London - To the untrained eye it might seem like just another cheeky bit of monkey business.
But when Milly the mandrill covers her face with her hand she is actually sending a serious message to her fellow primates: “Leave me alone!”
Experts believe the 15-year-old female invented the gesture to warn other monkeys at Colchester Zoo to give her some space.
And, remarkably, the signal has been picked by other members of the group, who use it when they too require solitude.
Evolutionary biologist Mark Laidre believes the behaviour is evidence of social culture among the mandrills, which are the largest species of monkey and are best known for the eye-catching colouring on their faces.
Significantly, the sign language is unlikely to have been influenced by human activity because, unlike apes, dolphins and dogs, mandrills do not mimic humans.
Mr Laidre expects further research will uncover other monkeys using cultural gestures - suggesting “the capacity to communicate with the hands in a meaningful way may have existed a long time before humans came on the scene”.
Zookeepers first noticed Milly, a shy, low-ranking member of her group, covering her eyes - mimicking the famous “see no evil” gesture - in 1999, when she was three.
At first, staff thought she was plagued with poor eyesight or simply shading her eyes from the sun.
It was only when Mr Laidre, of the University of California, spent 100 hours studying the mandrills in 2007 and 2008 that he noticed the significance of the gesture.
“I’d never seen this before - I knew it was very interesting,” he said.
“By covering their eyes with their hands, individuals possibly conveyed to others that they wanted to be left alone and this message may have been respected as a ‘do not disturb’ sign.”
The signal, which can last up to 30 minutes, involves placing the hands loosely over the eyes but with the fingers parted to keep watch.
While the hand is in place, the rate at which other monkeys approach or touch the individual was seen to drop dramatically. Those who rank lower in the social order also use the technique to avoid attacks from more dominant group members.
As the discovery appears to be unique to Colchester Zoo, it suggests it is a local phenomenon that arose naturally in the community of 25 mandrills.
Curator Sarah Forsyth said: “We believe Milly made up the signal and over the past five years some of the younger mandrills have picked it up. We’re not sure why she started doing it but it could be as simple as ‘I can’t see them, so they can’t see me’. It really does show you how intelligent mandrills are.”
The monkeys are found in the wild in Africa, mainly in the rain forests of Cameroon, Gabon, Equatorial Guinea and Congo, where they live in large groups often hundreds strong. - Daily Mail