Proboscis Monkeys of Borneo: The Big-Nosed Primate
- devanandpaul
- 3 days ago
- 3 min read
The first time I saw a proboscis monkey in the wild, perched on a riverside tree in Kinabatangan, Borneo, I couldn’t help but smile—with its rotund belly swaying gently and its long, drooping nose hanging like a pendulum, it appeared clownish. Locals call it monyet belanda (‘Dutch monkey’) in Malay and bekantan in Indonesian—inspired by its ‘resemblance’ to Dutch colonial settlers with large noses and potbellies.
Habitat of Proboscis Monkeys
Endemic to Borneo, proboscis monkeys (Nasalis larvatus) are river specialists, thriving in coastal mangroves, peat swamps (waterlogged forests—a unique ecosystem with nutrient-poor thick acidic peat soil formed from semi-decomposition of plant matter due to low dissolved oxygen), and riverine forests (dense woodlands along rivers).
The Legendary Nose
The nose of male proboscis monkeys can grow very large, hanging over their mouth, even flapping sometimes as they vocalize. Females have a smaller, but still distinctive, nose.
Scientists believe that the large nose of this monkey is a marker of fitness and dominance. Males with a bigger nose produce louder calls, making them more appealing to females. In the dense forest, where sight is limited, sound is key to communication—and the nose serves as nature’s built-in amplifier.
Life by the Water
Skilled swimmers, the proboscis monkeys often dive into rivers with dramatic splashes. Their long limbs help them glide smoothly through the water, while their partially webbed fingers and toes help them stay afloat and add extra speed, allowing them to cross rivers when necessary, dodging crocodiles.
Life along Bornean rivers is not without danger. Clouded leopards, saltwater crocodiles, pythons, and water monitors prey on proboscis monkeys. Roosting near water facilitates quick escape.
Feeding Habits
The proboscis monkeys mainly consume leaves, seeds, and unripe fruits, young leaves being their top choice—soft, easy to digest, and less bitter. But when fruiting season arrives, they feast on both unripe and ripe fruits. Apart from more than 180 plant species to choose from, they also peck at termite nests found high on trees.
They prefer the lush riverine floodplain forests, moving into mangroves only when trees in those forests lose their leaves. The leaves and the green, tangy fruit of the perepat (Sonneratia alba) tree, also known as mangrove apple, are their favourite food.
These monkeys have a complex four-chambered stomach filled with specialized bacteria (helping in natural fermentation) designed to digest tough, cellulose-rich plant matter. After foraging, they ruminate—bringing up and rechewing partly digested leaves to help break them down further, a habit once thought unique to cows and deer. This process allows them to absorb more nutrients and spend long hours grazing through the forest. And the seeds they swallow are scattered across the forests through their droppings.
The feeding area of the proboscis monkeys ranges between 100 and 900 hectares. And being nonterritorial, the alpha males protect their harem from rival males but do not defend their feeding range. Also, troops of proboscis monkeys often share the same trees with those of langurs and macaques without conflict.
Infanticide in Proboscis Monkeys
The proboscis monkeys live in harem groups, typically led by a dominant male, with several females and their young. When a new male takes over, he may kill the infants—a harsh but evolutionary tactic that forces the mothers back into breeding sooner, allowing him to pass on his own genes.
Family Ties
The proboscis monkeys have a gentle daily routine. Females groom each other, which promotes social bonding; playful youngsters swing and clamber through the branches, learning and exploring in the process; and the alpha male watches over its group. Unlike many other primates, these monkeys live together peacefully and harmoniously.
Shrinking Populations: A Call for Conservation
For centuries, Bornean rivers have been the lifeblood of its rainforests. Once flourishing along every waterway, the proboscis monkeys are now endangered; hunting (for food and body parts used in traditional ‘medicine’) and habitat loss have pushed them into shrinking patches of mangroves. Today, their populations are threatened by fish and prawn farming, and expanding industries.
The current distribution of the proboscis monkeys is uneven across Borneo, their numbers varying widely depending on habitat quality, protection, and restoration.
Sabah: approximately 5900 proboscis monkeys (best sightings along the Kinabatangan River, Klias wetlands, and Labuk Bay)
Brunei: around 1500 individuals (mainly near Brunei Bay)
Sarawak: likely fewer than 500 (occasional sightings at Bako National Park)
Kalimantan: significant drop in populations from 250,000 historically to under 15,000 today (Tanjung Puting National Park hosts the largest protected group)
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