Leaf-Eating Monkeys: Treetops and Limestone Cliffs

Dusky langur

Although the term "langur" is perhaps most associated with the monkeys of India, the term has been used more widely to refer to a range of similar monkeys found to the east. In this sense, the term is really one of convenience, reflecting both the resemblance and the relatively close evolutionary relationship between the various monkeys in the group. However, the word "langur" itself comes from Hindi and Urdu which are, of course, not native languages of the lands to the east, so there has been a move in recent times to use more local names for some of the species. I'm going to stick with the names that seem to be most common in English language sources, but there is considerable variation and not always much consistency.

One of the better studied of these species is the dusky langur (Trachypithecus obscurus). These live in the Malay Peninsula, from the southernmost parts of Myanmar in the north, through southern Thailand, and across essentially the whole of Peninsular Malaysia. Over the last few years, a small number have also been spotted in residential areas of Singapore, presumably having swum across the Johor Strait from the mainland, but they are not native there. Although definitive genetic evidence is lacking, seven different subspecies are currently recognised, three of which occupy different regions along the peninsula, with the other four found only on certain small islands off the coast. 

As the number of possible subspecies indicates, there is some variation in the appearance of dusky langurs across their range. In general, however, they have dark grey to black fur across most of their bodies, with paler undersides. Perhaps their most obvious features, however, are the circular patches of bright white skin around their eyes, giving the alternative name of "spectacled langur"; the muzzle is also much paler than the rest of the face. The young are born bright orange in stark contrast to their parents.

They prefer to inhabit dense forests, which can reach anywhere from sea level up to 1,800 metres (6,000 feet) although they can at times enter managed plantations and, as those individuals in Singapore indicate, gardens. In some areas, they have even been reported to make nuisances of themselves by entering people's houses and rifling through their rubbish for discarded food. About 50% of their diet consists of leaves, although they supplement this with plenty of fruit when it is available. Although they will move about throughout the trees, they prefer to spend most of their time in the highest branches, well clear of the ground.

They live in groups of ten to twenty individuals, typically including just one to three adult males - the surplus males are either struggling to survive on their own in the hopes of finding an available group, or, at best, sharing their time with one or two other young males. The groups seem to be fairly egalitarian and peaceful, by the standards of monkeys. When aggression does erupt, it is often followed by often immediate offers of reconciliation, in the form of hugging - where such an offer is rejected, the victim often seeks and receives hugs from other members of the group, in a relatively rare example of "consolation" in non-human primates.

Phayre's leaf monkey

Phayre's leaf monkey (Trachypithecus phayrei) lives further north, being found in most parts of Myanmar as well as eastern parts of India and Bangladesh and probably just across the border with China. They are not found at especially high altitude, with the result that the Arakan Mountains of western Myanmar divide the monkeys into two groups that meet up near the Irrawaddy delta to the south. Usually regarded as subspecies, it has recently been proposed that these two populations are actually distinct species.

Phayre's leaf monkey looks very similar to the dusky langur, but the coat has less extreme contrasts in colouration with, for example, the fur on the head not being quite as dark. The young are pale yellow rather than the brighter orange of their southern kin. 

Further east, we come to the Indochinese grey langur (Trachypithecus crepusculus), which inhabits northern Thailand, Laos, and Vietnam as well as a few neighbouring regions. It was long considered to be a third subspecies of Phayre's leaf monkey, but was raised to species level in 2009 after the discovery of some genetic anomalies. Further investigation of those anomalies confirmed the earlier suspicions that Indochinese grey langurs originated as a hybrid over 30,000 years ago, with males of a different species mating with the ancestors of the current animals to produce the form we have today.

As the name suggests, Indochinese grey langurs are predominantly grey in colour, becoming silvery in paler parts of their body, although there are some patches of brown around the face. However, they are easily distinguished from the grey langurs of India by the white facial markings and the lack of a clear crest, which emphasise their relationship (at least on their many-greats-grandfather's side) to the dusky species. 

Indochinese grey langur

The result of this is that, because most studies on "Phayre's leaf monkey" were conducted on populations in Thailand, not Myanmar, most of them refer to what we'd now regard as a different species, barring a few dietary studies conducted on the isolated populations in India and Bangladesh. In fairness, there probably isn't a great difference in much of their behaviour, which is why they were grouped together for so long. Both species live in groups that run from around 12 to 24 individuals, with one to three adult males and five to ten adult females, although these can be even larger where food supplies are plentiful.

