Orang-utans have confounded naturalists by learning to swim across rivers and to fish with sticks.

Naturalists were shocked to see the apes swim across a river to gain access to some of their favourite fruits at a conservation refuge on Kaja island in Borneo. Orang-utans were previously thought to be non-swimmers. The wildlife experts were equally surprised to see an orang-utan pick up a tree branch and stun a fish before eating it. Other apes introduced to the island were seen trying to spear fish with sticks after watching fishermen using rods. The naturalists also noted that the apes quickly worked out that it was even easier to steal fish from unattended lines used by the humans on the island. The unexpected behaviour has been captured in photographs published in the book Thinkers of the Jungle — the Orang-utan Report, by Gerd Schuster, Willie Smits and Jay Ullal, of the Borneo Orang-utan Survival Association. The pictures are thought to be the first to show an orang-utan using a tool for hunting. The apes live in Borneo and Sumatra and are regarded by some as second only to humans in intelligence. They are threatened with extinction as their habitats diminish.

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