A zoo in the Netherlands is trying to understand what has spooked a group of baboons, with the monkeys huddled together and hardly eating.

The troop of baboons has become inexplicably withdrawn, apparently fearful of an unseen threat.

The 112 east African monkeys started behaving strangely on Monday and even turned their backs on visitors at Emmen Zoo in the northern Netherlands.

Zoologist Wijbren Landman said he has no idea what is traumatising the monkeys, who were all born in captivity.

"They were hysterical, not jumping around, but behaving strangely," he said.

"On Tuesday and Wednesday they just sat in the trees or on the ground, hardly moving or eating."

No other animals at the zoo have shown similar signs of hysteria.

Staff said it was the fourth time in 20 years that the baboons have behaved this way, but such behaviour has not been seen elsewhere.

"I've heard lots of theories but nothing that holds water," Mr 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.

There are also suggestions they felt an earthquake or other natural disaster.

"The craziest suggestion was that it was caused by a UFO," Mr Landman said.

"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."

Despite the lack of an explanation for the behaviour, the baboons have at least started to eat a little.

"Some of them started eating little bits of apple and we gave them pellets with vitamins and minerals," Mr Landman added.

"They're not yet 100 per cent 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."

ABC/AFP