Nilgiri Langur, Semnopithecus johnii
NILGIRI LANGUR
Semnopithecus johnii
Geographic Distribution and Habitat
The Nilgiri Langur (Semnopithecus johnii) is a monkey of many names. It is also known as the black leaf monkey, hooded leaf monkey, Indian hooded leaf monkey, John’s langur, Nilgiri black langur, and the Nilgiri leaf monkey.
Its common name is after the Nilgiri mountains, which is a part of the Western Ghats mountain range in southwestern India. They are endemic to this area, meaning they are found nowhere else.
Here, these monkeys inhabit the various types of forests found throughout the region, including evergreen, semi-evergreen, and moist deciduous forests, as well as sholas. The latter are found only in Southern India, and take the form of narrow forests surrounded by grasslands in high-elevation valleys. Their home range is about 2.5 acres (1 hectare) on average.
Because Nilgiri langurs are fairly shy, they prefer to occupy remote habitats with few humans nearby. As human developments expand, this squeezes the monkeys into smaller and smaller ranges.
Scientists encounter the most Nilgiri langurs at higher elevations. They are found between elevations of ~980–6,560 feet (300–2,000 meters).
Classifying the Nilgiri langur has been quite a challenge for scientists. They are a member of the colobine monkeys, a subfamily of Afro-Eurasian monkeys, and were originally sorted into the genus Trachypithecus, with dusky langurs. However, research began to suggest that they belong in the genus Semnopithecus, with the gray langurs. Recent genetic analysis performed in 2022 confirmed they, along with the rest of the Semnopithecus species, are a part of the same clade: a group of organisms with a common ancestor.
There has also been discussion of possible hybridization—reproduction with members of other species. Scientists have recently observed Nilgiri langurs living with tufted gray langurs (Semnopithecus priam) and even spotted possible hybrids among these mixed troops. These hybrid offspring have brownish coat colors that are unusual for members of either species. More surveying is needed to confirm whether these are indeed hybrid langurs, and to what extent the two are interbreeding.
Size, Weight, and Lifespan
Sexual dimorphism—i.e., physical differences between the sexes—can be seen in Nilgiri langurs. Adult females have a head and body length of 23–24 inches (58–60 cm), while adult males measure 30–31 inches (78–80 cm). Their tails extend their total length by 27–39 inches (68–98 cm). Females weigh an average of 24–26 pounds (10.9–12 kg), and males weigh 9.1–33 pounds (9.1–14.8 kg).
Nilgiri langurs have been known to live as long as 29 years in captivity. However, there aren’t yet any official data on their lifespans in the wild. Other species in the Semnopithecus genus tend to live into their early 30s.
Appearance
The Nilgiri langur is a striking monkey. Their bodies are covered in a dense black fur, all the way up to their heads, where a very distinctive hair-do is worn. The fur here becomes a lighter brown, occasionally even red. It grows in a large tuft over the crown, much like a human’s. Sometimes, this hair can extend all the way around the face, resembling a beard and fluffy sideburns. Wisps of longer fur may fall to the sides, and can even create the look of some very dramatic eyebrows!
Their faces have black skin, with very rounded orbital bones. Noses are flat and small. Opening their mouths reveals large canines at the top and bottom.
Aside from size, the only difference in appearance between the sexes is seen on the thighs. Here, female Nilgiri langurs have white patches that are never found on males.
Diet
Like most colobine monkeys, Nilgiri langurs are primarily folivorous, with a diet consisting mainly of leaves. They are known to consume both young and mature leaves from a variety of plant species, though they have a preference for younger leaves—especially from the teak tree. When dining on mature leaves, they will often eat only the stems, targeting these parts for their dense nutrients.
This is typical of their meticulous approach to eating any leaf: slowly and thoughtfully, they first bite off the tip, then carefully peel off each side, and finally eat the middle. Given that it takes them an average of 30 seconds to eat a single leaf, it’s no surprise that the langurs devote half of their waking hours just to consuming food.
Overall, their diet is highly varied—it also includes flowers, buds, seeds, bark, stems, mushrooms, and even insects, allowing them to adapt to the availability of food resources in their habitat. In one 9-month study period, these langurs were observed feeding on as many as 115 different plant species alone. Nilgiri langurs have even been caught raiding vegetables and other plants from human farms.
Additionally, Nilgiri langurs are known to consume soil, a behavior known as geophagy. It is believed to help neutralize both stomach acidity and toxins found in their leafy diet. This is important because Nilgiri langurs have a rather specialized digestive system with multiple stomach parts, including a fore-stomach with intense acids for breaking down the tough cellulose in the vegetation they eat. Such an adaptation allows them to extract maximum nutrients from fibrous foods.
Generally speaking, these monkeys are highly selective feeders. They will choose favored food items whenever possible. Therefore, their foraging behavior is influenced by the seasonal availability of food, with a preference for fruits and flowers during seasons when these are abundant.
