Keepers at a Dutch zoo are baffled by the strange behaviour of a troop of baboons, which appear to be fearful of an unknown threat.
According to AFP, the primates at Dierenpark Emmen in the Netherlands are so spooked that they are turning their backs on visitors and huddling together.
Zoologist Wijbren Landman of Emmen Dierenpark said: "They became panicked at the end of the day on Monday, they were hysterical, not jumping around but behaving strangely.
"On Tuesday and Wednesday they just sat in the trees or on the ground, hardly moving or eating."
The zoo said that it was the fourth time in 20 years that the 112 east African monkeys have behaved this way. But the behaviour has not been seen elsewhere, in captivity or in the wild.
Landman says he has no idea what has traumatised the animals, which live on an island in the zoo.
Sky News reports that theories for their unusual behaviour include a predator wandering into their enclosure or an image on a visitor's T-shirt scaring them.
The baboons may also have felt an earthquake or another natural disaster.
Landman added that the "craziest suggestion was that it was caused by a UFO."
Experts have been called in for their opinions but nothing has yet been explained.
Landman said the baboons are at least starting to eat a little.
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