"Various news sources reported last week that handlers at Kushiro Municipal Zoo in northern Japan tried, and failed, to mate two polar bears before realizing that both Tsuyoshi and Kurumi are female. Tsuyoshi had been misidentified as a male three months after birth, and it took zookeepers six months (looking for signs of amorous activity while the two lived together) to recognize their mistake. A representative from the zoo, Masako Inoue, noted that it's not uncommon to mistake a polar bear's gender. Why is it so difficult to distinguish boy polar bears from girl polar bears? Because they're so furry. The polar bear penis is similar to a dog's: It is nublike, with a baculum (a bone) that extends when the animal is excited. But long hairs cover polar bears' reproductive organs, making it hard to determine gender by sight alone. (Even scientists who observe polar bears having sex may find that fur obscures the penis.) Males do have slightly longer hair than females at the tip of their penile "sheaths" (skin that surrounds the penis). And females, in turn, have long vulvar hairs underneath their tails. But unless a polar bear is anaesthetized, zookeepers can't get close enough to make out these differences." [Slate]