Leaf-Eating Monkeys: The Colourful Doucs of Vietnam

Red-shanked douc

Modern genetic analysis has shown that the leaf-eating monkeys of the Old World consist of three, related, evolutionary branches. Two of these, the colobus monkeys of Africa, and the langurs of Asia, had long been recognised as distinct, but the third was not always so. The monkeys belonging to this branch had, for the most part, previously been considered to be langurs, or at least part of the langur line, but it turns out that they split off from the others early on. Because of the comparative recency of this discovery, this third group doesn't have any distinctive common name and nobody seems to have come up with a scientific one, either. ("Rhinopithecinini" would seem the obvious choice to me, but one can see why nobody bothered).

So, instead, this group, when scientists need to refer to it, ends up being called the "odd-nosed monkeys". Even then, not all of them have noses that look (at least to me) especially odd, and this is most clearly true of the doucs. 

The word "douc" is French, and thus pronounced "dook" as it would be in that language. But it derives from a word for the monkey in the local Muong language of northern Vietnam, and it's here that the monkeys were first identified. That was back in 1771, and, as such an early date for a remote jungle-living animal might imply, that's because they are distinctive-looking animals. 

We'd now call this particular species the red-shanked douc (Pygathrix nemaeus) because of the reddish fur on their shins, but this is just part of their complex colour pattern. The shoulders, back, hands, feet, thighs, and upper arms are black, the belly and top of the head are grey, and the lower arms, tail, and scrotum are white. There are white whiskers around the face, which itself has unusual orange skin except around the muzzle, and a white bib with thin bands of orange and black running around it over the chest.

Red-shanked doucs are found in the forested interior of central Vietnam, and also across the border into the eastern half of southern Laos and the extreme north of Cambodia. Once thought to be found on Hainan Island, this is almost certainly the result of a mistaken zoo record, since there is no other evidence of their existence there. They prefer relatively dense forest, where they spend most of their time up in the canopy, although they do descend to the ground from time to time. Much of the terrain where they live is hilly or even mountainous, and the monkeys can be found at high elevations, especially during the dry season, and in rugged limestone terrain.

About 80% of their natural diet is said to consist of leaves, with the low nutrient content requiring them to spend much of their time either eating or resting and digesting. They eat something like 20% of their own body weight in leaves each day, although they are fairly picky, choosing fresh leaves with low fibre content; when kept in zoos, they leave a surprising proportion of the leaves they are offered as food untouched. Like other leaf monkeys, their ability to digest this low-quality food is due to their compartmentalized, fermenting, stomach. In their case, this has four chambers (it's just three in most other species, including some of their closest relatives) with exceptionally deep mucus-secreting glands in the second chamber.

They live in groups of around five to ten individuals, typically with a single adult male. One report estimated that each group inhabits an area of about 36 hectares (90 acres) but that they do not travel enough each day to be actively defending it against trespassers. Indeed, neighbouring groups often meet up and travel together, forming larger associations; 18 monkeys seems to be the average for these "bands", but some much larger bands have been reported. The bands are apparently stable over time, indicating a multi-level society, although the individual groups within them are always breaking away before meeting up again. Despite being able to form such associations, they seem to have remarkably poor self-control (which monkeys living in larger groups usually do have), perhaps because leaves are so plentiful where they live that more sophisticated cooperation just isn't needed.

Another unusual feature of doucs as compared with other leaf monkeys is their method of locomotion. Langurs, colobus monkeys and so on mostly move about in the way that most other monkeys do; they walk about on all fours, often gripping onto branches, and sometimes leaping about between different supports. But doucs spend about half of their time swinging through the trees by their arms in the manner that a gibbon or spider monkey would. This is associated with skeletal features of the arms and shoulders that parallel those in gibbons, and is more common in infants than adults.

The young are born with a grey and ginger coat and a comparatively dark face; the fur darkens and the face lightens over the first ten months until it reaches the adult pattern. Pregnancy lasts about six months, with each mother giving birth to an infant once every year or two. 

