Swimming



"Orang-utans 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 orang-utans have drowned.



"One day we saw an adolescent orang-utan 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.



(Image: Anne Russon)

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 orang-utan, the structure is used by all the orang-utans on Kaja. "Nothing like this has been seen anywhere else," says Russon.



(Image: Anne Russon) Advertisement

Fishing



"Orang-utans 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 orang-utans were seen grabbing live fish from streams and eating them.



"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 orang-utans the idea of developing more complex tactics to actively hunt down fish. "The orang-utans probably tasted the dead fish, thought, 'Hey, this isn't too bad,' and tried to figure out how to get some more," says Russon.



(Image: Alain Compost)

More fishing



"There can't be more than half-a-dozen observations of orang-utans 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.



She is sceptical of claims made in 2008 that orang-utans hunt fish using wooden harpoons. "The orang-utan 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."



(Image: Alain Compost)

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 orang-utans prefer to trawl underwater, despite the risk of crocodiles and water snakes.



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



(Image: Anne Russon)