Japanese zookeepers have wondered for a while why two polar bears that they had hoped would mate have shown little interest in each other.

The answer came this month when the zoo discovered both the bears were female, the zoo said Wednesday.

Tsuyoshi, a 4-year-old polar bear, and 11-year-old Kurumi have been living in the same enclosure at the Kushiro Municipal Zoo in Hokkaido in northern Japan since June.

The zoo said it thought Tsuyoshi was male because of the bear's appearance. It did not explain further. Male polar bears are generally significantly larger than females.

"Observing his behaviors, we got suspicious as to whether Tsuyoshi was really a male," the zoo said in a statement Wednesday.

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Experts say when polar bears are young, it is difficult to determine their sex because their long hair covers their reproductive organs.

Tsuyoshi was determined to be male at 3 months old.

But earlier this month, the zoo put Tsuyoshi under an anesthetic and learned he was a she.

"I have mixed feelings considering the need for breeding," said Yoshio Yamaguchi, head of the zoo.

Tsuyoshi is very popular with visitors, and Kyodo News agency said the bear would keep her name, even though Tsuyoshi is a common Japanese name for boys.