Orangutans communicate intelligently using an unspoken language of gestures, research has shown.
British scientists who spent nine months observing the great apes in three European zoos identified 40 frequently-used body language signals.
These were employed repeatedly to send messages such as "I want to play", "give it to me", "go away", "follow me", or "stop doing that".
"Play" gestures involved a range of clowning antics, including back rolls, placing objects on the head and blowing raspberries.
"Nudge and shoo" movements meant an ape wanted to be left alone, while a hand-to-mouth "begging" gesture requested food.
Other gestures included hitting the ground, hair pulling, biting the air, swatting, grabbing and walking in tandem with another individual.
Although studies of great ape body language have been carried out before, none have focused on the intentional meanings of specific gestures.
Two scientists from the University of St Andrew's in Scotland observed 28 orangutans at Twycross Zoo in the UK, Apenheul Primate Park in the Netherlands and Durrell Wildlife Conservation Trust in Jersey.
In each case, video recordings were made over a period of three months.
A total of 64 gesture types were identified, 40 of which were used frequently enough for their meaning to be analysed.
The gestures were used to achieve one of six social goals, to initiate contact, grooming or play, request objects, share objects, instigate joint movement or "co-locomotion", cause a partner to move back, or halt an action.
The intention of gestures was tested with a "goal-outcome matching" technique by looking at what an ape did when its signals were ignored or misunderstood.
Apes were found to be more persistent when the response to their gestures was not what they expected.
Richard Byrne and Erica Cartmill reported their findings in the journal Animal Cognition.
They wrote: "Orangutan gestures are made with the expectation of specific behavioural responses, and thus have intentional meaning as well as functional consequences.
"The level of specificity we were able to identify in the intentional meanings of orangutan gestures resulted from our novel use of goal-outcome matching as a means of incorporating signaller intentions into the analysis of signal meaning.
"When paired with a high frequency of intentional use, goal-outcome matching is a strong tool for identifying intentional meaning."