Why Do Dogs Have Those Weird Ridges on Their Lips?

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Have you ever noticed the ridges on your dogs lips? If you look closely at your dog's mouth next time they yawn, especially near the back corners, there's a row of small wavy bumps. They're easy to miss until you notice them, then you can't unsee them. Vets call these rugal folds, but most people just call them serrated lips. But, what is their purpose?

Honestly, nobody knows for certain the purpose of the serrations. There's no single confirmed function, with no formal study settling the question. But, like all mysteries, there are several theories, and most of them sound convincing. Some, well, not so much.

Theory 1, A conveyor belt for food

A popular explanation is mechanical. The folds act almost like the ridges on a conveyor-belt. Those tiny bumps help guide food toward the back of the mouth as a dog chews. Some vets compare the ridges to crampons you'd put on your boots in the winter. They give the mouth extra grip on food so it doesn't slip out mid-chew.

The same shape may also help with drinking. Dogs don't sip water; they curl their tongue backward and use it like a small ladle. The folds along the lips may help create just enough suction to make that flinging of water into their mouths process more efficient. Some also suggest the ridges make the water flinging slightly less messy, though anyone with a dog knows "slightly" is doing ~a lot~ of work in that sentence.

Theory 2, Built-in floss

An idea, from about 100-years-ago credited to anatomy professor Frederick Wood Jones back in 1918, is that the ridges help keep a dog's teeth clean. As the lips move against the teeth during chewing, the bumps may act like tiny scrapers. The same way ridges and nubs on modern dental chew toys do. It's a neat parallel: we design chew toys with ridges to clean teeth, and it's possible dogs came with a built-in version.

Theory 3, A grip for carrying things

Dogs don't have thumbs, so their mouths do a lot of the handsy-work: carrying toys, retrieving a ball, hauling a stick twice their size across the yard. The serrations may give a bit of extra traction, helping an object stay put between the lips instead of sliding loose.

This theory sometimes gets extended to puppies, with the idea that the ridges help a mother carry her pups gently by mouth. It's a nice thought, but most experts are skeptical on this theory. The doggy-mama's instinct to control bite pressure is probably doing the real work, not the shape of her lips.

Theory 4, Protection from their own teeth

Dogs' teeth, especially the canines, overlap when the mouth closes. Some researchers think the folds exist to cushion the lips from that overlap, keeping soft tissue from getting nicked or irritated every time the jaw shuts.

Theory 5, Extra sensory real estate

The ridges are packed with nerve endings, giving dogs another way to gather information about all sorts of things. Texture, temperature, and taste before something even reaches their tongue. Combined with a dog's whiskers, this makes the whole mouth area a surprisingly sensitive sensor array and not just a set of teeth.

Theory 6, Probably all of the above

Traits like this rarely evolve for a single reason. Natural selection loves to make things multi-purpose. So, a structure that helps with grip, cleaning, and sensitivity all at once is exactly that kind of multi-purpose feature.

What we do know: not every dog shows equally pronounced serrations. Breeds with looser, jowlier lips, like Bloodhounds or Mastiffs, tend to show them more dramatically than breeds with tighter mouths. Next time you're scratching behind your dog's ears, take a peek at those lips. You're looking at one of those small anatomical mysteries that's been sitting in plain sight the whole time.

Questions about your pet's health or behavior? We're not vets, but we are pet people who love talking about this stuff. Contact your vet if you have any health concerns for your pet.