Zookeepers had been baffled by 2 polar bears that wouldn’t mate.  They finally got to the bottom of the matter by putting one of the bears under anesthesia and looking at its, well, bottom.  They found that the male polar bear isn’t a male at all—he is a female.  That explains why s/he won’t mate with the other female.  Astonishing!  Good thing it only took 5 months to solve this “mystery.”