Leaf-Eating Monkeys: The Leaf Monkeys of Sumatra

Black-crested Sumatran langur

When the system of scientific naming that we now use was first devised, all monkeys were placed in a single genus, Simia. The initial listing of names, from 1758, does not include any species we would now recognise as leaf monkeys but, as these were "discovered" over the next few decades, most were placed in Simia, although a few were placed in newly-created genera that contained other, more typically fruit-eating, monkeys. The first naturalist to create a genus specifically for what we would now describe as "leaf monkeys" was Johann von Eschscholtz, a naturalist who had served on a Russian expedition that circumnavigated the globe. His write-ups of that expedition included the first scientific description of the plantlife of California, but they also included the description of a new kind of monkey, which he felt was distinct enough to give its own genus: Presbytis.

That was in 1821, and, while various other naming schemes were suggested through the 19th century, for much of the 20th, all langurs were placed in Eschscholtz's genus as a group distinct from the colobus and other leaf monkeys. During the 1980s, however, the grey and golden langurs, and their various close relatives, were split off into the genera they now occupy, leaving relatively few species still with their older designation.

According to the rules of such things, one of these species was, of course, the one that Eschscholtz had described when he created the genus in the first place, the Sumatran langur. So far as he knew at the time, the monkey he had discovered lived only on the island of Sumatra, west and south of the Malay Peninsula. It's a huge island, larger than Washington state in the US, and covered with jungles and forested mountains that make it easy to hide all manner of animals. It was later discovered that the monkey also seemed to live on the mainland, but in 2011, genetic evidence demonstrated that what had previously been thought to be a fairly widespread species was, in reality, multiple similar species living close by one another. Eschscholtz's original specimen had been collected from what is now Bengkulu Province of Indonesia, so it's the species that lives there which got to keep his original 1821 scientific name.

We now call this the black-crested Sumatran langur (Presbytis melalophos). It lives in the western half of southern Sumatra where it inhabits both lowland and hilly forests. Even within the one species that remains with the name, there is a surprising amount of variation in colour, with some individuals being reddish, others tan-yellow or near-white, and some with having black hands and feet rather than the usual red-brown. The one thing that doesn't tend to change is the pointed crest of black hair on their head that gives them their name. The young are born greyish before changing to the adult colour, and while the different colour morphs are not found randomly mixed up with one another, there does not seem to be any good evidence that they represent different subspecies - although the variation does make identifying any subspecies that might exist difficult at best. The comparatively bright colours, compared with many other langurs, may serve to confuse predators as groups of the monkeys flee through the treetops, often making as much noise as they can.

Not much is known about the species, which has never been studied outside of a few protected reserves. One study showed that their preferred food, at least in the one geographic area where they were watched, was the leaves of the terap tree, a close relative of breadfruit. They also like eating the leaves of fig trees and rubber plants and seem able to survive on plantations of the latter as well as in more untouched woodland. They live in small groups of more no more than eight individuals, with the usual leaf-monkey pattern of a single adult male and multiple females.

Robinson's banded langur

One of the main reasons that they have not been studied more extensively is that they were previously thought to live elsewhere, so most of the studies in the literature refer to what are now regarded as different species. Among these are what's now called Robinson's banded langur (Presbytis robinsoni) which lives in the Malay Peninsula, from northwestern Malaysia, through Myanmar to southwestern Thailand.

They are usually dark coloured with patches of white on the belly and the inside of the limbs, and typically range from grey-brown to almost black, although a few very pale grey individuals have been seen. They spend most of their time in tall trees, typically in marshy areas such as mangrove swamps, although they are capable of surviving elsewhere and are one of the few species in the group not considered to be imminently threatened - although the advance of agriculture in their forests does not place them entirely in the clear. Like other leaf monkeys, they spend a lot of their time resting, in order to help digest their low-quality diet.

Here, too, however, there can be confusion in the literature as to which species we're talking about. That's because Robinson's banded langur was only raised to full species status in 2020 when genetic analysis split the former banded langur species into three. The other two are Raffles' banded langur (Presbytis femoralis) which lives in the extreme south of peninsular Malaysia and in the few parts of Singapore that aren't covered by the city, and the East Sumatran banded langur (Presbytis percura) which lives just across the Malacca Strait from Raffles' species in an equally small area of eastern Sumatra. (To confuse issues even further, it was proposed in 2018 that Raffles' species be renamed P. neglectus... but this idea seems not to have withstood further scrutiny).

Raffles' banded langur

The former looks most like Robinson's species (although, in fact, it turns out to be more closely related to the black-crested one) but has only a single small patch of white on the belly, and darker grey on the  areas that would otherwise be white. Possibly because it has little choice in the matter, it tends to live in much drier lowland forests than its northern cousin. The East Sumatran species, on the other hand, is paler than either of the other banded langurs, although it's still more of a dark grey than anything else.

