Geographic Distribution and Habitat
Francois’s langurs (Trachypithecus francoisi), also known as the Tonkin langur and the Tonkin leaf monkey, live in the forests that grow along the steep ledges of limestone mountains, known as karst, which are a signature of the landscape of northern Vietnam and southern China. Historically, their distribution spanned across Vietnam’s northeastern provinces, as far east as Lào Cai and Yên Bái, and as far south as Tuyên Quang, Thái Nguyên, and Lạng Sơn. Today, the full extent of their distribution is unknown, but groups are no longer found in several of these provinces. Remaining Francois’s langur troops are concentrated mostly in Viet Nam’s Hà Giang province and in China’s Guangxi and Guizhou provinces.
Size, Weight, and Lifespan
Francois’s langurs stand approximately 2 feet (60 cm) tall. Males weigh between 14 and 17 pounds (6.5–7.8 kg), slightly larger on average than females who range between 12 and 17 pounds (5.5–7.9 kg).
The average lifespan for this species is not known in the wild. Though not at all suited for a life in captivity, some captive Francois’s langurs have been recorded to live up to 25 years.
With slender bodies, Francois’s langurs are agile and acrobatic monkeys. Their dark, furry forms look like mere shadows as they clamber up and down gray karst cliffs and leap between the trees that overhang their steep ledges. Lanky limbs and long muscular fingers are their best insurance against a deadly slip and fall down the side of their mountain homes. A tail longer than their torso provides them exceptional balance.
The skin on their faces, hands, and feet is as black as their fur. Above a gull-shaped brow ridge, a tuft of hair rises to a triangular peak. Their ears, also black in hue, are rimmed at their tops with bright white fur that extends down their cheeks as white sideburns—their most striking feature. Females also have a distinctive white patch of fur close to their genitals.
Infant Francois’s langurs are born a conspicuous golden-orange color. Their bright coats, which darken to black as they mature, make it easier for adults to keep a close eye on their juveniles as they learn to navigate the hazardously steep karst forests they call home.
Half of a Francois’s langur’s diet is composed of both young and mature leaves. Though leaves grow in abundance throughout the year in the tropical and subtropical climates they live in, these langurs are not entirely folivorous, and regularly eat other cellulose-rich plant material such as shoots, flowers, and even bark. Langurs, like other species of leaf-eating monkeys, are equipped with multi-chambered stomachs. In their forestomaches, specialized bacteria breakdown any cellulose, making further digestion more efficient. Certain fruits, and the occasional insect, serve as infrequent—but no less nutritious—snacks.
Behavior and Lifestyle
Francois’s langurs are diurnal, spending much of their day foraging for food. Between feedings, in order to digest their cellulose-rich meals, they take long rests. High on the shadowy ledges of karst mountains, beyond the reaches and eyes of most predators, they live with more than enough safety to relax. Caves formed in the limestone offer uniquely secure, dry places to bide time out of the rain or to sleep.
To navigate their habitat, Francois’s langurs have evolved into uniquely agile and sure-footed monkeys. They can scale the rugged limestone cliffs with ease; clear cavernous drops with daring and acrobatic leaps between gnarled branches and jagged rock faces; or brachiate swiftly through the trees with incredible precision.
Highly social creatures, Francois’s langurs are adept at keeping group politics generally peaceful. This is achieved through prosocial, affiliative behaviors such as allogrooming. Additionally, the ritual of alloparenting instills the group with the common goal of raising offspring—even when some females do not currently have offspring of their own. The solitary male of the group tends to territory and matters of his harem’s protection. He regularly vocalizes to warn other groups of monkeys not to wander too close and makes flamboyant physical displays to intimidate any who do.
Francois’s langurs are quite mobile, traveling frequently and through quite an extensive range in order to locate certain preferable foods. They tend to travel longer distances in the dry season than the wet, when some of their staples may become harder to locate.
In general, the lives of Francois’s langurs remain somewhat mysterious since their difficult-to-navigate karst habitats make thorough research nearly impossible.
What Does It Mean?
Social interactions that function to reinforce social bonds with a group or which are of mutual benefit to all animals involved in the interaction.
Social grooming within a species.
Individuals other than the biological mother of an offspring performs the functions of a mother (as by caring for an infant temporarily).
Also called arm swinging, it’s a form of arboreal locomotion in which primates swing from tree limb to tree limb using only their arms.
Molecule found in plant matter that gives structure and strength to the cell walls of plants and provides dietary fiber.
A type of social hierarchy that arises when members of a social group interact, often aggressively, to create a ranking system. In social living groups, member are likely to compete for access to limited resources and mating opportunities.
A recurring period of sexual receptivity and fertility in many female mammals.
Having a diet that consists of leaves.
