Geographic Distribution and Habitat
The Nilgiri langur (Semnopithecus johnii), also known as the black leaf monkey, hooded leaf monkey, Indian hooded leaf monkey, John’s langur, Nilgiri black langur, or Nilgiri leaf monkey, is endemic to India. Specifically, Nilgiri langurs occur in a patchy distribution across the Western Ghats mountains in southwestern India. They live at elevations between 980 and 6,500 feet (300–2,000 m) and occupy a variety of forest types, including evergreen, semievergreen, moist deciduous, mountain, and riparian. A favorite habitat type of theirs is sholas—narrow stretches of forest nestled within high-elevation valleys and surrounded by grasslands. These unique habitats are found only in South India. The langurs tend to prefer locations that are close to water and far from humans.
TAXONOMY IN TRANSITION
The taxonomy of Nilgiri langurs has been debated in recent years. Nilgiri langurs were originally sorted into the genus Trachypithecus with dusky langurs. More current research suggests that they belong in the genus Semnopithecus, with the gray langurs.
Size, Weight, and Lifespan
Adult females have a head and body length of 23–24 inches (58–60 cm) and adult males measure 30–31 inches (78–80 cm). Tails add between 27 and 39 inches (68–98 cm). Females weigh between 24 and 26 lbs (10.9–12 kg) on average, and males weigh 9.1–33 lbs (9.1–14.8 kg). They can live up to 29 years in captivity, and it is unknown what their average lifespan is in the wild.
Nilgiri langurs have long, glossy black hair over most of their body. Their head is framed by a crown of tan fur that resembles hair on a human’s head. They have round brown eyes and large canine teeth. Their long tails are essential to maintaining their balance, and their long legs are limber and agile. The only form of sexual dimorphism they exhibit is the females’ white patch on their thighs.
What Does It Mean?
An individual other than the biological parent of an offspring that performs the functions of a parent (as by temporarily caring for an infant).
Physically adapted to living primarily or exclusively in trees.
Active during daylight hours.
An animal who primarily eats leaves.
The process by which a large, continuous stretch of habitat gets divided into smaller, disjointed patches of habitat.
The killing of young offspring by a mature animal of the same species
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Nilgiri langurs are primarily folivores, although they supplement their leafy diet with flowers, buds, seeds, bark, stems, insects, and even soil, which, it’s believed, they consume to balance the pH of their stomach like an antacid. Their propensity for eating leaves has given them the alternate common name of leaf monkeys. Their favorite leaves are those from teak trees. They spend about half their waking hours eating, and they are very meticulous about the way they eat. When eating a leaf, they first bite off the tip, then they carefully peel off the sides of the leaf, and finally they eat the middle. It takes them about 30 seconds to eat a single leaf. They have also been known to forage from agricultural fields for cabbage, potatoes, cauliflower, and ornamental poppies.
Behavior and Lifestyle
Nilgiri langurs are arboreal and diurnal. During the day, they alternate between foraging and resting, and often travel in the late afternoon when they begin to head to their sleeping sites. Before going to sleep, they engage in social activities like grooming, play, and bonding with infants. They tend to sleep in the middle or lower section of the canopy. Their home range is about 2.5 acres (1 hectare) in area on average, although this fluctuates seasonally with the availability of food. Their sleeping and resting sites tend to be in the center of their home range and close to water.
It’s believed that Nilgiri langurs may hybridize with another species in their genus, the tufted gray langur (S. priam).
Daily Life and Group Dynamics
Nilgiri langurs live in unimale or multimale groups with nine or ten group members on average, although groups with up to 27 members have been observed. Unimale groups are more common and they are usually smaller, as a single male cannot protect and exert control over larger groups of females.
Nilgiri langurs have subtle dominance hierarchies within their groups, one for males (if there are multiple males in the group) and one for females. These roles are not rigid, and are defined more by the number of times that an individual behaves dominantly or submissively. For example, the highest-ranked female will act dominantly over other females the majority of the time, but she still may sometimes act submissively. Conversely, a lower-ranked female will usually behave subordinately, but she may still act dominantly on occasion. The “perks” of being the alpha female include things such as preferred feeding and sleeping sites. For males, the stakes are higher. The alpha male controls the travel for the entire group and he is able to eat and socialize with any group member that he likes.
Nilgiri langurs bond and communicate with their groupmates through a wide variety of behaviors, such as grooming, playing, chasing, fighting, mounting, and aggression. While dominance hierarchies exist, dominance behaviors are not performed frequently, and aggression among groupmates is rare. Group life tends to be peaceful.
Intergroup conflict often occurs when two groups come into contact with each other. Nilgiri langurs are territorial, and males defend their home areas through vocalizations, physical displays, and chasing other groups away. This may escalate to direct fighting, resulting in adult males often having scars on their bodies. In extreme cases, it’s been known for males to take over another group and perform infanticide of the other males’ offspring. These extreme conflicts tend to occur in areas with multiple groups in close proximity, which are not common as Nilgiri langurs tend to avoid other groups before conflict occurs.
