To the untrained eye it might seem like just a bit of a monkey trick. But when Milly the mandrill (, ) covers her face with her hand she is actually sending a serious message to her fellows: "Leave me alone!" Experts believe the 15-year-old mandrill invented the gesture to warn other monkeys at Colchester Zoo to give her some space. And, surprisingly, the signal has been picked by other members of the group, who use it when they too require _ . Biologist Mark Laidre believes the behavior is evidence of social culture among the mandrills. Importantly, the sign language is unlikely to have been influenced by human activities because mandrills do not copy humans. Mr. Laidre expects further research will uncover other monkeys using cultural gestures -- the ability to communicate with the hands in a meaningful way. "By covering their eyes with their hands, possibly conveyed to others that they wanted to be left alone and this message may have been respected as a 'do not disturb' sign," said Mr. Laidre. While the hand is in place, other monkeys are not likely to approach or touch the monkey. Those who rank lower in the social order also use the technique to avoid attacks from more powerful 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." According to Mark Laidre,   _  .
human activities affect mandrills' sign language animals are slow in picking up information mandrills' social culture has nothing to do with humans mandrills are the largest species of monkey in the world