Zookeepers at the Emmen Zoo said the baboons have stopped eating, fighting and grooming each other, Spiegel Online reported Monday.
The officials said the animals now spend most of the day sitting motionless with their backs toward zoo visitors.
"For us it's a mystery, it's the fourth time this has happened and we haven't got a clue what could have caused this," zoo biologist Wijbren Landman said. "If the leader is upset then the whole colony is upset. The leading men in the group were shocked by something. What it was we don't know."
Zookeepers said the latest problems began July 29, when the baboons became hysterical. The animals calmed down Tuesday and the depression seemed to have set in.
Landman said the zoo saw baboons in similar states in 1994, 1997 and 2007.
"In 2007 they were all looking in the same direction. It wasn't like that this time," Landman said.
"Some people suggested that maybe foxes or an escaped snake got close to them, but we're not missing any snakes," Landman said. "This hasn't happened at other zoos. But it has been seen in the wild that baboons are upset for a few days after a confrontation with a predator. But we have no lions or hyenas on our baboon island."
He said the baboons seem to be slowly emerging from their apathetic state.
"They are searching for food again and some youngsters are being punished by the older ones for indecent behavior," he said.