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Some species of felines make a sound which is called a purr. 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. Some cats purr so strongly that their entire body vibrates. Cats purr at 27 - 44 hertz.
Cats produce the purring noise by vibrating their larynx, or voice box, in a particular manner. They have a timing mechanism in the brain which sends neural messages to a muscle in the larynx, rhythmically opening and closing the air passage several times per second. Combined with the steady inhalation and exhalation of air as the cat breathes, a purring noise is produced.
Cats can either purr or roar, one exception being the tigers which can purr but only in one direction. In general, small cat species purr, while larger ones roar, although pumas and cheetahs purr and do not roar.
One theory held 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.
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.
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.
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 seem to be able to meow without interrupting the purring sound.
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