Sunday 6 November 2022

Leaf-Eating Monkeys: Monkeys on the Mountainsides

Golden snub-nosed monkey
Much of eastern China is dominated by two great river basins: the Yellow River in the north and the Yangtze in the south. In the western parts of the country, the upper reaches of the two basins are separated by the Qinling Mountains, running eastward from the vast Tibetan Plateau. And here, in addition to many other unique animals and plants, we find a rather strange species of monkey.

This is the golden snub-nosed monkey (Rhinopithecus roxellana) and its oddity merely begins with its unusual appearance. As the name implies, its fur is mostly golden, ranging from a brownish-red hue to a much brighter golden-yellow. Contrasting stripes of black fur run down the outer edges of the limbs and there are white patches on the back of the thighs. All of these colour patterns are brighter in the males, which are also noticeably larger than the females, indicating that these are likely used in sexual signalling - something supported by the fact that the genitalia also have a contrasting dark colour.

It's their face that looks particularly odd to human eyes, however. Most of the skin is hairless, and blue in colour (again, it's brighter in the males), except for a strip of narrow hairy skin running across the upper cheeks like a thin handlebar moustache, giving the face what has been described as a 'trefoil' appearance. Adult males also have fleshy red flaps at the corners of their mouths which, again, it's hard to see as anything other than a sexual signal, since they have no other apparent purpose. Most obviously, perhaps, the nose has flaring nostrils and is extremely short and upturned, enhancing what's already a somewhat skull-like look. The nasal bones, which would normally form the bridge of the nose, are extremely short, and are missing altogether in some individuals.

The monkeys live in the high-altitude forests around the mountain slopes, where there are plentiful bamboo thickets and rhododendron woodlands in addition to the mix of broadleaf deciduous trees and conifers. This is the same area that pandas are found in the wild, but it's really not somewhere you'd expect to find monkeys. In fact, the golden snub-nosed monkey has a colder natural habitat than any other primate species apart from humans and, unusually, has a thick winter coat that it sheds in the spring.

During the summer they ascend the slopes to altitudes as high as 3,000 metres (9,800 feet) where they feed on a fairly even mix of leaves, seeds, and lichen. During the winter, snow covers the ground for up to six months of the year, and this does seem to be going a bit too far even for these cold-adapted primates, so they often (but not always) head downhill into the valleys - although even these are no lower than 1,400 metres (4,600 feet) above sea level. As many of the local trees shed their leaves at this time of year (something that's not true in the tropical and subtropical forests where most other monkeys live) they instead switch to bark and buds, resorting to things like pine cones if they get particularly hungry. They spend more time walking along the ground than most other leaf-eating monkeys, although young individuals are more often in the trees, possibly to make it easier to hide from predators.

The monkeys have a two-tiered social structure. Most live in one-male units, with each male leading an average of eight females; the excess males gather together into bachelor units of six or seven, consisting of younger individuals yet to establish a harem. However, these units are not isolated from one another, and associate into much larger troops with anything up to 340 monkeys often travelling together in a relatively organised fashion led by the most dominant units. Different units seem to be able to leave and rejoin these larger groups as and when they desire, and females have a very fluid social network, even switching between different units if they happen to prefer a different male. Indeed, even when a male establishes a new harem, the process is surprisingly non-violent, with female choice playing more of a role than fighting between the males.

Given the changes in weather through the year in the mountains, it should not be surprising that there is a distinct mating season, which lasts from October to December, with the single young being born six months later. Mating is overwhelmingly initiated by the females and, while the male obviously does get to mate with multiple partners, paternity testing shows that over half of the young in a given group may not actually be fathered by its resident male - suggesting that the "harem" system is a lot less robust than one might assume. Once the young are born, female cooperation continues, in that many help to look after young that are not their own, giving the mother more time to search for food or take part in grooming activities.

The Qinling Mountains are, unfortunately, no more safe from human development than other parts of the world, with many forests being cut down to make way for agricultural land. Climate change is an additional factor and will likely force the monkeys to move to higher altitudes as the lowlands warm, something that will inevitably reduce the area they can live in. Some studies have shown that the monkeys were relatively widespread at the end of the Last Ice Age but gradually split into separate populations (which may not wholly match up with the three recognised subspecies). Due to a significant decline in the estimated population over the last few decades, it is officially recognised as an endangered species.

Black snub-nosed monkey

The golden snub-nosed monkey was first scientifically described by Alphonse Milne-Edwrads in 1870. He initially identified it as a type of langur, but gave it its own genus two years later. In 1897, he split it into two species, with one living further south than the other. They were merged back together in the 20th century, but were separated out again in the 1990s. This other species is known as the Yunnan or black snub-nosed monkey (Rhinopithecus bieti) and it lives in the Yunling mountains that separate the upper stretches of the Yangtze and Mekong rivers in northern Yunnan Province and southeastern Tibet.

