Zoologists are baffled by the strange behaviour of a troop of baboons at a Dutch zoo which have become inexplicably withdrawn, apparently fearful of an unseen threat.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.