Delacour’s Langur, Trachypithecus delacouri
DELACOUR'S LANGUR
Trachypithecus delacouri
Geographic Distribution and Habitat
The Delacour’s langur (Trachypithecus delacouri) is a critically endangered monkey species native to Vietnam and found in the north central parts of the country. With a geographic range spanning about 1,931 square miles (5,000 km^2) of area, the Delacour langur, locally known as “vooc mong trang”, lives among various limestone karst landscapes, including the limestone karst forest. Limestone karst is a unique type of dissolving bedrock that can create sinkholes, streams, caves, springs, etc., with elevations of up to 1,076 feet (328 meters).
Since the entire world population of Delacour’s langurs is between 200-250 individuals and is localized to Vietnam, the majority live in small pockets of broadleaf evergreen forest, closely tied to the country’s limestone mountain ranges. Across Vietnam, the five protected areas they occur in include Cuc Phuong National Park, Pu Luong Nature Reserve, Hoa Lu Cultural and Historical Site, Huong Son Cultural and Historical Site, and Van Long Nature Reserve. Smaller populations are distributed across various Vietnamese provinces, including Ninh Binh, Nam Ha, Hoa Binh, and Thanh Hoa.
Northern Vietnam has both a rainy and dry season, so temperatures range between 75.2 to 82.4 degrees Fahrenheit (24 to 28 degrees Celsius). During colder days, temperatures may drop to 41 degrees Fahrenheit (5 degrees Celsius). With such a unique climate combination, these landscapes provide large amounts of biodiversity and create healthy environments for different plant and animal species.
Unfortunately, limestone karst is surrounded by limestone quarries and mining sites, frequently used for the country’s cement production. As a result, mountains are torn down for construction and development purposes, reducing fertile forest areas to flat degraded land. Since each karst creates its own distinct microhabitat, animals native to each environment have evolved within that particular environment. Since these animals rely on their environment’s uniqueness, they are unable to survive without it.
Named after French-American ornithologist Jean Théodore Delacour, this langur was previously considered a subspecies of the Francois’ langur (Trachypithecus francoisi). The Delacour’s langur was promoted to species level in 1984, a decision later proven in 2001 molecular genetic studies.
In Vietnam, there are 12 species of leaf-eating primates identified between the country’s North and South. They are categorized into three genera: Pygathrix, the Douc langurs; Rhinopithecus, the snub-nosed monkeys; and Trachypithecus, or crested langurs. The Delacour’s langur belongs to the latter category.
Size, Weight, and Lifespan
Delacour’s langurs are sexually dimorphic in weight and some of their markings. Male Delacour’s langurs are slightly larger than females, weighing 16.5 to 23.2 pounds (7.5 kg to 10.5 kg). Females weigh between 13.7 to 20.3 pounds (6.2 to 9.2 kg) and have a pubic patch of white hair and pale skin towards the front of their sitting pads. Both genders sport sitting pads, referred to as ischial callosities, that are thickened layers of skin covering the hip bones. Evolutionarily, these pads allow the Delacour’s langur to sleep and sit in upright positions on thin branches, preventing potential falls. They also offer a measure of comfort while seated on hard surfaces.
Both males and females have a body length of about 23.6 inches (60 centimeters) and a tail length of about 35.4 inches (90 centimeters). For both sexes, the average life expectancy is around 20 years.
Appearance
The Delacour’s langur has a predominately black upper body with white markings covering the area between their hips and knees. To many, it may look as if they sport a pair of fitted white shorts, an accessory that can come in handy for a fun night out on the karst! Coming out through the rear of their “pants” is a long, bushy tail with hairs that stick out at various angles, also black in color. Although their tail is long, and like those of all African-Eurasian monkeys, it is not prehensile and cannot be used to hold or grip objects.
Like many other langurs, Delacour’s langurs have a tuft of long, upright hair sitting on top of their heads, like a mohawk. This hair expands down their necks and behind their ears, while white whiskers cover their cheeks. With the vast amount of hair wrapping around their face, it looks as if the Delacour’s langur has a well-developed beard!
Similarly to their sitting pads, Delacour’s langurs have specially adapted pads on their hands and feet that allow them to run and jump on the karst, which can feel very sharp, especially to sensitive skin. These pads may also act as a form of protection from predators when engaging in aggressive behaviors.
When birthed, infants are born with orange fur. Although their full coat pattern will not develop until they are at least three years old, their coat will turn from orange to black when they are about four months old.
