The purrsuit of happiness. Why (and how) cats purr.
Perhaps a better question might be why humans can't purr. Purring is one of those mysterious things that cats do, and as is often the case with feline physiology, the more we learn about it, the more we realize that cats got the better deal from nature.
Start with a wild mother cat. Her kittens are blind and deaf, but they need to know where mum (and the milk bar) is. Fortunately even a two-day-old can work out if its warm and vibrating the milk is there somewhere. I say a two-day-old because that is the time when the kitten starts purring back. This has advantages for mum, because it tells her where the kitten is. If there is a hazard out there, mum can keep her beady eyes fixed on the danger, and know from the vibrating little furballs against her skin that the kittens are present and accounted for.
Compare this with, say chicks and ducklings, whose constant piping is of great interest to passing foxes. (And to passing cats for that matter). Don't you have your phone set to vibrate when you want to stay in touch without being heard?
Since those cosy hours of mother-child bonding are a happy memory for both, it is probable that cats purr in times of contentment, just as they sometimes peform the same kneading movement with their forepaws that they once used to get mother's milk flowing. It is no co-incidence that they do this particularly when a human is stroking them in the same way that mother's tongue used to do.
Yet cats don't just purr when they are happy. Cats purr when they are in the grip of any strong emotion that does not involve adrenaline. Cats purr when frustrated, when in pain, or even when near death.
After centuries of bafflement, scientists have managed to put the why and how together. Basically, an emotionally-charged cat gets a reflex from its central nervous system that signals the muscles in its voice box, (the laryngeal muscles) to tighten to the point that they vibrate with air coming in and out of the lungs. (Which is why a cat purrs continuously, whether breathing in or out. However, cats with damaged or missing laryngeal muscles can't purr.) No-one seems certain whether purring is a voluntary or involuntary sound.
It turns out that cats purr at between 25 and 150Hz. This gently vibrates the body at a frequency which seems to help with bone growth and muscle stimulation. Very useful for a growing kitten. But it is also useful for a cat which has been injured in some way. It not only emits a sound which has associations with happy times in the past - thus easing mental stress, but the same sound also provides gentle, all-over body therapy. Cats are remarkably long lived for their size. This is in part (as any cat owner can tell you) because cats don't strain themselves unduly. Yet, even a sedate cat is both fit and well toned. Why? It purrs.
Because this 25 and 150Hz frequency helps bone growth, this may be why cats suffer less from dysplasia or osteoporotic conditions that are more common among dogs. It is also interesting that humans who live with cats tend to have lower blood pressure. It cannot be the shredded curtains or inappropriate toilet habits that cause this. It might be the purr. Certainly doctors are convinced enough of the benefits of associating with cats to allow them as therapy animals in some hospitals.
So why do cats purr? Because, like racoons, pumas and mountain lions, they can. Since humans can't purr, having a purring cat on hand is the next best thing. A mountain lion just would not have the same effect.
Recent studies have shown that a hungry cat use purr/cry combination which is know as 'solicitation purr'. This sound has been shown to work on humans in a similar way sa baby cry, which apparently makes us to rush for a tin opener. You can find out more about the 'solicitation purr' here.