Orangutan communication resembles a game of charades, a study suggests.
Orangutans intentionally modified their gestures to get what they wanted
Researchers from St Andrews University have shown that the animals intentionally modify or repeat their signals to get their messages across.
The scientists said they believed all great apes could have this capability, suggesting that the skill may have evolved millions of years ago.
The study, which is published in the journal Current Biology, involved six orangutans living in two zoos.
Richard Byrne, an evolutionary psychologist and an author on the paper, said: "We were interested in the intentions behind communication.
"When humans communicate, we routinely use our knowledge of what our audience knows and what they don't know automatically.
"We wanted to find out whether the great apes, that have so much flexibility with their communication, do the same thing."
Bananas v leeks
To find this out, the researchers set up a situation where six captive orangutans from Durrell and Twycross zoos were presented with a keeper who had treats, such as bananas, and blander food, such as leeks or celery.
The animals gestured to attract the keeper's attention so the tasty treat would be passed to them.
However, once the orangutans had done this, the keepers did one of three things: they either handed them the treat, handed them the bland food or handed them half the treat. The scientists then recorded their reactions.
"When the keeper gave the orangutan the really nice food, understandably, that was the end of it," explained Professor Byrne.
"But when the keeper pretended to fail to understand the original gesture and gave the wrong food, the orangutans stopped using the gestures they had used before and started using some different gestures," he explained.
"And when the keeper half understood and gave the orangutan part of the treat, the orangutans started to repeat the same gestures that they had used, but they would repeat them even more enthusiastically."
Professor Byrne likened it to a game of charades.
He said: "Part of the skill is to do the miming and the gesturing in the cleverest way - but also you are paying attention to what your team is guessing, and you tailor what you do next to what they are doing." Effectively, the orangutans were able to take into account the states of knowledge, ignorance and partial knowledge of the keeper and react, said Professor Byrne.
Given that orangutans are the most distantly related great ape to humans, the scientists believe that all great apes would prove to have this skill.
"This ability to take into account how much individuals have understood you, and to modify what you do next, is probably quite an ancient one in the human lineage," explained Professor Byrne.