Leaf-Eating Monkeys: Sacred Langurs of India

Sacred langur

In Hinduism, the monkey god Hanuman is a companion of Rama, one of the more popular incarnations of Vishnu, and represents, among other things, loyalty and virtue. The specific type of monkey most associated with him is known by various names, including "Hanuman langur" and the rather uninspired "northern plains grey langur", but I'll stick with sacred langur (Semnopithecus entellus).

As currently defined, the sacred langur is found across almost the whole of northern India where suitable habitat exists. The biggest limitation to that habitat is elevation; sacred langurs are very much a lowland species, not found above about 400 metres (1,300 feet) - which cuts out rather a lot of the more northerly parts of the country as it reaches towards the Himalayas. Other than that, they require forest, but they seem adaptable to different types. 

In practice, they mostly live in dry tropical deciduous woodland because that's largely what there is in that part of the world, but they extend into thorny scrubland in the northwest near the deserts of Rajasthan and into damper forests in the east. They are also common, probably due in part to their reputation as a sacred animal, on the outskirts of densely settled urban areas. A population is also found in southwestern Bangladesh, but they aren't thought to be native there and were likely brought across by Hindu pilgrims in the late 19th century.

For much of the 20th century, all langurs in and around India were considered to belong to the species. While this had long been suspected, in the early 1980s M.L. Roonwal of the Zoological Survey of India noted a key distinction between what were then thought to be, at best, different subspecies of the same animal. Those in the southern part of their range held their tail behind their bodies while walking, much as cats or most dogs would, but those in the north held their tail upwards and curving forward, looping over their backs, rather like a long-tailed Pomeranian.

Shorn of the other species, the sacred langur proper belongs to this northern group, with the tail looping right round so that the white-coloured tip dangles level with their back. They have black faces, hands, and feet, but the longer fur elsewhere is greyish tinged with yellow or brown, and they have a neat natural parting to the hair on top of their heads. Like many leaf monkeys, the males are much larger than the females, weighing from 17 to 20 kg (37 to 44 lbs) as opposed to just 10 to 16 kg (22 to 35 lbs). 

While they primarily eat leaves, they also eat a significant amount of seeds and other plant parts and probably also insects during the monsoon season. In many areas, they will raid cropland, somewhat protected from retaliation by their sacred status, and many temples actively feed them, giving them little fear of humans. They are active during the day, spending around a third of their time on the ground even when trees are readily available.

It may be partly because of this terrestrial travel, which exposes them to predators such as leopards and tigers, that sacred langurs travel in such large groups. Most have between 20 and 80 members, and, where food is plentiful, there may be over 100. Females stay within the troop of their birth, but the males soon leave, often living in smaller (15 or so) single-sex groups until they find a new troop to join. 

Despite their relatively large size, most mixed-sex troops contain only a single adult male, mating perhaps with over 15 females. The takeover of a troop by a new male is a violent affair, consisting of an extended battle with the existing troop leader that may last for over a day; if successful, the newcomer is likely to kill at least some of his predecessors' children to ensure that their mothers mate again. On average, he can expect to keep the position for two-and-a-half years before being ousted and likely dying in the process. (One 2008 study evaluated male langurs on a modified version of the personality tests used by psychologists on humans, showing not only that this gave similar results, but that dominant males were, perhaps unsurprisingly, more extroverted than others).

This, however, is not universal, and some of the larger troops do, in fact, have multiple adult males, helping to keep a lookout for predators while the females feed, care for the young, and groom each other. While the females in such troops do have their own internal hierarchy, it is looser and less stable than those of single-male groups, and the females regularly mate with different males, even when they aren't fertile - presumably to sow doubt among the males as to which children are likely to be theirs. Even so, there is an "alpha" male who sires over half the children.

