DELACOUR'S LANGUR

Trachypithecus delacouri

​Geographic Distribution and Habitat
The Delacour’s langur is endemic to a small section of northern Vietnam just south of Hanoi. Their few remaining subpopulations mostly inhabit open subtropical rainforests, spending most of their time on limestone rocks and in caves.

Delacour’s langur geographic range, IUCN 2020

Size, Weight, and Lifespan
Males and females are around the same size, with a body length of about 23 in (59 cm) and a tail length of 33 in (84 cm). Males are heavier than their female counterparts at 18.7 lb (8.5 kg), compared to females at 16.5 lb (7.5 kg).

In the wild, Delacour’s langurs can live to be around 20 years old.

Appearance
Delacour’s langurs are noted for the distinct white patches of fur over their thighs, which is in contrast to their otherwise black fur. Females also have a white patch of fur near their genitals. Like their relatives in the lutung and langur families, the Delacour’s langur has gray tufts of fur on their cheeks as well as a tuft on the top of their head.

Also like many of their relatives, the babies of the Delacour’s langur have bright orange fur for their first four months of life. This odd trait has been the subject of numerous scientific studies and theories. One would think that bright colors would just make the baby more noticeable to predators, but the color actually seems to help the babies. When danger is spotted, every adult in the group can look around, quickly spot the babies, and move to protect them.

Photo courtesy of ©Nguyen Van Truong. Used with permission.

What Does It Mean?

Allomothering:
Individuals other than the biological mother of an offspring perform the functions of a mother (as by caring for an infant temporarily).

Arboreal:
Physically adapted to living primarily or exclusively in trees.

Colobinae:
A subfamily of the Old World monkey family that includes 61 species in 11 genera.

Diurnal:
Active during daylight hours.

Endemic:
Native or restricted to a certain area or country.

Folivore:
An animal who primarily eats leaves.

Hierarchy:
A system of organization in members of a group who are ranked one above the other according to status or authority.

Sacculated:
Having a series of different sections (e.g., langurs have sacculated stomachs, or stomachs with three sections).

Visit the Glossary for more definitions

Diet
Delacour’s langurs are folivores, with leaves making up the vast majority of their diets. They may also eat fruits, flowers, and tree bark. Like other colobines (lutungs, langurs, and colobus monkeys), these langurs have evolved a complex digestive system to accommodate their plant-only diet. They have developed large salivary glands, sacculated stomachs (which work similarly to a multi-chambered stomach), more stomach acid, and a longer intestinal track. In many ways, the colobine stomach is more like a cow’s stomach than that of a typical primate.

Behavior and Lifestyle
The Delacour’s langur is diurnal. They may switch between arboreal and terrestrial lifestyles depending on their environment. When traveling through the trees, they leap from branch to branch, as opposed to swinging. When on they ground, they walk on all four limbs. Delacour’s langurs prefer rocky habitats, living in what are called “karst forests,” which are essentially tropical forests filled with limestone cliffs and caves.

Delacour’s langurs are one of the few mammals in the world that can be called trogloxenes—animals that spend much of their lives in caves. A group of langurs will generally control a territory with a few caves in it. They sleep in these caves at night to protect themselves from the elements and predators. They switch caves every few nights.

Fun Facts

The Delacour’s langur’s color patterns lend the monkey its local name, “Voọc quần đùi trắng,” which is Vietnamese for “the langur with white pants.”

Daily Life and Group Dynamics 
These langurs emerge from their caves just before sunrise. Although they are diurnal, they spend the majority of the day resting. The rest of the day is mostly spent foraging and traveling, with some time dedicated to social grooming.

Delacour’s langurs live in two different social systems. The main group type is a uni-male system with a single male, a few breeding females, and their offspring. Each group has a dominant female, but little is known about the female hierarchy. Bachelor males join together to form all-male groups that stay on the periphery of the uni-male groups, each one waiting for the opportunity to replace the uni-male leader.

Ideally, a typical group of Delacour’s langurs will contain about 9 individuals. However, due to hunting and habitat destruction, most groups fall short of this number.

Communication
Delacour’s langurs have a relatively simple vocal repertoire with 15 different vocalizations. Most of these calls are related to defense or intimidation. Like many other primates, groups maintain social bonds with each other through grooming.

Reproduction and Family
The dominant male mates with the females of the group between January and June. Mothers are pregnant for 6–7 months. Mothers usually give birth to a single baby, although twins have been observed on rare occasions. Delacour’s langurs practice allomothering, where all females and juveniles in the group assist the mother in raising her baby.