When differentiating males from females in the wild, there are some distinct phenotypic, that is observable characteristic, differences that may help researchers identify them in the wild. One difference between males and females is that the white coloring of a female’s hip can change from white to yellow, or dark brown. Additionally, because females have white hairs on their pubic patches, these hairs may also change color, just like their hips. In both circumstances, these color changes are due to sweat staining. It is unknown why this occurs only in females, and not males.
Diet
The Delacour’s langur is a primarily folivorous primate that specializes in eating leaves, which also makes them herbivores. Although 80% of their diet consists of leaves (60% being young leaves and 20% mature leaves), Delacour’s langurs may also eat fruit, flowers, plant shoots, and tree bark. Within their home range, some plants they may frequently consume include dragon plum (Dracaena marginata), wild rhododendrons (Rhododendron irroratum), powdered giant bamboo (Dendrocalamus barbatus), or red-fox orchids (Bulbophyllum andersonii). Although they can venture into the forest for food, some of their time is also spent foraging for food on the ground.
In order to comprehend how they consume food, it is important to consider that mature leaves, plant shoots, and tree bark are hard to digest, due to the amount of cellulose these plant products contain. Cellulose is a complex carbohydrate, or polysaccharide, made of over 3,000 glucose units. Since cellulose is a fiber that creates the cell walls of plants, it can be hard for animals to digest. In fact, when cellulose is digested, and because some primates lack the necessary enzyme to break it down, its shape will stay intact as it passes through the digestive system. To combat this and to digest these products, folivorous primates like Delacour’s langurs evolved particular adaptations that help with their diet. They have evolved broad molars, with sharp cusps, to properly break cellulose down while chewing. In addition, they have a ruminant-like digestive system. Their multi-chambered stomachs break down the cellulose and prevent the leaves’ bacterial fermentation. This is an energy-intensive process and long periods of rest between feedings help facilitate their digestive process.
Behavior and Lifestyle
As terrestrial primates, Delacour’s langurs lives mostly on land and rock formations, occasionally traveling into the forest and spending time amongst the trees. They travel between trees by swinging their hands from one branch to another and have even been recorded leaping almost 40 feet (12 meters)! Because they are diurnal, most days are spent exploring home ranges, often traversing dangerous routes along the terrain of the karst. To prevent falling, they rely heavily on their tails for balance support.
At night, troops sleep on open rock surfaces, or within limestone caves. Interestingly, each limestone cave provides a unique environment in which light, heat, moisture, and nutrient conditions differ from one another. Because Delacour’s langurs evolved to adapt to these unique circumstances, caves are utilized consistently by single groups across generations. Because each microhabitat is unique to each troop, it is unlikely for a troop to abandon their cave unless it becomes too dangerous or unsafe. Taking advantage of this, hunters often target these caves, especially the ones that are easily accessible from the ground. As a result of these exploitations, humans have nearly eliminated this species through hunting practices.
Besides humans, potential predators of the Delacour’s langur may include clouded leopards (Neofelis nebulosa), reticulated pythons (Malayopython reticulatus), crested serpent eagles (Spilornis cheela), or mountain-hawk eagles (Nisaetus nipalensis). Animals that may share cave space with the primates include camel crickets (Rhaphidophoridae), giant Vietnamese centipedes (Scolopendra dehaani,), bumblebee bats (Craseonycteris thonglongyai), or semi terrestrial crabs (Tiwaripotamon bellum).
During winter months, when temperatures significantly drop, these caves may provide extra protection from predators and the elements for all of the species living in the caves.
Move over Beatles! Previous researchers have reported seeing all male, Delacour’s langurs troops. With the maximum number of individuals being 4, these bachelor troops prove that bromances are real. Just like the song, sometimes all you need is love!
Delacour’s langurs are polygynous and can consist of anywhere from 4 to 16 individuals. Typically, troops are made of about 10 individuals: one adult male, four adult females, two to three juveniles, and two to three infants. In previous years, when the Delacour’s langur populations were more robust, there could be up to 30 individuals within a single troop.
Social bonds between troop members are created and maintained through grooming and play. Grooming may establish friendships, trust, and alliances among troop members. Additionally, social grooming can establish hierarchies through dominance, especially among females and juveniles. Research shows that play behaviors help langurs practice and improve their sensory, cognitive, and motor skills, which can be very beneficial when it comes to foraging, avoiding predators, raising offspring, etc.
When males reach sexual maturity, they may leave their natal troop (the troop they were born into) in pursuit of creating their own family unit. After traveling alone, these sub-adult males may attempt to separate females from troops, or attempt to take over a new troop entirely. Often, these infiltrations create tension and lead to aggressive outbreaks.