Terai grey langur

Sacred langurs have been considered a lowland, plains-dwelling species since 2001, when those living further north were split off into their own distinct species. As we move north out of the Indo-Gangetic Plain, the first such species we come to is the Terai grey langur (Semnopithecus hector) inhabiting hills between about 150 and 1,600 metres (500 and 5,250 feet) elevation. The foothills at this height form a band sweeping across the whole of southern Nepal, and stretching from at least Uttarakhand in the west to Bhutan in the east. 

The woodlands here are dominated by oak, cedar, and sal, and include, at their southern margins, the marshy grasslands and open savannah of the "Terai", for which the species is named. The monkeys look similar to the regular sacred langurs (and are, therefore, considered equally sacred) but with slightly paler fur and significantly paler hands and feet. The tip of the tail arcs up, rather than down, giving the overall shape less of a complete loop. 

Perhaps because the land is less fertile, troop sizes are smaller, with around 20 members being typical, but are more likely to include multiple males - usually two or three, but it can be more. The all-male bands seen among sacred langurs don't seem to be long-lasting in the Terai species, and bachelors spend much of their time alone. When they do find a troop to join, they tend to replace just one of the existing males at a time, rather than launching a full-blown coup; perhaps because of this, they are less likely to be infanticidal, but this does happen.

Nepal grey langur

Further north, and further uphill, we come to the Nepal grey langur (Semnopithecus schistaceus) found from Pakistani Kashmir to Bhutan, including much of northern and central Nepal and just across the border into China/Tibet. These are higher foothills of the Himalayas, with the monkeys found between 1,500 and 4,000 metres (5,000 to 13,000 feet) where pine trees become more common alongside the cedar and sal, and there are frequent cliffs and rocky outcrops. Over the course of a year, a given troop will occupy an enormous range of up to 12,500 hectares (50 square miles) compared with a maximum of 90 hectares (0.35 square miles) for the lowland species; this is driven largely by the need to move up and down slopes as the weather changes with the seasons.

Troop sizes among this species are even smaller than those in the belt to the south, with a dozen or so being typical, including up to four males, although the most dominant individual gets most of the mating opportunities. Infanticide apparently hasn't been observed among them at all, which is quite different from the sacred species. On the other hand, they do have comparatively long-lasting all-male groups. 

A study published last year confirmed that the Nepal grey langur is, indeed, a distinct species from the sacred langur of the lowland plains but could find no evidence that the other two putative northern species merited even subspecific status. Until this is confirmed, however, the Tarai grey langur remains listed in most sources, as does the endangered Kashmir grey langur (Semnopithecus ajax). 

This is known with certainty only from the Chamba section of the Ravi river valley in Himachal Pradesh.  As its common name implies, there are also some reports from nearby Pakistani Kashmir; these are probably the same animal, but could potentially be members of the Nepal species, or even hybrids. Despite living at elevations of 2,200 to 4,000 metres (7,200 to 13,000 feet) it looks very similar to the sacred langur, with the same looping tail. Its total population has been estimated at less than 1,500 - and this is including those in Kashmir - and is thought to be declining.

Black-footed langur

The southern forms of the sacred langur were split off as a separate species in the late 20th century and, after some confusion about their exact status and inter-relationships, were again divided into two during the 21st century resurrecting species first named in the 19th century. The most widespread of these is the black-footed langur (Semnopithecus hypoleucos) which lives across much of central and southwestern India from just north of Mumbai to northern Kerala.

This has a much browner colour than its relatives to the north and has the "southern" tail posture, stretching out behind it, arcing upwards and then dangling towards the tip. It inhabits a range of moist subtropical forests, including lowland plains in the north and towards the coast, and up into the hills of the interior, reaching as high as 1,200 metres (4,000 feet) in the mountainous terrain of the Western Ghats. They eat leaves and, to a lesser extent, seeds and fruit, from a wide range of plants, although the most common seem to be fig trees, emblic, and Malabar plum. 

