Primate Paradise of Gunung Leuser, Sumatra

Gunung Leuser National Park is not very far from Kuala Lumpur as the crow flies. A hop over the Makasar Strait and a journey of around 120 km from the metropolis of Medan up to the forested hills.

In practice, the journey takes much of a day, including an hour in the air and four to five hours to cover the ground betwen Medan and Bukit Lawang. We made the journey last weekend for a couple of days of trekking and wildlife watching.

A short hike from the friendly village of Bukit Lawang, over one of the bridges suspended across the river, past rubber trees in a small plantation, takes you to the entrance of the Gunung Leuser National Park.

A short while later we met the first of three new (to me) primate species, the North Sumatra leaf monkey (Presbytis thomasi), charming with their crests and impish faces.

Alfred Russel Wallace visited Sumatra from November 1861 to January 1862, but further south, where he also encountered Presbytis monkeys (presumably the mitred leaf monkey).

"In Sumatra, monkeys are very abundant, and at Lobo Kaman they used to frequent the trees which overhang the guard-house, and give me a fine opportunity of observing their gambols. Two species of Semnopithecus were most plentiful — monkeys of a slender form, with very long tails. Not being much shot at they are rather bold, and remain quite unconcerned when natives alone are present; but when I came out to look at them, they would stare for a minute or two and then make off. They take tremendous leaps from the branches of one tree to those at another a little lower, and it is very amusing when a one strong leader takes a bold jump, to see the others following with more or less trepidation; and it often happens that one or two of the last seem quite unable to make up their minds to leap until the rest are disappearing, when, as if in desperation at being left alone, they throw themselves frantically into the air, and often go crashing through the slender branches and fall to the ground."

Wallace was too far south to see the Sumatran orangutans, however.

"As the Orangutan is known to inhabit Sumatra, and was in fact first discovered there, I made many inquiries about it; but none of the natives had ever heard of such an animal, nor could I find any of the Dutch officials who knew anything about it. We may conclude, therefore, that it does not inhabit the great forest plains in the east of Sumatra where one would naturally expect to find it, but is probably confined to a limited region in the northwest part of the island entirely in the hands of native rulers."

The Sumatran orangutans are a distinct species from their Bornean cousins (Pongo abelii). They are a delight to encounter in the forest. Over the course of two days we came across a mother with her son, an impressive bearded male, looking like a giant orange forest Odin, and a mother with her daughter.

Perhaps my favourite of the primates on this journey were the white-handed gibbons, hard to photograph given their height in the canopy and their speed of movement. Eventually, we found a group relaxing within clear view from the ground.

Long-tailed macaques (like this mother below) were abundant, as were the solidly built pig-tailed macaques, these ones often with a sort of forlorn expression on their faces.

These photos illustrate just a few of the primate encounters on this weekend journey. The Thomas's leaf monkey, white-handed gibbons and Sumatran orangutan were new to me, the long-tailed and pig-tailed macaques old familiars from other countries.

On our return to Kuala Lumpur, we learned of the newly designated orangutan species Pongo tapanuliensis, which lives further south on the other side of the Toba supervolcano. ".. the deepest split in the evolutionary history of extant orangutans occurred ∼3.38 mya between the Batang Toru population and those to the north of Lake Toba, whereas both currently recognized species separated much later, about 674 kya."

Here's Gunung Leuser (front left), home to Pongo abelii, in relation to the home of the newly recognised species (Pongo tapanuliensis) at Batang Toru, with the Lake Toba supervolcano separating the two groups. That's Borneo in the distance at top right, home to Pongo pygmaeus.


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