Indochinese Gray Langur, Trachypithecus crepusculus
INDOCHINESE GRAY LANGUR
Trachypithecus crepusculus
Geographic Distribution and Habitat
The Indochinese gray langur (Trachypithecus crepusculus) is a species of lutung found throughout mainland southeast Asia, the region formerly known as Indochina. Home countries include China, where the species occurs east of the Salween River extending to its western limit south of the autonomous prefecture of Xishuangbanna in southwestern Yunnan province; northern Myanmar (the country formerly known as Burma); northern Thailand, where the species occurs south to Raheng and west to the Bay of Bengal; northern and central Laos; northern Vietnam (where its range bumps into the range of Phayre’s Langur (Trachypithecus phayrei) which occurs just south); and Cambodia.
Habitat is provided by many types of forests, including primary and secondary evergreen and semi-evergreen, mixed moist deciduous, light woodland, bamboo, and the limestone (karst) forests of Laos. These leaf-eating monkeys also occupy areas near plantations. Lower elevation limit for the species within inner forests and wetlands is 164 feet (50 meters); upper elevation limit for those residing on inland cliffs and mountain peaks is 3,281 feet (1,000 meters).
Prior to its reclassification as a distinct species, a result of later genetic studies, the Indochinese gray langur had been classified as a subspecies of Phayre’s langur.
Similar nomenclature for other langurs can seem like a game of “who’s who” and leave one scratching her head. To clarify this jargon, the Indochinese gray langur is not to be confused with the Indochinese silvered langur (Trachypithecus germaini), also known as Germain’s langur or Indochinese lutung; nor to be confused with the silvery lutung (Trachypithecus cristatus), also known as the silvered leaf monkey or the silvery langur; and finally, not to be confused with the gray langurs within the genus Semnopithecus.
And just to make the name game even more ambiguous, leaf-eating monkeys of the genus Presbytis of Malaysia and Indonesia are known by the local name “surili.” Whereas Indochinese gray langurs, who also eat a lot of leaves, go by the name “lutung.”
Size, Weight, and Lifespan
With a head-to-body length of 21 inches (51 centimeters) and weighing up to 15.6 pounds (7.1 kilograms), males are slightly larger than females, who have a head-to-body length of 19 inches (49 centimeters) and weigh up to 14.1 pounds (6.4 kilograms). Males also have larger canine teeth. A long, nonprehensile tail adds another 33 inches (83 centimeters) to the body of males and 32 inches (82 centimeters) to the body of females.
Lifespan for the Indochinese gray langur has not been documented, though wildlife biologists posit that it is likely similar to that of other langur species, who live between 20 and 30 years in their respective natural habitats.
Appearance
Cuteness overload. Seriously . . . how irresistibly adorable is this fuzzy, little forest sprite?! Let’s talk about this monkey’s beguiling face. Bluish-gray skin colorizes its muzzle, anchored by a narrow nose and accented by patches of unpigmented skin above and beneath thin lips. Deep, chestnut-colored eyes appear even darker by a ring of pale-colored skin that surrounds each orb. Long, white whiskers punctuate the chin, and untamed plumes of blue-gray fur feather out from each side of the face, concealing the monkey’s ears. A thick cap that begins immediately above the brow ridge flourishes into a wild whorl of fur on the crown of the head. Back, limbs, and tail are cloaked by a long, blue-gray pelage (fur coat); fur color on the underside ranges from silver-gray to white. Feet and hands are dark gray, and a modest tassel decorates the tip of the tail. Mother Nature has given females a pubic patch of white fur and pale skin, anterior to (in front of) the ischial callosities, those hard, bare buttock pads that make sitting for long periods of time more comfortable for these monkeys.
Infants are born cloaked in golden fur. At about seven to eight months of age, the pelage becomes darker. By the time the young langur is one year old, the pelage attains its blue-gray color.
Across the species’ geographic range, Indochinese grey langurs can exhibit subtle variations in coat color. As example, the pelage of individuals residing in the northeastern region is often darker, especially with regard to their limbs and tail.
Diet
Indochinese gray langurs are primarily folivores, which is a fancy way of saying they eat mostly leaves—with a penchant for young leaves. Their primary source of nutrition, however, comes from complementary food sources that include figs (an excellent source of fiber), bamboo shoots, flowers, seeds, and tree gums. The langurs’ diet adjusts with the change of seasons. Wildlife biologists have identified 148 plant species as food sources. These monkeys also engage in geophagy; that is, they eat dirt. This intentional practice of eating earth or soil-like substances is believed to provide important minerals and aid in detoxification.
A multichambered stomach, similar to that of a cow or a sloth, contains unique gut flora (specialized microorganisms) that break down and digest leaf cellulose and other plant matter, thereby allowing essential nutrients to be extracted.
