Purr

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 cat purring shares a common frequency range of 25-150 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 when touching the cat's throat. In addition, some are able to meow or hiss without interrupting the purring sound.

How Felines Purr

Despite being an universally recognized phenomena, 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 theory is that 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. Combined with the steady inhalation and exhalation of air as the cat breathes, a purring noise is produced with strong harmonics.
• Another possibility is that another area of soft tissue or muscular tissue in the neck or torso (e.g., the diaphragm) similarly vibrates.

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 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

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. 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, 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. 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.

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 as Communication Between Cats

It is not quite clear how and when purring is used between cats themselves, which may be more indicative of its evolutionary significance than why it happens when humans are involved. It is likely that purring is a cat's way of communicating to other cats that it is in a sociable mood, or that it has no aggressive intent toward the other cat(s). It may also indicate to other cats in its social group that it is healthy and well.

Purring may also be a signalling 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.

References

• 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.

External links

1. Why and how do cats purr?
2. Why do Cats Purr?