A brood of baboons living in a Netherlands zoo have been spooked for days. Staff at Emmen Dierenpark say the primates have been huddled together, barely eating or moving.

It’s the fourth time in 20 years that the 112 east African bamboos have been traumatized for some unexplained reason, BBC reports. The hamadryas baboons have also turned their backs on zoo visitors.

"They became panicked at the end of the day on Monday, they were hysterical, not jumping around but behaving strangely," zoologist Wijbren Landman told Agence France-Presse on Friday.

Landman consulted with a baboon expert who said the hysteria could be triggered by seeing a predator. Since the primates were born and raised in captivity, they could have seen an image of one on a visitor’s T-shirt or felt an earthquake or natural disaster.

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

Zoo officials first saw a change in the baboon’s behavior on Monday evening. "We were going to bring them to the night enclosure -- it normally takes a minute for all 112 to enter, but it took more than an hour to get them all inside. Then the next morning it was a problem to get them out, and then they were immediately sitting in the trees and on the rocks doing nothing at all," he said.

Now, the baboons have started eating apples – a sign of progress. "They're not yet 100 percent but they're improving,” Landman said. Nearby animals at the zoo including lemurs, elephants and kangaroos are acting normal, he added.

Hamadryas baboons, native to semi-deserts of northeast Africa and the Arabian peninsula, spend most of their time on the ground. They never stray far from watering holes, living in clans led by male leaders.

"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 behavior," Landman said. "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."