Females leave the group of their birth on reaching adulthood, gathering together with unrelated females to form a new one, establishing a clear dominance hierarchy amongst themselves when they do so. This hierarchy seems to favour younger (and possibly fitter) females, a reversal of the normal situation among female relatives where you would expect mothers to be dominant over their own children. Compared with more distantly related species with similar behaviour, the females are unusually friendly with their unrelated peers (and much less so with their sisters), and the males seem to be more helpful with childcare than one would expect. Perhaps less surprisingly, since they are larger, they also tend to lead the defence of the group, although both sexes help with this, and have been observed mobbing a clouded leopard to discourage it from attacking.

The species to which those long ago male ancestors of the Indochinese grey langurs belonged still exists today. Known as Tenasserim langurs (Trachypithecus barbei) they survive today only in one small patch of forested hills along a short stretch of the Thailand/Myanmar border near the northern end of the Malay Peninsula. They are darker in colour than their more widespread descendants, but so few have been seen alive that we know virtually nothing about them - beyond the DNA that was used to confirm their relationship. 

Perhaps because of the remote nature of those hills and the fact that they do include some nature reserves, the Tenasserim langur is not officially listed as an endangered species, although it is considered close to that status. Its three relatives, despite being far more widely distributed, all are, largely because of the continuing loss of native forest to agriculture and urban development. Their overall populations may well be larger, but it's likely that they are declining far more rapidly than the more isolated species, a major criterion for deciding whether or not a species is endangered. The closely related Popa langur (Trachypithecus popa), first described only in 2020 from central Myanmar, has yet to be formally evaluated, but is already thought to be virtually extinct.

Francois' langur

The female ancestry of the Indochinese grey langur lies, however, in a separate branch within the genus, known collectively as "limestone langurs" or "karst langurs". The most widespread of the remaining species in this branch is also the most northerly of all langur species, Francois' langur (Trachypithecus francoisi), which is native to southern China and the far north of Vietnam.

Visibly, this looks quite different from the members of the spectacled group, primarily since it lacks the white patches around the eyes and on the muzzle. In fact, it's almost entirely black, the dark face offset with a white handlebar moustache. However, as in other members of the genus, the young are born with a bright orange coat. This starts to darken earlier than in some other related species, and shifts rapidly to black over the course of a single week between about 125 and 130 days of age.

Francois's langurs live in forested hills up to about 600 metres (2,000 feet) and in a climate that's significantly cooler than that inhabited by most of their neighbours, with mixed broadleaf trees rather than anything subtropical. The terrain tends to be dominated by limestone cliffs and similarly rocky ground. They often sleep in caves or on ledges that are difficult for predators to reach; they have some preferred sites that they use over and over again, but switch between them every few days to avoid becoming too predictable.

Unlike the dusky langurs and their kin, the "limestone langurs" spend a significant proportion of their time on the ground, taking advantage of the uneven terrain for protection. Nonetheless, they primarily eat leaves, and climb into the tree canopy to do so. They have distinct preferred foods, not simply eating whatever is most abundant, although exactly which foods they prefer seems to vary amongst those living in different areas. Their behaviour also changes through the year, with them devoting more time to seeking out food in the dry season, when high-quality young leaves may be in short supply.

Groups are typically smaller than those of the spectacled species, with about seven individuals being average. These tend to include only a single adult male, and travel about 540 metres (590 yards) a day in an area of around 70 hectares (170 acres). 

Delacour's langur

The species was also thought to live further south, but the southern population was raised to species level in the 1980s, an impression confirmed by modern genetic studies since. Delacour's langur (Trachypithecus delacouri) lives in a relatively small patch of forested hills south of Hanoi in northern Vietnam. It can readily be distinguished from Francois's species by the fact that its hindquarters are white, the colouration being sharply divided from the black of the rest of the body, and stretching down to a similarly sharp line on the thighs, making it look almost as if the animal is wearing a pair of shorts. The tail is also said to be much bushier, resembling a bottle brush. 

They live in similar habitat, but perhaps to even greater extent, spending more of their time on the ground and leaping less often than their relatives - perhaps because of the dangers of unstable ground or sharp pebbles. On the other hand, they are even more dependent on leaves than other langur species, with as much as 80% of their diet consisting of the foodstuff, and a correspondingly small amount of fruit. 

