Two female polar bears fail to procreate - Japanese zookeepers surprised

Fom The Telegraph:

Tsuyoshi the polar bear is seen at Kushiro Municipal Zoo in northern Japan. Puzzled Japanese zookeepers have cleared up a mystery over a lack of chemistry between a couple of polar bears, as both have turned out to be female.

Experts had initially thought the problem could be due to bi-polar disorder. But the discovery that both bears were female provided zoo employees with a more likely explanation.