Despite the relative richness of the habitat, black-footed langurs live in slightly smaller troops than their northern cousins, with no more than about 40 members. These can be either male-only or consisting of multiple females with a single male. Conflict tends to result whenever the two different types meet up, with the dominant male of the mixed sex troop threatening the intruders by whooping, jumping, and making mock biting motions. This often works but sometimes result in violent counterattacks that, if successful, may temporarily leave the troop with multiple males until they sort things out amongst themselves.

Tufted grey langur

The other southern species is the tufted grey langur (Semnopithecus priam), which lives in the much drier deciduous forests east of the black-footed langur, primarily in Andhra Pradesh and Tamil Nadu. It has been barely studied there, but is also found in the eastern, drier reaches of Sri Lanka where it has received more attention. It too, has the "southern" tail posture, although the tip is said to dangle less than it does in its western relative. It's the smallest of the species once assigned to the wider sacred langur group, although not by very much, and there is considerable overlap.

This is, however, another species with reportedly large troop sizes. Although 20 seems typical in some areas, in others troops up to three times that size have been reported and there is a corresponding tendency for troops to have multiple adult males. Like other langurs, they primarily eat leaves, but they have also been reported to rely heavily on fruit, such as jackfruit, at certain times of the year when these are abundant and fresh leaves less so.

While this completes the list of the species formerly grouped together as the "sacred langur", two other closely related species live in the extreme south, both of which are different enough in appearance to have long been considered distinct. Indeed, they were long placed in a separate genus, until DNA evidence made their true relationships clear.

Nilgiri langur

The Nilgiri langur (Semnopithecus johnii) lives along the southern edge of the Western Ghats, primarily in Kerala state, where it inhabits moist evergreen forests and, at times, teak plantations. It is black in colour over most of the body, but with a shock of hair around the head that can vary from medium grey to vivid orange. The tail is held out to the rear, but without taking on the arcing shapes seen in the grey-coloured species. 

Nilgiri langurs live in groups that rarely exceed nine members, each with a single male. Unlike the sacred langurs, females do sometimes leave larger groups, presumably to avoid competition with their sisters, and bachelor males are usually solitary although very occasional all-male bands have been reported. They seem, if anything, to be even more dependent on leaves than the sacred species; apparently, this results in them spending more of their time digesting tough foodstuffs and less on socialising than their kin.

Finally, we come to the purple-faced langur (Semnopithecus vetulus) which is entirely absent from India, inhabiting only the rainforests of western Sri Lanka where it ranges from lowland plains to montane forests as high as 2,200 metres (7,200 feet). It's not really accurate to say that its face is purple, although there is a vaguely purplish sheen to it, offset by dramatic white facial whiskers and swept-back pale brown hair on the top of the head. The rest of the body and tail are broadly dark grey, although there are paler patches on various areas, which vary with the subspecies and even with the individual.

Purple-faced langur

Although preferring fresh leaves when they can get them, their diet seems to be flexible, changing with the seasons, the availability of fruit, and (in the central parts of the island) the presence of the larger tufted grey langurs, which can displace them from their preferred trees. Relations between the two species where they overlap are not always so hostile, however, with some troops being reported that consisted of a male tufted grey living with several female purple-faced langurs. Some of the infants in these groups appear to be hybrids although the fact that they aren't more common suggests that they are sterile or at least of limited fertility. The same phenomenon has been reported, if less frequently, among Nilgiri langurs on the mainland.

The population of purple-faced langurs is unknown, but there is strong evidence that it is rapidly declining, not least because the amount of woodland in the areas where it is found have halved in the last sixty years. As a result, and considering that, not being a sacred animal, it is occasionally hunted, it is officially listed as an endangered species and there seems little chance of this being reversed any time soon.

The term "langur", however, is not applied solely to these species, but to a number of other, less closely related, leaf-eating monkeys across southern Asia. I will be turning to some of these others next time...

[Photos by Tisha Mukherjee, Ross Huggett, Bernard Gagnon, Sankara Subramanian, Steve Garvie, "Ashwinsakaran", and Dan Lundberg. Cladogram adapted from Springer et al. 2012.]