Cats Don’t Just Purr When They’re Happy

By Debra Kelly on Wednesday, January 1, 2014
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“In ancient times cats were worshiped as gods; they have not forgotten this.” —Terry Pratchett

In A Nutshell

Most people associate a purring cat with one comfortably curled up on the lap of someone who’s petting her. But cats will also purr when they’re scared or in pain, bringing up the question of why the same sound is made in such opposite circumstances. The answer might lie in the frequency of the cat’s purr, and the comforting, healing ability associated with it. The frequency of a cat’s purr is the same frequency that heals broken bones and muscles, making it likely that cats have an ulterior motive in their purring, too.

The Whole Bushel

Cats can purr because they have amazing control over the muscles in their larynx. When their laryngeal muscles twitch, this pulls their very stiff vocal cords apart and causes the sound of purring as they breathe in and out. Most of us are familiar with the happy, content cat purring away on someone’s lap, but cats also purr when they’re scared, upset, or injured. Cats will often purr when they’re in a stressful situation like waiting in the vet’s office, or when confronted with a scary new creature.

So why do they make the same sound in completely opposite situations?

While no cats are telling, scientists have made some educated guesses—and it starts with what’s hidden inside the sound of a cat’s purr.

Since the purr is generated by the cat’s breathing, it’s a sound with a regular rhythm. It also has a frequency that’s somewhere between 25 and 150 Hertz. Scientists don’t think that’s a coincidence.

That’s the same frequency that has been found to encourage healing in bones and muscles, and it might explain why injured cats purr—they’re helping their own body to heal. And as an added bonus, since the purring sound is generated by their own breath, it’s a very low-energy form of therapy. And oddly enough, it’s not just beneficial for the cat. The University of Minnesota Stroke Center has found that people who own a cat or two are as much as 40 percent less likely to have a stroke; it’s thought that the calming effects of having a cat around, coupled with the healing properties of the purr, might have something to do with that.

Not all cats can purr; those that can roar can’t purr (due to the different structure of their vocal cords). But a variety of cats can purr, not just your domestic housecat. Cheetahs, ocelots, and other “small” big cats can also purr, and recordings of their sounds have shown that they have the same frequency. This gives credence to the theory that they’re not just purring for you.

So why do they purr when they’re happy?

This is where they turn into the manipulative little creatures they have a reputation for being. Researchers at the University of Sussex have gone a bit farther in examining just what the sound of a purr is, and they’ve found something surprising. Inside the 25–150 Hertz range of the purr is another sound—namely, a hidden cry that registers between 220 and 520 Hertz.

As a comparison, that’s a frequency very similar to the cries of a human baby.

Purring is an important part of cat communication from the time they’re kittens, purring at their mother for attention. And they might be doing the same to you, hiding a rather annoying plea for attention and more petting deep inside the sound of their more pleasant, calming, and therapeutic purr. It makes them extremely hard to ignore, and pretty much guarantees that you’ll keep doing whatever you’re doing to make them happy.

Some cats have been known to purr when they want to be fed as well. This secret purr might be the reason they’re so hard to ignore when they wake you up in the morning for their breakfast.

Show Me The Proof

Scientific American: Why do cats purr?
Mother Nature Network: Why do cats purr?