Kitten Approved Technology

• purr
• cats
• health
• macro

What better use for the interwebs and podcasting than to broadcast soothing purrs around the world? This is not purely ear candy: purring is officially productive.

The purr is used as a social communication to indicate 'all is cool' — kittens purr to their mother from a day old, and cats may purr when approaching strange felines to signal their peaceful intent. Purring also indicates the cats' emotional state; but while the most common instance of purring is to indicate contentment, cats also purr when injured, stressed, or giving birth. In a stressed situation or when an animal is hurt, where is the motivation to purr? Using precious energy to hum to oneself would seem to be a bit of a useless evolutionary strategy.

And it's a widespread one: all so-called 'small cats' purr (which includes many species, from the domestic moggy through the long-legged caracal to the not-so-small mountain lion). Lions and leopards, officially 'big cats', also like a bit of purring. The only difference is that while small cats can maintain a continuous purr whether inhaling or exhaling, big cats only purr on the outward breath. As to how they make the sound, it's definitely produced by vibrating air in the cat's throat - but what exactly is vibrating is still not clear. Some people suspect the vocal chords, and others thinks the epiglottis may be responsible (in humans, this is what causes snoring.)

For over 40 purring felines, Purr analysis (what a great job!) shows that purrs hit frequencies conducive to stimulating or healing bones, muscles and tendons. Frequencies of 25 - 50 Hz increase the density of growing bone by one fifth in animals and humans, and make fractures heal quicker. Higher frequencies (50-150 Hz) are used in medicine and physiotherapy to stimulate muscles and tendons, preventing them dying.

These low frequencies can also have a less physical effect; when played to sufferers of acute pain, 82% of patients reported feeling calmer and generally happier. But if you're going to use infrasound to affect your moods, be careful to get the right frequency. There is evidence that some frequencies below 50Hz can induce chills and feelings of fear and sorrow. It's been suggested that the production of these sounds by old buildings may be responsible for the sensation of 'haunting'.

It is well known amongst vets that cats recover rapidly from injuries — and the folk adage 'cats have nine lives' derives from the common observation that cats tenaciously grip onto life. The healthful properties of the purr could explain this: when injured, a cat can soothe their stress and stimulate new growth in the affected muscles and bone. When healthy, the purr gives continuous gentle stimulation to a body that spends most of its time lying around in sybaritic bliss, but needs to snap into action when it is time to hunt.

The jury is out on whether purring is a conscious decision on the puss' part, though. While vet Neils C. Pederson claims the purr is controlled by the nervous system, others think it could be involuntary and triggered by the presence of endorphins in the brain. Endorphins are the neurochemicals produced as painkillers, but are also present during pleasurable sensations, so both theories explain the multiple and varied appearances of the purr.

All in all, the ability to purr seems to have been a real bonus for the feline world. And since several studies suggest the effects extend to humans, you might want to consider adopting a cat — or learning to purr.