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Only four species in the cat family can roar: lions, tigers, leopards, and jaguars. And zoologists are pretty sure those four species can’t purr. But why?

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I have a cat.

Her name is Cameo, she’s adorable, and recently she peed inside of a potted plant. But can you imagine how amazing it would be if she could roar… while peeing inside of a potted plant?

She’d be like, “give me some treats” and then like ferocious roar... it would be so cute! But domestic cats can’t roar. Only four species in the cat family can: lions, tigers, leopards, and jaguars.

Here’s the weird thing, though: zoologists are pretty sure those four species of cats can’t purr. So even though my cat can’t roar, the reason probably has a lot to do with the things in her throat that let her purr. Now, we still aren’t - this is amazing - totally sure how cats purr.

There’s no, like, “purr box” that we can locate in a cat, and no one’s ever stuck a purring cat in an MRI to find out exactly what’s happening. But we’ve known for a while that it probably involves the larynx, aka the voice box. Back in 1834, a British zoologist named Richard Owen noticed that there was an anatomical difference between the cat species that purred and the ones that roared: roaring cats had a more flexible hyoid.

The hyoid is a structure that supports the tongue and larynx. In humans, it’s horseshoe-shaped. It’s basically the first bone under your chin in the front of your neck—although you shouldn’t be able to feel it from the outside, so maybe don’t go squeezing up around in there.

On the murder shows, it’s how they know people called strangled when they got killed. They’re always like, “the hyoid is broken! That’s how we know that the murder was the squeezy neck kind.” In cats, the hyoid is more of a hook that hangs down and connects the back of the skull to the front of the larynx and the base of the tongue.

In most species of cat, including the domestic cat, the hyoid bone is very … bony. It’s said to be completely ossified, meaning that it’s fully hardened bone. And all of the cats that have a completely ossified hyoid can purr, but don’t roar.

The cats that do roar don’t have a fully ossified hyoid, meaning that it hasn’t fully hardened into bone, so it’s a lot more flexible. The tissue is more like the ligaments that normally connect bones to each other. Owen thought that flexibility was the key to roaring.

Roaring is a low, deep, resonant sound, so cats need long vocal folds to do it, just like people with longer vocal folds have deeper voices. And Owen figured a more flexible hyoid was what let a cat’s vocal folds stretch enough for them to roar. For a long time, pretty much everyone agreed with this idea.

They also assumed that hardened, bony hyoids were the reason why all the species of cat that couldn’t roar could purr. But there is one exception: the snow leopard. Snow leopards don’t have a fully hardened hyoid — they have the more flexible kind that lions do.

But they can’t roar. And even though we’ve known about the snow leopard exception since at least 1916, scientists didn’t really question the hyoid idea until the late 1980s. That was when researchers realized that there is another difference between cats that can roar and cats that can’t: roaring cats have thick pads of tissue on their vocal folds.

The pads make their vocal folds longer and heavier, which allows them to vibrate more slowly and make a lower-pitched sound. Only lions, tigers, leopards, and jaguars have these pads, with no exceptions — not even snow leopards. So roaring isn’t entirely about the hyoid bone.

Sure, the extra flexibility might help, but they also need that extra tissue that our domestic cat friends don’t have. These pads also help explain why cats that roar can’t purr. When cats purr, they vibrate their vocal folds about 26 times per second.

There seems to be some mechanism in their brains that controls the vibrations. As they inhale and exhale, the vocal folds open and close, which is what makes the purring sound. It’s like how you can make that motorboat sound by vibrating your lips, except they do it inside of them with their vocal folds.

But the extra padding that allows lions, tigers, leopards, and jaguars to roar would dampen the constant vibrations that they’d need to purr. The females of those species do make purring-like noises when they’re in heat, but those noises don’t seem to be quite the same as true purring. They’re closer to a growl.

So, my cat can’t roar because her vocal folds just aren’t shaped for it. But she can purr, and that’s pretty dang cute and I like it. Thanks for watching this episode of SciShow, and thanks to Patreon patron Janet Neidlinger for asking about how cats purr!

If you’re curious about why cats purr in the first place, check out our video where I explain that it’s not necessarily because they’re happy.