Orang-utans behave as if they are playing charades to express what they want, according to research published today.

Scientists at St Andrews University found the animals use gestures to get their point across, like humans playing the game.

If they are not understood, the animals change their movements to try to get their point across.

Researchers studied six orang-utans at Jersey and Twycross zoos in England, offering the animals tempting food such as bananas and bread, and less tasty food such as celery and leeks, which the orang-utans could reach only with human help. The animals used gestures such as pointing and waving to communicate which food they wanted.

The researchers sometimes pretended to misunderstand the animals and gave them the wrong food, or only half of the tempting food they wanted.

The orang-utans changed their range of gestures to express what they wanted when their first attempt to communicate failed.

Erica Cartmill, a PhD student, said: "The orang-utans made a clear distinction between total misunderstanding, when they tended to give up on the signals they'd used and use new ones, and partial misunderstanding, when they tended to repeat the signals that had partially worked, keeping at it with vigour.

"The result was that understanding could be achieved more quickly."

Professor Richard Byrne of the school of psychology, who was also involved in the research, said: "In effect, they are passing information back to the audience about how well they are doing in understanding them, hence our charades analogy."

The findings are published in Current Biology.