March 27, 2008

Cats Purring

Stupid Question ™
Dec. 21 2000
By John Ruch
© 2000

Q: Why do cats purr?

A: Why cats purr is as mysterious as how they purr—and it’s probably impossible to solve one of these mysteries without solving the other.

Cats start purring at 1 week of age. Purring is a low, rumbling sound that cycles 26 to 30 times per second and continues whether the cat is inhaling or exhaling. It occurs only in the presence of other cats or humans.

Purring appears to be exclusively a domestic-cat sound, at least in its regularity and its occurrence in both inhaling and exhaling. Big cats like lions and tigers sometimes make a similar sound, but only when exhaling. Other mammals also supposedly purr; the raccoon indeed makes a convincing “purr,” but again only on the exhale.

There is moderately convincing evidence that purring is a voluntary act. Why a cat would choose to purr, however, is not clear.

Most people associate purring with happiness or contentedness, because those are the only situations in which they see domestic cats. But cats also purr when they’re injured, giving birth or dying. And a feral cat purrs when it’s in a stand-off with another cat.

It’s safe to say that purring is a form of communication, given as a response to emotional arousal, with several possible meanings. It might signal submission. It could also be a self-reassuring sound, like humming in humans.

It appears especially significant as communication between a mother cat and her kittens, which are born blind and deaf and may feel purring rather than hear it. The mother’s purring may reassure kittens, while their purring lets her know their location. All adult uses of purring may derive from this childhood purring.

There’s even less agreement about how cats purr. The confusion seems to have arisen from the difficulty in locating the sound; it can be felt in both the throat and the chest of the cat.

The most obvious theory is that it involves two additional folds in the cat’s larynx, right behind the vocal cords. These are called “vestibular folds” and appear to add resonance to the “meow.” Their vibration may also cause purring.

Another theory involves the whole breathing system, noting that purring is physiologically associated with rapid contractions of the larynx and diaphragm. These contractions might produce extremely rapid changes of air pressure in the windpipe that in turn produce the purring sound.

Still another theory is that’s a vibration in the blood vessels caused by raised blood pressure, in response to emotional arousal. The most plausible culprit is the vena cava (the body’s major vein) causing turbulence where it narrows to pass through the diaphragm and liver. Its vibration is supposedly transmitted to the diaphragm and resonates in the sinuses.

The blood vessel theory explains the chest-rumbling part of purring. But so does the larynx/diaphragm theory, which also is more plausible if purring is indeed voluntary; it’s generally easier to willfully control breathing than blood pressure.

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