Why do cats and dogs eat fur?

The full narration of the video.

The moment a cat hops off your lap, it licks every spot you just petted, stroke by stroke. Most of the fur it licks off gets swallowed. Dogs sometimes gnaw at one patch on a leg and just won't stop. Fur doesn't taste good. So why swallow it? Let's pick one hair and see where it goes.

Stop one: the cat's tongue. Up close, it's covered in hundreds of tiny backward-curving hooks, like a comb growing the wrong way round. Every hook points toward the throat, like a one-way street. Fur can only travel inward; it can't back out. And a cat has no hands to pick the fur off, so it goes the only way it can: down.

So a cat eating fur isn't greed. It's a side effect of grooming. The longer the coat and the more it grooms, the more it swallows. Stop two: the stomach. Fur is a protein cats can't digest, so it doesn't break down. Most of it drives on down the road of the gut and leaves with the poop.

A little stays behind and tangles with other hairs. It grows and jams at the exit. Here the road idea breaks down: stuck cars can't turn around and drive the wrong way. A cat's stomach can. One heave, and the clump comes back out the mouth. That's the soggy hairball on your floor. It's long and thin because the food pipe squeezed it into shape.

Now dogs. A dog's tongue is soft, with no backward hooks. Dogs groom much less, so fur rarely slips in by accident. When a dog eats fur, something's usually up. Most often it's an itch: flea bites, or an allergy to pollen or dust mites. So it scratches with its teeth. Another is boredom. Home alone all day, bored or anxious, a dog may lick and chew on and on.

Cats that lick themselves bald are often itchy too. One study checked twenty-one cats thought to be doing it from stress. Only two really were. Eating fur has older roots, too. Wolves and wildcats swallow their prey skin, fur and all. Researchers found that in wolves, swallowed fur wraps around bone shards and carries them out. In Belgium, cheetahs fed whole rabbits made less smelly gut gas than ones fed plain meat.

But only about a dozen cheetahs were tested, so the evidence is still thin. So when should you see a vet? A hairball every week or two isn't unusual. Retching with nothing coming up, or a day or two without eating, don't wait. A dog that chews a bald, red patch? Get the skin checked; don't just blame the dog.

And brush them often. Fur that ends up in the brush never has to make the trip. Cats swallow fur because their tongues comb too well. Dogs chew it because something's bothering them. Where did your cat leave its last hairball?