LAOTIAN LANGUR

Trachypithecus laotum

Geographic Distribution and Habitat

The Laotian langur, also called the Lao langur or Lao leaf monkey, is endemic to the Lao People’s Democratic Republic (Lao PDR). They are found in the Bolikhamxai and Khammouan provinces, in rugged terrain and dry rainforests. They live in several protected areas, including the Nam Kading National Protected Area, Nam Sanam Provincial Protected Area, and Phou Hinpoun National Protected Area. In Phou Hinpoun National Protected Area, they share their forested habitat with the Hatinh langur (Trachypithecus hatinhensis), and there is some overlap in appearance. Because of this, their habitat boundary is not strictly known in this area.

TAXONOMIC NOTES

The Laotian langur belongs to the Cercopithecidae family, which includes many monkeys in both Asia and Africa. Langurs, as a group, live exclusively in Asia. As mentioned previously, the Laotian langur shares part of its habitat with the Hatinh langur (Trachypithcus hatinhensis). There is some speculation about hybridization between the two monkeys, however, that has not been confirmed by any genetic testing.

Laotian langur range, IUCN 2023

Size, Weight, and Lifespan

The Laotian langur is similar in size to other langurs. They are mildly sexually dimorphic, with females being smaller in body size than males. Males weigh approximately 19 pounds (8.6 kg), and females weigh 16 pounds (7.2 kg). In length, they are roughly 21-23 inches (53-58 cm), and their long, slender tails add another 30 to 35 inches (79 to 90 cm). 

Not much is known about their lifespan in nature, and because of the Laotian langur’s diet, they are not often kept in captivity. It is estimated, based on other members of their genus, that they live to be approximately 25 years old. 

Appearance

The Laotian langur is a stunning, slender monkey. Adults have black faces, however, they have a sort of white cap over the rest of their heads—except for a small tuft of black hair at the top. They have night-black fur from their necks down to their toes and the tip of their tail. Sub-adults look similarly, although they are slightly smaller in body size. Juveniles often have yellow, orange, or black and white fur all over, as well as on their heads. Infants have a golden fur color that contrasts against their mothers’ dark fur. While it is not uncommon for infant monkeys to have different colored fur than adults, this golden coloration is unique to Trachypithecus monkeys. This coloration is thought to be a signifier to adults of their immature status. This physical attribute affords them prolonged patience and more tolerance when they are playing or vocalizing loudly. There is also evidence that the bright-colored fur stimulates greater maternal care, which allows young monkeys to pass through the infant stage quickly. This allows the monkeys to be weaned relatively early, which is a benefit, as infanticide can happen when monkeys are still nursing. 

The Laotian langur, along with all langurs and many other leaf-eating monkeys, have a reduced thumb size. It may appear as though this monkey only has four fingers, however, their thumb is simply short. This is thought to be an adaptation of their highly arboreal lifestyle.

Photo: © Pierre-Louis Stenger/iNaturalist.Creative Commons
Diet

This monkey is herbivorous, more specifically folivorous. Their preferred food is young leaves, but, they will feed on unripe fruits, seeds, and flowers. Foods such as seeds and flowers are important fallback foods for the Laotian langur. When young leaves are not in abundance, these foods sustain them. Interestingly, the Laotian langur has been seen as sympatric, or sharing a distribution, with the Assamese macaque (Macaca assamensis). They have been observed feeding in the same trees, although the langurs preferred to be higher in the canopy than the macaques.

Because of this monkey’s leaf-heavy diet, they have developed a special adaptation. Much like a cow or antelope, the Laotian langur has a sacculated stomach. This multi-chambered organ allows them to digest the compounds found in the cell walls of plants. An abundance of plant matter is difficult for many animals to eat, as they lack the key adaptation these monkeys have. This allows the Laotian langur to occupy a crucial niche in their environment. 

Behavior and Lifestyle

Laotian langurs have unique sleep-sites that only a few other primate species are drawn to. These sleep sites are small caves, naturally carved into limestone. Assamese macaques also enjoy these sleep sites, and they have even been known to sleep in the same cave systems or cliff faces. These sleep sites keep them safe from predation, as well as cool, rainy weather that is typical for their habitat. They often reuse these sleep sites, which is not especially common among monkeys. 

