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When orangutans use gestures to get their point across, they rely on the same basic strategy that people follow in playing the game of charades. Captive orangutans intentionally modify or repeat hand or other signals selectively based on the success or failure of their first attempt, according to a August 2nd study in Current Biology, a publication of Cell Press.
"We were surprised that the orangutans' responses so clearly signaled their assessment of the audience's comprehension," commented Richard Byrne of The University of St. Andrews. "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," he continued. "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--Erica Cartmill and Richard Byrne presented six captive orangutans 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, the researchers provided a catch. Rather than play along all the time, the experimenter sometimes purposefully misunderstood the orangutan's requests. In some cases, they provided only half of the delicious treat; in others, they handed over the yuckier alternative instead.
When the person with whom they were trying to communicate did not meet the orangutans' aims, the apes persisted in further tries, the researchers reported. When partially understood, the animals narrowed down their range of signals by focusing on gestures already used and repeating them frequently. In contrast, when completely misunderstood, orangutans elaborated their range of gestures and avoided 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 is that understanding can be achieved more quickly."
The orangutans' charades-like strategy is one way to construct a shared lexicon from learned or ritualized signals, 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 language.
The researchers include Erica A Cartmill and Richard W Byrne of University of St Andrews in Fife, Scotland, UK. This research was supported by The University of St Andrews School of Psychology and The Russell Trust.
Cartmill et al.: "Orangutans Modify Their Gestural Signaling According to Their Audience's Comprehension." Publishing in Current Biology 17, 1-4, August 7, 2007 DOI 10.1016/j.cub.2007.06.069. www.current-biology.com