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.

Leaf-Eating Monkeys: The Leaf Monkeys of Sumatra

Black-crested Sumatran langur

When the system of scientific naming that we now use was first devised, all monkeys were placed in a single genus, Simia. The initial listing of names, from 1758, does not include any species we would now recognise as leaf monkeys but, as these were "discovered" over the next few decades, most were placed in Simia, although a few were placed in newly-created genera that contained other, more typically fruit-eating, monkeys. The first naturalist to create a genus specifically for what we would now describe as "leaf monkeys" was Johann von Eschscholtz, a naturalist who had served on a Russian expedition that circumnavigated the globe. His write-ups of that expedition included the first scientific description of the plantlife of California, but they also included the description of a new kind of monkey, which he felt was distinct enough to give its own genus: Presbytis.

That was in 1821, and, while various other naming schemes were suggested through the 19th century, for much of the 20th, all langurs were placed in Eschscholtz's genus as a group distinct from the colobus and other leaf monkeys. During the 1980s, however, the grey and golden langurs, and their various close relatives, were split off into the genera they now occupy, leaving relatively few species still with their older designation.

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.

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. 

Leaf-Eating Monkeys: Sacred Langurs of India

Sacred langur

In Hinduism, the monkey god Hanuman is a companion of Rama, one of the more popular incarnations of Vishnu, and represents, among other things, loyalty and virtue. The specific type of monkey most associated with him is known by various names, including "Hanuman langur" and the rather uninspired "northern plains grey langur", but I'll stick with sacred langur (Semnopithecus entellus).

As currently defined, the sacred langur is found across almost the whole of northern India where suitable habitat exists. The biggest limitation to that habitat is elevation; sacred langurs are very much a lowland species, not found above about 400 metres (1,300 feet) - which cuts out rather a lot of the more northerly parts of the country as it reaches towards the Himalayas. Other than that, they require forest, but they seem adaptable to different types. 

In practice, they mostly live in dry tropical deciduous woodland because that's largely what there is in that part of the world, but they extend into thorny scrubland in the northwest near the deserts of Rajasthan and into damper forests in the east. They are also common, probably due in part to their reputation as a sacred animal, on the outskirts of densely settled urban areas. A population is also found in southwestern Bangladesh, but they aren't thought to be native there and were likely brought across by Hindu pilgrims in the late 19th century.