Although females reach sexual maturity after four years, they usually don’t mate until they’re around six years old, when they are more physically and mentally prepared for motherhood. Males reach sexual maturity at around five years old, at which point they leave their family and join an all-male group.

Photo courtesy of ©Nguyen Van Truong. Used with permission.

Ecological Role
Because of their diet, Delacour’s langurs are important for seed and nutrient transport in their environment. In addition, by foraging, they may play a role in the natural pruning of plants, thereby allowing these plants to increase their potential leaf and budding areas for flower and fruit production.

The likely natural predators of the Delacour’s langur, including the Indochinese leopard and the Ussuri dhole, have disappeared from Vietnam in the past few decades.

Conservation Status and Threats
The Delacour’s langur is listed as Critically Endangered by the International Union for Conservation of Nature (IUCN, 2015). Researchers estimate that there are between 200 and 250 Delacour’s langurs remaining in the wild, along with around 19 in captivity. The species has been listed on every iteration of the IUCN’s “World’s 25 Most Endangered Primates.”

According to the Vietnamese Journal of Primatology, of the 16 populations of Delacour’s langurs that existed in 2004, at least 6 had been eradicated by 2011. The majority of the remaining 10 populations only consist of one or two families, meaning that they are unlikely to survive in the long term without human help. Only the populations in Van Long Nature Reserve and possibly in Pu Luong Nature Reserve have enough monkeys to persist on their own.

The langur’s greatest threat is human hunting. Both local and Chinese poachers hunt these monkeys so that their bones and tissues may be used in traditional medicines. The Vietnamese government has done very little to enforce their laws. Although hundreds of poachers are arrested each year, only 10% are prosecuted.

They are also threatened by habitat destruction via the mining of limestone for cement production, although the IUCN states that this is only a minor threat as the Delacour’s langur’s biggest habitats are not under much pressure from deforestation.

Conservation Efforts
Vietnam’s “Endangered Primate Rescue Center” (EPRC), primarily funded by the Leipzig Zoo in Germany, is dedicated to rescuing, breeding, and rehabilitating the many endangered primates native to Vietnam. The Delacour’s langur is just one of the 15 primate species represented at the center.

In 2011, the EPRC began reintroducing Delacour’s langurs into the Van Long Nature Reserve. The center is also working with the reserve to educate the local communities about the species, as well as other endangered animals in the area. Ecotourism has become a valuable source of income to locals who live around the Van Long Nature Reserve.

Researchers also suggest the creation of migration corridors to connect forest patches and give previously isolated langur populations access to one another, allowing them to interbreed. With such a small remaining population, conservation groups are doing everything they can to boost the genetic diversity of the Delacour’s langur.

OUR GRATITUDE

Many thanks to primatologist Nguyen Van Truong of Fauna and FIora International Vietnam for generously permitting us to use his beautiful photos of Delacour’s langurs and their habitat.

References:

  • http://www.eprc.asia/our-animals/langurs/delacours-langur/
  • http://www.iucnredlist.org/details/22043/0
  • https://www.arkive.org/delacours-langur/trachypithecus-delacouri/
  • https://www.newscientist.com/article/2081626-on-the-trail-of-some-of-the-worlds-rarest-monkeys/
  • Ebenau A, Nadler T, Zinner D, Roos C (2011): “Genetic population structure of the critically endangered Delacour’s langur (Trachypithecus delacouri) in Van Long Nature Reserve, Vietnam.” Vietnamese Journal of Primatology. 1(5): 1–15.
  • Elser, S., Nguyen, Hong Chung (2013): “A survey to evaluate public opinion about the reintroduction of the ‘Critically Endangered’ Delacour’s langur (Trachypithecus delacouri) in Van Long Nature Reserve, Ninh Binh Province, Vietnam.” Vietnamese Journal of Primatology. (2(2), 27–35.
  • Nadler, T. (2012): “Reintroduction of the ‘Critically Endangered’ Delacour’s langur (Trachypithecus delacouri) – a preliminary report.” Vietnamese Journal of Primatology. 2(1), 67-72.
  • “Monkey Planet” (2014): https://www.bbc.co.uk/programmes/p01r52gr
  • https://www2.palomar.edu/anthro/primate/prim_6.htm
  • https://science.howstuffworks.com/life/biology-fields/cave-biology1.htm

Written by Eric Starr, October 2018