Behavior and Lifestyle
Being diurnal, Nilgiri langurs are active during the daytime and through the night. They spend their days alternating between foraging and resting—a necessity since digesting leaves takes a lot of energy. As they rest, they will usually engage in social activities, such as grooming and play. Feeding sessions typically take place 2 to 3 times per day, in the morning and late afternoon, or morning, noon, and late afternoon to evening.
Like many arboreal (tree-dwelling) monkeys, Nilgiri langurs choose their sleeping spots carefully. For them, it works best to stick to the middle and lower levels of the canopy, where vegetation is dense and provides plenty of cover. They also prefer to sleep and rest in the middle of their home range, near a source of water.
Nilgiri langurs live in very organized social groups known as troops. These troops can take two forms. One is the harem, which is a group consisting of one adult male, several adult females, and their offspring. The other is the unimale group, all-male “bachelor” groups, usually consisting of young and subadult males. These unfortunates will have been kicked out of their natal (birth) groups upon reaching sexual maturity. They can also include older males who have lost dominance battles in their previous groups.
Troop sizes vary depending on the location of the population. In some areas, the average troop size may have fewer than 6 langurs. In others, like the Mundanthurai Plateau, average troop size can be as high as 18–19 individuals. Larger troops tend to have larger home ranges, in less densely populated areas. Smaller troops are typical of more densely populated regions, and their home ranges stay smaller too.
Within groups, there is a clear dominance hierarchy. The adult male is normally the dominant individual in a harem. Females also have their own hierarchy, usually based on age, with older females being more dominant. However, they do tend to have strong bonds, and dominant behaviors can ebb and flow. With an established hierarchy in place, aggression is rare and troop life is mostly peaceful.
Dominant females get access to preferred food and sleeping spots, while dominant males have freedom to socialize with everyone. They are also in charge of the group’s comings and goings, deciding where and when the monkeys go to forage and rest.
Among male groups, more competition can be expected. The strongest and most experienced male will usually win dominance over the group. Bachelors will continuously look for opportunities to take over a troop from the dominant male of a harem. Unimale troops may form alliances with one another (coalitions) to increase their chances of successfully taking over a harem.
Nilgiri langurs are highly territorial. Males will often climb to the treetops to scan for trespassers. When troops of any kind encounter each other, conflict erupts. This can be expressed through threatening vocalizations, physical displays, chasing, or even outright fighting. It is not uncommon to see males with battle scars, especially if they are the dominant leader.
If competing males are able to overthrow a harem’s dominant male, infanticide can occur. The new male will kill the offspring of the previous male. This is a reproductive strategy that works in his favor. Without babies to nurse, the females will stop producing milk. Their hormones will adapt, eventually sending them back into estrus, commonly known as “heat.” This is a natural cycle during which females are ready to mate again. Consequently, the new male will be able to mate with the females and produce his own offspring far sooner than if he had let the infants live.
Nilgiri langurs employ a multifaceted communication system to convey information, maintain group cohesion, and alert troop mates to potential threats. These can be categorized as 3 main types:
Vocalizations: Vocal communication is crucial for this species. They produce a variety of calls and vocalizations to maintain contact within the group, attract mates, resolve disputes, and warn of predators. Different alarm calls can signify the presence of different predators, allowing group members to respond appropriately. On the rare occasion that females do get feisty with one another, they will engage in screeching sessions that can drag on until the dominant male gives a specific call. One can only imagine it means “Be quiet already!”
Body Language: Physical expressions play a significant role in conveying the langurs’ emotional states, intentions, and social information. Submissive or aggressive postures and facial expressions help in establishing and maintaining social hierarchy. They can also be used to resolve conflicts. Additionally, Nilgiri langurs might engage in visual displays like shaking branches or showing body parts in attempts to intimidate.
Olfactory Signals: Although not as prominently studied as vocalizations and body language, scent marking, and other olfactory signals are likely to play a role in territorial marking, mate attraction, and conveying information about reproductive status, especially during the breeding season
Nilgiri langurs have a year-round breeding season, allowing them to give birth at any time of the year. The birth rate of Nilgiri langurs increases between May and June and again between September and November (albeit to a lesser degree). So far, scientists think this could be because food scarcity prevents females from ovulating (producing eggs) at certain times of the year.
Little is specifically known about their mating system or behaviors, but typically in langur species, mating rituals can involve displays, vocalizations, and grooming behaviors. There is some evidence that birth rates are higher in harems where there is only one male, indicating that competition between males results in lower reproductive success.
Currently, their generation length is measured at around 12 years, and sexual maturity is reached between 3 and 5 years. Their gestation period, or the length of their pregnancy, is not precisely known but is assumed to be around 200 days (6–7 months), similar to the closely related gray langurs. Females will give birth to a single, tiny infant, weighing only about 1.1 pounds (0.5 kg).
Nilgiri langur females probably aren’t going to win a “Mother of the Year” award anytime soon. Although they do attend to all of their baby’s needs in its first 10 days of life, observation has revealed that they are fairly disinterested mothers. They will wean their infants earlier than other langurs, after about one year. The mothers engage in relatively little grooming of their babies and have been seen ignoring their cries.