The population of red-shanked doucs has declined so rapidly in recent years, due to human encroachment on their forest habitat and widespread hunting, that they are currently considered critically endangered. The great majority of those alive today are probably in remote parts of Laos and even they are at high risk. While the Vietnamese population may well have once been larger than that in Laos, today only around 2,000 are thought to be living in the country, mostly in a single national park. The population in Cambodia is lower still, if it remains at all.

Grey-shanked douc

In 1997, the population of red-shanked doucs at the southern end of their range were identified as a distinct subspecies. Since then, they have become regarded as a full species, the grey-shanked douc (Pygathrix cinerea) due largely to genetic evidence. Physically, they look very similar to the red-shanked species, except for fur that is paler on average, and notably, the fact that their shins are grey, not red.

Grey-shanked doucs live almost entirely in Vietnam, in a few provinces just to the south of the region inhabited by the red-shanked species. They are probably also found across the border in Cambodia, although the issue is confused by the fact that the two species can interbreed, and do so fairly commonly in the zone where they meet up. 

Grey and red-shanked doucs are similar in many respects beyond the merely physical, behaving and living in much the same way. Their diet is similar, but is said to have a slightly lower proportion of leaves, while they are even more likely to move by swinging through the trees by their arms, although this not reflected in any obvious physical difference - it's something they prefer rather than something they are inherently better adapted to do. Their calls are also very similar, with eight different types being identified, although precisely what each of them "means" is not yet clear. We also know, purely because someone happens to have checked, that grey-shanked doucs, like many other monkeys, are either right or left-handed - although neither is more common than the other.

Like its red-shanked cousin, the grey-shanked langur is a critically endangered species. Estimates of its surviving population vary, but a total of 2,000 is very much the upper limit. Somewhere around half of these may be living outside protected areas, and there is some evidence that climate change may force them into less protected, higher altitude areas in the coming decades, making things even worse for them.

Black-shanked douc

Further south still we come to the black-shanked douc (Pygathrix nigripes), first identified as a separate species back in 1871, but merged with the red-shanked as a single species for much of the 20th century before changing fashions split it back out again. This lives in a wider region than the grey-shanked species, reaching just north of Ho Chi Minh City in the south of Vietnam and across much of southern Cambodia east of the Mekong.

Their colouration is less dramatic than the red-shanked species, with darker fur across much of the body, including shins that are black, rather than red or grey. The bib seen in the other species is replaced by a narrower strip of orange fur, and the face and scrotum are blue, with wide yellow rings around the eyes. Some variation of the general pattern is sometimes observed, including some (non-hybrid) individuals with red fur on  parts of their legs. Nonetheless, as expected, genetic analysis shows that they are not quite as closely related to the grey and red-shanked species as those two are to each other.

The black-shanked douc is apparently the least folivorous of the three species, although leaves still form over half of its diet, and the least likely to swing by its arms. In other respects, it is similar to the other two species.  For example, they have the same multi-level society as other doucs, with groups of five to ten individuals led by a single male meeting up to form larger bands of up to 25, and younger males either travelling alone or in small bachelor groups. Within the male-led groups, the various females help one another out, even sharing childcare duties.

The great majority of the population is thought to be in Cambodia, primarily in the Keo Seima Wildlife Sanctuary. As with their northern kin, future warming of the climate is thought likely to force them away from the lowlands and into the more mountainous interior, rather than simply heading northward into entirely new territories. Their population is in steep decline due to hunting for use in alternative medicine, as well as illegal logging. In Vietnam, they live in the area that was most heavily affected by the war with the US, which saw a dramatic post-war population expansion. As recently as 2013, analysis of faeces left by the monkeys showed that they had been eating food that was still contaminated with agent orange, which is hardly likely to have been good for them.

The three species of douc seem to be distributed along the north-south gradient that they are due to highly conserved habitat preferences, with each preferring a specific climate temperature. Northern Vietnam is therefore presumably too cold for them, since they clearly don't reach that far. However, another species of "odd-nosed" monkey does live there and it is to that, and its relatives, that I will be turning next time.

[Photos by Bjørn Christian Tørrissen, fPat Murray, and "Khoitrain1957" from Wikimedia Commons.]