Why cats purr

Cats purr most often when they are content, but they also purr to ask for food or attention, to calm themselves and sometimes when they are hurt, frightened or ill. A purr tells you that a cat is making a signal. It does not tell you the cat is fine. Read the purr together with the rest of the body and the situation.

Kittens purr within days of birth, and mothers purr when they nurse, so the sound is linked to closeness from the start. Adult cats keep using it in many settings.

How a cat makes the sound

The sound comes from the larynx, the voice box, and the muscles around it. Scientists have described two main ideas. In one, the brain sends a repeating signal to the muscles of the larynx, which twitch the vocal folds as the cat breathes in and out. The air passes over the twitching folds and makes a steady hum. A cat can purr as it inhales and exhales, which is why the purr sounds continuous, unlike a meow, which comes on the out-breath.

The second idea concerns the hyoid apparatus, the small chain of bones that supports the tongue and the voice box. In the big cats that roar, such as lions, tigers, jaguars and leopards, parts of that structure are flexible and the larynx is built for loud roars. These cats can purr only on the out-breath, if at all. Small cats and cheetahs have a more rigid structure and can purr on both breaths, but they cannot roar. The picture is more complicated than a simple rule, and researchers still discuss the details. The glossary entries on purring and the felid family give more background.

A commonly quoted range for a purr is about 25 to 150 vibrations per second. That is a low, steady hum.

Kinds of purr

Cats do not purr for one reason. Some of the main contexts are below.

• Contentment. A relaxed cat on a lap or a warm cushion often purrs. The eyes are half closed, the body is loose and the cat may knead. See kneading.
• Nursing. A nursing kitten purrs, and the mother purrs back. Newborn kittens cannot see or hear well, and the vibration may help the mother find them and may help them find her.
• Soliciting. Some cats purr in a different tone to ask for food or attention. Researchers have found that a purr with a high-pitched cry mixed in is more urgent and more annoying to listeners than a plain one. Owners who live with a cat that has learned this trick often recognise it.
• Greeting. Many cats purr and trill when you come home. A short, rising trilling sound often goes with this friendly purr.
• Self-comfort. Some cats purr when they are anxious, in pain, at the vet or recovering from illness. The purr may be a way to comfort themselves, much like a person humming. Cats have been known to purr during labour or when badly injured.

Does purring heal?

There is a popular claim that the frequency of a purr heals bones and tissue. The idea is interesting, but it is not proven. Some researchers have noted that the frequencies fall in a range that is linked to bone and tissue effects in other studies. A cat's purr has not been shown to heal a cat or a person. Do not treat the claim as a reason to delay care for an injured or ill cat.

How to read the purr

Look at the whole cat. A purr by itself says little.

A relaxed purr usually comes with:

• Soft, half-closed eyes or slow blinks.
• A loose body that may be lying on its side or sprawled.
• A tail that is relaxed or curled.
• Kneading, rubbing and seeking contact.
• Easy breathing.

A purr that needs a closer look may come with:

• Hiding, crouching or a hunched posture.
• Dilated pupils or squinting eyes.
• A tense body, flattened ears or a tucked tail.
• A cat that does not want to be touched, or flinches when you touch one area.
• Poor appetite, vomiting or changes in the litter box.
• Fast, laboured or open-mouth breathing.

The same sound can mean opposite things. A cat that purrs while hiding under a bed after a fall is not content.

What to do when your cat purrs

• Enjoy it. If the cat is relaxed, you can pet it or leave it alone.
• Check the body. If the cat seems off, look at its eyes, posture, breathing and appetite.
• Notice patterns. If a cat purrs loudly at the same time every day, it may be asking for food.
• Respond to the request. A purr that comes with meowing at the bowl probably means food.
• Leave a purring cat alone if it seems to want space. A purr is not always an invitation. Some cats purr and then bite when overstimulated, so watch the tail and ears.

Common mistakes

• Assuming a purr always means happy. Cats also purr when stressed or sick.
• Ignoring breathing. Purring can hide laboured breathing at first glance.
• Believing a purring cat is not in pain. Some cats purr through pain.
• Waiting for purring to stop before calling a vet. The purr is not a measure.
• Teaching a cat to purr or forcing it. You cannot, and handling a stressed cat makes things worse.
• Using purr claims to treat a cat at home. The healing idea is unproven.

When to call a vet

Call your vet if purring comes with any of the following:

• Trouble breathing, open-mouth breathing or fast breathing at rest.
• Hiding, refusing to move or crying when touched.
• Not eating for a day or more.
• Vomiting more than once, or diarrhoea that persists.
• Straining to urinate. A male cat that strains or passes little or no urine needs an emergency vet at once.
• Weakness, limping or a swollen belly.
• A kitten that is cold, limp or purring weakly and not feeding.

Also ask your vet if the purr changes. A cat that has always purred and suddenly stops, or a cat that purrs far more than usual along with weight loss or restlessness, deserves a check.

Summary

• A purr is a signal. Contentment is the most common meaning.
• The sound comes from the voice box and its muscles.
• Cats also purr to ask for food and to soothe themselves when hurt or afraid.
• The idea that purring heals is unproven.
• Read the whole cat, and call your vet if a purring cat looks unwell.