Handlers of a popular polar bear, brought to mate with a female in a zoo in northern Japan, found their breeding plan was doomed when they noticed that he, in fact, was a she.
Tsuyoshi, a four-year-old, 200kg cream-coloured polar bear, had been living in harmony with a female polar bear since June.
Masako Inoue, a zookeeper at the Kushiro Municipal Zoo, said the two often play together.
'We thought he was a male, so we never had any doubts as we took care of him,' she said.
'But one day we realised that the two bears urinated in the same way, and we thought, is that how males do it? And once we started to look at things that way, we weren't quite so sure.'
After two DNA studies of Tsuyoshi's hair and a manual examination, the zoo found out the bear was female.
'We do have mixed feelings,' said Inoue. 'But because Tsuyoshi was supposed to be a male, she came here, and because she came here, we were able to take care of her since she was very small.'
It is not uncommon for the sex of polar bears to be misread, Inoue said, as their long hair makes it difficult to distinguish, especially when the bears are young.
Tsuyoshi was pegged as a male three months after birth.
The Kushiro Municipal Zoo will talk with other zoos in the area to see what to do about their breeding plan, she added.