We think we know what a cat’s purr means.

It is arguably the most recognisable sign of animal contentment: a pleasurable rasp that erupts whenever a cat is tickled or petted, the soundtrack to countless sessions sprawled on an owner’s lap.

But that’s not quite the full story. There is a lot more going on with the cat’s purr than you might reasonably expect.

Even the ‘how’ was long a subject of debate. Some thought it was linked to blood flowing to the inferior vena cava, a vein that carries deoxygenated blood to the right side of the heart. But with more research it seemed likelier that the noise came from the muscles within the cat’s larynx. As they move, they dilate and constrict the glottis – the part of the larynx that surrounds the vocal chords – and the air vibrates every time the cat breathes in or out. The result? A purr.

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Even though science is now fairly sure this is the process, there’s no definitive answer as to what triggers the response. The biggest clue is a neural oscillator deep within the cat’s brain, one that otherwise has no clear purpose.

But if that neural oscillator is triggered, is it just when a cat is happy?

Sometimes. But only sometimes.