Ah, the thrill of the hunt. The trusty hounds at your side, howling and panting and dragging you toward your quarry: a lumbering beaver not accustomed to moving on land. You close in. You raise your spear. The beaver suddenly stops, looks over its shoulder at you, and lifts a back leg. It bears its teeth, stares you right in the eye, and proceeds to chew off its own testicles. Then it throws them at you.

Perhaps a bit taken aback by this gesture, you let it scurry away sans gonads, because a medieval hunter like you is only after the precious oil, known as castoreum, those organs bear. The beaver has quite cleverly just saved its own life.

Or at least according to any number of medieval bestiaries, often gorgeously illustrated tomes that cataloged nature’s critters—the real, the totally imagined, or the slightly embellished. (Interestingly, bestiaries noted that when pursued the wolf similarly chews off a tuft of hair on its back that humans covet as an aphrodisiac.) And like many creatures in these bestiaries, the beaver carried a moral lesson: If a fella wants to be chaste, he has to cut off his vices and throw them at the devil, who will then leave him alone. Which goes to show that they just don’t make moral lessons like they used to.

This tale begins with the ancient Egyptians, who had a hieroglyphic depicting a beaver chewing off his testicles as a representation of the punishment for adultery among humans in their society. In the West, it was Aesop who first wrote of the myth in his famous fables: “When pursued, the beaver runs for some distance, but when he sees he cannot escape, he will bite off his own testicles and throw them to the hunter, and thus escape death.” Pliny the Elder, the first great naturalist (though also a fairly reliable peddler of untruths), echoed this in his encyclopedia Natural History, which for hundreds and hundreds of years served as a trusted scientific authority.

A beaver's default demeanor is deviousness. Some guy named Steve/Wikimedia

So we find ourselves in 1188, when one Gerald of Wales embarks upon a much more detailed account of the beaver’s strange lifestyle in his Journey Through Wales. He confirms in rather eloquent language that the beaver will “ransom his body by the sacrifice of a part,” and adds that once castrated, “he has the sagacity to run to an elevated spot, and there lifting up his leg, shows the hunter that the object of his pursuit is gone.” While the beaver is hunted in the East for the medicinal properties of its testicles, Gerald notes, in the West it of course is also hunted for its pelt. So the beaver "cannot save himself entirely, yet, by a wonderful instinct and sagacity, he endeavours to avoid the stratagems of his pursuers.”

Then there’s the matter of hunting beaver for food, which according to Gerald tastes like fish. This is super convenient if you’re Catholic, and you’re banned from eating any meat other than fish on Fridays. So according to Gerald, “in Germany and the arctic regions, where beavers abound, great and religious persons, in times of fasting, eat the tails of this fish-like animal, as having both the taste and color of fish.” (It’s worth noting that the same trick was supposedly once applied to the capybara of South America, the world’s largest rodent, weighing in at up to 150 pounds. It spends its days largely wading through swamps, so many Venezuelans consider the critter to be more fish than mammal. Indeed, legend goes that clergy there in the 1700s asked the Vatican to officially classify it as such.)

That vaguely beaverish thing is in fact a beaver. You can tell from its insistence on biting its testicles off. The British Museum

And just one more of Gerald’s beaver oddities before we get back to the testicles: He claims that when constructing their dams, beavers “make use of the animals of their own species instead of carts.” A few individuals obey “the dictates of nature” and “receive on their bellies the logs of wood cut off by their associates.” Holding tight with their feet, and having “transverse pieces placed in their mouths,” the unfortunate workers are "drawn along backwards, with their cargo, by other beavers, who fasten themselves with their teeth to the raft.” They are, in essence, living skis.

OK, the testicles. Along comes the 17th century and with it a polymath by the name of Sir Thomas Browne, who had somewhat of a nose for sniffing out nonsense and ripping it to pieces. He notes quite rightly that a beaver’s testicles do not hang outside the body as ours do—they’re situated internally. “And, therefore, it were not only a fruitless attempt, but an impossible act, to eunuchate or castrate themselves; and might be an [sic] hazardous practice of art, if at all attempted by others.”

Sir Thomas Browne: the look of a man who's sick and tired of your silly misconceptions. National Portrait Gallery, London/Wikimedia

It turns out what our ancients had confused for testicles were in fact small external bumps that connect to internal castor sacs, which produce the oily castoreum that hunters so prized. By rubbing its bum all over logs and rocks and such, the beaver marks its territory with the oil’s musky, vanilla-esque scent. And if you were thinking, if it smells like vanilla, why don’t we use it in our food?, then you’d have something in common with the first sick turkey who 100 years ago started putting it in vanilla flavorings. And if you were thinking, I hope that’s not happening anymore, then I hate to break it to you, but it sometimes is. Oh, and also: Manufacturers need only mention it on the package as a “natural ingredient,” so good luck knowing when you’re eating it.

Anyway, the very word "castoreum" seems to have an obvious cousin: castration. But here, says Sir Thomas Browne, is what helped us get into this mess in the first place. The Latin word for beaver, castor (and by extension, castoreum), doesn’t share the same root as castration at all. Castor comes from the Sanskrit for musk. Browne reckons this etymological confusion played no small part in helping perpetuate the myth.

So our apologies for the mixup, good beavers. We promise we won’t go around accusing you of biting off your own testicles anymore. And we won’t confuse you for a fish any longer, or assume that you use each other as skis. Though that last one is pretty excellent. You can keep that if you like.

References:

Brown, T. (1894) The Works of Sir Thomas Browne. George Bell and Sons: London

Gerald of Wales (1912) Journey Through Wales. Dent