When using gestures to get their points across, orangutans rely on the same basic strategy that humans follow when playing the popular game and intentionally modify or repeat hand (or other) signals based on the success or failure of their first attempt.
Professor Richard Byrne of the School of Psychology said, "We were surprised that the orangutans' responses so clearly signaled their assessment of the audience's comprehension. Looking at the tapes of the animal's responses, you can easily work out whether the orangutan thinks it has been fully, partially, or not understood, without seeing what went before."
"This means that, in effect, they are passing information back to the audience about how well they are doing in understanding them, hence our 'charades' analogy. In playing the game, you want primarily to convey your meaning non-verbally - as does the orangutan - but secondarily to help the team get your meaning by giving them hints as to how well they are doing."
To find out whether orangutans intentionally communicate with people through gestures - a skill earlier attributed to chimpanzees - PhD student Erica Cartmill and Professor Byrne presented six orangutans in Jersey and Twycross Zoos with situations in which one tempting and one not-so-tempting food item had to be reached with human help.
But to test the orangutans' strategy, there was a catch. Rather than play along all the time, the experimenter sometimes purposefully misunderstood the orangutan's requests, providing them with only half of the delicious treat in some cases and, in others, handing over the less pleasant alternative instead.
When the person they were trying to communicate with did not meet orangutans' aims, they persisted with further attempts, the researchers reported. When partially understood, the animals narrowed down their range of signals, focusing on gestures already used and repeating them frequently. In contrast, when completely misunderstood, orangutans elaborated their range of gestures, avoiding repetition of 'failed' signals.
"The response showed that the orangutan had intended a particular result, anticipated getting it, and kept trying until it got the result," Cartmill said. "The orangutans made a clear distinction between total misunderstanding, when they tended to give up on the signals they'd used already, and use new, but equivalent, ones to get the idea across, and partial misunderstanding, when they tended to repeat the signals that had already partially worked, keeping at it with vigor. The result was that understanding could be achieved more quickly."
The orangutans' charades-like strategy is one way to construct shared meaning from learned or ritualised signals in the absence of language, the researchers concluded. Further investigation of communication among apes may therefore provide insight into the pre-linguistic devices that helped construct the very earliest forms of hominid language.
The findings are published in Current Biology (2 August 2007).
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