Behavior and Lifestyle
Despite the species-wide geographic distribution, Indochinese gray langurs are rarely seen. Shy and elusive, they retreat whenever a threat is perceived. Thus, much of the information gleaned about their behavior and lifestyle is taken from other langurs in the genus Trachypithecus.
Indochinese gray langurs are largely arboreal, meaning that they spend most of their time in trees. They use the higher strata (layer) when feeding and resting and the lower strata when traveling. A slender body, hands and feet with opposable thumbs and toes that allow them to grip branches, and a long tail that helps them to balance are physical adaptations that allow the langurs to move easily through their forested environment with great agility, either quadrupedally (on all four limbs) or by impressively leaping from tree to tree. While eating they typically sit on their rump, making good use of those ischial callosities, a considerate adaptation from Mother Nature. These langurs only descend to the ground to drink water from a stream or river or to get a “mineral fix.” A group studied at Mt. Wuliang, in Yunnan, China, spent a significant amount of time on the ground at saltlick locations, where they engaged in licking rocks to absorb the minerals within. Even when a group is on the ground, a scout always remains in the trees to keep a lookout for tigers and other ground predators.
The exception to the arboreal lifestyle may be those individuals residing in karst (limestone) forests—defined by ridges, cliffs, crevices, fissures, caves, and underground streams. We can conjecture that these langurs likely exhibit more terrestrial tendencies, similar to those langur species [(white-headed black langur (Trachypithecus leucocephalus)] who reside exclusively in such habitat.
Active during daylight hours, langurs are diurnal. Some species are known to be crepuscular (most active during the hours of dawn and dusk), but little information is available specific to the Indochinese gray langur’s daily activity pattern.
When foraging, langurs embark as a group and feed on leaves and fruits at a particular tree height before moving on to the next tree. Wildlife biologists describe this foraging behavior as a “browse line” and speculate that the practice promotes successful foraging by minimizing competition for food sources among members of the group. Foraging expeditions are orderly and effective.
Overnight, these arboreal langurs sleep in the dense canopy of trees, concealed from potential predators. Those dwelling in karst environments might seek overnight refuge in a cave, or perhaps on the ledge of a limestone cliff to minimize predator threats.
Natural predators of langurs include leopards (Panthera pardus), wild dogs known as dholes (Cuon alpinus), tigers (Panthera tigris), marbled cats (Pardofelis marmorata), along with aerial predators such as hawk eagles and black eagles.
In the Hindi language, the word langur means “long tail.”
An Indochinese gray langur group (known as a “troop”) usually comprises several families. Characterized by an intricate social structure, group size ranges from 14 to 30 individuals, including 1 to 5 adult males with one being the alpha (dominant) group leader, 2 to 12 adult females, and the group’s youngsters.
A field study in Wuliangshan, China, recorded a group as large as 90 individuals.
Unlike other langur species, wherein males disperse from their natal (birth) group upon reaching maturity, female Indochinese gray langurs disperse. In fact, they do so regularly. Shortly after transferring to a new group, a female often successfully installs herself as the dominant female of her new group.
Home range covers a territory of 86.5 to 98.8 acres (35 to 40 hectares), often overlapping with other primate species, including that of the western black crested gibbon (Nomascus concolor furvogaster), a subspecies of the black crested gibbon (Nomascus concolor). Although western black crested gibbons are known to be territorial, the “neighborliness” of Indochinese gray langurs assuages any tensions between the two primate species. Violent clashes are rare. The langurs’ flexible eating regimen helps to reduce any rivalry or competition for food sources.
Indochinese gray langurs begin foraging shortly after waking in the morning, covering a distance between 0.3 to 0.5 mile (500 to 800 meters) in the course of a day. They follow fixed routes, moving swiftly through the canopy, shaking tree branches and making quite a racket as they advance, leaving their leafy travel route in disarray. These langurs don’t have the best dining etiquette; they routinely drop half-eaten fruit to the forest floor below. But the food doesn’t go to waste; grateful animals on the ground quickly consume the discarded morels. Afternoon naps in the shade of the tree canopies follow the langurs’ feeding sessions. These long rest periods provide the time necessary to digest all that consumed plant matter.
Depending on the region, sympatric primate species include François’ langur (Trachypithecus francoisi), red-shanked douc (Pygathrix nemaeus), and, of course, Indochinese gray langurs’ neighbor, the western black crested gibbon. Other mammals include tigers (Panthera tigris), snow leopards (Panthera uncia), and large deer known as sambar (Rusa unicolor). One of the largest species of snakes, the Burmese python (Python bivittatus), is found in the langurs’ range.
