Why Do Cats Purr? What the Sound Means and How It Is Made

For a long time the standard account was that a cat purrs by twitching the muscles of its larynx twenty-five to fifty times a second, opening and closing the gap between the vocal folds so that each breath in and out is chopped into a rapid pulse. That is why purring continues on both the in-breath and the out-breath, which no other cat sound does, and why it can go on for minutes without the cat pausing.

A 2023 study led by Christian Herbst complicated the picture. Larynxes removed from cats that had been put down because of terminal illness produced purring-frequency sounds when air was simply pushed through them, with no muscle activity at all. The team found pads of connective tissue embedded in the vocal folds that let them vibrate at very low frequencies passively, much as a large animal's folds do. The brain may still switch purring on and off, but the sound itself may need less continuous effort than anyone thought, which would explain how a cat can keep it up for so long.

Kittens purr within days of being born, while they are nursing, and their mother purrs back. Newborn kittens are blind and deaf for the first week or so, but they can feel the vibration, and the purr seems to work as a signal between mother and litter that everything is in order and that this is the place to feed. Adult cats appear to carry that association with them, which is the simplest explanation for purring in the presence of a trusted person.

Cats also purr at each other, when grooming one another or resting in contact, and adult cats that have never lived with humans still purr. It is not a sound cats invented for us.

The awkward fact for the contentment theory is that cats purr when they are frightened at the vet, when they are seriously injured, while giving birth, and sometimes as they are dying. One reading is that purring is self-soothing, a behaviour the cat produces when it needs to calm itself, in the way people hum or rock. Another is that it is an appeasement signal, a way of telling a larger animal that the cat is no threat.

A more surprising suggestion is that the purr helps the cat heal. Vibration in the range of roughly 25 to 50 hertz has been shown in laboratory and clinical studies to encourage bone density and speed the repair of some tissues, and cats' purrs sit squarely in that range. Cats also recover from falls and fractures notably well for their size. The idea is suggestive rather than proven; nobody has shown that a purring cat heals faster than a silent one, and the frequencies could be a coincidence. But it would explain why a cat in pain does something that costs energy.

In 2009 Karen McComb at the University of Sussex recorded cats purring at their owners in two situations: when they were relaxed and when they wanted to be fed. Listeners, including people who had never owned a cat, rated the feeding purrs as more urgent and less pleasant. Analysis showed why. Inside the low purr the cats had embedded a higher-frequency cry at around 220 to 520 hertz, close to the frequency of a human baby's cry.

The solicitation purr, as it was named, seems to be learned. Cats living with people discover that this particular sound gets a response, and they keep it. It is a small example of how a signal that began between mother and kitten has been repurposed for a different audience.

The big cats divide in two. Lions, tigers, leopards and jaguars roar, and their larynx is built for it, with a partly flexible hyoid bone and long, fleshy vocal folds; they can produce purr-like sounds only while breathing out. Cheetahs, pumas, lynxes, ocelots and the small wild cats have a fully bony hyoid and purr as domestic cats do, on both breaths, and cannot roar. A cheetah's purr is loud enough to be felt through the ground. The rule of thumb, roaring or purring but not both, holds across the family with few exceptions. When you hear a purr, then, the situation tells you as much as the sound:

• •On a lap or beside another cat: contentment and contact
• •Nursing kittens and their mother: everything is in order
• •Beside a food bowl, with a whine inside it: a request
• •At the vet or after an injury: self-soothing, and possibly repair

Purring makes most sense when you see it alongside the rest of a cat's vocabulary. Adult cats rarely meow at one another; the meow is mainly a kitten's call to its mother, and adult cats living with people keep using it, often far more than feral or wild cats do. Cat behaviour researchers widely regard much adult meowing as directed at humans, a sound that persists because it gets results. Hisses, growls and yowls, by contrast, are used between cats and other animals, and they carry warnings that any listener understands.

Much of a cat's communication is silent, which is why a purr is best read together with the body. A relaxed purring cat usually has half-closed eyes, forward ears and a loose body, while a frightened one at the vet may purr with flattened ears, wide pupils and a tense, crouched posture. The slow blink is another quiet signal: a study from the University of Sussex found that cats were more likely to approach a stranger who slowly narrowed their eyes at them. If you want to know what a purr means, look at the ears, eyes and tail before you decide.

A purr is a nuisance in the consulting room. When a vet listens to a cat's chest with a stethoscope, the steady vibration fills the earpieces and masks the sounds that matter: the beat of the heart, the faint murmur that can signal heart disease, and the crackles or wheezes that point to trouble in the lungs. Heart sounds are low-pitched, so they sit in the same general range as the purr, and a cat can keep it up for an entire examination. The very behaviour owners find reassuring can hide the evidence a vet is looking for.

Vets have a small repertoire of tricks for stopping a purr long enough to listen, and owners are sometimes surprised by them. Turning on a nearby tap sometimes works, apparently because the sound of running water catches the cat's attention. Others hold a piece of cotton wool dampened with surgical spirit near the nose, since the sharp smell makes many cats pause, or gently cover the nostrils for a moment, which interrupts the breathing rhythm the purr rides on. Each pause may last only a few seconds, so the vet listens quickly in short bursts, sometimes trying several tricks in turn on the same patient.

Before the larynx was firmly identified as the source, some researchers proposed something stranger. For part of the twentieth century one theory held that purring came from turbulence in the bloodstream: as blood rushed through the large vein that carries it back to the heart past the diaphragm, the theory went, the flow set up vibrations that travelled through the chest and windpipe to be felt and heard at the throat. It seemed plausible because a purr can be felt all over a cat's body, not just at the neck, and because the sound does not obviously come out of the mouth.

The evidence that settled it came from recording the muscles. In the early 1970s physiologists placed fine electrodes in the muscles of the larynx and diaphragm of purring cats and found bursts of electrical activity at the purr's rhythm, timed with sharp changes in air pressure in the windpipe as the vocal folds closed and opened. The muscle bursts and the sound started and stopped together, which a blood-flow theory could not explain. The 2023 work on passive vibration does not overturn those findings, but it raises a question still being studied: how much of each pulse is driven by the muscles, and how much by the tissue itself.

Cats purr with a low-frequency vibration of the vocal folds that runs on both the in-breath and the out-breath, and they do it when content, when nursing, when asking for food and when hurt. The sound began as a signal between mother and kitten and has been put to several uses since, including, quite possibly, helping the cat mend.