A group of monkeys at Colchester Zoo have been observed using gestures and sign language to communicate with each other.
The troop of mandrills can tell each other when they do not want to interact by covering their eyes with their paws, experts have noted.
Evolutionary biologist Mark Laidre explained: "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."
It is thought the gesture was developed by one member of the group, Millie, who passed it on to the others.
Experts believe the signals are a sign of an advanced social culture within the group, as it is something that has never been witnessed before.
The primates at Colchester Zoo are not the only mandrills to exhibit intelligent behaviour, as one animal at Chester Zoo was recently observed using a twig to clean dirt from under its nails.