A group of females guarded by a male, who prevents other males from mating with them; an animal group consisting of one or two males, a number of females, and their offspring. The dominant male drives off other males and maintains the unity of the group. If present, a second male is subservient to the dominant male.
A topography formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves.
Visit the Glossary for more definitions
Daily Life and Group Dynamics
Groups of Francois’s langurs have a single male whose primary responsibilities include mating with the female members of his group and protecting this harem, and their offspring, from threats.
Females operate according to a dominance hierarchy, but relationships generally remain peaceful between them. Females nurture their bonds by grooming one another, and they readily look out for each other’s infants as if they were their own. Average group sizes—once estimated at up to 13 members—now vary by location, with seven members being the average for groups living in China and five for groups in Vietnam. Such a decrease is an unfortunate trend caused by increased human activity within the monkeys’ ranges, rather than poor prior research.
Almost as soon as the group wakes, they begin foraging for food, chowing down on leaves, or traveling to find potentially tastier options. Afterwards, they rest for long stretches of time while they digest their fiber-rich meals. Eating, traveling, and resting make up the bulk of their routine, but juveniles find plenty of time in between to play together and adults enjoy grooming each other. At night, the group sleeps, huddled close on a remote cliff ledge or tucked inside a dark karst cave.
High in the trees atop these towering karst formations—and sometimes concealed inside them—the daily life and group dynamics of Francois’s langurs have proven difficult to study in more significant detail.
The Francois’s langur is one of the least studied species of langurs.
They are referred to by some researchers as Francois’s lutungs; “lutung” is a Sundanese word for “blackness.”
Francois’s langurs are known less officially as white side-burned black langurs.
A male Francois’s langur vocalizes to ward off any potential intruders, emitting a substantial “whoop” sound. In the event that an intruder comes too close for comfort, he makes an intimidating visual display, using his facial expressions and body language to persuade the threat to flee.
The ways in which Francois’s langurs communicate is a subject needing more thorough study in the wild. Studies of captive populations have revealed some communication patterns among this species, including a variety of alarm calls. However, such research can only suggest what sorts of behaviors researchers may see when they are finally able to conduct studies among wild populations. Until such a time, the meaning of these signals cannot be understood as interchangeable between captive and wild groups. Like other langur species—and primates in general—it is more than likely Francois’s langurs have sophisticated visual, acoustic, kinesthetic, and olfactory means of communicating that only require proper study to illuminate.
Reproduction and Family
A group of Francois’s langurs consists of a single male, several females, and their many offspring. Mating can occur at any time throughout the year, with the average female giving birth every 20 months. Her estrus cycle lasts for 24 days. During this time she initiates copulation with the male. After six to seven months, she births a single offspring. Her newborn already sports a thick orange coat and, though quite clumsy, is surprisingly mobile for being so new to the cliffs and trees. Their bright fur makes their infants stand out, allowing adults to keep a good eye on them and better ensuring their safety as they develop. As they mature, their orange pelage is gradually overtaken by black fur.
Though infants are cared for primarily by their biological mother, every female in the harem readily takes responsibility and looks out for every other female’s offspring. This ritual of allomothering not only mitigates the dangers of growing up on the side of steep cliffs, but gives mothers the time to forage for themselves. It also has the added benefits of providing other females with genuine opportunities to practice being nurturing, and of bonding infants to the entire group. In general, such a gesture must go a long way towards maintaining peaceful group relationships among the langurs.
The male may occasionally help out with parenting, but he rarely does anything more chivalrous than carrying a tired juvenile or a still-too-clumsy infant on his back. Apart from mating, he provides his group with protection. Though he certainly watches out for predators, his focal worry is the potential that a rival male might kill him and his offspring, thus claiming the harem as his own. Infanticide is necessary when taking over a group, as it causes mothers to re-enter the estrus cycle more quickly and ensures the new male doesn’t waste his energy providing for needy juveniles outside his gene pool.
A male Francois’s langur reaches sexual maturity at four years of age, and a female at five. Currently, the circumstances by which female or male langurs take their leave of their natal groups—if they do at all—and how they go about joining another, is not well understood. While in other langur species a freshly matured male joins an all-male group until he is able to supplant a rival to claim his females, such behavior has not been observed in Francois’s langurs.
Once again, the difficulty of tracking Francois’s langurs in their wild habitats leaves some significant gaps in our knowledge of their reproductive cycles and family dynamics.
The leaf-heavy diets of Francois’s langurs have an effect on foliage coverage in the forests they frequent. Otherwise, the particular roles that these langurs play in their ecosystems are not well-understood at this time.
Research has shown that Francois’s langurs’ diets vary from group to group. Additionally, forest fragmentation—caused by human activities—limits their ability to travel in search of certain foods. Langurs in more continuous habitat have been observed to eat a less strictly folivorous diet than those subsisting in fragmented habitats. Such patterns muddy our understanding of these monkeys’ diets and, in turn, make it difficult to understand the roles they may have naturally played in their ecosystems before humans began degrading them.