Although it is further south, and not quite so cold as the Qinling Mountains, the area is even higher in altitude, making this the highest-elevation species of primate, although it does overlap with the range of some local macaques in the valleys. The habitat is similar to that of its golden cousin, but here, 3,000 metres (9,800 feet) is about the lowest it is willing to go, and it often ascends to 4,700 metres (15,400 feet). They appear to favour the sunniest places in winter, probably because the snow melts most quickly there, even if it means moving to high, relatively exposed, elevations.

As their common name implies, black snub-nosed monkeys have a dark coat, although the underparts are distinctly white, leading to the alternative (and arguably more accurate) name of "black-and-white" snub-nosed monkey. The skin on their face is mostly pink, although often with yellowish or greenish tints around the eyes and the moustache-like strips of fur are less prominent. In other respects, they look quite similar.

Their behaviour and biology is generally similar, too. For instance, they eat much the same foods, although lichen can be far more important, comprising the bulk of their diet throughout the year. Since this wouldn't be very nutritious, it has been argued that this is a sign of them being forced to rely on more marginal habitats than they would naturally prefer. The two-tier social structure is also apparent in the black species, with the larger groups often breaking up when bamboo begins to sprout, providing readily available nutritious food, and reuniting again a few weeks later. For much of the year, however,  quality food is harder to obtain, with the result that the monkeys travel over a wider area than most other primate species, with one group of 400 monkeys estimated to use 56 km² (22 square miles) of forest.

At night, the monkeys sleep in evergreen trees, mainly conifers despite the fact that these aren't the sort of trees they tend to feed on. Since they pick the largest ones that they can, and tend to stick with the same trees night after night, this is probably because they provide the best shelter from snow and freezing cold rain - not something most other primates have to worry about. Mating peaks in August, rather earlier than in the golden species, so that births coincide with new plant growth. As in the golden species, the mothering style is relatively relaxed and egalitarian.

In the 1990s, the total population was estimated at less than 2,000, many of which would be infants that, even without interference, would be subject to a natural death due to predators or disease before reaching maturity. Due to the ongoing decline in their natural habitat, this is unlikely to have improved since and this, too is considered an endangered species.

Serious though that is, the remaining species of snub-nosed monkey are faring even worse. The grey snub-nosed monkey (Rhinopithecus brelichi) also known as the Guizhou snub-nosed monkey, is the only other Chinese species, first being identified as such in 1903. Today, it is only found in an area of approximately 28 km² (11 square miles) in the Fanjingshan National Nature Reserve on the slopes of a peak sacred to Buddhism in the Wuling Mountains, some way to the east of the other two Chinese species. While a census is difficult to perform because of the steep terrain and foggy climate, the latest estimate suggests that no more than around 200 adults are currently alive, as a result of which the official rating for the species was changed from "endangered" to "critically endangered" earlier this year.

Tonkin snub-nosed monkey

The Tonkin snub-nosed monkey (Rhinopithecus avunculus) is slimmer and darker than the other species. It lives in northern Vietnam, in a range of limestone hills and low mountains east of the Red River (which flows past Hanoi and into the Gulf of Tonkin). This is an area that has been subject to extensive redevelopment over the last few decades, with little of the original forest remaining - even more of a problem for this species than it might otherwise be due to an apparently low tolerance for disturbance. As the forest declines and is broken up into smaller sections between which the monkeys cannot travel, hunting has also increased, further hastening their decline. A contender for the unenviable title of "world's most endangered primate", there may be as few as 100 adults left.

The remaining species is the Myanmar snub-nosed monkey (Rhinopithecus strykeri). This was only identified as a distinct species in 2011, previously having been thought to be part of the black species. It is, however, distinguished by being almost entirely black, without the white underparts seen in the other species... which means that sometimes it is the one called the "black snub-nosed monkey" causing general confusion when we need to distinguish the two. It lives in the mountains on the western side of the Mekong, just across from its relative and on the other side of the China/Myanmar border. Even when discovered, it was thought that no more than 330 individuals existed, although if recent claims of its presence in China are correct, the true figure may not be quite so low. Nonetheless, it too is considered Critically Endangered, at high risk of extinction in the relatively near future.

For a significant part of the 20th century, snub-nosed monkeys were placed in the same genus as the doucs. Although they are now considered distinct, the two groups are closely related, together forming the "odd-nosed" clade of leaf monkeys. However, there are two other odd-nosed species that don't quite fit among either the doucs or the snub-nosed monkeys and it is to these, the last two species in my survey, that I will turn next time.

[Photos by Jack Hynes, Rod Waddington, and Quyet Le, from Wikimedia Commons. Cladogram adapted from Liedigk et al. 2012.]


  1. The timing of this post is rather apt, as the first known photographs of a captive Rhinopithecus strykeri were uploaded to the ZooChat forums fewer than 24 hours ago! The images show the distinctive appearance of the species rather well.

    For instance:

    1. Thanks! And it is, indeed, a complete coincidence, as (since it's part of a series) I've known this particular post would go up on the day it did since January...

  2. Readers who watch either HBO or the BBC might be familiar with the golden snub-nosed monkey as the daemon of Marisa Coulter in "His Dark Materials." This is fantasy, not science, but it makes for a good pop-culture reference.