulbophyllum andersonii). 

In order to effectively consume leaves, shoots and bark, which are generally difficult to digest due to their high cellulose content, Delacour’s langurs and other folivorous primates have evolved specialized adaptations to sustain their diet. They possess high-cusped, broad molars that are designed for crushing and grinding food, thoroughly breaking down cellulose when chewing. They also have a ruminant-like digestive system, comprised of a multi-chambered stomach armed with mutualistic microbes, which can digest cellulose, neutralize chemical defensive compounds, and prevent bacterial fermentation. This digestive method is a high-energy, resource-intensive process that requires lengthy rest periods between feedings.

Habitat and Behaviour 

The Delacour’s langur is endemic to Vietnam, occurring only within a geographic range of approximately 5,000 square kilometers in the country’s north-central region. At present, the species occurs in five protected areas: Cuc Phuong National Park; Pu Luong Nature Reserve; Hoa Lu Cultural and Historical Site; Huong Son Cultural and Historical Site; and the Van Long Nature Reserve. Some smaller, fragmented populations can be found across the Vietnamese provinces of Ninh Binh, Phu Tho, and Thanh Hoa with their distribution closely associated to the presence of limestone mountain ranges.

Limestone karst is a remarkable landscape, formed over millennia by the weathering and dissolution of the soluble bedrock, limestone, by acidic rainwater. Characterized by geological features such as caves, streams, sinkholes, underground rivers, springs, and rocky terrain, and with elevations of 200 to 2,000 meters, limestone karst forests are typically regions of high biodiversity. In Northern Vietnam, where rainy and dry seasons occur annually, temperatures can range from 28C in warmer months, to 5C in the winter months.

Delacour’s langurs are currently restricted to limestone karst habitats with broadleaf evergreen forests, although some records indicate a historic presence in secondary limestone forests as well. Bowl-shaped indentations on the surface of limestone peaks retain rainwater for drinking, and caves and crevices offer shelter from inclement weather. Spending most of their time on land and rock formations, Delacour’s langurs are diurnal, terrestrial primates, only entering the forest occasionally and travelling between trees by swinging or leaping from branch to branch. Once night falls, troops will sleep on open rock surfaces or sheltered within limestone caves.

Research has shown that each limestone cave contains a distinct environment, with unique light, moisture, heat, and nutrient conditions. Having adapted to these microhabitats, specific caves are used consistently by single troops over numerous generations, only abandoning a cave if it becomes unsafe. Aside from human hunters, Delacour’s langurs may fall prey to clouded leopards (Neofelis nebulosa), reticulated pythons (Malayopython reticulatus), crested serpent eagles (Spilornis cheela), or mountain-hawk eagles (Nisaetus nipalensis).

Delacour’s langurs are polygynous, and troops typically consist of 10 individuals: one adult male; four adult females; a few juveniles; and a few infants. If two males are present in a group, these individuals tend to be related (father and son, or brothers). The largest group observed in the wild in more recent times comprised 17 individuals, and groups of up to four bachelors have also been found to exist. Historically, when the Delacour’s langur population was greater, troops of 30 individuals were noted. 

Social bonds are established and sustained through play, which improves sensory, cognitive and motor skills, as well as social grooming, which establishes alliances and hierarchies. Once a male becomes sexually mature, he will typically leave his natal troop to create his own, often attempting to separate females from, or trying to gain control over, other troops. Inter-group aggression is therefore commonplace, especially given that home ranges often overlap. Delacour’s langurs give birth to precocial young with open eyes, a robust upper body, and orange fur, allowing mothers to identify infants amongst dense foliage when exploring. Troops often practice allomothering, where all females participate in the rearing of infants.  

Ecological Importance 

By virtue of their folivorous diet, Delacour’s langurs serve as seed dispersers within their ecosystem, dropping seeds whilst consuming fruit or plant products, as well as releasing them through their excretion. When foraging, Delacour’s langurs further support the vitality and growth of plants by pruning various species, removing unhealthy, injured, or dead parts from a plant’s main structure. As a result, these herbivorous primates contribute to the regeneration of native flora and the overall restoration of forest landscapes.

