wanted to be left alone, and this message may have been respected as a 'do not disturb' sign." He found that mandrills who covered their eyes gesture were generally left alone not approached or touched - by their peers. It is believed the signal was first used by Milly and she has passed it on. Scientists have known incidents when monkeys have used sign language for genetic or environmental reasons but this is the first example of its kind with cultural roots. Sarah Forsyth, Colchester Zoo curator, 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 just be as simple as I can't see them so they can't see me." At first the team thought Milly was suffering from bad eyesight and called in vets to check her over. When they eventually discovered she was communicating the team was astounded. It was not until American researcher Mark Laidre visited the zoo in Maldon Road he realised it was a unique gesture. Mrs Forsyth said: "It really does show you how intelligent mandrills are. "We were teaching the dominant male Dume to open his mouth and he picked it up in only a few days. Now he comes up to us with his mouth wide open because he knows he will get a treat. "The hand over the eyes gesture is the only one invented by