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 died of natural causes 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

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.