PURPLE-FACED LANGUR

Semnopithecus vetulus

Geographic Distribution and Habitat
Purple-faced langurs (Semnopithecus vetulus), also known as purple-faced leaf monkeys, are endemic to Sri Lanka. In Sinhala, one of the official languages of Sri Lanka, they are known as “the black monkey of Sri Lanka.” Located near the southern tip of India, the tropical island of Sri Lanka is teeming in biodiversity. This is thanks in part to its varying landscapes and climates. The perimeter is dusted with sandy beaches and the valleys and rivers of the plains make up a large portion of the island. In the south-central area lies the highlands, also known as the wet zone, with its mountain ridges and tall rainforests—this is where purple-faced langurs call home.

Purple-faced langurs live in the lush high canopies of a variety of forests including tropical rainforests, evergreen rainforests, semi-deciduous forests, and montane forests. Their range includes the Sinharaja Forest Reserve, the small town of Kitulgala, Galle, and Horton Plains National Park. While they prefer life in the tops of the trees, deforestation and other threats have forced groups to survive in more urbanized and farmed areas where they live wherever they can find shelter and cover such as in home gardens or rubber plantations.

TAXONOMY IN TRANSITION

Previously, purple-faced langurs were considered part of the lutung genus Trachypithecus, but later DNA evidence pointed toward a closer relationship to gray langurs, which reclassified them. There are four subspecies of purple-faced langurs: the southern lowland wetzone purple-faced langur (Semnopithecus vetulus vetulus), the western purple-faced langur (Semnopithecus vetulus nestor), the dryzone purple-faced langur (Semnopithecus vetulus philbricki)and the montane purple-faced langur (Semnopithecus vetulus monticola). There is an additional possible subspecies, Semnopithecus vetulus harti, but it is not yet confirmed.

Purple-faced langur range, IUCN 2020

Size, Weight, and Lifespan
Male purple-faced langurs tend to weigh more and have longer bodies than females. On average, males weigh somewhere between 15 lbs (7 kg) and 21 lbs (9 kg) and females weigh between 11lbs (5 kg) and 17 lbs (8 kg). Males range in length from 20 to 26 inches (50–67 cm) from their heads to the base of their tails, and their tails can measure between 26 to 34 inches (67–85 cm) long. Females are 18–24 inches (45–60 cm) long from their heads down to the base of their tails and their tails average around 23–32 in (59–82 cm).

Their average lifespan is 23 years.

Appearance
Contrary to its name, the purple-faced langur actually has a dark brown to black face. The area around their eyes, nose, and mouth is hairless, but fluffy cream-colored fur borders their jawline like a beard. On top of their heads is a crown of light to medium brown fur. The color of the fur on the rest of their bodies varies but can range from cool-toned dusty gray to warm dark brown or even black. Their coat tends to get darker around their hands and feet, which are hairless and match the color of their faces.

Infants generally have a lighter coat than adults and can be a warm medium brown to light gray. At around 12 to 16 weeks old, their coat transitions to its adult color.

The eyes of purple-faced langurs are large, round, and dark amber in color. Like all primates, their eyes face forward, which provides the stereoscopic vision necessary for the depth perception required to leap through the canopy of the forest.

Purple-faced langurs are long and slender from their arms and legs to their fingers and toes and even their tails. With tails longer than their bodies, they have excellent balance for running across branches.

The dentition of purple-faced langurs makes their leafy diet possible: their premolars and molars have high crowns and pointed cusps that easily slice through tough foliage. 

Diet
Purple-faced langurs are primarily folivorous and favor immature leaves for their higher protein and lower lignin content. Their digestive systems are well-adapted with multi-chambered stomachs filled with specialized stomach bacteria that helps them to extract nutrients from complex carbohydrates like leaves. Some their favorite trees to eat leaves from include Holoptelea integrifolia (also known as Indian Elm), Hydnocarpus venenata (known as Makulu gaha in Sinhala), and Manilkara hexandra (known as Rayan in Sinhala), among many others.