Within limestone karst environments, where habitat loss is largely due to human activity, the Delacour’s langur plays a crucial role in promoting the regeneration of lost forest cover. Since these habitats are regions of high biodiversity, the Delacour’s langur is also regarded as a flagship species, with conservation measures implemented for the primates also helping protect the ecosystems they inhabit as a whole.

Threats

Currently classified as Critically Endangered under the International Union for Conservation of Nature (IUCN) Red List, the Delacour’s langur has suffered a population decline of more than 80% over the past three decades due to poaching, habitat loss and fragmentation. 

Studies carried out between 1990 and 2000 identified the existence of 19 populations of Delacour’s langurs in northern Vietnam (270 to 302 individuals), half of which were found to occupy unprotected areas and were thus exposed to a high threat of hunting. Surveys conducted between 2000 and 2010 saw a slight increase in the total population of Delacour’s langurs to between 281 and 317 individuals. However, the degree of habitat fragmentation had also increased, with numerous smaller subpopulations having been eradicated. 

Between 2012 and 2015, the Endangered Primate Rescue Center and Vietnam’s National Forestry University carried out surveys of all known Delacour’s langur populations, finding a significant decline in numbers to between 234 and 275 individuals. Although the majority of remaining groups were found to occur in the Van Long Nature Reserve, with approximately 164 to 191 individuals residing within the protected area’s boundaries, none of these populations were deemed sufficiently large to guarantee the species’ security, even if the threat of hunting were to be eradicated. Whereas inbreeding tends to constitute an issue of concern with small, highly fragmented populations, researchers noted that the species was likely to go extinct from other pressures before inbreeding became a significant threat, indicating the rapid speed at which Delacour’s langurs have declined. 

Hunting has long posed a significant threat to Delacour’s langurs across their endemic range, primarily targeted for traditional Vietnamese medicinal practices on a local and national scale. Occasionally captured for local consumption, or for the pet trade, hunters exploit the fact that troops utilize the same caves over generations, preying upon caves that are easily accessible. 

Rapid habitat loss, resulting from industrial limestone quarrying, mining, and agricultural conversion, has further contributed to the decline of Delacour’s langurs, given the specificity and restrictive nature of the species’ habitat needs. Commonly used in the production of cement, aggregate for roads, and building stones, limestone mountains are drilled or blasted into smaller blocks, then removed with bulldozers, excavators and front-end loaders. In addition to disturbing natural habitats with noise and air pollution, disrupting streams and springs, and degrading fertile forest areas by removing topsoil, the destruction of karst forests eradicates numerous microhabitats, each with a unique, reliant ecosystem.

As limestone mountains and broadleaf forests are increasingly cleared, separating ecosystems that were once connected, populations of Delacour’s langurs have become significantly fragmented into small, isolated groups. As mentioned, troops of up to 30 individuals were once commonplace for the species, prior to the rapid population decline and habitat loss it has suffered. Now, with most troops consisting of 10 individuals, and with limited mobility between habitats, remaining Delacour’s langur groups are inevitably exposed to a greater threat of poaching, disruptions to their social structures, dynamics and natural behaviours, as well as to the risk of inbreeding should these subpopulations survive in the long term. 

Conservation 

Delacour’s langurs are currently classed as Critically Endangered in the 2007 Vietnam Red Data Book, and are protected under national law by Decree 32/2006 ND-CP: 1B, further receiving special conservation considerations in Decree 160/2013. The species is also listed under Appendix II of the Convention on International Trade in Endangered Species (CITES).

