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.
A slightly more reliable, but still visual, way to determine the sex of a polar bear is to watch it pee. If urine seems to be coming from the belly area, it might be a male; from the tail, a female. (The would-be breeders at Kushiro Municipal Zoo first suspected they were dealing with two animals of the same gender upon noticing that they urinated in the same way.) A zookeeper can also check for urine spotting—if the belly-area hair gets wet, probably a male; if the tail-area hair does, a female. But these methods aren't foolproof. Some polar bears—of both sexes—squat when they pee, so it's hard to tell where the urine's coming from. And bears are often wet from swimming, which makes the spotting technique rather difficult.
You might assume a manual check performed while the animal's anaesthetized would do the trick every time—but it doesn't. A poorly trained handler might lift up the tail, not see the vulva (which is quite small if the animal's not in heat), and quickly conclude that the bear is a male without turning it over to feel around for the penis. Or he might mistake an unprotracted penis for a bellybutton (polar bears have outies) and conclude that it's a female. Checks are often done when the bear is young and its genitalia, including the baculum, are still small.
Tsuyoshi isn't the only polar bear experiencing gender trouble: Another zoo adopted a bear it thought was Tsuyoshi's brother—but it's actually her sister. In 1999, the Wildlife Society Bulletin published a paper on using genetics to verify the gender of polar bears. The authors found that, after a harvest in the southern Beaufort, Chukchi, and Bering seas of Alaska by native hunters, "sex was incorrectly determined" for 19 of 139 bears.
Verifying gender isn't usually so difficult with mammals—in fact, it's not hard at all with other, shorter-haired bears. But zookeepers and researchers have trouble with birds, which have a cloaca, or multipurpose opening, rather than gendered genitals. Some rodents can also be problematic.
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Explainer thanks Donald Moore of the Smithsonian National Zoological Park.