DUSKY LANGUR

Trachypithecus obscurus

Geographic Distribution and Habitat: 
Dusky langurs—also known as spectacled langurs, dusky leaf monkeys, and spectacled leaf monkeys–are found primarily on Asia’s Malay Peninsula. Their range includes southern Burma (Myanmar), parts of Thailand, and the Malaysian islands of Langkawi, Penang, and Perhentian Besar.

These arboreal (that is, tree-dwelling) primates prefer tall trees, 114 feet (35 meters) or higher, of dense forests. But they also inhabit coastal forests, riparian forests, botanical gardens, national parks, and even urban areas.

Dusky langur range, IUCN 2020

Size, Weight. and Lifespan
Head-to-body length in male dusky langurs is about 26.5 inches (67 cm), with a weight of 18.3 pounds (8.3 kg).

Head-to-body-length in female dusky langurs is about 23.5 inches (60 cm), with a weight of 14.3 pounds (6.5 kg).

The species’ long, non-prehensile tail adds another 20 to 33.5 inches (50 to 85 cm) to their bodies.

Lifespan data is elusive for wild dusky langurs; for captive dusky langurs, the data is limited and includes conjecture related to a similar species. For a captive female dusky langur, the recorded average lifespan is 15.3 years. However, the recorded captive lifespan of 31 years for the silvered langur—a primate who shares the same genus, Trachypithecus, as the dusky langurhas led some scientists to hypothesize that a similar captive lifespan is possible for the dusty langur.

Appearance
“Adorable.” adjective ador·able \ə-ˈdȯr-ə-bəl\ : very appealing or attractive : very lovable.
If an image of a dusky langur were to accompany the dictionary definition of this word, it’s hard to imagine anyone taking dispute.

White rings that surround deep, dark eyes—for a “bespectacled” appearance—give the species its alternate name of spectacled langur or spectacled monkey.

At birth, these Old World monkeys have pink faces, and their bodies are covered with bright yellow or orange hair. Within their first six months of life, however, an amazing and colorful metamorphosis occurs: their hair color changes to shades of gray, brown, or black with paler fur on their underside, hind legs, and tail. A demurely spiked lighter-colored hairdo adorns the crown of their heads. Adults’ faces are gray, except for their pale-pinkish lips that are encircled by a white patch of hair.

The hands and feet of dusky langurs resemble that of human primates and are capable of easily grasping objects. Fingers are well-developed, and the species has an opposable thumb. Palms and soles are hairless and are usually black.

Although their long tail is incapable of grasping, it helps the langurs to balance as they leap from tree to tree in the upper canopy of the rainforest.

To embark in a favorite activity—sitting for long periods of time—Nature has given dusky langurs a hard, calloused pad on their derrieres that allows them to pursue this pastime in comfort.

Diet
The species’ dietary fondness for leaves gives the dusky langur its alternate name of dusky leaf monkey. Considered a folivorous species; that is, an herbivore who specializes in leaf-eating (thanks to teeth adapted for this purpose), dusky langurs feed from 87 different species of trees. They also consume shoots, seedlings, flowers, ripe and unripe fruits (with a penchant for figs)–eating an impressive 4-1/2 pounds (2 kg) of food a day.

Enlarged salivary glands help to break down leaf cellulose, which the bacteria in their multi-chambered stomachs help to absorb. These bacteria also aid digestion by neutralizing any toxicity in unripe fruit or seeds.

Captive dusky langurs, who have no access to rainforest canopies nor to the foods found in their natural environment, are typically fed a diet of sweet potatoes, cabbage, lettuce, carrots, green beans, and various soft fruits. They’ve accepted certain insects, on occasion, but have rejected meat, when offered by their keepers.

Behavior and Lifestyle
Dusky langurs are a diurnal species (most active during daylight hours), spending virtually all their time leaping, running quadrupedally (that is, on all fours), and climbing through the rainforest’s high canopy. The motion of their tail allows them to balance, and their superior eyesight and depth of field helps them to easily advance.

Large groups often split into subgroups when foraging. Juveniles remain within sight of their mothers. When feeding, dusky leaf monkeys use their hands to pluck leaves and shoots. They also pull down leafy branches to directly munch their lunch.

But the day isn’t only about finding and eating food, which dusky langurs do for about four hours from late morning to early afternoon. Time is allotted for rest, grooming, and play. The monkeys often chase, wrestle with one another, or even pull each other’s tail for fun.

Come nightfall, dusky langurs return to their treetop roosts to sleep.

Fun Facts

The name “langur” comes from Hindu word that means long-tailed.

The amount and strength of ambient light; that is, natural lighting in the environment, affects the shade color of the dusky langur’s fur.

Seven subspecies of the dusky langur are recognized. Their ranges include Malaysia, Burma (Myanmar), and Thailand.

Daily Life and Group Dynamics 
Dusky langurs are social animals who live in groups, known as troops, of 5 to 20 individuals. A troop is comprised of multiple females and one or more adult males; only one adult male is considered dominant, however. His job is to keep the troop together, patrol the boundaries of a troop’s home territory, and  alert troop members of potential predators (who include snakes, large birds of prey, and humans). He also gets to mate with the females.

