The Complicated Truth About a Cat’s Purr

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

　　It is arguably the most recognizable 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.

　　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.

　　Marjan Debevere is a cat shelter photographer in London who is currently studying for a degree in feline psychology. She is also the owner of four cats – Clive, Hula, Luigi and Archie – who are something of an Instagram sensation (33,000-odd followers and counting, Instagram fans).

　　Part of the mystery around the purr is that we often only notice cats purring “when we tickle them in places that they like to be tickled”, says Debevere. Yet they also purr when we’re not around, and the extent of that purring varies between individuals. “All cats are different, some never purr and some will purr constantly,” she says. She draws the comparison between her cat Luigi – a stray who followed someone in to their office and was subsequently taken to a shelter – and Archie, who “moved in from next door” and became part of the family. Luigi purrs little, and Archie a lot.

　　“I’ve photographed more than 3,000 cats so far [at shelters] and no two are the same,” Debevere says. “I’ve witnessed a lot of cats purring when they’re dying, and when they’re being put to sleep. The vet will say something like ‘They were purring right up until the end’, and people assume they’re happy when they’re purring. That’s just not always the case.”

　　The study of cats’ behaviour and communication has lagged behind that of dogs, which are usually more willing participants, especially if there is a reward of food involved. But in recent years more light has been shed on the purr.

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