BLACK-SHANKED DOUC LANGUR
Geographic Distribution and Habitat
Found only in northeast Cambodia and southern Vietnam, the black-shanked douc langur is geographically restricted by the Mekong River. The Mondulkiri province in Cambodia boasts the largest population of these beautiful monkeys, around 42,000.
They live in semi-evergreen deciduous forests and coastal dry forests. They are highly arboreal—however, they still descend to the forest floor for certain needs. Due to habitat loss, their geographic region has been greatly restricted and fragmented. This, along with poaching, has resulted in an estimated 50–80% decline of the black-shanked douc langur, as well as other douc langur species in the region, in just the past 30 years.
Black-shanked douc langurs share their habitat with other primates, and their home ranges often overlap with other troops.
There are two other species of douc langur that the black-shanked douc langur shares their habitat with. The red-shanked douc langur (Pygathrix nemaeus) and the gray-shanked douc langur (Pygathix cinereus). It is suspected that the black-shanked douc langur has hybridized with the red-shanked douc langur; however, genetic analysis has yet to confirm this suspicion.
Size, Weight, and Lifespan
The black-shanked douc langur is a sexually dimorphic species, meaning their body size and characteristics are dependent on the sex of the individual. Males are larger, weighing approximately 18–24 pounds (8–11 kg). Females weigh around 13–18 pounds (6–8.2 kg). Male black-shanked douc langurs have a body length of 22–26 inches (55–65 cm), and a tail length of 27–33 inches (69–85 cm). Females’ body length measures 21–24 inches (54–60 cm), and their tails 26–24 inches (65–80 cm). Their long tails are used for balance (as well as communication), which is an asset to this canopy-dwelling species.
Their lifespan is approximately 24 years; however, in captivity they live to be over 30 years old.
The black-shanked douc langur is easily distinguished from other douc langurs, and it is considered by some to be one of the most beautiful monkey species. Although they may be easy to tell apart from their douc langur relatives, adult black-shanked douc langurs are difficult to spot in nature, due to their excellently camouflaged pelage. Adult males have dark blue faces, with cream-colored rings around their eyes. Females also have a blue face with similar eye rings; however, the blue is not as pronounced. Infants and juveniles have gray-blue faces, which become more brightly pigmented as they mature. They have light gray mid-sections, and black or gray hair over their arms and legs. Males have light gray, wiry hair that creates a beard, just below their chins and up the edges of their lower faces, and it becomes longer and more prominent with age. Their noses are flat, with forward-facing nostrils.
There is variation in the coloration of their bodies. Some individuals have partial red coloration on their legs, which is generally an attribute of other douc langurs. Genetic investigation indicated that they had not commingled with other species in these cases.
The black-shanked douc langur has a specialized diet. This means that they are not able to adapt to food sources outside of their habitat, yet another reason the species is suffering due to human influence. They are herbivorous, and greatly prefer young leaves to other options. While leaves make up about 82% of their diet, they will also consume seeds, flowers, and unripe fruit. They have a sacculated stomach, which allows them to process and break down the large amount of cellulose (a sugar found in plants, one that humans are unable to break down in their digestive system) that their diet requires. This stomach anatomy is one of the reasons the black-shanked douc langur has a protruding, round belly. They are rarely observed drinking water, instead acquiring most of their hydration needs from the plants in their diet.
Behavior and Lifestyle
The black-shanked douc langur is diurnal, which means they are awake during the day and sleep at night. They move quadrupedally through the trees, which is where they spend much of their time. They move horizontally by swinging from branch to branch, a mode of movement called brachiation. When moving vertically, they jump to lower branches, landing feet first. They often move through the forest in a single file line. While other douc langurs rarely, if ever, leave the safety of the canopy, the black-shanked douc langur has been known to spend up to 20% of their day on the forest floor.
Previously classified as a subspecies of the genetically similar red-shanked douc langur, only recently has the black-shanked douc langur received due recognition, and respect, as a distinct langur species.
The black-shanked douc langur is a highly social primate, relying on their group members for protection, socialization, care, and companionship. They live in troops (or groups) of 4–50 individuals. They are multi-male, multi-female groups; however, there are generally more females in a troop than males. Males and females both disperse from the group when they reach maturity, but some do remain in the group. Infants and juveniles engage in considerable play in their family and are well tolerated by the adults of the troop. Adults also play with one another, enjoying the company the large group provides.
