Why do cats purr?

Your cat is on your lap, eyes shut, and a low rumble rolls out of its throat. Most people say that means "happy". But a mother cat purrs while feeding her kittens, and kittens purr at two days old, before their eyes are even open. Stranger still, cats purr while shaking with pain, and while terrified on the vet's table. Happy, frightened, hurting — the same sound every time. So what is the cat actually doing?

Purring is not really an expression of mood. It is closer to a built-in vibration therapy.

A cat's purr does not come from the vocal cords. The work is done by the diaphragm and the muscles of the larynx, contracting in a steady rhythm about 25 to 150 times a second, which makes air vibrate in and out at the glottis and produces a low-frequency hum. The frequency range is the interesting part: many studies put the main power of a purr at about 25 to 50 hertz, and that band happens to sit inside what researchers describe as an ideal window for stimulating bone healing and tissue repair. It also seems to ease pain and steady breathing.

So a mother cat purring while nursing may be giving her kittens a kind of physiotherapy for bones that are still forming. A sick or injured cat purring looks much more like self-soothing and self-repair. There is a simpler explanation too: a purr is a contact signal, a low rumble that tells other cats nearby "I am here, and it is safe."

Next time your cat starts its engine on your lap, you could read it as doing two things at once: giving itself treatment, and checking in with you.

In one line: a purr does not mean "I am happy" — it means "I am repairing myself, and I am telling you where I am."