Sunday 11 September 2022

Leaf-Eating Monkeys: Borneo and Beyond

Hose's langur
Borneo is the third largest island in the world (after Greenland and Papua New Guinea), being about 10% larger than Texas, and around three times the size of the entire UK. The interior is rugged and mountainous, especially in the north, and, in its natural state, almost entirely covered in dense jungle. This is a perfect environment for spawning new species, which can easily become isolated from one another and still have plenty of lush vegetation to feed upon. It's also ideal for hiding these species from naturalists so that it's possible that even the species we currently recognise from the island are not a complete list.

As was common at the time, several species were named in the 19th century, often by naturalists unaware that what they'd just described had already been named in some obscure source by somebody else. Even after the general taxonomic tidying up of the 20th century, however, no fewer than four species of the genus Presbytis - loosely speaking, the Indomalayan group of langurs - were still recognised as living on Borneo.

One of those four was Hose's langur (Presbytis hosei), first identified in 1889. This lives in the central northern parts of the island, primarily in Sarawak and the Sultanate of Brunei, but also in significant neighbouring regions of both the Malaysian and Indonesian parts of the island. They are dark grey monkeys with whitish underparts and black hands and feet. The fur on the head is mostly white, but with a prominent black crest. On adult females, this black colour extends around the side of the face to the ears. It had previously been reported that some females in the west lacked this pattern, and a subspecies was erected on this basis, but it seems that the individuals in question were juveniles that simply hadn't developed it yet, so the subspecies is probably not valid.

Their home includes some of the most mountainous parts of the island, and the langurs live from the lowland forests well up into the hills, to at least 1000 metres (3,300 feet) elevation, and probably 1600 (5,200 feet). The relative isolation of this terrain may have helped protect them against some of the problems facing other species of Borneo, and they are not currently considered endangered. Which is not to say that they are entirely safe either. Certainly, their habitat is shrinking as agriculture, palm oil plantations and the like expand. However, while they clearly prefer older, untouched forests with tall trees, they seem relatively tolerant of disturbance and sometimes even enter and feed in the plantations.

A more significant threat comes from hunting. While this is partly for meat, as one might expect, the real value in the monkeys to hunters comes from their bezoar stones. These are concretions of material that form in their stomachs after eating mud to gain extra mineral nutrients, and they can fetch a high price locally for use in alternative medicine. The monkeys are reported to live in groups of six to eight adults, with the single male typical of langurs, each group occupying perhaps 1.3 km² (320 acres) of territory. Males make loud calls to other groups, and the monkeys also make at least three other call types, although these are said to be less complex than those of other langurs.

In 2014, Hose's langur was split into three species. Unusually, this was not done on the basis of any new genetic or molecular evidence, but purely because the two "new" species happen to look a bit different. 

One of these, the Sabah grizzled langur (Presbytis sabana) had originally been named just four years after Hose's species, and by the same man, Oldfield Thomas. As such, it is a clear example of changing fashions among zoologists when it comes to naming species - nothing new had come to light that caused either its demotion to subspecies or its subsequent resurrection. It's purely a "how different does it have to be?" question.

Found, so far as we know, solely in Sabah, the easternmost part of Malaysian Borneo, the answer in this case is "not very much". It looks pretty much the same as Hose's species, with the only significant difference being that it has patches of blackish skin on its cheeks, rather than being a more uniform pale pink there. The little we know of it suggests that it's behaviour is much the same as that of its cousin. One report has it engaging in the unusual practice for a specialist leaf-eater of consuming bird's eggs it had stolen from a nest... but it seems probable that its relatives do much the same on an equally rare basis.

Miller's langur
Miller's langur (Presbytis canicrus) is also distinguished largely on the basis of its facial colouration. Here, the face is almost entirely black, except for the pink chin, and the hair either side of the black crest is brown, rather than grey. It lives further south than the other two species, in the central-eastern parts of the island, entirely within Indonesian territory. While the Sabah grizzled langur is also considered to be an endangered species, Miller's is even more so. It used to be found primarily in Kutai National Park, where it was protected, but a survey in 2008 failed to find any evidence of its continued existence inside the Park, with just one group living nearby. For a while, it was even thought that it might be extinct, but some were discovered in Wehea Forest, close to Kutai, in 2012, where they frequently visit salt licks.

Rarer still is the Bornean banded langur (Presbytis chrysomelas) which is also known under a variety of other common names, including "Sarawak langur" and "cross-marked langur". The latter name comes from the fact that some - but by no means all - adults of the species retain the black cross pattern across their backs that are found on the otherwise whitish infants (as they are in other members of the genus). Even apart from this pattern, there is considerable variation in the colour of this species of monkey. Most are predominantly black or grey, but some are brown; only these latter seem to have the cross pattern, although even here, it is not universal. 

Previously considered to constitute one or two subspecies of the black-crested langur, the Bornean bander langur used to live in lowland and mangrove forests across northern Borneo. Rarely observed in the wild, it is now exceptionally rare, and is known definitively to exist only at five sites dotted across the Sarawak (Malaysian) region of the island, although it is just about possible that some survive in Indonesian territory just south of the border. In 2020, the population was estimated at no more than 500 adult individuals and declining so rapidly that extinction was likely within just a few generations.

The closest relative of the Bornean banded species is probably the white-fronted langur (Presbytis frontata) so-named because of the white spot on its forehead. This is one of two species that were distinctive enough not to have undergone any significant taxonomic changes through the 20th century. Although they are widespread across many parts of Borneo, including both lowland and hilly forest, they are relatively little studied. They are common enough to avoid "endangered species" status, although their population is thought to be declining for the same reasons as those of their relatives. They are reported to live in smaller groups than other Presbytis langurs, with no more than four adults (one male and three females) being typical, which may also help them hide from observers.

