Orangutans normally steer clear of water. In the wild they rarely go near rivers and lakes, to avoid the crocodiles and snakes that lurk there. So it came as a surprise to conservationists when a group of orphaned orangutans that had been relocated to Kaja Island in Borneo started getting wet for all sorts of reasons: one pair was even seen having sex in water.

"My guess is that the male chose the location because there was less chance of him being interrupted by other, more dominant males," says Anne Russon of York University in Toronto, Canada.

Swimming

"Orangutans are famous for their fear of water," says Russon. "They have high body densities and can't help but sink." They're such lousy swimmers that some zoos have stopped surrounding enclosures with moats - too many orangutans have drowned.

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© Anne Russon
"One day we saw an adolescent orangutan called Sif wade into deep water, hunker down and then lunge forward making simple paddling movements with her arms and legs," says Russon. "It was kind of like a bad dog paddle." Sif didn't get all that far - about a metre.

Bridge-building

Others in the group have found drier means of crossing water: they've learned how to build bridges. "They deliberately bend slender trees over and use them as bridges to travel over broad stretches of water," says Russon. "The trees remain partially bent after the first use, and after several uses they stay permanently bent into these positions." And although each bridge is engineered by a single orangutan, the structure is used by all the orangutans on Kaja. "Nothing like this has been seen anywhere else," says Russon.

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© Anne Russon
Fishing

"Orangutans aren't supposed to eat fish, let alone hunt them," says Russon. "They're primarily fruit eaters, and they rarely hunt." So it came as a surprise when the Kaja orangutans were seen grabbing live fish from streams and eating them.

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© Alain Compost
"In 2005, we saw them scavenging for dead fish that had washed up on the shore during the dry season," says Russon. That probably gave the orangutans the idea of developing more complex tactics to actively hunt down fish. "The orangutans probably tasted the dead fish, thought, 'Hey, this isn't too bad,' and tried to figure out how to get some more," says Russon.

More fishing

"There can't be more than half-a-dozen observations of orangutans hunting in the wild, and in all those cases they were targeting grey tree rats, slow lorises, gibbons or young birds - never fish," says Russon.

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© Alain Compost
She is sceptical of claims made in 2008 that orangutans hunt fish using wooden harpoons. "The orangutan that was photographed sticking a 'spear' into the water was not observed catching or eating any fish," she says. "They use sticks to test the depth of water before wading into it, so that's a more likely explanation."

Fishing for sunken fruit

It's the wet season and Markisa, a 7-year-old dominant female, is groping for sunken fruit in the mud beneath a Rengas tree. Kaja Island has seasonal floods that submerge some of the Rengas trees on the island. Although dry fruit are available, some orangutans prefer to trawl underwater, despite the risk of crocodiles and water snakes.

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© Anne Russon
"The best explanation I could come up with was that the water may soften the typically hard, nut-like Rengas fruit, so that they're easier to eat," says Russon.

For some orangutans such as Leonora, an adolescent, subordinate female, sunken fruit is a critical source of food. "Markisa and other orangutans often steal Leonora's food on dry land," says Russon. "But out there in deep water, her dinner is safe."

Drinking from rocks

The orangutans of Kaja Island use everything they can get their hands on as cups. "I've seen them crinkle up plastic bags, place them in water and then shake the water out of the plastic crevices into their mouths," says Russon. In this picture, Yuni, a 4-year-old female, is showing Erika, a 2-year-old, how to drink water from a stone.

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© Anne Russon
Journal reference: Journal of Comparative Psychology, DOI: link