Sunday, 17 July 2022

Leaf-Eating Monkeys: Golden Langurs and Silvery Lutungs

Golden langur
During the Ice Ages, it is thought that the early grey langurs, ancestors of the modern sacred langur and its kin, sheltered in southern India where the climate allowed the warm forests on which they rely to continue to flourish. A second group of langurs, which had originally diverged from the grey langurs over five million years ago, sheltered instead further east, and their descendants include the spectacled and "limestone" langurs that still live in that area today. At some point, however, both moved northward again, and they met up somewhere near Assam in northeastern India.

In this region, they remained separated by mountains and rivers, but the barriers were not perfect, and there is some evidence that the two hybridised, leaving a genetic trace in the non-grey langurs of the region. Certainly, while these descendants are close enough to the spectacled and "limestone" langurs to be placed in the same genus, they form the oldest branch within it, somewhat distinct from the others.

Perhaps the most distinctive of these animals is the golden langur (Trachypithecus geei), sometimes called "Gee's golden langur", although it's not as if there's any other sort to confuse it with. Gee had discovered references dating back to 1907 that mentioned langurs living in Assam that were not the usual grey colour of such animals, and in 1953 he went to look for them, successfully managing to capture some on film. Four years later, the animal was officially written up as a new species by H. Khajuria of the Zoological Survey of India.

That's surprisingly late for something that is, after all, relatively distinctive. Once scientists knew it existed, they were able to discover even earlier references to it dating back to the early 19th century, but still, it seems that nobody had paid them much attention. This may be because the animals are not common, and are found only in a very small area. Essentially, they live in a wedge of land between the Manas and Sankosh rivers, tributaries of the Brahmaputra that are no more than about 100 km (60 miles) apart. 

In the south, the monkeys reach the north bank of the Brahmaputra, and from there they can be found northward through Assam into the foothills of the Himalayas and across the border into Bhutan. Even then, human activity has extirpated them from the middle of this range, splitting them into two populations, one in the south and the other largely inside Bhutan.

Like their relatives in Southeast Asia, golden langurs are born with a yellowish coat, albeit somewhat darker in their case than for the spectacled and "limestone" species. Unlike those others, however, their coat actually becomes brighter as they age, ranging from a creamy white to a distinctive golden yellow, offset by the black face. They live in forests of tall trees, anywhere from the moist river valleys at the south of their range to as high as 3,000 metres (900 feet) elevation in the Himalayan foothills. They seem to spend their time as high up in trees as they can manage, hidden by the foliage and out of reach of tigers and other menaces, as well as providing a good place to find the young leaves and buds that they prefer. Golden langurs have also been reported from rubber plantations, and are known to raid crops of tapioca, cardamom, and guava when their preferred wild habitat has been destroyed.

Active mostly in the morning and evening, resting in the heat of the midday sun, golden langurs live in groups averaging around a dozen individuals, usually led by a single adult male, although there is considerable variation. Births are most common in the rainy season. Most of the surviving population is thought to be in Bhutan, where there are more protected parks within its range than there are in India, but encroaching human development threatens its continued survival in both countries, and it is formally listed as an endangered species.

Capped langur

In 2003, it was suggested that the Indian and Bhutanese populations of golden langur might represent distinct subspecies, but it now seems that the animals sampled to make this determination were actually hybrids between true golden langurs and their closest relatives, the capped langurs (Trachypithecus pileatus). In Bhutan, these live east of the Manas River but they are much more widespread than that, being comparatively common across Assam and neighbouring states in northeast India east and south of the Brahmaputra, as well as in neighbouring regions of Bangladesh, Myanmar, and China.

The common name of the species comes from the presence of a cap of dark hair on top of the head and the monkey also a belly that's distinct in colour from the grey of the rest of its body. Sometimes this is a paler yellowish-grey, but in other subspecies it can be a bright orange-red, making the animal easier to distinguish from its relatives. Like the golden langurs, they prefer to stay high up in the trees, with average heights of 30 to 35 metres (100 to 115 feet) being reported. They seem to live in slightly smaller groups than golden langurs, but these are more territorial, being less inclined to share feeding areas with one another. Nonetheless, the male's focus seems to be more on attacking any females who try to leave his own group than on fighting the males that he fears they might be trying to join. The females are more cooperative, and may share some of the duties of looking after their younger children.

