A community of mandrills at a zoo in the UK has been seen doing something no other members of their species appears to do, covering their eyes with one hand.
The mandrills at Colchester Zoo have been doing this for more than a decade, and it is believed they may be using the gesture as a sign that they do not want to be disturbed by others.
All 23 members of this group have been observed performing this gesture, often with their elbows raised in the air, sometimes for more than 30 minutes.
Researchers believe one individual began using the gesture, which was copied by others, and it developed a meaning.
"The creation of this gesture by monkeys suggests that the ability to cultivate shared meanings using novel manual acts may be distributed more broadly beyond the human species," states Mark Laidre, an integrative biologist at the University of California, Berkeley, in a research article in PLoSONE.
Laidre studied 19 other communities of mandrills in Europe, North America and Africa; and found no other incidents of eye covering.
Other primate observers he spoke to reported that this was something they had never seen.
"While I imagine that other primate species might occasionally cover their eyes, the behavior does appear fairly unique," he told National Geographic.
"And at least in mandrills, very few of the hundreds of individuals I have observed around the world do this behavior. So it is obviously special in mandrills, and may not be very common in other species either."
Zookeepers first noticed a female mandrill called Milly make the gesture in 1999, but its significance was not realized until Laidre visited in 2007.
He spent 100 hours studying the animals and noted that they were always stationary when making the gesture, and that the rate at which others approached dropped dramatically during this time.
Only seven mandrills were seen covering their eyes, and all were at the low end of the social hierarchy.
"Mandrills have never once, in the lifetime of this gesture, been observed using this in a way that is directed toward humans," Live Science quoted Laidre as saying. "It is done irrespective of humans."
Mandrills, the largest of the monkeys, live in the rain forests of Africa. The are very colourful. with blue and red skin on their faces and bright rumps.