Why Do Cats Purr?

How do cats make the purring sound?

According to Barbara Sherman, a behavior expert at NC State University's College of Veterinary Medicine, purring is set off by a signal from the central nervous system, and as the cat breathes in and out, the muscles of the vocal folds in its larynx vibrate to produce short bursts of sound.^[2] Leslie A. Lyons, writing in Scientific American as an assistant professor at the University of California, Davis, said the purr comes from intermittent signals to the muscles of the larynx and diaphragm.^[1]

Cats have long been thought to purr differently from most mammals: in this view, nerve signals drive bursts of muscle activity, typically 20 to 30 a second, so that muscles inside the larynx contract in cycles and modulate the flow of breath.^[3]

Sources describe a purr's speed differently. Sherman says the sound bursts come every 30 to 40 milliseconds, and that purr frequency does not depend on a cat's age, size, weight or sex.^[2] Lyons gave a wider range of 25 to 150 hertz.^[1] A 2023 study in Current Biology puts the basic frequency of domestic cat purring at 25 to 30 hertz.^[3]

What did the 2023 cat larynx study find?

A team including researchers at the University of Vienna tested the nerve-driven explanation, which predicts that a larynx cut off from nerves and working muscles should not vibrate.^[3] They used larynges from eight domestic cats that had been euthanized in veterinary clinics because of terminal illnesses unrelated to the airway, with their owners' consent.^[3]

Mounted on a tube that supplied heated, humidified air, all eight larynges vibrated at typical purring rates with no nerve signals or muscle contractions.^[3] The researchers also found pads of connective tissue, up to 4 mm across, embedded in the cats' vocal folds, and suggest that these pads let the folds vibrate at the unusually low frequencies of a purr.^[3]

The authors write that their data do not fully reject the muscle-contraction explanation.^[3] They propose that in a living cat both mechanisms work together: the vocal folds vibrate passively while nerve-driven muscle contractions keep them in step.^[3]

Do cats purr only when they are happy?

No. Cats are thought to purr to keep in contact with other cats or with people, and a cat purring on a familiar person's lap may look content.^[2] One researcher who put throat microphones on free-ranging cats recorded purring in many situations: when the cat's caretaker was near, when greeting a familiar cat, while rolling or rubbing, and in drowsy sleep.^[2] Mother cats purr while nursing, and so do their kittens.^[2] Some situations seem to switch purring off, including aggressive or sexual encounters, hunting and a cat's first exposure to catnip.^[2]

Cats also often purr under duress, such as during a visit to the veterinarian or while recovering from an injury, so a purring cat is not always a content one.^[1] Cats may even purr when they are in pain: veterinarians treating a cat that has been hit by a car may find it purring.^[2]

What is a solicitation purr?

In 2009, Karen McComb of the University of Sussex published a study in Current Biology on the purrs cats make when they want food.^[4][5] Her team recorded 10 cats purring in different situations and played the recordings to 50 people.^[4] Listeners rated the purrs of food-seeking cats as more urgent and less pleasant than purrs made while resting, sitting or being petted, and even people who had never owned a cat could tell them apart.^[4]

The difference was a high-pitched sound, similar to a cry or a meow, hidden inside the low purr, at a frequency of around 380 hertz, very similar to that of a crying baby.^[4][5] The Sussex researchers suggest that people find this sound hard to ignore because humans are predisposed to care for their own infants.^[5] When the team removed the cry-like element from recordings, listeners could no longer tell solicitation purrs from other purrs.^[4] These purrs seemed to be used most by cats that had the most one-to-one contact with their owners.^[4]

Which big cats purr and which roar?

Charles Darwin noted that the puma, cheetah and ocelot purr as well as the house cat.^[2] The cheetah does not roar, but it purrs much like a house cat, sometimes for minutes, continuously while breathing in and out.^[8]

The large roaring cats of the genus Panthera, such as the lion, jaguar and leopard, do not purr.^[2] Smithsonian's National Zoo says lions and tigers cannot purr, and that lions often show contentment with growls, grunts and moans instead.^[6] The clouded leopard can do neither: the zoo says it cannot roar or purr, and that it lacks the fully ossified hyoid bone that lets small cats purr.^[7]

The explanations differ. The zoo says small cats have a hyoid bone in the throat that can vibrate against the larynx, while in big cats that bone is replaced by a softer length of muscle and cartilage, which lets them roar but is too soft to vibrate.^[6] The authors of the 2023 study credit a classic demonstration by Richard Owen with tying the split between roaring and purring cats to the structure of the hyoid.^[3] They add that more recent work shows the roar depends on the unusual structure of big cats' vocal folds, while the elastic hyoid and a longer vocal tract lower the roar's resonant frequencies.^[3]

Does purring help cats heal?

That purring helps cats heal is a hypothesis, which Lyons presented as a possibility rather than a demonstrated effect.^[1] She pointed out that various investigators have found sound frequencies in the 25 to 150 hertz range can improve bone density and promote healing.^[1] She suggested that purring might be a low-energy way for cats, which rest for long periods, to stimulate their muscles and bones, and called purring a potential source of self-healing.^[1]