In the last 20 years, one of the subspecies of capped langur has been raised to full species as Shortridge's langur (Trachypitechus shortridgei). It's distinguished from its relative by having an almost perfectly uniform grey coat, lacking the offset colour of the belly seen in the more widespread species. Whereas that is, if not entirely safe, widespread enough not to be considered fully endangered, Shortridge's langur lives in a much smaller area and is unlikely to have a high population. It inhabits a 200 km (125 mile) stretch of mountainous hills along the Myanmar/China border between the Chindwin and Dulong Rivers. At one point, a large portion of this area on the Myanmar side was planned to be flooded for a hydroelectric project; this was suspended in 2011 although, with political changes in the country it's unclear whether or not it might eventually be resumed.

Other than a report stating that the monkeys live in small family groups and that they eat much the same food as their close relatives, almost nothing is known about the species as distinct from its kin.

While the ancestors of the golden langur group moved north from some refuge in Southeast Asia, another group of comparatively close relatives stayed behind. The colour of their fur leads to them often being collectively referred to as "silver langurs", although some of the individual species are more commonly known by other names.

Germain's langur

Germain's langur (Trachypithecus germaini) for instance, while most commonly known as such, is sometimes called the "Indochinese silver langur" or some variation on that name. It is most common in Cambodia, where it lives across almost the whole of the country except for the area around Phnom Penh, but it is also known from neighbouring parts of Laos, Thailand, extreme southern Vietnam and Phu Quoc Island, and in a narrow belt across the northern end of the Malay Peninsula, reaching as far as the Myanmar coast.

Germain's langur is light grey over much of the body, although only the underparts are truly "silver", and has black hands and feet with an unusually long, dark-coloured tail. The face is dark, but surrounded by a halo of creamy hair. As with many of their relatives, however, they are born with golden-yellow fur and much paler, almost white, skin on the face, hands, and feet. They have a similar diet to other langurs, feeding on the young leaves from a wide range of trees, along with some fruit - especially figs. They live in lowland forests, where they spend most of their time in the understory, rather than high up in the trees, and often come down to the ground in sunny weather. They can live in large groups, with up to 30 members, although about half this is common. It's likely that, as with their relatives, there are only a few adult males in each of these groups but there doesn't seem to have been much detailed study of their behaviour and social organisation.

In 2003, it was proposed to raise what was then thought of as the eastern subspecies of Germain's langur to full species status, something later confirmed by genetic studies. It has a variety of common names, although some variation of Annamese langur (Trachypithecus margarita) seems to be the most common; "Elliott's silver langur" is another. They are separated from their sister species largely by the Mekong River, living in eastern Cambodia, south-central Vietnam, and southern Laos. They are very difficult to tell apart from Germain's species, being distinguished by fine details of the hair on their heads, and paler faces, often with rings of unpigmented skin around the eyes. 

One study found that they mostly eat leaves of trees in the mulberry family (which, besides the obvious, also includes figs), but concluded that was probably just because there were more of those around in the research area than due to any inherent preference. Living in lowland forests that tend to be cut down to make way for agriculture, their total population is likely well below 10,000, and continuing to decline, making them another species considered to be endangered.

Selangor silvery langur

The only other continental species in the group is the Selangor silvery langur (Trachypithecus selangorensis), which lives along the west coast of peninsular Malaysia, including, but not limited to, the Malaysian state of Selangor. It, too, is a relatively uniform dark grey colour, but can be distinguished by a high pointed crest of hair on top of the head as well as long, straight, whiskers. Only identified as a subspecies in 2008, and raised to full species status within the last decade, the little that is known about them specifically suggests little difference from their close relatives. A recent study, conducted over a 12-year period, showed that their groups are stable over this time period, remaining about the same size, even as individuals come and go, and occupying essentially the same geographical patch of land, although this often overlaps with those of their neighbours.

The species that they were split off from happens to be one of the two "original" species of the group, and was once thought to include what are now Germain's and the Annamese langurs as well. It is now most commonly known, not as the "silver langur", as it used to be, but as the silvery lutung (Trachypithecus cristatus). The second half of the name comes from a Sundanese word that is felt to be more appropriate than the Hindi word "langur" for monkeys from the area. Indeed, many of the other silver langurs and, for that matter, the golden, spectacled and "limestone" langurs, are sometimes now referred to as "lutungs", regardless of where they happened to live.

