As any cat owner—or even someone who’s seen a cat—knows, cats often purr when they’re being petted or getting their chin scratched. They also purr when they want food and when they’re eating. Sometimes, they’ll purr as they drift off into sleep. Purring is most known to be a sign that cats feel comfortable, safe, and content.

However, cats also purr at other times: mother cats purr when feeding their young, and it is likely that purring is one way of communicating with their kittens. Nursing kittens will purr as well—it is thought that the purr may have developed at such a young age so that kittens would have another audible way to communicate with their mothers, letting them know that everything was fine, and for the mothers to comfort their kittens.

Something slightly less well known is that cats will also purr when they are in pain or in stressful situations, not just happy ones. Going to the vet is a good example: the cat has been taken out of its comfort zone, gone for a bumpy ride in the car, and is now being poked and prodded by someone in a white coat. Still, he’s purring—even though he isn’t at all content. Some vets have suggested that it could be the cat’s way of attempting to comfort himself, like a human feigning a smile when he’s nervous, but this is just speculation.

To answer your “how cats purr” question, to purr, a cat’s brain will send a signal to their laryngeal (or voice box) and diaphragmatic muscles. The laryngeal muscles then twitch, restricting airflow before relaxing to let air through again. The twitches occur at 25 to 150 vibrations per second and cause a brief separation of the vocal chords, which allows purring to occur during both inhalation and exhalation. Each cat has a consistent pattern, but if you’ve been around many cats you probably noticed that every cat has a slightly different purr. Purrs can vary in their sound and volume and they are technically considered a “muscular tremor” rather than an actual vocalization.

As to the “why” question-mostly for communication. Many cat owners will be familiar with purrs becoming less cute and more annoying. Many cats develop a way of purring that feels “insistent” or “obnoxious” to the human ear, with some scientists relating it to the cry of a newborn baby. This is called called a solicitation purr. They add a vocalization—a cry or meow—into the purr in order to solicit a response from their human carers, basically telling us “I need something!” It taps into the human need to nurture something in distress—or, if you’re a bit less empathetic, just stop the crying.

There is also another surprising reason.  As mentioned, cats will also expend energy purring when they are sick, injured, or in some sort of pain. The interesting thing about this is that there have been many studies done on the apparent “healing powers” of purring, and it’s believed that the frequency of the cat’s purr may act as a type of physical therapy on the cat.  Specifically, studies have shown that a cat purring can aid in increased bone density and healing muscles. The same frequency treatment is sometimes used in humans to speed up the healing of certain types of wounds and has been proposed as a way to help humans maintain bone density and muscle mass while in space.  In the same way, it is thought that cats purring may help them maintain bone density despite spending the majority of their lives either sleeping or in a resting state (often seeming to be asleep).

Potential healing powers aside, purring is mostly just a nonverbal way for a cat to communicate with other cats and people. They purr in order to bring attention to their needs and desires, or, if they’re anything like my new kitten, simply to let their humans know that they exist… at all times. In the end, though, all cats are different and will purr for different reasons, just like humans will laugh for different reasons or get nervous about different things. (As an interesting aside, studies have shown that, contrary to popular belief, most of the time people laugh, it’s not because we think something’s funny.  Rather, we generally laugh in non-humor related social interactions, with the laughter functioning as social feedback.)

If you liked this article, you might also enjoy our new popular podcast, The BrainFood Show (iTunes, Spotify, Google Play Music, Feed), as well as:

• When the CIA Attempted to Use Cats as Spies and the Disaster That Ensued
• Why Cats Like Catnip
• Domestic Cats Can Fall from Any Height with a Remarkable Survival Rate
• Why Black Cats are Considered Bad Luck
• Violin Strings Were Never Made Out of Actual Cat Guts

Bonus Facts:

• Cats are the only known animal that prefers to get its food with no effort, rather than working for it.  Mice, rats, humans, dogs, dolphins, etc. that have been scientifically tested for this preference, if given the choice in the experiments run, all prefer to do something to get food.  In a sense, cats then are the only known perfectly rational creatures, at least when it comes to caloric acquisition. 🙂
• Cats aren’t the only animal that can purr.  Squirrels can purr too.
• The longest lived domestic cat was named Creme Puff.  She lived from August 3, 1967 to August 6, 2005, a span of 38 years and 3 days.  This is well over double the normal life span for domestic cats, which is typically around 12-14 years for males and 13-15 years for females. Interestingly, the owner of Creme Puff, Jake Perry, also raised a sphynx cat which was born in 1964 and didn’t die until 1998, a span of 34 years and 2 months.  The cat’s name was “Grandpa Rexs Allen”.  Why Perry’s two cats lived so long isn’t entirely known, however, he didn’t typically feed them store bought cat food.  Rather, he raised them on a variety of “natural” foods; prominent among these foods were: bacon, eggs, asparagus, and broccoli, among other things.  This can be a somewhat dangerous practice normally as cats require certain nutrients they won’t always get if they are just eating “human” food.  For instance, cats will go blind fairly quickly (and permanently) if they don’t get enough taurine, found in muscle.  Cats also require a high amount of protein and calcium.
• This high amount of protein consumed by cats is thought to be why dogs like cat poop so much, with it being very protein-rich.
• Ever wonder why your little house cat can’t roar like a lion? It isn’t because of their size. A purring cat has stiff structures around its larynx that allows it to twitch, while a roaring cat, in the style of a lion’s roar, has structures that are less stiff. Roaring cats roar to communicate across distances and protect their pride, while purring cats are more likely to live alone and not need to communicate in such ways. (In fact, domestic cats in the wild need so little communication that upon reaching adulthood, they almost never meow to other cats.  Meowing seems to simply be something domestic cats do to communicate with humans.)
• When a cat is purring, it’s very difficult for a vet to hear its heartbeat. If your cat is purring at the vet’s, the vet will sometimes turn a faucet on to get him to stop—the sound of running water will often stop a purr in its tracks!
• Studies have shown that cat owners are 40% less likely to have a heart attack than people who don’t own cats.
• Cats have an inner eyelid that helps protect their cornea from injury as they move through tall grass or whatever other activities the cat might get up to. It also acts as a “windshield wiper,” wiping away any debris or bacteria in the eye.
• That cute way your cat rubs up against you when you get home? Yeah, he might be showing affection, but he’s probably also trying to mark you as his territory with scent glands on his face. His tail and paws also carry scent glands, so every step he takes leaves territorial footprints.  Now I know why my cat becomes extra friendly and seems to like to rub his face on me more after I’ve just taken a shower.  I just thought he was traumatized that I was being pelted with water. 🙂
• Are you a cat lover? You could also be called an “Ailurophilia.”
• Contrary to popular belief, many cats are lactose intolerant. While they might enjoy milk and cream, it can give them an upset stomach, gas, diarrhoea, or cause them to throw up.  Cats also shouldn’t be given chocolate, garlic, or raisins, among other “human food” items.