Why do cats purr? (It isn't always happiness)

"Why do cats purr?" is the most searched why question about cats on Google. The short answer: usually because they feel good. The longer answer is more interesting, because a purr can mean several different things.

How cats purr

Purring is produced in the voice box (larynx). Muscles around the vocal folds twitch very rapidly, opening and closing the airway, and the vibration happens both when the cat breathes in and when it breathes out. That's why a purr can go on and on without a pause. Kittens can purr when they're only a few days old, which helps them communicate with their mother while nursing.

The happy purr

Most of the time, purring means a relaxed, content cat: curled up on a lap, being stroked in a favourite spot, dozing in the sun. You'll usually see the rest of the "happy package" too: soft, half-closed eyes, a relaxed body, maybe slow blinks and kneading.

The "feed me" purr: a cry hidden inside

In 2009, researchers at the University of Sussex recorded cats purring in two situations: while asking their owners for food, and while relaxing. When they played the recordings to people (including people who had never owned a cat), listeners rated the food-asking purrs as more urgent and less pleasant.

The reason: hidden inside the normal low rumble was a higher-pitched sound, similar to a cry or meow, and similar in frequency to a human baby's cry. When the researchers removed that component, the purrs no longer sounded urgent. In other words, your cat may have learnt exactly which purr gets you out of bed at 6 a.m.

Purring when stressed, unwell or in pain

Cats also purr in situations that are anything but happy: at the vet, when frightened, when injured or very ill, and even while giving birth. In these cases purring seems to work as self-soothing, a bit like humming to yourself when you're nervous. It's one reason cats are so good at hiding pain: a purring cat doesn't always mean a comfortable cat.

How to tell the difference

Look at the whole picture, not just the sound:

• Content purr: relaxed body, soft eyes, normal appetite, seeking contact.
• Worried purr: hiding, hunched posture, not eating, reluctant to move, squinting, flinching when touched, or breathing faster than usual.

When to see a vet

A purring cat that is also hiding, not eating, drooling, breathing with effort or suddenly less active needs a vet check. Purring doesn't rule out pain or illness. For breathing problems, see our guide to asthma in cats.

In our rescue, many of the shyest street cats purr for the first time weeks or months after arriving. When it finally happens, it's the best sound in the world.