The white-headed langur (Trachypithecus leucocephalus) is another species previously thought to be a Francois' subspecies, in its case split off rather more recently, in 2001. It lives in China, just to the south of the Francois' langur, and the two species are capable of interbreeding, so the distinction is largely made on the grounds of their appearance. In a reversal of the pattern seen in Delacour's langur, here it is the head and shoulders that are white, with some white on the upper arms and tail, and greyish, rather than black fur, on the rump. The bushy tail is also absent.

Groups number usually have a single dominant male, but sometimes a pair of brothers may temporarily share leadership. Unlike the Indochinese langurs, but like most other monkey species, the females stay with their sisters and other close relatives in the group of their birth, while lone adult males seek to take over such groups, or at least abduct some of the females from larger ones. As is often the case, such takeovers are violent affairs, with the victorious male killing any young infants sired by his predecessor, although those around a year or so old tend to survive his attentions. 

Weaning is unusually late in white-headed langurs, and can be as much as 21 months after birth. Although the males are less helpful than in the Indochinese langurs, young females often help to look after young that are not their own, apparently to gain practice for the future rather than from a cooperative instinct, since they stop once they're older and have got the hang of it.

The limestone cliffs where the white-headed langurs live are less extensive and more fragmented than those of their northern Francois counterparts, often forming small patches that are now entirely surrounded and separated by farmland. Studies of the way that they move across the rugged landscape show that they are less adaptable, too, which further limits any possible expansion to new territory even were it available. The result has been that, while both species are endangered, the white-headed langur has suffered an even more extreme population crash, and as few as 250 adults may remain alive today.

The Laotian langur (Trachypithecus laotum) lives only along a 100 km (60 mile) stretch of the east bank of the Mekong in Laos, where a range of limestone hills suits its requirements. It differs from the other "limestone langurs" in having white hair across most of the head, except for the black face. It is said to be more arboreal than its relatives, although it likely still requires the rugged limestone terrain for shelter and protection. 

Split off from Francois' langur in the 1990s, the Laotian langur has itself been split into two. The "new" species is the Hatinh langur (Trachypithecus hatinhensis) lives a little further south, in the hills along the Laos/Vietnam border and has only a narrow band of white hair along its cheeks. Ironically, it doesn't live in Ha Tinh, which is a province of Vietnam, although this unfortunate state of affairs is due to a governmental reorganisation since it was named rather than any actual change in its status. A subspecies in the northern part of its range, briefly recognised as a separate species in its own right, but since reunited with it due to a lack of conclusive evidence to the contrary, is distinguished by being entirely black, without a trace of the white patches normally seen - this may, however, be little different than the colour variation in black panthers.

Cat Ba langur

All of these species are endangered, living in relatively restricted habitats in areas where forests are being lost. Rarer still, and perhaps one of the rarest primates anywhere in the world is the Cat Ba langur (Trachypithecus poliocephalus). They are found solely on the eponymous island, which lies just off the coast of Vietnam, and is largely covered with rugged forest with a few tourist resorts along the beach. It's not an especially large island, and the langurs are currently only found in one small area within it, perhaps occupying just 22 km² (8½ square miles). 

Uniquely among the "limestone langurs", the Cat Ba langur never fully loses the orange colouration it has as an infant. The head and shoulders remain golden in colour throughout life, with the rest of the body chocolate brown, rather than the more typical black. Their lifestyle appears similar to that of their fellows and is equally reliant on limestone cliffs and caves. 

In the 1970s, their population is thought to have been around 600 or so, already a significant decline from what it must have been hundreds of years ago. Since then, it has continued to decline still further but the area in which they live is part of a National Park that has recently been expanded, and there is some evidence that their population is starting to recover. The latest census, in 2019, estimated the population at 67, which we can put further into perspective by recognising that they live in groups of six or seven individuals each. Dire as this sounds (and it is) that's up from about 40 twenty years ago and represents a small, but almost certainly real, year-on-year increase. Unfortunately, it probably wouldn't take much to reverse the trend.

But the "limestone" and "spectacled" langurs do not form the entirety of their own genus, let alone of langurs as a whole. Next time I will be continuing to look across Southeast Asia to describe some of the others...

[Photos by Tontan Travel, Md. Tareq Aziz Touhid, "Rushen", "Becker1999", "Olevy", and "ALOnIShOnETH". Cladogam adapted from Springer et al. 2012.]