The Real Story Behind a Cat's Purr

We tend to assume we already understand what a cat's purr means. It is arguably the single most recognisable sign of animal contentment, that pleasurable rasping sound that starts up whenever a cat is tickled or stroked, the soundtrack to countless afternoons spent curled up on an owner's lap.

But that is not quite the whole picture. There is considerably more going on with a cat's purr than most of us realise.

Even the mechanics of how cats purr were long debated. Some scientists once thought it was linked to blood flow through the inferior vena cava, the vein carrying deoxygenated blood back to the heart. More recent research points instead to the muscles within a cat's larynx, which dilate and constrict the glottis, the part of the larynx surrounding the vocal cords, causing air to vibrate every time the cat breathes in or out. The result is the purr.

Even with the mechanism reasonably well understood, there is still no definitive answer to what actually triggers it. The strongest clue is a neural oscillator deep within the cat's brain that otherwise appears to serve no obvious purpose.

So is that oscillator only triggered when a cat is happy? Sometimes, but only sometimes.

Marjan Debevere, a cat shelter photographer in London currently studying feline psychology, and owner of four cats with a considerable Instagram following, says people wrongly assume purring always signals happiness. Part of the confusion, she explains, is that owners usually only notice their cat purring when tickling it somewhere it enjoys, yet cats also purr when nobody is around, and how much an individual cat purrs varies enormously. Some barely purr at all, while others purr almost constantly. She contrasts her cat Luigi, a stray who wandered into an office and ended up at a shelter, with Archie, who moved in from next door and settled easily into family life: Luigi purrs rarely, Archie purrs a great deal.

Having photographed more than 3,000 shelter cats, Debevere says no two are ever quite the same. She has also witnessed cats purring while dying or being put to sleep, with vets often remarking that an animal was purring right up to the end, which leads people to assume the cat must have been happy. That, she says, is simply not always the case.

Feline behaviour and communication have been studied far less than canine behaviour, partly because dogs tend to be more cooperative research subjects, especially with the promise of a food reward. In recent years, though, more has come to light about the purr.

Gary Weitzman, a veterinarian and CEO of the San Diego Humane Society, says researchers are only beginning to understand purring, with far more questions still unanswered than answered. While a purr generally reflects contentment, he notes it can also express nervousness, fear and stress, though contentment remains the more common explanation. It has been suspected for decades that purring serves as a form of communication, and work carried out in the early 2000s by researchers including Elizabeth von Muggenthaler and Karen Overall has since deepened understanding of its purpose, suggesting it likely serves communicative, appeasement and healing functions all at once.

Purring begins within the first few days of a kitten's life, helping the mother locate her kittens at feeding time. Some adult cats continue purring while feeding, or beforehand as a way of persuading their owner that dinner is due. Others purr loudly while cautiously exploring somewhere new, and cats may also purr after being startled or after a stressful episode such as being chased by a dog.

The deeper researchers dig, the more layers they seem to uncover. Author and cat behaviour expert Celia Haddon points to research distinguishing "ordinary purrs" from purrs used specifically to solicit food from an owner. Even people without cats could tell the two apart, since hidden within the low, ordinary purr was a higher-pitched cry, not unlike a meow. This particular sound closely resembles the isolation cry of kittens, and indeed the human infant distress cry, to which humans are naturally attuned, which may explain why we respond so readily to the cry embedded within the purr.

Sam Watson, scientific officer at the RSPCA, says there is still very little understanding of how cats purr with one another in the wild, though it is clear they purr while grooming each other. He speculates there could be different purrs for different messages, perhaps one meaning "I want that" and another meaning "let's share," and stresses that feline communication remains a genuinely under-studied area deserving far more research attention.

A form of self-healing

One theory holds that purring is a powerful act of physical healing, with the vibrations helping a cat recover and rejuvenate after stress. The frequency range involved, roughly 20 to 150 hertz, is thought to promote bone growth, since bone tends to harden in response to this kind of pressure, and other frequencies within that range may have a similar effect on soft tissue.

Weitzman notes that purrs in the 25 to 100 hertz range correspond closely with frequencies already used therapeutically in human medicine, with bone responding well to 25 to 50 hertz and skin and soft tissue to around 100 hertz.

This may explain why cats often appear to purr contentedly while dozing; in reality, it may be a low-energy form of self-repair. Because cats spend so much of the day resting, this behaviour may have evolved partly as a way of maintaining bone and tissue condition while avoiding the risk of injury through overexertion.

The benefits may not stop with the cat itself. Stroking a cat has long been recognised as a stress-reliever, and cat ownership has even been linked to a reduction in the risk of stroke or heart disease of up to a third. Since the frequencies cats purr at overlap with therapeutic ranges used in humans, it is possible the purr does us some good too.

Weitzman believes the purr carries real benefit for people, beyond any physiological effect, simply through its calming psychological impact, comparable to the sound of waves on a beach. Humans, he suggests, may even have unconsciously selected for cats with a greater tendency to purr over generations of domestication.

Haddon agrees, noting that a cat winding around your feet, glancing towards you, or eyeing the food cupboard or fridge, all while purring loudly, is impossible to misread as anything other than a demand for dinner right now. In the morning, loud purring combined with face-patting or rubbing is often enough to rouse a sleeping owner, prompting breakfast, which, she points out, shows just how effective a cat's communication really is, given that most owners feed their cat before themselves.

Ultimately, fully understanding what a purr means may depend on reading a cat's wider body language alongside it, from the raised, periscope-like tail of a sociable cat to the wide eyes and flattened whiskers of one ready to fight. With that broader understanding, the bond between cat and owner can only deepen.

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