Conservation Status and Threats
The Francois’s langur is classified as Endangered by the International Union for Conservation of Nature (IUCN, 2015), appearing on the IUCN Red List of Threatened Species. In the last 40 years, their general population has fallen by over 50% and is likely to continue declining at its current rate.
Groups of Francois’s langurs have disappeared entirely from areas where they were once quite common. In the entire country of Vietnam, no more than 200 mature adults are estimated to be left, each subpopulation having no more than 50 individuals. In China, population estimates have fallen by a full 70%—from 6,000–7,000 individuals down to 1,600. In specific areas, these statistics become even more bewildering. In total, there is estimated to be no more than 2,500 Francois’s langurs left in the wild. Researchers fear that, very soon, this species may unfortunately make it on the list of the world’s most highly endangered primates, joining the ranks of several of their closest relatives: the Cat Ba langur, the white-headed langur, and the Delacour’s langur.
The pressures Francois’s langurs face are layered and diverse. Each is directly related to increased and unabated human activity within their natural environments. Mining—not uncommon in the karst mountains these monkeys call home—and firewood extraction are two practices that contribute to the overall steady degradation of their habitats. Additionally, forests are readily leveled for grazing and to plant crops like rice and maize. The typical slash-and-burn technique locals use to clear forest greatly reduces the foliage. Leaves being the staple of Francois’s langurs’ diets, this practice easily causes severe food shortages for the monkeys.
Furthermore, agricultural projects like these not only further degrade habitats, but fragment the once-continuous forest as well. Francois’s langurs therefore become less capable of going in search of food. When the groups of any species of primate become isolated in this way, their entire gene pool is potentially compromised. The genetic bottlenecks that form make more precarious an already precarious situation. In only a few generations, offspring become less viable, more prone to disease and parasites, and generally less capable of thriving in their already dwindling and degraded habitats.
A recently observed trend of Francois’s langurs and white-headed langurs interbreeding where their ranges overlap in Guangxi, China, south and east of the River Zhou, may create other genetic problems for both species in the future. More research needs to be conducted to find out.
On top of these already serious pressures, Francois’s langurs are the prized targets of illegal poaching. Occasionally hunted for their meat, they are most often used for traditional Chinese medicine—being the vital ingredient to produce a concoction known as “black ape wine.” Poaching is especially severe in China. Populations in Guangxi Province, for instance, have declined by 85% due largely to this illegal trade. But specimens are also imported illegally from Vietnam, fetching the seller a large sum.
Though many protections exist in both international and national law to conserve Francois’s langurs, their poor implementation in the real world renders them essentially useless—so far. Additionally, the gaps in our knowledge of this species severely limits our knowledge about how to best help them so they can someday thrive in the wild once more.
As evidenced by the bulk of laws they are technically protected under, the plight of the Francois’s langur is recognized internationally as well as nationally in both countries where this species lives. It is listed on the Convention on International Trade in Endangered Species (CITES) Appendix II, included as a Category I species of the Wildlife Protections Act of 1989 in China, and enjoys the highest protection level and is recognized as a species for special conservation considerations in Vietnam under Decrees 32/2006 and 160/2013, respectively. Hunting and capturing Francois’s langurs is illegal, but effective implementation and enforcement of these protections are still lacking.
Francois’s langurs are found in several protected areas and conservations projects for this species are under way in both China and Vietnam. In China, Francois’s langurs live in 21 protected areas, three of which are nationally run. Additionally, a captive breeding program, in Wuzhou, China, has been successful at introducing individuals born in captivity into the wild.
In Vietnam, the Francois’s langur occurs in two national parks. Two other conservation areas have been established to specifically protect the Francois’s langur groups that live in them: Sinh Long-Lung Nhoi Species and Habitat Conservation Area and Nam Xuan Lac Species and Habitat Conservation Area.
In Vietnam, several projects, run through nonprofits, are working toward Francois’s langur conservation. The Endangered Primate Rescue Center, in Ninh Binh, for instance, works to rescue and rehabilitate the endangered primates of Vietnam, including the occasional Francois’s langur. They also educate locals about the importance of conserving primates in the wild.
The Vietnamese chapter of the People Resources and Conservation Foundation (PRCF) was established in 2004 and works to protect endangered species of primates, specifically those living in karst forest habitats. Some of their work has focused on a small population of Francois’s langurs living in Ba Be National Park. In addition to conducting population surveys, threat assessments and general conservation planning, they are also developing and implementing a comprehensive Conservation Action Plan to bring this species back from the brink of extinction. The organization encourages community involvement in their efforts, and leads educational programs at local grade schools as one way to raise awareness for this species’ plight.
Written by Zachary Lussier, April 2021