Nilgiri langur’s main form of communication is vocalization—and they have many in their repertoire. They are used to maintain the social hierarchy, during territory disputes, during conflicts with groupmates, and to warn of predators. Their alarm calls are high-pitched barks, which can either be short and quick, for minor threats, or continuous for more severe threats. Other forms of communication include: tactile, such as biting, licking, and stroking; and facial expressions and body posture, such as submissive head shaking, an outstretched hand from a mother to an infant, or a threatening open-mouth gesture. Nilgiri langurs have also been known to use olfactory communication.
Reproduction and Family
The birth rate of Nilgiri langurs increases at two points in the year: between May and June and, to a lesser degree, between September and November. This may be a result of seasonal food scarcity, which may affect ovulation. It is believed that Nilgiri langurs breed year-round, but only conceive offspring when there are enough resources available to support a pregnancy. There is evidence that higher birth rates occur in groups with only one male, suggesting that male-male competition results in overall lower reproduction. Their gestation time is likely about 200 days or 6 to 7 months, after which they give birth to a single offspring weighing, on average, 1.1 lbs (0.5 kg).
For the first 10 days of their life, infants are cared for solely by their mothers, after which she permits other females in the group to alloparent while she forages. One female can “babysit” up to three unrelated infants. Not only do adult males not play a significant role in child rearing, young Nilgiri langurs will only rarely come in direct contact with adult males for the first year of their life. Interestingly, Nilgiri langur mothers are known for being somewhat inattentive to their youngs’ cries, ignoring their distress calls for no discernible reason. One theory is that the relatively low risk of predation makes an infant’s distress call less of a threat. In all other ways, however, female Nilgiri langurs are nurturing mothers. They carry their infants on their bellies even when jumping and have been seen sheltering their babies from the rain. The infants are nursed for about a year, after which they are weaned and are largely independent. They are believed to reach sexual maturity between 3 and 5 years of age.
Nilgiri langurs have relatively few predators, one of which is the Indian wild dog. They are also hunted by humans. Nilgiri langurs feed on the largest number of plant species of all the primates of the Western Ghats, their home mountain range. This makes them vital to the ecosystem as they help spread the seeds of the 115 plant species they consume.
Conservation Status and Threats
The International Union for Conservation of Nature lists the Nilgiri langurs as Vulnerable (IUCN, 2015), appearing in the IUCN’s Red List of Threatened Species due to their relatively low population of under 10,000 mature individuals. No populations contain more than 1,000 individuals. They exist in severely fragmented populations, and their habitat quality and area is in decline. The overall Nilgiri langur population increased from the early 1970s through the early 1990s, at which point the population remained stable. However, current research suggests that the population is again in decline.
Historically, Nilgiri langurs were hunted for use in traditional medicine, although this practice has declined in recent years due to community participation in conservation and better protection. Current threats include habitat loss due to agriculture, mining, dams, and human settlements, as well as habitat fragmentation, hunting, road kills, deliberate fires, flooding, and landslides. Human disturbance, which is increasing as humans encroach on Nilgiri langur territory, correlates positively with the rate of infanticide among Nilgiri langurs. Without humans present, females are able to peacefully leave their group if and when they choose to. When humans encroach, the ecosystem becomes stressed to the point that the holding capacity of Nilgiri langurs is lowered—the ecosystem simply can’t support the number of Nilgiri langurs present. When that happens, groups are much more susceptible to takeover from new males, who sometimes perform infanticide when taking over a group.
Nilgiri langurs are protected by the Indian Wildlife Protection Act. Before its passing in 1972, it was not uncommon to find medicines made from Nilgiri langur body parts advertised openly. After the passing of the act, Nilgiri langur populations increased until the early 1990s. They are also listed on Appendix II of the Convention on International Trade in Endangered Species (CITES). About 50% of their range is protected by a number of conservation areas, including Aaralam Sanctuary, Mukurthi National Park, and Periyar Sanctuary. Top priority areas for conservation are further research into taxonomy, life history, and ecology of the species, public education, habitat management, poaching control measures, and the prevention of conversion of forest areas to private land.
- Joseph, G. and K. Ramachandran. 2003. Distribution and Demography of the Nilgiri Langur (Trachypithecus johnii) in Silent Valley National Park and Adjacent Areas, Kerala, India. Primate Conservation 19:78-82.
- Kavana, T., J. Erinjery, M. Singh. 2014. Male Takeover and Infanticide in Nilgiri Langurs Semnopithecus johnii in the Western Ghats, India. Folia Primatologica 85(3).
- Nag, K. S. 2020. A new report on mixed species association between Nilgiri Langurs Semnopithecus johnii and Tufted Grey Langurs S. priam (Primates: Cercopithecidae) in the Nilgiri Biosphere Reserve, Western Ghats, India. Journal of Threatened Taxa 12(2):12975-84.
Written by K. Clare Quinlan, August 2021