Though not reported as occurring with Indochinese gray langurs, infanticide is common among other langur species, particularly in those societies where males disperse their natal groups and install themselves as leader of another group. After systematically killing the new group’s unweaned infants (the brightly colored pelage of many infant langur species makes these babies easy targets), the murderous male mates with a group’s reproductive females, thereby ensuring that only his genes are carried by new offspring.
Indochinese gray langur vocalizations include loud, staccato calls used to intimidate and scare away a rival or potential predator. Males may curl their upper lip and bare their canine teeth to add to this threat. When a tiger is spotted, they “cough” to warn others. To call out to other members of a group, langurs emit throaty, reverberating chirps. Squeaks often accompany their daily activities. And after a big meal, these leaf-eating langurs often belch—just like human primates.
Mothers attentively groom their young, reinforcing the parent-child bond.
Because the species is so elusive, other ways that wild Indochinese gray langurs use to communicate with one another remain a bit of a mystery. More field studies by intrepid wildlife biologists are necessary to discover the langurs’ full repertoire of expressions.
The mating system of langurs varies: societies can be monogamous (exclusive male-female mating partners); polygynous (males mate with multiple females), or polyandrous (females mate with multiple males).
Reproductive maturity for females (able to conceive and bear young) is between 3 and 5 years of age. After a pregnancy of 6 to 7 months, a single infant is born. Twin births are rare. Births occur year-round, but peak is between November and April. A mother’s interbirth interval (time period between births) is about two years, assuming her previous infant survives.
Females initiate copulation by presenting their anogenital region to a prospective male partner. In one study, female Indochinese gray langurs behaved in a more receptive (or inviting) manner toward adult males during the peri-ovulatory period (when the females were most fertile and likely to conceive). When the females were experiencing their post-conceptive period (least likely to conceive), they were more receptive to adolescent males. Adult males appeared to hone in on the fertile females, whereas adolescent males remained more oblivious to the females’ heightened reproductive condition. Researchers have posited that, depending on their reproductive state, the females deliberately chose their copulation partners as a strategy to confuse paternity.
Further studies into the reproductive lives of Indochinese gray langurs are lacking; hence, like other areas of their lives, these elusive monkeys remain an enigma.
Besides feeding other animals with all that half-eaten fruit they drop to the forest floor, Indochinese gray langurs help to influence the composition and structure of local plant communities. Through the seeds of the fruits they drop and also through the seeds excreted in their poop, dispersed throughout their environment as they travel, the langurs help to regenerate their forest habitat. They also help to prune the forest canopy through their folivorous dining habits.
The Indochinese gray langur is classified as Endangered by the International Union for Conservation of Nature (IUCN, November 2015), appearing on the IUCN Red List of Threatened Species. Fewer than 2,500 mature individuals remain, with the population having declined more than 50 percent over the last 36 years (three generations).
Hunting has taken a grave toll on the species; the langurs are killed for their flesh (known as “bushmeat”) and for use in Asian medicines; specifically, they are sliced open for the large gallstones these primates tend to develop, believed to hold healing properties.
The species is also threatened by habitat loss, especially in Vietnam and Laos, due to human encroachment. Although these langurs are known for their adaptability, the destruction and transformation of forestland into human settlements, farmland, and into pulpwood plantations displace the langurs and destroy or significantly reduce their food sources—negatively impacting their nutritional intake, reproductive success, and survival.
The Indochinese gray 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. In Vietnam, the species is protected at the highest level under the government’s wildlife protection law, Decree 32/2006; in China, it is listed as a National Class I Protected Species under Chinese Wildlife Conservation Law.
Indochinese gray langurs reside in the following protected areas:
- Vietnam: Pu Mat and Ben En National Parks, Pu Luong, Xuan Lien, Muong Ne Nature Reserves, and Mu Cang Chai Species and Habitat Conservation Area
- Laos: Phou Dendin, Nam Et-Phou Louey, Phou Khaokhoay, Nam Kading National Biodiversity Conservation Areas, Phou Gnouet Production Forest
- Thailand: Nam Nao National Park, Huai Kha Khaeng, and Phu Khieo Wildlife Sanctuaries
- China: Ailaosjan, Huanlianshan, Wuliangshan, Xishuangbanna, Fenshuiling, Niuluohe, Caiyanghe, Lancangjiang National Nature Reserve
Those residing outside these areas are afforded no protections, however. And no species-specific conservation plan currently exists for Indochinese gray langurs. Wildlife biologists express a glimmer of hope in that the intensive conservation efforts to protect the black crested gibbon, a sympatric species, may incidentally help protect these langurs. But to ensure the Indochinese gray langurs’ survival, serious conservation action is needed.
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Written by Kathleen Downey, April 2024