Dusky Langur,Trachypithecus obscurus
DUSKY LANGUR
Trachypithecus obscurus
Geographic Distribution and Habitat
Tucked away in the forest canopies of Southeast Asia lives the dusky langur, Trachypithecus obscurus, also known as the dusky leaf monkey, spectacled langur, and spectacled leaf monkey or lutung (because they look like they’re wearing glasses). This highly adaptive species is spread throughout various areas across the Malay Peninsula. Located in the Andaman Sea of the Indian Ocean, their territory is composed of Myanmar, Malaysia, Singapore, Thailand, and adjacent islands. The 700-mile (1,127-km) strip of land consists of a variety of primary and secondary old-growth forests, as well as subtropical and tropical moist lowland, montane, riparian, and disturbed forests. They are also found residing in human settlements, including plantations, rural gardens, national parks, and urban areas.
In select areas, they are sympatric with Selangor silvered langurs (T. selangorensis) and Raffles’ banded langurs (P. femoralis).
Size, Weight, and Lifespan
The dusky langur’s head and body length ranges from 19.7–27.5 inches (50–70 cm) and their tail length adds another 27.5–31.4 inches (70–80 cm). On average, males weigh 18.3 pounds (8.3 kg) and can grow up to 26.5 inches (67 cm); females weigh 14.3 pounds (6.5 kg) and 23.5 inches (59.7 cm). The dusky langur’s estimated lifespan is 25–30 years in the wild.
Appearance
It’s difficult to look at a dusky langur and not have your heart just melt. Their sweet faces feature dark eyes encircled by white rings, giving them the look of wearing a pair of spectacles (thus their nickname). The centered placement of their eyes is an evolutionary adaptation that provides excellent depth perception, allowing them to move about agilely.
They have pink lips and petite chins that sport delicate whiskers. Fuzzy black-gray pelage covers their body, extending from the crown of their head to the tip of their considerably long tails. Bellies are a soft silver color, palms are hairless, and bottoms are equipped with calloused sitting pads called “ischial callosities” for comfort while sitting on thin tree branches.
Babies are born with striking, tangerine-colored fur—the utmost contrast compared to older troop members. Their vibrant, silky pelage almost glows against the dark bosoms of their mothers, while they are often seen burying their small, pink faces into her fur. At three weeks old, biology prompts them to assimilate to the appearance of the rest of their troop, and they begin to sprout darker pelage. By the time they are six to ten months old, they lose their orange coat completely, although their new darkened appearance is still lighter than mature adults. At this time, young grow a secondary layer of white, fluffy hair.
Diet
Dusky langurs are folivores (leaf-eaters) and consume a wide variety of leaves, sourcing from over 87 kinds of trees. Their specialized physiology, shared by all monkeys of their genus, allows them to eat an abundance of vegetation, making them well-known for their adaptability. They are equipped with large salivary glands and three-chambered “sacculated” stomachs (similar to cows and antelopes) that help break down the cellulose found in plants. Their digestive adaptations have also made it possible to break down unripe fruit—which often have chemical defenses—to expand their food selection. Leaf shoots, seeds, seedpods, tree bark, and occasional insects are included in their nutritional medley. Their keystone food source is the ficus, and some adults have been known to descend from trees to eat low shrubs as well. Dusky langurs eat up to 4.5 pounds (2 kg) a day, which is about a quarter of their body weight.
Groups that live near humans are known to eat other fruits and vegetables, such as shoots, sweet potato shoots, maize, fruits, green beans, cabbage—and sometimes even processed foods.
Behavior and Lifestyle
Arboreal (tree-dwelling) and diurnal (primarily active during daylight hours), dusky langurs are agile quadrupeds. They are playful and highly social, and traverse canopies taking courageous leaps from one tree to another using their nonprehensile tail to help them balance.
A typical day consists of 0.6 miles (.95 km) of travel throughout the canopy. Dusky langurs venture out early morning and evening to feed for roughly four hours, sometimes breaking off into smaller subgroups. They use their slender, dexterous fingers to grab leaves and pull them to their mouths. After this, about half of their day is spent resting, as their leaf-based diet does not afford them a ton of energy. Naps are taken in elevated but sheltered spots to avoid the hot sun. Intermittent periods are dedicated to grooming and engaging in social play, such as wrestling and chasing. Researchers have also observed them playing with items such as leaves and sticks. The majority of their daily activity takes place high up in the tree canopies at 114.8 feet (35 m).
At night, they return to their roosts and curl up for bed, where they retire until the next morning.
- “Langur” is a Hindi word that means “the one with a long tail.”
- Dusky langur’s stomachs resemble that of… an antelope!? Yes! Their stomachs have three chambers to help digest tough plant matter—specifically cellulose found in plant cell walls.
- The amount of ambient light in the environment affects the shade and hue of dusky langur’s fur.
Their groups are known as troops, tribes, barrels, or “cartloads.” The monkeys are highly territorial and guard their territories, which reach up to 81 acres (33 ha) in size.
A single male leads a troop size of 5–20 individuals. This is known as a one-male unit (OMU). Males that become troop leaders are usually the strongest and most aggressive. These traits come in handy because it is their job to protect the group from predators (such as raptors or snakes) or other unwelcome troops. When provoked, they are not opposed to using force to protect their families.
