Why cats purr: it does not always mean they are happy
Purring is probably the most characteristic sound a cat makes, and one of the ones that makes us feel best. Almost everyone takes it to mean happiness, and almost always it does. But cats also purr when they are ill, frightened or giving birth, and knowing that can help you avoid missing a problem.
How they do it
For a long time people speculated about how cats purr, with theories involving the blood or vibrations along the spine. We now know that purring is produced in the larynx: muscles around the vocal cords contract and relax very quickly, 25 to 150 times a second, opening and closing the airway. Because this happens both on the in-breath and the out-breath, the sound is continuous, without breaks, and a cat can purr for a long time while breathing normally. It is controlled by the nervous system, which is why a cat can start and stop at will. Recent research suggests that the tissues of the larynx can also vibrate on their own, which would explain very low-frequency purrs.
It starts with the mother
Kittens start purring a few days after birth, while they are still blind and deaf, and they do it mostly while suckling. The mother purrs too while she nurses them. That exchange seems to work as communication: the kittens tell her they are fine and feeding, and the mother reassures them and guides them towards her. That is why many experts believe the original meaning of purring is a signal of “all is well, stay close”, rather than simply an expression of pleasure. The adult cat keeps that signal and uses it with its people in moments of contact and calm, which look a lot like those moments with its mother.
The contented purr
The best-known purr is the one that appears when the cat is relaxed: lying in your lap, being stroked, curled up in the sun, before falling asleep or while kneading a blanket. It usually comes with other signs of calm: half-closed or closed eyes, a soft, stretched-out body, ears in a neutral position, slow blinks and sometimes a little drool. In this context, yes, purring means wellbeing and trust. Cats also often purr when they greet you, while rubbing against your legs, or when you are reunited after time apart. If the context is relaxed and so is the body, there is not much more to interpret: your cat is happy with you.
They also purr in pain, fear or illness
What many people do not know is that cats also purr in situations that are anything but pleasant. They purr at the vet, when they are injured, while giving birth, when they are seriously ill and even in their final moments. In those cases purring seems to work as a way of calming themselves, or as an appeasing signal to whoever is nearby. So a purring cat is not proof that it is not in pain. If your cat is purring a lot but is also hiding, not eating, hunched up, staring fixedly or breathing strangely, the purring should not reassure you: take the cat to the vet.
The food-soliciting purr
There is a kind of purr many owners recognise without being able to explain it: the one a cat makes first thing in the morning to get you up to feed it, more insistent and a bit more irritating than the usual one. A study published in 2009 analysed these soliciting purrs and found they carry a hidden high-pitched component, similar to a baby's cry, which makes them feel more urgent and harder to ignore. People listening to the recordings, even those without cats, rated them as more pressing than normal purrs. In other words, cats have learned to adjust their purr to get what they want from us. Pretty clever, it has to be said.
Does purring heal? What is known
The idea that purring heals bones or has therapeutic effects circulates widely, because its frequencies match those used in some vibration therapies to help bones knit. It is an interesting hypothesis, but it needs saying clearly: there is no proof that purring heals fractures, either in cats or in people. What is well documented is that living with animals and spending calm time with them has positive effects on many people's mood and stress, and purring is part of that experience. There is no need to exaggerate its powers to enjoy it. It is a relaxing sound, and that is enough.
Cats that do not purr
Some cats purr rarely, very quietly or almost never, and that does not mean they are unhappy or do not love you. How strongly cats purr varies a great deal: some sound like an engine you can hear from the next room, and others can only be felt if you put a hand on their throat. If your cat has never purred much, that is just how it is. What does deserve attention is a change: a cat that used to purr a lot and stops, or one that never purred and starts doing it constantly and at odd moments, may be telling you something has changed. Any unexplained change in behaviour justifies a check-up.
Big cats and purring
This is a curiosity people often ask about. Domestic cats and many small and medium-sized wild cats, such as the lynx, the puma or the cheetah, purr continuously. The big cats that roar, such as the lion or the tiger, do not purr like a cat, because the structure of their larynx and of a bone in the throat, the hyoid, is different: it lets them roar but not produce that continuous sound on both the in-breath and the out-breath. They can make similar sounds as they breathe out, but not the classic purr. That is why, in the cat world, it is often said that you either roar or purr. Your cat is firmly on the purring side.
How to read a purr in context
On its own, a purr does not tell you much. What tells you something is the purr plus everything else. A cat purring with a relaxed body and half-closed eyes, seeking you out, is content. A cat purring hunched in a corner, with dilated pupils, not eating and moving away when you approach, may be suffering. A cat purring very loudly in front of you at six in the morning while treading on your face probably wants its breakfast. Learning to read the cat's body language as a whole, and not just one sound, is what lets you really understand what it means and when it needs help.
Cats purr with the larynx, at 25 to 150 vibrations a second, and learn it while suckling. It nearly always means wellbeing, but cats also purr in pain, in fear, while giving birth or when very ill, to calm themselves. Read it together with the rest of the body: a cat purring while hiding and not eating needs a vet. The claim that it heals bones is not proven.
Educational content from Michipedia, translated from the Spanish original. It does not replace the advice of a vet or a feline behaviour specialist. Every cat is an individual.