Within troops, although females outnumber males, males are the ones who will defend their territory. By standing watch around their established home, sitting along the perimeter of the rocky terrain, males will wait for trespassers to appear and engage in aggressive behaviors before taking action, such as when single males attempt to steal away females or take over troops.
When it comes to defending their territory, Delacour’s langurs communicate through visual displays and by producing various vocalizations. For example, as a scare tactic, they may choose to physically chase or fight predators to get them to submit and abandon their attempts. Additionally, when leaping between trees during a pursuit, Delacour’s langurs may decide to tear down branches along their path. As an intimidation tool, this action creates loud crashing sounds that effectively send their opponent into a panic. When visual displays do not work, vocalizations like soft nasal grunts, guttural snorts, barks, and/or hoots are aggressively used as a form of communication against predators. Additionally, since home ranges often overlap, within an individual troop, members produce fast and short snorts as a form of an alarm call, allowing others the time to get to safety from potential danger.
After a gestation period of between 180 to 196 days, females give birth to a single infant. After an interval of 19 to 25 months, females may give birth again.
When they are born, infants are precocial, meaning they are relatively mature and mobile from the moment they are born. They are born with open eyes and with a strong upper body.
Since langur troops practice allomothering, with all female troop members participating in care of infants and juveniles. Evolutionarily, since young langurs love to explore and roam within their home ranges, their orange coloring makes it easier for mothers and other troop members to identify and find them in the dense foliage. As time goes on and they become more independent, their coloring will also mature and change to that of the adults.
Compared to adults, infants have facial skin that is yellowish and it will darken with age. After 5 weeks, the hair on an infant’s hands and feet begins to turn from dark orange to black. Around 4 to 6 months old, their “shorts” become more distinct and their orange hair turns to a dark brown and dark gray. When they reach 19 to 21 months, infants are typically weaned, no longer relying on their mothers for complete dependence. During this time, their hair continues to change to black and their “shorts” start to become light gray. In their next stage of development, at around 36 months, their full coat pattern is completely achieved and their tail reaches its final shape.
Females achieve sexual maturity at 4 years old, while males achieve sexual maturity at five years old.
Because of their folivorous diet, Delacour’s langurs play an important role in seed dispersal. Whether they are physically dropping seeds after eating plant products, or releasing them through excretion, primates assist plant regeneration and forest restoration.
Additionally, when foraging for various plant species, primates such as the Delacour’s langur become natural pruners. Pruning is the process of removing dead, diseased, or injured plant parts from a plant’s main structure. By removing these parts, Delacour’s langurs help maximize the potential for a plant to grow and flourish. In a habitat such as a limestone karst, where much of the habitat is lost due to human encroachment, the Delacour’s langur is vital to promoting environmental success.
The International Union for Conservation of Nature lists the Delacour’s langur as Critically Endangered (IUCN, 2015), appearing on the IUCN Red List of Threatened Species.
The Delacour’s langur is listed as Critically Endangered because, in the last three decades, its population has declined by more than 80%. According to IUCN, this species is hunted primarily for the purpose of traditional “medicine” in local and national capacities. Although they are also hunted for food and the pet trade, other threats such as habitat destruction by deforestation, agricultural conversion, and industrial quarrying have driven this species to near extinction.
As these threats separate connecting ecosystems, this species has a fragmented population. Fragmentation limits wildlife mobility with disastrous effects. If individuals cannot move between habitats and interact with different troops of Delacour’s langurs, the likelihood of inbreeding increases. Unfortunately, inbreeding leads to the loss of genetic diversity and decreases the long-term health of the population, making this species more vulnerable.
The Delacour’s langur is listed in Appendix II of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival.
Delacour’s langurs occur in 5 protected areas across Vietnam, Cuc Phuong National Park, Pu Luong Nature Reserve, Hoa Lu Cultural and Historical Site, Huong Son Cultural and Historical Site, and Van Long Nature Reserve. In 2001, Van Long Nature Reserve was established for the conservation of this species, allowing populations to grow within its borders.
Unfortunately, when attempting to create more protected areas, populations are often too small and have little chance of becoming viable long-term. Organizations such as Fauna & Flora conduct research and monitoring projects in order to discover more Delacour’s langurs populations, helping to establish more protected areas. Additionally, reintroduction projects by the Endangered Primate Rescue Center and the Frankfurt Zoological Society have positive reception by locals, allowing the community to engage in protection and conservation activities for this species.
Besides research, conservation actions that are needed include more education and awareness, law and policy, species management, land/water management, and land/water protection.
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Written by Brooklynne Mitchell-Arno, June 2024