Purple-faced langurs will also eat fruit, flowers, and seeds. Increased urbanization and habitat loss have increased their fruit intake, as they often take fruits such as jackfruit, bananas, and mangos from farms in cultivated areas. However, these foods are not preferred when there are sufficient quantities of their typical diet, and too much of these cultivated fruits can be detrimental to their nutritional needs.

Purple-faced langurs love wild fruits such as fruit from Dimocarpus longan trees that is similar to lychee. 

Behavior and Lifestyle
Purple-faced langurs are highly arboreal and only descend to the ground very briefly. They are diurnal and very active. As a gregarious species, they spend much of their time socializing with each other. When it is time for bed, they sleep in high trees with plenty of cover. Often, they choose sleeping sites that are close to water sources. They run quadrupedally and can drop 50 or more feet when leaping through the tree branches.

Daily Life and Group Dynamics 
Generally, purple-faced langurs live in groups consisting of one male and multiple females (usually up to 7) as well as their offspring. In these troops, usually one male is dominant over the rest of the group and is responsible for defending the females from predators and other threats. They have a range of about 1640–2625 ft (500–800 m) that they spend large amounts of time foraging within throughout the day as a group.

Additionally, some groups are made up of only males—2 to 14 bachelors will live together in a troop. A single male will sometimes join another troop by ousting the dominant male and killing any infants that are not yet weaned. Using this strategy, the bachelor gains dominance over the troop and the females will soon go into estrus so they can mate.

Due to the overlap in their geographic locations, purple-faced langurs sometimes live in troops with tufted gray langurs (Semnopithecus priam). There is a possibility that this rare living dynamic may lead to a unique hybrid species.

Other primates that live in Sri Lanka include the toque macaque (Macaca sinica), the northern plains gray langur (Semnopithecus entellus), and the red slender loris (Loris tardigradus).

What Does It Mean?

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

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

Canopy:
Beneath the emergent layer, the canopy layer is the primary layer of the forest and forms a roof over the two remaining layers (the understory and forest floor). Many animals live in this maze of leaves and branches, where food is abundant.

Cultivated areas:
Agricultural areas of land that are harvested every year. 

Deforestation:
The permanent cutting, clearing, and removal of trees to convert forest land for other use, such as pasture, cropland, or plantations.​

Dentition:
The arrangement or condition of the teeth in a particular species or individual.

Diurnal:
Active during daylight hours.

Endemic:

Native or restricted to a certain area or country.

Estrus:
A recurring period of sexual receptivity and fertility in many female mammals.

Folivorous:
Having a diet that consists of leaves.

Homeostasis:
The tendency of a system, especially the physiological system of higher animals, to maintain internal stability, owing to the coordinated response of its parts to any situations or stimulus that would tend to disturb its normal condition or function. Any self-regulating process by which biological systems tend to maintain stability while adjusting to conditions that are optimal for survival.

Hybridization:
The act or process of mating organisms of different varieties or species to create a hybrid.

Lignin:
A complex oxygen-containing organic polymer that, with cellulose, forms the chief constituent of wood.

Montane forest:
A forest that grows on the slope of a mountain, regardless of altitude or latitude, within a specific climate, just below the subalpine zone.

Semi-deciduous:
A botanical term referring to plants that lose their foliage for a short period of time; as leaves fall, new ones start growing.

Stereoscopic vision:
Combines different inputs of the images received by each eye into a succinct, singular image with the proper size, depth, and proximity.

Subspecies: 
A population usually restricted to a geographical area that differs from other populations of the same species, but not to the extent of being classified as a separate species.

Tropical forest:
A type of forest found in areas with high regular rainfall and no more than two months of low rainfall, and consisting of a completely closed canopy of trees that prevents penetration of sunlight to the ground and discourages ground-cover growth. 

​Quadrupedal:
Using four limbs to move about. This word comes from the Latin meaning “four feet.”

Visit the Glossary for more definitions

Communication
The vocal calls of the purple-faced langur are distinct from those of any of the subspecies. Individuals can also be recognized by their calls. Adult males tend to be the most vocal among troops.