Male leaders typically pair up with two mature females and sire all offspring. They work to keep their family close and resolve any inter-group disputes or spats that may occur. This species is noted for reconciliation and consolation and, aside from threats, is actually quite docile in nature.
Females remain with their family from the time they are born. These tight-knit groups are closely bonded and use grooming, grabbing, ventro-to-ventro (belly-to-belly) embracing, and social time to strengthen ties between individuals. When engaging in play, they may tackle, wrestle, jump kick, and mount one another.
Other males are seen as rivals and are pushed out of the troop upon maturation. If a group does not have a male leader, a new one steps in. During this transition, the new male may commit infanticide to reset females from a caregiving state to one of fertility. Although this may appear as harsh or cruel, in the animal kingdom it is done so the male can create his own line and instill the best chance for reproductive success.
Dusky langurs have a wide array of communication methods. They voice over twelve different calls to communicate with their group members, all for different purposes.
Common vocalizations include snorts, squeaks, murmurs, and hoots. Males have distinct alarm calls for territorial and threat purposes. To alert their troop members of a perceived threat, such as another troop or predator, they will softly “whoo” and then emit a “chengkong” call, earning them the nickname “chengkongs.”
Communication is also demonstrated through physical touch, bodily movements, and posturing. Sniffing, the licking of genitals, and ventro-to-ventro (belly-to-belly) embracing are all ways dusky langurs communicate non-verbally with one another.
Dusky langurs become sexually mature between the ages of three to four years old. They are polygynous (one male mates with multiple females) and do not have a seasonal mating period. Females experience a three-week estrous cycle after which their genitals swell to indicate that they are receptive to copulation. Gestation lasts about 150 days until a single baby is born (twins are incredibly rare).
An infant is fully dependent on his or her mother for the first 20 days and clings to her body, nursing almost constantly. Mothers dote upon and tenderly kiss, scratch, groom, and grasp their newborns. At one month old, the offspring gain their confidence and begin to learn to climb and play. This is also when they start to eat solid foods. To help their small tummies digest, mothers will chew leaves for them, using their saliva to break down the vegetation. After two months, the young start to engage in social activities, such as grooming, with other group members. By eight months of age, they are completely independent from their mothers.
Upon maturity, males leave the troop and either form their own troop or assimilate into temporary “bachelor groups.” These are groups of solitary, rogue males who have yet to find their own females to start families with.
Dusky langurs are credited with cultivating their ecosystems from their dietary patterns. As foragers, they disperse the seeds of plant species throughout the forest either by dropping seeds or excreting them in their travels. They are prey to raptors, snakes, and other carnivores that can reach them.
The International Union for Conservation of Nature lists dusky langur as Endangered (IUCN, 2020), appearing on the IUCN Red List of Threatened Species.
Although the species appears to be widespread, as of the IUCN’s assessment in 2015, forest loss exceeded an estimated 70% over 3 generations (36 years) and has led to a suspected decline in population of over 50% in that time from hunting, habitat loss, fragmentation, habitat conversion to agriculture, and pet trade.
Hunting for food is a major threat, as is forest fragmentation and habitat loss and degradation due to expanding oil palm plantations, agriculture, urbanization, and development for tourism. In Peninsular Malaysia the langurs are frequent victims of road-kill.
The dusky 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.
Several protected areas offer refuge for dusky langurs. Malaysia has three protected areas (Penang Botanical Gardens, Krau Wildlife Reserve, and Taman Negara National Park) and Thailand has four (Kaeng Krachan National Park, Khao Sok National Park, Taratau National Park, and Khao Sam Roi Yot National Park). Additionally, a local air force base at Khao Lommuak protects an isolated population.
International legislation and trade controls exist to help mitigate illegal hunting and poaching, although this is often a difficult feat.
Researchers recognize the ecological importance of dusky langurs and have conducted many studies to help improve conservation efforts while promoting primate diversity. To help support declining populations and habitat loss, they have proposed the widespread implementation of canopy bridges and food plant cultivation in non-risk areas to help salvage habitat loss. Experimental attempts to prevent roadkill and offer safe food sources are ongoing, as well as public education to promote awareness.
The Langur Project Penang (LPP), a Malaysian conservation group, is noted for the construction of the first official urban canopy bridge. They also use education, research, and community involvement to help promote human-wildlife coexistence. Future conservation efforts are focused on highlighting the dusky langur’s ecological role as seed dispersers.
- https://www.iucnredlist.org/species/22039/17960562
- https://www.britannica.com/place/Malay-Peninsula
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- Leen, Y., Ruppert, N., & Rosely, N. F. N. (2019). Activities, habitat use and diet of the wild Dusky langurs. Journal of Sustainability Science and Management, 14(4), 71-85.
- Palombit RA. Infanticide as sexual conflict: coevolution of male strategies and female counterstrategies. Cold Spring
Harb Perspect Biol. 2015 May 18;7(6):a017640. doi: 10.1101/cshperspect.a017640. PMID: 25986557; PMCID: PMC4448612.
Written by Dana Esp, November 2023