In 1991, the Frankfurt Zoological Society commenced a project in Cuc Phuong National Park, Vietnam’s oldest and largest nature reserve, to determine the status of Delacour’s langurs in the region. Led by German primatologist Tilo Nadler, the project represented a significant step in collaborative efforts between Germany and Vietnam in the field of conservation, focused on reorganizing the local forestry protection department and increasing patrol operations to reduce instances of poaching and illegal logging. However, at the outset of the project in 1993, rangers were alerted to the confiscation of two juvenile Delacour’s langurs in poor physical health, requiring urgent medical care for injuries caused by hunters.

As law enforcement efforts strengthened over the following months, Nadler’s team were confronted with a rapid influx of confiscated endangered primates similarly requiring refuge and medical care. As a result, an agreement was reached between the Ministry of Forestry Vietnam, the IUCN Primate Specialist Group, and foreign organizations to establish the Endangered Primate Rescue Centre (EPRC) in Cuc Phuong National Park. 

Having operated with basic, temporary cages and structures since its inception, in 1995 the EPRC was granted a one-hectare area of land by the national park to develop secure enclosures for confiscated primates. With successful breeding programs, aimed at bolstering declining populations of endangered species, creating additional demands for space, in 1997 the EPRC was granted a two-hectare semi-wild patch of primary forest land for training primates prior to their release. This also allowed conservationists to study the behavior of elusive species under semi-wild conditions. Since then, the EPRC has expanded to cover nine-hectares of both enclosed and semi-wild land, providing refuge for 230 animals.

In 2011, the release of four Delacour’s langurs within the Van Long Nature Reserve marked an incredible step in the species’ recovery, with subsequent monitoring efforts carried out by the EPRC providing valuable insights into the success of breeding and release initiatives.

Although managed and funded by Zoo Leipzig, in collaboration with Cuc Phuong National Park, the primates housed at the EPRC are protected by the Ninh Binh Forest Protection Department, as well as local community rangers, illustrating the importance of cooperative efforts and local involvement in the field of conservation. The EPRC has facilitated research initiatives carried out by global institutes, helping to shed light on primate locomotion, behavior, nutrition, genetics and systematics, later sharing these findings in both national and international publications.

In the field of primate rescue and rehabilitation, the EPRC works with local government bodies, Forest Protection Departments, NGOs, and local communities to locate langurs in dangerous situations, remove them safely and provide the care necessary for future release. To date, this collaborative effort has resulted in the rescue of more than 300 primates. Fieldwork aimed at identifying suitable habitats for primate release, and the protection of these areas from hunting and habitat depletion, has also been supported by the EPRC, funded by the Frankfurt Zoological Society and The Thin Green Line Foundation.

At the commencement of the above-mentioned Cuc Phuong National Park project in 1993, Tilo Nadler discovered a population of approximately 50 Delacour’s langurs residing within an unprotected area of limestone karst forest on the northeastern border of Ninh Binh Province. Having observed local communities in the area clearing forest land for the production of firewood and charcoal, as well as for agricultural and pastoral farms, Nadler entered into discussions with various municipalities regarding the ecological effects of over-exploiting natural resources. Within a few months, the municipality with the greatest number of grazing goats had removed them all from the mountain, heedful of Nadler’s advice. By 2001, Vietnam’s government had officially designated a 22-square kilometer area within the region as the Van Long Nature Reserve.

Despite its status as a protected area, the Van Long Nature Reserve continued to be afflicted by poaching in the months and years that followed its establishment. To counter this threat, a group of approximately 30 volunteers from surrounding communities was recruited to form the Community Protection Unit, serving as guards for the nature reserve and tasked with patrolling the area for illegal activity. In an attempt to reduce local reliance on hunting and the exploitation of natural materials as means of support, initiatives to develop the reserve into a tourism highlight have transformed former poachers into guides, hosts and conservationists, providing ethical sources of income. As a result of these efforts, the population of Delacour’s langurs within the Van Long Nature Reserve has been able to increase from 50 individuals to approximately 176 to 184 individuals over a span of 20 years, now constituting the largest, most viable population of Delacour’s langurs in Vietnam. Conservation successes such as these have consequently earned the Van Long Nature Reserve a spot on the IUCN Green List of Protected Areas.