The black-shanked douc langur spends the majority of their day resting, around 62%. They feed for 27% of their day, travel for 6%, and socialize for 3%. Their socialization is primarily composed of allogrooming, in which two or more individuals will groom each other. This strengthens group bonds and can be used to form hierarchical alliances.
When a troop encounters an opposing group, males will display aggression and dominance by slapping their thighs or brachiating between two branches in an attempt to make themselves appear larger and intimidate their opponent.
As well as humans, the black-shanked douc langur has other predators in their environment. They are known to fall prey to large birds and big cats that share their habitat.
The black-shanked douc langur is not an especially vocal species, usually only making soft vocalizations when communicating. They communicate through nonverbal means primarily, such as facial expression, body movement, and tail position. Play is initiated by looking at a partner with a slightly open mouth, avoiding showing the canines. When an individual is not feeling social, they will glare at others to warn them to stay away.
Although they are a quieter species, they will let out alarm calls when a predator is spotted, which sound like loud barks. After a predator is spotted, they will hide in the trees, instead of fleeing through the forest canopy. Others will join in on the alarm calls, and in some cases experience “panic diarrhea” during the commotion.
As mentioned in the previous section, allogrooming is a vital communication tool for these, as well as most, primates. It communicates social bonds, reinforces relationships, and establishes the relationship between family members and relatives.
The earliest that both males and females reach maturity is at 4 years old. Females can reach sexual maturity anywhere between 4 and 6 years old, and males from 4 to 9 years old. They are polygynous, meaning that one male may have multiple female partners.
Both males and females engage in courting behavior, but often it is the female who approaches a male with the intent to copulate. The female will intently stare at the male, with her chin jutted out and mouth closed. She will sway her head back and forth as she approaches the male. If he is interested in her, he will return the gesture and they will copulate. During this time, the female has visible signs of estrus, the time in which she is receptive to mating. These signs include red and swollen genitals.
If the female becomes pregnant, she will become quiet and calm, spending much of her time resting and in solitude. She will be pregnant, or gestate, for approximately 6.5 months and give birth to a single infant. The infant is weaned at around one year old, and the mother may become pregnant again within 11 to 38 months after giving birth.
Black-shanked douc langurs play an important role in their environment. By eating fruit, they act as vital seed dispersers. While many non-primate species are responsible for dispersing seeds to regenerate and maintain forests, primates such as douc langurs are important because of their size. They are able to consume larger seeds, thereby they are capable of ensuring the distribution of larger foliage.
The International Union for Conservation of Nature lists the black-shanked douc langur as Critically Endangered (IUCN, 2015), appearing on the IUCN Red List of Threatened Species.
The black-shanked douc langur’s biggest threat is humans, and the IUCN lists their population numbers as decreasing. They have been in a relatively rapid decline in the past 30 years, losing over 80% of their population due to poaching and habitat loss.
While the black-shanked douc langur does occur in some protected areas, they are still threatened by habitat loss and poaching. Pressures that can be attributed to logging and agricultural development have contributed to the habitat fragmentation these primates experience.
They are affected by poaching, although the motivation for this illegal activity changes. Adults are often hunted for food purposes and traditional medicine. Those who poach for traditional medicine mistakenly believe that the black-shanked douc langur’s stomach holds medicinal value. Often, when an adult douc langur is killed, their young are taken and sold into the pet trade. Black-shanked douc langurs in the pet trade have grim prospects and outcomes, as they are wholly dependent on their natural habitat for their diet.
Habitat fragmentation caused by the construction of roads, for example, is an advantage to poachers, allowing them easier access to these primates. If a poacher is caught, they may face up to five years in prison, or pay a fine. However, laws put in place to protect all douc langurs are not always enforced. Between 2008 and 2013, it is estimated that 2,885 to 5,770 black-shanked douc langurs were killed or illegally traded.
The black-shanked douc langur is listed in Appendix I 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.
The black-shanked douc langur lives in several protected areas in Cambodia, including several forest reserves and sanctuaries. There are multiple sanctuaries that house black-shanked douc langurs, which is vital in the conservation of the species. While it is always the hope that they remain in their natural habitat, housing and care for individuals removed from their home is crucial to the species’ survival.
Several courses of action have been recommended to protect these primates. The first and most pressing is the control of illegal trading, which includes hunting them for bushmeat as well as the pet trade. It has also been advised that human development be planned strategically and thoroughly, to minimize impact on the douc langur habitat. In Cambodia, specifically, laws regarding illegal logging must be exercised.
Written by Robyn Scott, December 2023