Children wearing blue face paint have been told to stay away from a zoo display after it was found they were scaring the monkeys.
The monkeys are part of the zoo's large primate collection
Keepers at Twycross Zoo, Leics, were mystified by the reaction of some marmoset monkeys to certain visitors.
The small monkeys were upset and would sound warning calls without any apparent reason.
An observant zoo manager spotted the blue-faced children were causing the monkeys to react aggressively.
Kim Riley, zoo spokeswoman, said the attraction's face painter now has to warn children who want blue designs that they cannot visit the display.
She said experts could not explain why blue face paint in particular was scaring the small squirrel-like monkeys.
"Whatever it is all three marmosets did not like it. They were running up and down the branches. It was quite an extreme reaction.
"First of all they act all brave and try to warn people off, but then they run away.
"All the other children with their faces painted have not caused any problems whatsoever," she said.
The Geoffroy's marmosets form part of the zoo's walk-through South American tropical house exhibit.
The zoo has the world's largest collection of primate species and is a major world player in global breeding programmes to aid species survival.