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How felines purr
Cats produce the purring noise by vibrating their larynx, or voice box, in a particular manner. A timing mechanism in the brain sends neural messages to a muscle in the larynx, rhythmically opening and closing the air passage approximately 25 times per second and has strong harmonics. Combined with the steady inhalation and exhalation of air as the cat breathes, a purring noise is produced.
All cats are able to purr. However, the entire panthera genus is able to purr only while exhaling. Cats that roar lack the purring vocal chords, and use the vocal chords instead, making a growling sort of noise when they purr. As a result, the two sounds are often confused. The roar in these cats is a very loud growl with respect to the production method. Additionally, because these chords can only be used while exhaling, the purring equivalent sound can only be made while exhaling. Cats not members of the panthera genus, even larger ones such as the cheetah, purr.
Historical theories
One theory held that purring involved blood hitting the aorta. Another held that purring might have been caused by the vibrations 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 found.Why felines purr
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, combined with the experience that human children tend to enjoy stroking by their parents and interpret it as a gesture of affection. Consequently, most humans enjoy listening to or holding a purring cat.
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. Purring may also reduce pain, help a wounded cat to heal, or even help to keep a cat's bones strong. When cats purr while also lightly clawing the ground it may mean they are trying to relieve stress or comfort themselves. An example is when a female cat is nursing kittens; as humans may find with children, cats may also become stressed from the attention of their young and therefore start to purr and lightly claw the ground. This may also be associated with "kneading" behavior, in which the kittens' pawing helps release milk from the nursing mother's teats. Adult cats knead (also called paddy-pawing, making muffins, making bread, or treading paws) possibly as a sign of contentment and relaxation.
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.
Other examples of purring
It is not clear quite how and when purring is used between cats themselves, which is probably a more important issue bearing on its primary purpose than how and why it happens when humans are involved. One speculation is that it is a signalling mechanism between mother cat and nursing kittens. Female cats are known to purr while giving birth, and this may be to reduce the pain and also assist post-natal healing. Kittens purr while nursing, presumably as an "all's well" signal to their mother.
Some cats, usually the more vocal ones, are able to meow or hiss without interrupting the purring sound.
Effect on humans
A cat's purr has been shown in several studies to have effects on healing rates and in relieving stress. The fundamental frequency of the purr is remarkably constant across several purring species (at 25 Hz with substantial energy in its harmonics), with only cheetahs varying (by a few percent) .
References
"Purr"