Despite this, they aren’t totally cold-hearted. Mothers will allow their young to cling to their bellies, and they’ll use their bodies to shelter infants from environmental elements like rain.
Alloparenting, or as you may call it, “babysitting,” can sometimes be observed in Nilgiri langur troops. It helps in reinforcing social bonds within the group and also provides some relief to the mothers. All babysitting is done by other females in the troop, who will perform all of the same care tasks as the biological mother—even nursing. Females who are not lactating will still allow troop offspring to suckle so that they can relieve stress.
The offspring will spend their time exclusively with the troop’s females until they have weaned. Males play no part in raising offspring. In fact, baby langurs may go their entire first year without ever really interacting with an adult male.
By eating a wide range of plant parts, Nilgiri langurs help in maintaining the balance of the ecosystem. The fact that they are the most diversely foraging primates in the area makes their impact all the greater. Their eating habits help in spreading and fertilizing seeds, as well as stimulating leaf production. Both are important for the growth of new plants and their ongoing health, contributing to the diversity and health of their habitats.
In order to reduce competition for food with other species, such as macaques and Indian giant squirrels, Nilgiri langurs adopt a strategy known as resource partitioning. Their broad diet has helped them adapt to relieve competition pressure for specific food sources, allowing them to coexist with species who occupy overlapping ecological niches (their place and role in the ecosystem).
Nevertheless, macaques tend to take on a dominant role when the species encounter each other, effectively bullying the langurs out of choice foods. This will most often happen when fruits and flowers become abundant after monsoon-driven growth.
Aside from humans, Nilgiri langurs have a variety of natural predators. These include dholes (Indian wild dogs), as well as pythons, jungle cats, raptors, and occasionally even civets. Leopards are especially dependent on the langurs as a food source: one study in the Periyar Sanctuary found that 81% of their diet consisted of these monkeys specifically.
The International Union for Conservation of Nature lists the Nilgiri langur as Vulnerable (IUCN, 2015), appearing on the IUCN Red List of Threatened Species. This is just one step below being an endangered species.
While the population of Nilgiri langurs was considered stable in 2015, there has been a notable decline in the number of mature individuals. It is estimated that there are fewer than 20,000 of the monkeys in total, with just under half of them being mature adults.
One threat to the monkeys is human hunting: these beautiful primates are hunted for their fur, skin, and other body parts, both to use as ingredients in traditional medicines and to make crafts such as jewelry or musical instruments. Some locals will also hunt the langurs for food, and may even keep or sell them as pets.
Parts of the Nilgiri langur’s habitat are under protection. One example is the Nilgiri Biosphere Reserve, but it only comprises a patchwork of territories throughout the Nilgiri hills. Not all are connected. This results in habitat fragmentation, a term that describes how parts of a habitat can become isolated from one another.
Without protected connecting territories, the Nilgiri langurs living in the habitats of the Nilgiri Biosphere Reserve and other similarly affected areas are forced to split into separate populations. This is a trend throughout their entire habitat range, making the entire species population severely fragmented. No single population contains more than 1,000 members because of this.
Fragmentation is a very pressing threat, given that human development has steadily increased deforestation in and around the langurs’ habitats. Entire sholas—patches of stunted tropical montane forest found in valleys amid rolling grassland in the higher montane regions—are being cleared to make room for plantations that grow tea, coffee, and other crops.
A domino effect results, where even more habitat degradation takes effect. As farmers fertilize these crops, the soil becomes more porous, making it easier to wash away during heavy rains. Without the deep roots of forest trees to hold the soil in place, massive amounts can be dislodged. This creates prime conditions for landslides, which destroy more habitat and pose a threat to the lives of all of the animals living in the area.
Shola destruction has also resulted in the disappearance of perennial (permanent) streams, which are an important source of water for the Nilgiri langurs. Other human construction activities are forever altering parts of their habitat, often making parts inaccessible. There has been widespread construction of roads throughout the region, and the creation of the Kabini Reservoir has entirely submerged the valley that lies to the north of Bandipur National Park.
The Nilgiri 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.
Nilgiri langurs are also protected by the Indian Wildlife (Protection) Act of 1972. After this became law, their population began to experience growth, up until the early 1990s.
About 50% of the species’ habitat is under protection. More conservation projects have progressed that will have an impact on preservation and expansion. Around the Silent Valley National Park—a hot spot for Nilgiri langurs—the Indian government has increased surrounding “buffer zones,” and even converted them to protected wildlife refuges with limitations on human activity.
Scientists stress that top priorities for conservation are further research into taxonomy, life history, and ecology of the species, as well as public education that dispels common myths about the langur’s usefulness for traditional medicine. Habitat management, poaching control, and prevention of deforestation are also crucial for protecting this irreplaceable species.
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Written by Amanda Riley, October 2023