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Cat's Purring

The purr is more than a feline vocalization, more than a physiological oddity. Even among the cat's remarkable collection of unique attributes, the purr stands alone. It is singular, personal, distinctive true feline performance art. The mysterious, uncannily soothing vibration, sometimes felt but not heard, other times rafter rattling, is at once eminently practical and one of nature's glorious luxuries.

A cat purrs to soothe and flatter its human companions; to celebrate and reinforce the familial bond among its own kind; to lull its kittens to sleep or call them to dinner; to reveal its emotional state; to manipulate and cajole; to entreat; to acknowledge friendship or approval; to revel in joy, thanks and contentment; to please its ineffable self. As both quintessentially feline self- expression and an admirably versatile communicative device, the purr is unequaled. Yet the purr remains a mystery.

Ever since the Egyptians started worshipping the cat, philosophers, scientists and cat lovers worldwide have wondered why cats purr.

It is commonly believed that cats purr when content. However, cats also purr when they are severely injured, frightened or giving birth. So if cats were purring solely out of happiness they would not purr when injured, especially as the generation of the purr requires energy, and an injured animal will generally not expend precious energy needed for healing on an activity not directly connected with their survival.

Since the purr has lasted through hundreds of generations of cats, there must be a survival mechanism behind its continued existence. Suggesting that the purr evolved to function solely as a vocalisation of self-contentment goes directly against the basic tenets of evolutionary psychology and natural selection. Could the purr in any way link to the fact that vibration stimulation not only relieves suffering in 82% of persons suffering from acute and chronic pain but also generates new tissue growth, augments wound tissue strength, improves local circulation and oxygenation, reduces swelling and/or inhibits bacterial growth?

Throughout history, the cat has been the most worshipped and the most persecuted domestic animal. Perhaps the most popular cat saying is that they have "nine lives". This type of old wives' tale usually has a grain of truth behind it, especially since there is also an old veterinary school adage that states "If you put a cat and a sack of broken bones in the same room the bones will heal".

Most veterinary orthopaedic surgeons have observed how relatively easy it is to mend broken cat bones, as compared with dogs. Cats do not have near the prevalence of orthopaedic disease or ligament and muscle traumas as dogs have, and non-union of fractures in cats is rare. Researchers believe that self-healing is the survival mechanism behind the purr. There is extensive documentation that suggests that low frequencies, at low intensity, are therapeutic. These frequencies can aid bone growth, fracture healing, pain relief, tendon and muscle strength and repair, joint mobility, the reduction of swelling, and the relief of dyspnea, or breathlessness.

Although not strictly a vocalisation, the purr is an important means of communication and, depending on the cat's situation, it can convey contentment, pleasure or be placatory behaviour (i.e. "I am not a threat to you"). As well as purring when happy, cats also purr when severely injured, frightened or giving birth. A cat may even purr when close to death. As veterinary notice cats being restrained for veterinary procedures (blood samples or X-rays) frequently purred. The inference drawn from this behaviour was that the cats were indicating that they were tractable and co-operative and would not need to be forcibly handled.

The purr was therefore likened to the obsequious behaviour of a submissive cat when avoiding conflict with a larger, more powerful animal. It also noted that some cats, both male and female, gave low growl-like purrs as a warning when a stranger entered their territory. This is inaccurate, the "growl-purr" is in fact a low growl.

All the cats had purr frequencies between 20 Hz and 200 Hz. With the exception of the cheetah, which had frequencies 2 Hz from the rest, all the species had frequencies, notably 25 Hz, 50 Hz, 100 Hz, 125 Hz, and 150 Hz, that correspond exactly with the best frequencies determined by the most recent research for bone growth, fracture healing, pain relief, relief of breathlessness, and inflammation. All of the cats' purrs, including the cheetah, had frequencies 4 Hz from the entire repertoire of low frequencies known to be therapeutic for all of the ailments.

That fact that the cats produced frequencies that have been proven to improve healing time, strength and mobility could explain the purr's natural selection. After a day or night of hunting, purring could be likened to an internal vibrational therapeutic system, a sort of "kitty massage" that would keep muscles and ligaments in prime condition and less prone to injury. Additionally, the purr could strengthen bone, and prevent osteodiseases. Following injury, the purr vibrations would help heal the wound or bone associated with the injury, reduce swelling, and provide a measure of pain relief during the healing process.

Purring is caused by vibration of structures in the throat, though previous explanations have attributed the sound to the noise of blood turbulence in the chest! A truly ecstatic cat sometimes vocalises (uses its vocal cords) while purring, resulting in a shrill noise. Purring is also found in the cheetah, puma and most small cats such as the serval and ocelot. Big cats such as lions and tigers cannot purr because their throats are built for roaring. Although there are a few reports of purring-type sounds (which may be a breathy groaning sound) from lions and tigers, it seems that a cat can either purr or roar, but not both!

Since purring uses energy and has been passed on through many generations of cats, it must have some function. One puzzle was why a sick or injured would expend energy on purring, when it needs all its energy for healing? Researchers believed that suggestions that the cat's purr evolved solely to communicate self-contentment goes against evolutionary theory. The fact that cats purr when injured suggested that it had some survival value, for example a healing function. Cats close to death may also purr, suggesting a pain relieving function. Since many cats purr when on their own, the purr cannot merely be a form of communication - why would a cat purr when there is no-one around to communicate with?

Though this sounds far fetched, research in humans has shown certain frequencies of vibration relieved suffering in over three-quarters of test subjects suffering from acute or chronic pain. Ultrasound is often used alongside physiotherapy. Effects include (depending on the patient) generating new tissue growth, augmenting wound tissue strength, improving local circulation and oxygenation, reducing swelling and even inhibiting bacterial growth. Vibration at low frequencies and low intensities can aid bone growth/repair, tendon and muscle strength/repair, joint mobility, reduce inflammation and reduce breathlessness.

The soothing effect of a purring cat is well-known to cat lovers. Researchers believed that vibrating (purring) cats were communicating more than just a sense of well-being to their owners.

Though to humans, the purr is most often considered a sign of contentment or of a cat reassuring itself, the researchers concluded that after a strenuous activity (hunting, defending territory etc), a period of purring could act like a massage session and alleviate sprains and strains as well as speeding the healing of any wounds. The sense of relaxation many owners feel when cuddling a purring cat suggests that the therapeutic function of the purr can extend to humans.