Adults spend much of their time resting. Having a leaf-heavy diet requires a lot of digestive power and rest is essential. They spend other parts of their day foraging, socializing, and for sub-adults and juveniles, playing. 

Fun Facts

It is common for the Laotian langur to communicate via touch. Adult females love to snuggle up for a cuddle!

Daily Life and Group Dynamics

Laotian langurs live in groups of around 5 to 30 individuals. This estimate seems to be generous, as recent assessments record group sizes between 3 and 15. This drop in group size could be due to habitat loss, and groups being forced to fission, or break apart into smaller groups, in order to sustain themselves. As the quality of their environment improves, the group sizes increase. 

Their groups follow the social structure of one-male-units (OMUs). These OMUs are largely considered to be stable for the individuals involved. They are comprised of one to two adult males (in which a strict hierarchy is established), 2-4 females, and any offspring. 

Males are the sex that disperses, meaning they leave the group they were born into, and females are philopatric—meaning they stay in the same community. Dispersal is beneficial because it allows monkeys to avoid inbreeding, and maintain genetic diversity within the group. Some females may disperse, however, that does not appear to be a common behavior.

Communication

Not a lot of information has been recorded regarding the Laotian langurs’ communication. It is known that adult males will vocalize loudly in the morning, in order to establish their boundaries and make their presence known. In one study, local people were played a recording of the langurs’ vocalizations, and the monkey was easily recognized through these recordings. This implies that they have distinct sounds and calls, yet information regarding those characteristics has yet to be published.

Reproduction and Family

The Laotian langur is not an extensively studied species, therefore much of what we know is compiled from other langur species. With this in mind, males generally reach sexual maturity between the ages of 5 and 7. Females reach this stage of life earlier, at 4 years old. During copulation, it is not uncommon for other mature males to harass the individuals involved. This is thought to be a tactic for suppressing another male’s gene proliferation. When a female becomes pregnant, she will gestate for roughly 6 months, after which she will give birth to a single infant. She will nurse the infant for about one year, upon which the young monkey is weaned. A female will not become pregnant while she is still nursing. Therefore infanticide might occur if another male takes over the one-male unit.

Photo: © k.yoganand/iNaturalist/Creative Commons
Ecological Role

Since the Laotian langur’s primary food is leaves, they are considered leaf predators. Leaf predation often elicits a defense mechanism in plants that causes them to grow more, and faster. This helps maintain dense, lush forests. They act as prey for large felids in the area, such as clouded leopards and, occasionally, tigers.

Conservation Status and Threats

The International Union for Conservation of Nature lists the Laotian langur as Endangered (IUCN, 2015), appearing on the IUCN Red List of Threatened Species.

The Laotian langur is threatened primarily by habitat loss and hunting. Habitat loss is due to commercial logging, or felling forests in order to make room for agriculture. These monkeys are not considered to be flexible in their diet, and they rely heavily on their environment. Protecting their habitat is an urgent concern in their conservation. It does not appear that the Laotian langur is actively hunted by local people, but rather hunted opportunistically. It does not appear that hunted Laotian langurs are exported but rather are used locally for subsistence. However, langur bones are commonly used in traditional medicine, and demand in China and Vietnam may result in active hunting in the future.

Conservation Efforts

The Laotian 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.

The Laotian langur is fully protected by law. According to the Lao Wildlife and Aquatic Law, it is prohibited to hunt or harm these monkeys, because of their threatened status. This monkey does occur in protected areas and parks. It is difficult to maintain constant management and supervision, which is why they are still affected by hunting. 

It is imperative that this species be studied more, and that educational material be implemented in local communities. There is much we do not know about this species, and through more research, we will get a better understanding of their population numbers, distributions, and taxonomy, and further create laws to protect these beautiful animals.

References:
  • https://www.iucnredlist.org/species/22044/17959133
  • https://animalia.bio/laotian-langur
  • https://www.bioexplorer.net/animals/mammals/monkeys/laotian-langur
  • https://l8cd3c.a2cdn1.secureserver.net/hatinh-langur/
  • https://www.eprc.asia/langurs/
  • https://research.vu.nl/ws/portalfiles/portal/214610669/On_the_evolution_of_distinctive_natal_coat_coloration_in_primates.pdf
  • https://education.nationalgeographic.org/resource/herbivores/
  • https://link.springer.com/article/10.1007/BF02390470 

Written by Robyn Scott, March 2024