WESTERN PURPLE-FACED LANGUR
Semnopithecus vetulus nestor
Geographic Distribution and Habitat
The western purple-faced langur is endemic to the tropical rainforests in southwest Sri Lanka around Colombo, the country’s largest city. This region of Sri Lanka is called the wet zone because it receives up to twice as much annual rainfall as the rest of the island (98 in or 250 cm). This gives the langur its other name, the “north lowland wetzone purple-faced langur.” These Old World monkeys are one of four subspecies of the purple-faced langur.
Although the species is adapted to live in tropical forests, over 90% of their habitat has been deforested, forcing them to live alongside human communities in gardens and on rooftops. Untouched sections of forest are small in area and heavily fragmented.
Controversially, some researchers placed the purple-faced langurs in the genus Semnopithecus (gray langurs) while others place them in the genus Trachypithecus (lutungs). Mitochondrial DNA studies conducted in 2012 classify Trachypithecus vetlus and it subspecies under the genus Semnopithecus.
Size, Weight, and Lifespan
Purple-faced langurs can range from 17.5 in (44.7 cm) to 26.5 in (67.3 cm) from head to toe. Their tales can be as long as 33.5 in (85.1 cm). Males are slightly heavier than females, weighing in at just under 19 lb (8.5 kg) compared to females, who typically weigh just over 17 lb (7.8 kg).
On average, these monkeys can live to around 26 years in captivity.
Contrary to their name, purple-faced langurs actually have dark brown to black faces. Other than weight, there is very little sexual dimorphism between males and females. 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. Western purple-faced langurs can be differentiated from other subspecies by their lighter, more gray fur color.
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.
Like all monkeys in the subfamily Colobinae, western purple-faced langurs have a sacculated stomach, which is evolved to help break down cellulose in plant material. Ideally, leaves make up around 60% of the langur’s diet with fruits and flowers making up the rest; however, as their habitat continues to be converted for human use, western purple-faced langurs are now relying more on cultivated fruit that grows on farms and in gardens.
One study found that jackfruit, coconuts, and mangos were among the most preferred fruits of the purple-faced langur. Scientists do not know what the consequences of this diet change will be, but there is cause for concern as the species is physically designed for a folivorous (leaf-eating) diet.
Purple-faced langurs have also been observed eating soil from termite mounds. The soil contains minerals that aid digestion and defend against toxins.
Behavior and Lifestyle
Purple-faced langurs are diurnal and arboreal. This means that they are active during daylight hours and they are tree-dwelling monkeys. They are, by their nature, shy animals, which makes it difficult researchers to study their behavior.
The purple-faced langur is referred to locally as “kalu wadura,” Sinhala for “black monkey.”
Groups of western purple-faced langurs hold an average of 14 monkeys. This group is typically made up of one dominant adult male, a harem of about seven adult females, and their offspring that have yet to mature. Territories between these one-male units are not meant to overlap, but with suitable habitats shrinking, groups are being forced to share territory more and more. Because of this and the inconsistent density of forests in the region, it is hard to measure just how large an average group’s territory is; the best estimate is around 7.5 acres (3 ha).
In addition to the one-male units, these langurs may live in another type of group, called wanderers. Sometimes called all-male groups, these groups also carry an average of 14 monkeys and may occasionally have 1 to 3 females present, although these females generally do not mate with any of the males in the group. Wanderers spend most of the day alone before returning to a common sleeping location with the rest of the group. These monkeys live on the outskirts of one-male territories while their members look for opportunities to take over the one-male units.
Territorial disputes between langurs can turn violent, and takeovers by wanderer males often result in the infanticide of the previous alpha male’s offspring. With dwindling habitats bringing these monkeys into more contact with each other, and thus into more conflict, researchers have observed a new change to the species’ group dynamics. One-male units are being replaced by two-male units. The second male provides an extra sentry for predators, adds backup for territorial conflicts with other groups, and can help protect babies during attempted takeovers. With their surroundings rapidly changing, the alpha males must change too in order to adapt and prosper.
Langurs may use vocal communication to locate others, establish their territory, warn others of predators, and attract mates. Adult males often start their day after sunrise with a series of “whooping” calls lasting about 38 seconds to let other groups know their location. They will make periodic whooping calls throughout the day in an effort to ward off potential intruders. Purple-faced langurs can also communicate with “harsh barks” when dealing with nearby individuals.
Not only can the western purple-faced langur be distinguished from the other subspecies of purple-faced langurs through their calls, but researchers can sometimes distinguish individuals based on their calls. This is because of the severe forest fragmentation that makes it nearly impossible for some groups to mix with each other, leading separated groups to develop different tendencies and cultures over time.
Western purple-faced langurs typically live in polygynous groups, where one male lives and mates with a harem of females. A single infant is born to a mother after a pregnancy of 195 to 210 days. The baby is completely dependent on the mother for the first 12 to 20 weeks of their life. Fathers protect their offspring from outside threats, but do not take on any other parenting roles.
The leopard is the only natural predator of the western purple-faced langur. Humans have also been known to hunt these monkeys.
The western purple-faced langur is Critically Endangered (IUCN, 2015) and is listed as one of the world’s 25 most endangered primates as published by the International Union for Conservation of Nature along with the International Primatological Society and Conservation International in both the 2016-2018 and 2018-2020 reports. In 2009, Sri Lanka emerged from its 26-year civil war with one of the faster growing economies in the world, and that growth has triggered mass deforestation to make room for new developments.
Forest fragmentation can cause a plethora of issues for any species. Arboreal animals like the western purple-faced langur are unwilling to cross roads and large patches of bare ground. Thus, when a forest is physically cut off from other habitats, it becomes like an island to some of the inhabitants. As mentioned earlier, monkeys from different forests have developed different cultures and dialects because their populations have not mixed in years. This lack of gene flow leads to lower diversity, which makes the species as a whole more vulnerable to disease outbreaks and other threats. For many species, it is better to live in one large forest than several small ones.
Increasing clashes between humans and monkeys have caused these langurs to be labelled as pests who raid kitchens and damage roofs. Langurs can also be electrocuted from powerlines and attacked by dogs.
Although their numbers continue to fall, there are several potential bright spots for the western purple-faced langur. 70% of Sri Lankans identify as Buddhist, a religion that generally looks down on the killing of any living thing. Buddhists believe that any human may reincarnate as an animal and any animal may reincarnate as human, meaning that they believe all souls are related and should be respected. This may allow Sri Lankans to be more willing to help endangered animals than most other countries.
It should also be noted that large patches of the western-purple-faced langur’s remaining habitat are centered around major reservoirs that are more likely to be protected from further development. The government has expressed interest in expanding these forests rather than destroying them. The Sri Lanka Department of Forest Conservation has undertaken several initiatives to educate the public on the multitude of endangered species on the island including the purple-faced langur.
- Moore RS, Nekaris KAI, Eschmann C (2010) Habitat use by western purple-faced langurs Trachypithecus vetulus nestor (Colobinae) in a fragmented suburban landscape. Endang Species Res 12:227-234.
- Eschmann C, Moore R, Nekaris KAI (2008) Calling patterns of western purple-faced langurs (Mammalia: Primates: Cercopithecidea: Trachypithecus vetulus nestor) in a degraded human landscape in Sri Lanka. Contrib Zool 77: 57– 65.
- Rudran, R. 2007. A survey of Sri Lanka’s endangered and endemic western purple-faced langur (Trachypithecus vetulus nestor). Primate Conservation. (22): 139–144.
Written by Eric Starr, July 2018