The bizarre behaviour of a troop of 'hysterical' baboons at a Dutch zoo has been blamed on everything from an earthquake to a UFO.
Ordinarily seen scampering around on their island at the Emmen Dierenpark in the northern Netherlands, the gang of 112 monkeys have been sitting frozen on the ground and turning their backs on visitors this week.
The baboons have even lost their appetites, say baffled zoologists at the park, who have heard various theories as to what may be behind the animals' strange behaviour.
On Tuesday and Wednesday they just sat in the trees or on the ground, hardly moving or eating.'
Mr Landman described various theories put forward to explain the bizarre scenes at the zoo, with some suggesting the baboons may have spotted a predator either a real one or an image of one on a visitor's t-shirt or felt the rumblings of an earthquake.
The zoologist admitted he was yet to hear an explanation that 'holds water'.
'The craziest suggestion was that it was caused by a UFO,' Mr Landman said.
Baboons at the zoo, who were all born in captivity, have behaved this way four times in the past 20 years, the team at Emmen Dierenpark said.
But their activity has not been observed among any baboons anywhere else, either in captivity or in the wild, according to a report in the Huffington Post.
Mr Landman said the animals have been observed acting disconcerted if they come across a predator in the wild but nothing on the scale of the behaviour displayed by the creatures at the Dutch zoo this week.
Experts consulted by staff at Emmen Dierenpark have so far failed to shed any light on the mystery, but the animals have shown signs of returning to normal.
Some of the baboons managed to eat some small pieces of apple, and all were given pellets to provide them with vital vitamins and minerals. Watch the video below.