On the incredibly long, seemingly endless flight back from the Midwest Eye Conference (more on that later), the question came up: How do cat's purr? Not one of us four veterinarians could answer this. So here's some info off the Internet. Any other theories out there?
A purr is a sound made by some species of felines and is a part of cat communication. It varies in detail from cat to cat (e.g., loudness, tone, etc.), and from species to species, but can be characterized as a sort of tonal buzzing. All domestic cats purr in a frequency range of 22.4 to 30.2 hertz. Some cats purr so strongly that their entire bodies vibrate; conversely, other cats may purr so quietly that the only indication is a vibration felt when touching the cat's throat. In addition, some are able to meow or hiss without interrupting the purring sound. (Listen (help·info) to a domestic cat purring)
How felines purr
Despite being a universally recognized phenomenon, the exact mechanism by which the cat purrs has been frustratingly elusive for scientists. This is partly because the cat has no obvious anatomical feature unique to it that would be responsible and may also be partly because a cat placed in a laboratory for examination is unlikely to make the noise.
One hypothesis, backed up by electromyographic studies, is that cats produce the purring noise by fast twitching of the muscles in their larynx, which rapidly dilate and constrict the glottis, thus causing vibrations in the air both during inhalation and exhalation. There is also some contribution from the diaphragm. A timing mechanism in the brain sends neural messages to the muscles in the larynx, rhythmically opening and closing the air passage approximately 25 times per second. Combined with the steady inhalation and exhalation of air as the cat breathes, a purring noise is produced with strong harmonics.
It was once believed that only the cats of the Felis genus could purr; some older texts may still say this. In fact, all cats are able to purr. However, the entire Panthera genus is able to purr only while exhaling. Cats that are not members of the Panthera, even larger ones such as the cheetah, purr..
One theory held that purring involved blood hitting the aorta. Another possibility was that another area of soft tissue or muscular tissue in the neck or torso (e.g., the diaphragm) similarly vibrates. Another held that purring might be caused by vibration of the hyoid apparatus, a series of small bones connecting the skull and the larynx that nominally serves to support the tongue. Yet another theory held that cats might possess a special purring organ, though none was ever found.
Why felines purr
Above all, the purr is probably the cat's way of communicating to others (cats and humans) that it is in the mood to be sociable. The purring sound is frequently made at the same time that other 'sociable' signals are made, e.g. erect tail, slightly closed "contented" eyes, not walking away. Naturally, in most situations, this will also be when the cat is feeling contented, but it need not necessarily be so. Humans usually interpret the purring of a domestic cat as an expression of some type of friendliness or contentment. This assumption is based on the observation that cats often (though not always) purr when being stroked by humans.
It is, however, not entirely clear to scientists whether this really is one of the cat's reasons for making the sound; it is well-established that a cat also purrs when it is uneasy, nervous or in great pain, perhaps to comfort itself or to express submission. Other theories suggest that a cat purrs when it wants, needs, or is receiving attention, whether it be affection or medical treatment. When cats purr while also lightly clawing the ground it may mean they are trying to relieve stress or comfort themselves.
An intriguing possibility postulated by Roy Feinson in The Secret Language of Names (The Overlook Press, 2004) suggests that the question is best answered by asking 'when do cats not purr?' He writes "...a clowder of wildcats relax under a tree engaging in gentle purring when one of the cats spots a rabbit. The cat abruptly stops purring and the sudden silence immediately puts the other cats on alert without any audible or motion signal that might have alerted the prey. Thus, maintaining a low-energy purr at times of well-being allows the absence of that sound to become a communication device."
The theory is strengthened by the fact that cats tend not to purr when alone.
Ethologist Paul Leyhousen, in his book Cat Behavior, interprets purring as a signal meaning "I am not a threat" to explain the otherwise differing circumstances that elicit the sound.
Purring may also be a signaling mechanism between mother cats and nursing kittens. Female cats are known to purr while giving birth. Kittens purr while nursing, presumably as an "all's well" signal to their mother.
K.M. Dyce, W.O. Sack and C.J.G. Wensing in Textbook of Veterinary Anatomy 3rd Ed. 2002, Saunders, Philadelphia; p156
^ Purring for bone strength and healing
^ How A Puma Purrs
^ Overview of Felidae
^ Cheetahs can purr
Stogdale L, Delack JB. Feline purring. Compendium on Continuing Education for the Practicing Veterinarian 1985; 7: 551, 553.
Reprinted in: Voith VL, Borchelt PL (eds). Readings in Companion Animal Behavior. Trenton: Veterinary Learning Systems, 1996; 269-270.
From Wikepedia, so take it with a grain of salt......