Dolores has lost almost all her hair at Leipzig Zoo
Bears in a zoo in eastern Germany have lost their fur, but international experts cannot work out why.
Three spectacled bears in Leipzig Zoo are in various states of baldness, with the worst being hairless all over.
Zoo curator Gerd Noetzhold said he had discovered that zoos throughout Europe and further afield had encountered the same problem, but no-one knew why.
One expert suggested it could be caused by climate and the diet of the bears, whose native habitat is South America.
The bears come from the Andean mountains of Ecuador, Peru and northern Bolivia.
The bears - named Dolores, Bianca and Lolita - are suffering from itchiness as well, so animal keepers apply ointments to soothe their skin.
"This problem with the spectacled bears is not just in Leipzig," Mr Noetzhold told the BBC World Service.
"There are other zoos in Europe and overseas having the same problem.
"And so we've had an international working group of zoo vets looking at this for some time already."
Gerard Baars, director of the International Bear Foundation, said he had never seen such a condition before.
"I could hardly believe it is a bear although I have been dealing with bears all my life," he told the BBC's Europe Today programme.
Symptoms first appeared in the animals about two years ago.
"I suggested that feeding and climate can both be a factor because they are tropical animals that we have relocated to our climates," Mr Baars said.
Mr Baars, who said he was to travel to Leipzig Zoo to offer advice, urged the zoo to make special preparations for the winter.
"The bears should be kept indoors and they should make an imitation of a tropical climate with hot temperatures, humidity, straw and bedding on the floor."
The mountain bears have a seasonal pattern influenced by climate, behaviour and food, he said.
"We in zoos are not very good at imitating natural seasonality," he added.