Although attempts have been made at expanding the boundaries of the Van Long Nature Reserve and similar protected areas to encompass the habitats of all known Delacour’s langur populations, bureaucratic hurdles have hindered progress. Given that protection status is unlikely to be granted for small groups with little to no prospect of developing into viable populations, efforts are instead geared towards stabilising larger populations and protecting their habitat. These smaller, isolated groups would then be translocated to protected areas, thereby improving genetic flow in the process, although this would require a well-equipped team with significant funding and technical support to capture these elusive primates in the wild.

NGO Spotlight: Fauna & Flora

In 2016, the team at Fauna & Flora discovered a new population of approximately 40 Delacour’s langurs in Vietnam’s Ninh Binh Province, constituting the second-largest population of the species in the world after Van Long Protected Area. Subsequent surveys and interviews carried out in 2018 confirmed the presence of 73 individuals split amongst 13 groups, although unconfirmed reports have indicated that the population potentially holds as many as 100 Delacour’s langurs.

While most of these langur groups have been found to inhabit a small limestone karst outcrop and remnant forest near Van Long, this habitat does not receive the legal or national recognition of a protected area. In conversations with local communities, Fauna & Flora noted that snares, leg-hold traps, and hunting with handmade rifles posed the greatest threats to Delacour’s langurs in the area given the limited presence of law enforcement. The team further observed seven limestone quarrying companies operating on the eastern part of the forest, with the scale of the operation indicating a high risk of significant habitat loss for langurs in the near future.

Recognizing the importance of this population for the survival of the species, the Vietnamese provincial government agreed to institute a new protected area in Ninh Binh province specifically for the conservation of Delacour’s langurs and their habitat from the threats of poaching, logging and quarrying.

In September 2020, an emergency proposal was signed by Fauna & Flora, the Center for Nature Conservation and Development, WWF Vietnam, the IUCN Primate Specialist Group and other stakeholders, and sent to the Prime Minister of Vietnam for consideration. The proposal was supported by evidence of recruitment within this newfound population, with monitoring teams having observed several infant Delacour’s langurs in 2020. This proposed protected area, known as the Kim Bang Species and Habitat Conservation Area, and its exact boundaries have yet to be established. However, reports indicate that the project may be completed in 2026. Given the proximity of the proposed sanctuary to the Van Long Nature Reserve, FFI have also advocated for the establishment of a biological corridor to connect the two protected areas for the species’ long-term conservation.

Fauna & Flora has also worked with both government and local agencies in an attempt to reduce the extent of quarrying in Vietnam, and has supported the establishment of community-led patrol teams to protect Delacour’s langurs and their habitat. Across every Fauna & Flora project site in Vietnam, teams work closely with government partners to improve the technical capacity of local authorities, provide enhanced conservation technology, strengthen methods of monitoring biodiversity, and improve the efficiency of law enforcement. 

These initiatives, with the support of both local and global communities, have allowed Delacour’s langurs to take a step back from the brink of extinction. Although the species continues to face a precarious future, the dedicated efforts of organizations such as Fauna & Flora offer this elusive primate a significant chance at survival and long-term recovery, with strategic protection measures and community-based conservation strategies at the forefront.

How To Help

• Donate, fundraise or become a member: organizations such as Fauna & Flora and the Endangered Primate Rescue Centre are at the frontline of Delacour’s langur conservation, working hard with local communities to help protect this species and their habitat. Consider donating to help support these initiatives.
• Support sustainable tourism: When visiting nature reserves and national parks such as the Van Long Nature Reserve, opt for eco-tourism activities operated by local communities to support their efforts to conserve Delacour’s langurs and the limestone karst forest habitats they call home.

Featured image: Ryan Deboodt / Fauna & Flora.

Check out more from our Endangered Species Spotlight series