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Watch Monkeys Swim for Their Supper in Rare Video

Indonesia's proboscis monkeys are as at home in the water as they are in the trees—an unusual combination in primates.

These Web-Footed Monkeys Are Built For Swimming

Proboscis monkeys have an unusual trait not seen in many other species of monkey—they can swim. Using their partially webbed toes and fingers, they are able to escape predators and cross rivers to find food.

Proboscis monkeys might be best known for their giant, bulbous noses, but scientists are sniffing out another of these monkey’s unique attributes: their swimming abilities.

To discover more about these water-loving primates, conservation biologist Amalia Rezeki recently spent a few weeks on Indonesian Borneo's Bakut Island filming the animals and their behavior in mangrove forests. (Read about swimming pigs and other surprising animals that love water.)

"The main primate in the area that people think of are orangutans. We thought that a video about these monkeys' swimming abilities would help bring some positive attention," says Rezeki, who works with Sahabat Bekantan Indonesia, a nonprofit that works to protect proboscis monkeys, also called bekantan.

Due to loss of their mangrove habitat and hunting, proboscis monkeys are listed as endangered, with fewer than 7,000 animals left in the wild.

Rezeki's expedition discovered several monkeys on the island, suggesting the species is still hanging on.

Natural Swimmers

Proboscis monkeys likely took to the water because "it’s hard to live in a swamp without being able to swim,” says Lee Harding, chief scientist at SciWrite Environmental Sciences and an expert in proboscis monkeys. (Read more about mangroves, forests of the tide, in National Geographic magazine.)

For these primates, swimming serves a practical function: to get food. The animals have to travel far and wide to find the young, tender leaves that make up the bulk of their diet—and swimming gets the job done faster.

Propelled by partially webbed fingers and toes, the monkeys can even swim underwater—although no one knows exactly how long they can hold their breath, according to Liz Bennett, vice president of species conservation at the Wildlife Conservation Society in New York City. (Read more about the monkey with the outlandish nose.)

“The extent that they swim is quite unusual, especially since its close relatives don’t swim,” she adds.

Monkey Business

Harding, who wasn't involved in the recent expedition, has long been fascinated by the species, which live in harems with one territorial male and around eight females. (See an intimate portrait of a proboscis monkey in captivity.)

On his first visit to Borneo with his brother several decades ago, they found out the hard way how males mark their territory and intimidate those who encroach on their land.

“This big male, he was in a tree by the river, and he just started urinating on my brother,” Harding says.

But swimming isn't fun and games for proboscis monkeys. Traveling by water is risky: Crocodiles, pythons, and monitor lizards often prey on the swimming primates.

Vanishing Forests

Another danger to the species is Borneo's disappearing mangroves. Between one-third and one-half of these forests have already disappeared due to timber harvesting and drainage for oil palm plantations, according to the Mangrove Action Project. (See more pictures of mangroves from around the world.)

More and more of Borneo’s growing population is moving to the same lowland areas where the monkeys live.

That proximity—as well as the animals' habit of sleeping in exposed trees—makes the animals particularly vulnerable to sport hunters, Harding says.

Though the government has set aside some areas as refuges for proboscis monkeys, they're still vulnerable to hunting.

"They're becoming more and more threatened, and humans are their biggest threat," says the Wildlife Conservation Society's Bennett. "We need to do a better job of protecting them."

Follow Carrie Arnold on Twitter.