Non-dominant males often leave their birth group and form “bachelor” groups, consisting of other solitary males. Or they will search for a troop that has lost its dominant male, where they can exert their dominance. To ensure their success, these formerly non-dominant males will kill all the offspring of the troop’s last dominant male.

Apart from infanticide in the course of asserting dominance, dusky langurs live peaceably with one another in their established troops. If a dispute should occur, the focus is on reconciliation and not the perceived offense. These mostly docile primates practice what is known as “ventro-to-ventro” (or belly-to-belly) hugging during reconciliation. It’s not surprising that grooming is an important social and tactile behavior in their daily lives.

Communication
Dusky langurs possess a repertoire of vocalizationspostures, and tactile behaviors which they use to communicate with one another.

Vocalizations of males are typically more robust than that of female dusky langurs. Females emit similar calls as their male counterparts, but they use a softer tone.

As sentries, males sit high in the treetops to keep watch for predators. They will emit a “whoo” call to warn troop members of potential danger, followed by a two-part honk call known as “chengkong” to signify alarm and, some wildlife biologists speculate, to forewarn trespassers to keep away from a troop’s territorial border. (Alarm and territorial calls are similar.) Because this honk call is so distinct, local people refer to spectacled langurs as “chengkongs.”

The species’ vocabulary also includes soft warning and coughing calls and a variety of squeaks, hoots, snorts, and murmurs.

Postures hold their own significance in communication between dusty langurs. Tongue flicks and lunges are used to deter a perceived threat. Certain postures, such as mounting, are used to assert dominance over another langur.

By grooming one another, dusky langurs establish strong social bonds. Embracing, grabbing, wrestling, and the reconciliatory belly-to-belly (ventro-to-ventro) hugging are other examples of tactile behavior, each with its own significance.

Reproduction and Family
Both male and female dusk langurs are considered sexually mature between 3 or 4 years of age. Breeding can occur throughout the year; however, most births occur between January and March. Females usually give birth to a single infant, but occasionally to twins (only one usually survives) after a gestation period of 145 days. Interval between births is typically about 2 years.

The infants’ bright orange hair makes them stand out against the subdued coats of the adults.

Mothers are the primary caregivers to their offspring, who spend the first 20 days of life at their mother’s bosom, attached to her nipple. Besides nursing, mothers exhibit additional maternal behaviors such as scratching, kissing, grasping, leg pulling, and grooming.

The age that an infant dusky langur is considered weaned remains unknown, made difficult to ascertain because of a specfic maternal and biological behavior: mothers pre-chew leaves to pass necessary digestive enzymes to their babies through their maternal saliva.

Ecological Role
Dusky langurs feed on young leaves and occasionally on fruits, the seeds of which are then dispersed throughout the forest. This makes them important to the reforestation and biodiversity of their ecosystem.

Conservation Status and Threats
The dusky langur is classified as Endangered by the International Union for Conservation of Nature (IUCN, 2015), appearing on the IUCN Red List of Threatened Species.

Dusky langurs are widely hunted for food, making human predators the biggest threat to the survival of the species. These primates are also victims of the illegal pet trade.

Habitat loss—again, caused by humans—is another threat, as it is for all nonhuman primate species. Currently, the population of dusky langurs is on a downward trajectory due to habitat fragmentation, habitat loss and degradation as a result of palm oil plantation expansion, agriculture, urbanization, and touristic development.

Forest loss has exceeded 70% of 3 generations, the equivalent of 36 years. This has led to a suspected population decline of greater than 50% in that same time due to hunting, habitat loss, fragmentation, conversion to agriculture, and the pet trade. 

Conservation Efforts
International trade of dusky langurs is banned under its listing 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. Unfortunately, this ban is largely ignored by those individuals seeking to profit by the illegal trafficking of this near-threatened primate.

The Dusky Langur Conservation & Community Centre, based in Thailand, is one organization working to protect this species. Founded in 2005, the organization is committed to raising awareness of the dusky langur through field research, rehabilitation, and ecotourism that includes tourist volunteer programs. (A September 2016 Internet search on this organization revealed no new  information beyond 2007, however, and access to its website, www.conservationduskylangur.org, failed.)

A U.S.-based animal rights group called Annamiticus, “dedicated to ending the exploitation of endangered species for financial gain,” was instrumental in June 2016 for shutting down a social media site on Facebook that was illegally selling dusky langurs, along with other endangered species, as pets.

References:

  • http://theanimalfacts.com/mammals/dusky-langur/
  • http://animaldiversity.org/accounts/Trachypithecus_obscurus/
  • https://www.aboutanimals.com/mammal/dusky-leaf-monkey/
  • http://www.ecologyasia.com/verts/mammals/spectacled_leaf-monkey.htm
  • http://eol.org/pages/311201/details
  • http://thewebsiteofeverything.com/animals/mammals/Primates/Cercopithecidae/Trachypithecus/Trachypithecus-obscurus/
  • http://www.iucnredlist.org/details/22039/0
  • http://cleanmalaysia.com/2016/06/16/malaysian-facebook-group-shut-selling-exotic-pets/
  • http://www.animal-info.co.za/spectacled-langur/
  • https://twycrosszoo.org/animals/primates/monkeys/dusky-langur/
  • http://www.theprimata.com/trachypithecus_obscurus/

Written by Kathleen Downey, September 2016. Conservation status update, Jul 2020.