Both of these species have a very small population. A study published just three months ago estimated that the population of Raffles' species in Singapore consists of only around 70 adults, which tend to feed on cultivated plants (likely because there isn't much else available that's sufficiently nutritious) and there are likely no more than a few hundred living further north. On the bright side, at least some of them live in nature reserves but the population is so desperately low that they are considered critically endangered and they are becoming increasingly inbred, especially in Singapore. The East Sumatran species is so little studied that we don't really know how many of them survive, but since their home happens to be experiencing the highest rate of deforestation of anywhere in Sumatra, the future certainly doesn't look good, giving them the same conservation status as their mainland relatives.

While it now lives only in scattered and fragmented forests, the original northern boundary of the East Sumatran banded langur's range was the Rokan River. On the far side of this we come to the black Sumatran langur (Presbytis sumatrana), another animal formerly considered part of the island-wide "Sumatran" species. This has, as you might expect, mostly black, or (in some cases) dark brown, but the colour is sharply offset by white fur on the underparts, the throat, and the insides of all four limbs down as far as the wrists and ankles.

In the other direction, in a southeastern region of the island, we come instead to the black-and-white langur (Presbytis bicolor) which looks remarkably similar to the black Sumatran species but with a more prominent black stripe down the middle of the crest of white hair on its head. Despite this similarity of appearance and of the calls it makes, genetic analysis shows that it is more closely related to the black-crested Sumatran langur. Indeed, it was only recognised as a separate subspecies in 1992, let alone as a full species. Partly as a result of this, we don't really know across how large an area it lives, and therefore whether or not it would count as endangered - certainly parts of its home are at risk, but it may be safer elsewhere.

The southernmost part of Sumatra is home to the remaining Sumatran species to be definitively split from the black-crested species, the mitred langur (Presbytis mitrata). It has a similar coat pattern to the black and black-and-white langurs, but the "dark" parts are paler, varying from a medium grey to a pale tan. It apparently spends more of its time in the forest understory than its relatives, rather than up in the tops of the trees. It seems comparatively tolerant of disruption to its habitat and can survive well in rubber plantations, but outside of a couple of major national parks, the forests are still being cleared for agriculture. Combined with its popularity in the illegal pet trade, this leaves it as another species that is thought to have a declining population, albeit currently high enough to avoid being officially classified as "endangered".

Another Sumatran species has an even more convoluted naming history. In the 1940s, it was identified as a subspecies of banded langur, then got moved to the black-crowned species in the 1990s even after the banded langurs were split off again. It has now been promoted back to its 19th century status as a full species, and is known as the white-thighed surili (Presbytis siamensis). "Surili" is a local name that essentially means the same thing as "langur" (I'm unclear as to whether it's Malay or Indonesian; it could be both, since they are closely related languages) and is sometimes used to refer to the genus as a whole when it needs to be distinguished from the other types of langur.

The other part of the name refers to the pale fur on the outer surface of the thighs. It otherwise looks rather like the various banded species, but with pale brownish-grey fur over much of the body and jet black hands and feet. It lives in four different areas, two of them in Sumatra between the ranges of the East Sumatran banded langur and the black-and-white langur, with a third occupying most of Peninsular Malaysia north of the home of Raffles' species. The fourth population is even more isolated, living on the Riau Islands south of Singapore. Each population is currently thought to represent a distinct subspecies, and they probably have not been in contact with each other for centuries at the least.

White-thighed surili

The relatively little that is known about it suggests it is similar in most respects to its relatives, although it has been reported to live in slightly larger groups, with up to 18 individuals in total. It may, or may not, include the Natuna Island surili (Presbytis natunae) as a subspecies. This lives only on the named island, which is relatively remote and undeveloped so that, despite its small size compared with the likes of Sumatra (it's about half the size of Rhode Island or similar to the Isle of Skye in the UK) the monkey is currently considered threatened but not fully endangered, with around 10,000 individuals currently alive.

Many of these species are born with grey coats marked with a black cross over the shoulders and back, perhaps to aid easy identification by the mother. In the Sarawak surili (Presbytis chrysomelas), however, this sometimes remains until adulthood - although the majority of adults have an appearance closer to that of banded langurs. While not much is known about it, the one thing that is definitely surprising is that it lives not on Sumatra, but Borneo. Here it clings on to life in a few tiny patches of untouched forest in the Sarawak region of Malaysia, and possibly across the borders into Indonesia and Brunei. Even then, few of the records of its presence are modern, so that some of the reported populations may already have vanished. It is estimated that fewer than 500 adults remain alive, and since they are scattered over a wide area, the potential breeding stock for any individual is much smaller than that.

It's probably one of the world's rarest primates.

But it is not the only langur on Borneo, an even larger and more rugged island than Sumatra. To conclude the part of my survey of leaf monkeys that includes the langurs, I will have to head there next, visiting a few other islands along the way...

[Photos by Luke Makin, "Tontan Travel", Andie Ang, and Paul Prior. Cladogram adapted from Ang et al. 2020.]