Silvery lutung

Silvery lutungs live in Borneo and Sumatra, and on several of the smaller, neighbouring islands. The Natuna Islands, a small archipelago off the northwest coast of Borneo are home to the only remaining subspecies yet to be raised to be full species status - no doubt partly because its status even as a subspecies has been questioned

Silvery lutungs are greyish in colour, and look similar to the Selangor species of peninsular Malaysia, although the crest of hair on their head is not quite so pronounced, their fur is generally paler, and their whiskers more curved. As with many other langurs, about two-thirds of their diet consists of leaves, the younger the better, with the remainder made up primarily of fruit, fig trees being cited as the most common resource for both types of food. They live in lowland forests, often ones with substantial water supplies, whether it be a nearby river, or flooded mangrove swamps. Most studies seem to show that they divide their time almost equally between different levels of the forest from the canopy down to the undergrowth, although they sleep higher up whenever possible.

Groups range from 10 to 40 members, usually with a single adult male, although a few of the larger groups might have a second one. The females seem to lack any internal hierarchy, helping each other out with raising their young, as many other langur/lutungs do. The males doubtless compete rather more, since only a few will have mating opportunities at any given time - the others living solitary lives or in small bachelor groups outside the main, mixed-sex ones. Infanticide by an incoming male was once reported by a researcher in the 1970s, but hasn't been seen since, so it may not be a common occurrence. (Which, for what it's worth, it certainly is in some other langur species). They seem to spend over half their time resting, possibly to help them digest their relatively tough, leafy diet, and are most active in the morning and evening.

East Javan lutung (red morph)

Further south, we come to a couple of species with highly variable common names, partly caused by the two only having been distinguished from one another in 2008. The more widespread of the two is the East Javan lutung (Trachypithecus auratus) which lives across most of Java and on the islands of Bali and Lombok to the east, making it the most south-easterly of all leaf monkeys, since Lombok is on the far side of the Wallace Line that normally separates Asian from Australasian species. 

Another very common name for the species is the ebony langur, which refers to its dark coat of glossy black hair with grizzled tips. Another distinctive feature is the mop of forward-curling hair on the head that makes it look quite distinct from the two crested species living further north. Unusually, however, and helping to explain the auratus ("golden") part of the name, some individuals never entirely lose their infant coats, remaining a reddish-orange colour into adulthood, and also possessing pale skin on the hands and face. These are not noticeably genetically distinct from the black individuals (no more so than a red-headed human is from other people, anyway) but are found only in the easternmost part of Java, where they live happily alongside the darker form. Among those that do shift to black, the yellow colour of the infants prompts mothers to care for them, so it's possible that the unusual colour arose in a group that managed to scrounge off their parents for longer, giving them an unintentional leg up in life. (It's not clear whether the mothers of red offspring have gotten wise to this and use alternative cues to know when to stop sheltering infants, but it seems likely).

East Javan lutung (ebony morph)

The second, more recently distinguished, species is the West Javan lutung (Trachypithecus mauritius). This is found only in the extreme west of Java, and looks essentially like the dark form of the East Javan species, but lacks the white tips to the hair, making it look even blacker.

Both species live across the local forests, from the shoreline up into the mountains, seemingly unconcerned by any variation in the type of trees. They do wander into plantations when running short of food in their preferred wilderness habitat, and some have adapted to eating the leaves of teak trees, which are far more common in plantations than they are in the wild. Despite their preference for leaves over fruit, they have been noted as significant dispersers of seeds, which can survive unharmed on passage through the gut and are deposited in the animal's dung. Neither species is currently regarded as endangered, although both have some level of threat due to the ongoing reduction their native habitat.

These are by no means the only langurs found in Indonesia, however, since the many islands, with their rugged interiors have allowed the separation and creation of many additional species, forming a third group distinct from both the grey langurs and the various Trachypithecus species. Relatively little is known about most of them (many are, as you'll doubtless expect by this point, only recently named), but it is to them that I will turn next.

[Photos by M Swarnali, Lonav Bharali, "calflier001", Peter Gronemann, Bernard DuPont, Julie Langford, and "Ray", from Wikimedia Commons. Cladogram adapted from Springer et al. 2012.]

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