In research presented in Current Biology, PhD student Erica Cartmill and Professor Richard Byrne, both researchers in the University of St Andrews School of Psychology, presented six captive female orangutans with two of their favorite foods, bread and bananas, along with two unfavourite foods, celery and leeks.

When humans deliberately misunderstood or partially understood the apes' expressed preferences, the subjects showed a range of alternative gestures in order to improve communication and get what they wanted. Some of the large apes even tried to trade celery and leeks for the bread and bananas.

"This reaction to another's state of mind is an essential component of human language," says Erica Cartmill, "and the charades-like strategy illustrates how an individual in a prelinguistic society might still have been able to communicate their desires effectively."

Byrne says gestures used by the apes do not necessarily have "word-like meanings". Instead, they are more context-specific. Cartmill explains: "The orangutans in our study were all using natural gestures and had never been taught specific signs for different objects. The gestures they used varied from individual to individual."

This charade-like behaviour has so far only been observed in zoos, but Cartmill says she expects wild orang-utans to possess the same communication abilities: "A system for achieving common understanding more quickly by adjusting your communication to how well your recipient understands you would be particularly useful in a semi-solitary species," she says.