In a development that offers new insights into the intelligence of monkeys, animals at a research institute in Japan have used the branches of trees to catapult themselves over an electric fence to escape.

A group of 15 monkeys at Kyoto University's primate research institute in Aichi prefecture escaped from their forest home, which is encased by a five-metre electric fence. The monkeys made their break for freedom by bending and releasing tree branches to fling themselves one by one slingshot-fashion over the high-voltage fence. The pen held about 65 monkeys.

The trees were only two metres high, and there was up to three metres of open ground between the fence and the nearest trees to prevent monkeys from jumping from a tree, over the fence.

"Their jumping power was greater than we thought," said Hirohisa Hirai, deputy head of the institute, said. "We were extremely surprised by the intelligence and the power they used in order to escape."

Despite the intelligence demonstrated by their great escape, the primates then appeared unsure what to do with their freedom: They remained by the gates of the centre and were lured back by scientists with peanuts.

"We think that maybe there was some kind of dispute among the monkeys in the forest, and so this group decided to leave," Hirai said. "Fortunately, they stayed by the fence after escaping as they probably wanted to stay near to the other monkeys."

Scientists have trimmed the trees further in order to prevent a repeat escape.

The Kyoto institution is one of the world's leading primate research centres and has produced a series of internationally recognized studies exploring the social interaction, behaviour and evolution of primates.