Zoologists are baffled by the strange behaviour of a troop of baboons at a Dutch zoo which have become inexplicably withdrawn, apparently fearful of an unseen threat.
"They became panicked at the end of the day on Monday, they were hysterical, not jumping around but behaving strangely," zoologist Wijbren Landman of Emmen Dierenpark in the northern Netherlands told AFP on Friday.
"On Tuesday and Wednesday they just sat in the trees or on the ground, hardly moving or eating," Landman said.
The 112 east African monkeys also turned their backs on visitors.
The zoo said it was the fourth time in 20 years that the baboons have behaved this way. Nevertheless, such behaviour has not been seen elsewhere, either in captivity or in the wild.
The zoologist said he has no idea what happened on Monday to traumatise the baboons, who were all born in captivity and live on an island in the zoo.
"I've heard lots of theories but nothing that holds water," Landman said.
These include that the baboons saw a predator, either a real one or an image of one on a visitor's T-shirt, or that they felt an earthquake or other natural disaster.
"The craziest suggestion was that it was caused by a UFO," said Landman.
"They've been observed to be disconcerted in the wild if they've had a run-in with a predator, but not this type of behaviour," he said.
Despite the lack of an explanation for the behaviour, the baboons have at least started to eat a little.
"Yesterday evening some of them started eating little bits of apple and we gave them pellets with vitamins and minerals," Landman said.
"They're not yet 100 percent but they're improving.
"We've asked experts for their opinions, if they've ever seen anything like it or have an explanation, but that hasn't turned anything up yet," he said.