Maroon langur
The other species whose scientific name has remained unchanged is, if anything, even more widespread, and was named by German naturalist Salomon Müller in the same year, 1838, that he described the white-fronted species. This is the maroon langur (Presybtis rubicunda), also known as the red leaf monkey, and numerous variations on those two names. As its name suggests, this has a rich reddish coat, although the exact colour varies from orange or golden to dark wine-red. The bare skin of the face, which is typically pink or black in related species, is tinged blue.

Being found across much of Borneo, maroon langurs seem to be able to adapt to a range of different forest types, from lowlands to perhaps as high as 2,000 metres (6.,600 feet) in the mountains. In addition to leaves, they also eat a considerable amount of seeds when those are available, although they seem less picky about the precise plant species than they are when selecting leaves. As one might expect, when trees are not in fruit, leaves become more important, and, at least in Sabah in northeastern Borneo, over a quarter of their diet can come from just one species of liana. Other, smaller, dietary components include fungi and lumps of soil taken from termite mounds - presumably for their high mineral content.

Maroon langur troops contain up to eight individuals, and occupy areas of over 100 hectares (250 acres), travelling about 1650 km (one mile) per day. This is further than any other leaf monkey, and may be due to the relative lack of abundance of their preferred foods. Males regularly make loud calls, especially in the morning, which likely help both to keep other groups away from their territory and to maintain cohesion within their own group. Despite this desire to maintain their distance from others of their kind (with potentially rival males) they are often found living in the same areas as the three northern species - Hose's and its kin - without any apparent conflict. Even so, any given patch of trees will only be occupied by one or the other species, reducing any risk of competition.

Other species of langur live further afield. The island of Java, south of Borneo is, appropriately enough, home to the Javan langur (Presbytis comata). These are another species of dark grey to black langur with a comparatively tall black crest on the head. At 124,000 km² (48,000 square miles) Java is still about the size of the US state of Mississippi or almost the size of England, so it's hardly small and, in fact, the Javan langurs only inhabit the western half of it. 

Even more so than Borneo, Java is mountainous, and it's as well that the monkeys are comfortable with this terrain, because the very dense human population in the lowlands has all but forced them out of areas there where they were once common. As a result, they are really only found in the highlands today, where they inhabit slopes as high as 2,500 metres (8,200 feet) above sea level, although there are some even around plantations.

Despite this, at its last assessment in March of this year, the Javan langur remained numerous enough that it didn't quite fall into the criteria to be formally listed as an endangered species, although it's certainly close, and this may not remain the case for long. Like other langurs, they live almost entirely in single-male groups; a few males may have lucked out and ended up with only one female, and the younger males without groups of their own associate together in small all-male bands, but most troops follow the usual pattern. They prefer to spend their time high up in tall trees where they snooze three or four times each day, taking the opportunity to thoroughly digest their low-quality diet.

Siberut langur
At some point, perhaps around the beginning of the Last Ice Age when sea levels were lower, a population of monkeys related to the Javan langurs became separated from their kin and isolated on the Mentawai island chain off the west coast of Sumatra. Long regarded as a single species, these are now split into two. The Siberut langur (Presbytis siberu) lives only on the island of the same name. A heavily forested island, much of which consists of national parkland, this is nonetheless much smaller than Java, let alone Borneo or Sumatra; with an area of around 3,800 km² (1,500 square miles), it's a little smaller than Rhode Island, or, if you're British, about the size of Suffolk. The second species is the Mentawai langur (Presbytis potenziani) which lives on the even smaller islands of Sipora and North and South Pagai, just to the south of Siberut.

Both monkeys are mostly jet black, with a ring of white fur around the face, a tall crest, and reddish-brown fur on the underparts. They look virtually identical, although the Siberut species is said to have a slightly darker underbelly and a more distinct patch of white hair around the pubic area and on the scrotum. They were only distinguished, on the basis of genetic evidence, in 2011 and the habits of the two species are also very similar.

They turn out to be the most frugivorous of all leaf monkeys; at least half of their diet, and sometimes as much as two-thirds, consists of fruit and seeds, not leaves. Another oddity is that monogamy seems to be much more common among them than it does among their relatives, although it's possible that this is a factor of their recent population decline rather than any inherent behavioural tendency. 

Both species are endangered, living as they do on small islands where there is little room for agriculture to expand without felling forest, and subject to widespread hunting by the locals. For what it's worth,. the Siberut langur is the less threatened of the two, with the national park thought to be home to as many as 17,000 individuals in 2015, a considerable advance on the 2008 estimate of less than half this number. Unfortunately, we can be fairly confident that this is down to differences in the methods used to estimate the population rather than any genuine increase. The reality is almost certainly a decline, and probably quite a steep one. Hunting, in particular, is thought to have increased recently, partly due to increased access to air rifles (rather than bows) and to the loss of tribal hunting taboos as Christianity replaces the local religions.

Estimates of the total population of the Mentawai langur range as low as 1,200, although they are sufficiently good at hiding that this may be an underestimate. Even so, they are likely close to extinction, with a dramatic decline in their numbers over the last few decades.

Which at last brings me to the last of the 'true' langur species. Together with the colobus monkeys, they form one of the two main groups of leaf monkeys, but there is a third group, distinguished by a slightly strange physical appearance. There are far fewer species in this group than in the two main ones, and I will be looking at some of them next time...

[Photos by Mike Prince, Simon Fraser University, and "markindo", from Wikimedia Commons. Drawing from a book by Henry Forbes, in the public domain. Cladogram adapted from Springer et al. 2012.]

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