Most calls occur during the day when it is sunny and are primarily for defending territory. These are usually made during intergroup encounters and are characterized by deep and imposing whooping calls accompanied by impressive leaps between branches. However, calls can also be used to alert group members that they have spotted a predator, to attract mates, or to locate group members. The purple-faced langur’s loud barking call sounds similar to the roar of a leopard. When angry, they make short, repeated cough sounds.

Interestingly, the calls of purple-faced langurs are not different between troops living in urbanized areas and those living in natural habitats. In urban areas, purple-faced langurs do not respond vocally to villagers.

Reproduction and Family
Purple-faced langurs become ready to reproduce at 4 years old. Females give birth to one baby at a time and their pregnancy lasts approximately 6 to 7 months. The baby is nursed for 7 to 8 months, after which they are mostly independent. Mothers practice allomothering, where they allow other mothers within the group to care for their infants. They generally wait a year and a half after pregnancy before having another baby.

Ecological Role
By foraging leaves, purple-faced langurs naturally prune a wide variety of vegetation, which helps maintain the homeostasis of the forests. They also disperse seeds from the fruits that they eat when they defecate, which helps with forest regeneration. While perhaps not the most pleasant ecological role, purple-faced langurs also serve as food for the endemic predators of the region like the leopard. Nature requires balance, so the conservation of one species ultimately effects them all.

Conservation Status and Threats
Purple-faced langurs are classified as Endangered by the International Union for Conservation of Nature (IUCN, 2015), appearing on the IUCN’s Red List of Threatened Species. Like many primates, deforestation is one of the primary factors in the decline of purple-faced langur populations. In the last 65 years, Sri Lanka has lost more than 50% of its forest cover. The cultivation of crops such as tea, rubber, and coffee in addition to timber harvesting has reduced high forest cover by 68%. Now, many areas of their range leave them exposed to direct sunlight. The few forests left with adequate canopy cover for them are not in protected areas.

In addition to the issue of deforestation, 90% of the areas where purple-faced langur troops live is now populated with humans. Having humans in that much of their habitat threatens their survival due to croplands and road production taking over their natural range. Some are also poisoned due to farmers trying to prevent them from raiding their crops. Furthermore, living in urban environments among humans makes them more prone to infection with gastrointestinal parasites, electrocution, being killed by domesticated animals like dogs, and being hit by cars in traffic.

Conservation Efforts
There have been some considerable efforts towards conservation. Growing concern over the country’s environmental stability led President Mahinda Rajapaksa to form a plan to increase the forest cover using native species. This also included a program in schools to promote conservation awareness. Due to its success, the program has been used in other communities for further reforestation projects.

Moving forward, additional efforts towards conservation may include improving and expanding already protected areas, building rope bridges for purple-faced langurs to use in urbanized areas, as well as finding successful ways to mitigate human and langur conflicts within local communities.

Purple-faced langurs are listed in the Convention on International Trade in Endangered Species (CITES) Appendix II.

References:

  • https://www.dilmahconservation.org/about-animals/mammals–f9925cffabdd604dfab150bbcb49f2a8/purple-facedlangur–4d3d8e2f9ff0f0b28e8715756f4da56f/
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  • https://www.iucnredlist.org/species/22042/17959452
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  • https://pubmed.ncbi.nlm.nih.gov/16474091/
  • https://animalia.bio/purple-faced-langur
  • https://palmoildetectives.com/2021/01/29/purple-faced-langur-semnopithecus-vetulus/
  • https://eol.org/pages/128463
  • https://whc.unesco.org/en/list/1203/
  • https://www.britannica.com/animal/langur
  • https://www.jetwingeco.com/wp-content/uploads/2016/01/Primates-of-Sri-Lanka-Canon-2009-10-15-LP.pdf
  • https://pfaf.org/User/Plant.aspx?LatinName=Dimocarpus+longan
  • https://www.karger.com/Article/Fulltext/496025
  • https://pubmed.ncbi.nlm.nih.gov/4201908/

Written by Elizabeth Joslin, August 2021