Sunday, June 18, 2006

Apple #178: How Cats Purr

A while back, a3dmofo posted several requests of the Apple Lady. I've answered one, and now I'm going to do another.

I thought I knew what makes the purring sound in cats. I thought it was the blood traveling through their veins or something like that. Or anyway, that's what many people had told me many times. Turns out, that's a big fat lie!

The actual mechanism of purring is situated in the cat's voice box. Sounds obvious, doesn't it? A certain timing mechanism in a cat's brain transmits messages to the muscles around the voice box (a.k.a. larynx), so that the muscles open and close the air passage several times per second, or vibrate. This makes the air as it travels over the voice box vibrate also. The muscles vibrate whether the cat is inhaling or exhaling, so that's why it sounds like the purring happens continuously.

In a human larynx, the vocal cords are open when you're breathing, and closed and vibrating when you're talking.  In a cat, purring happens while breathing, during both inhaling and exhaling.
(Diagram from vocal clinic)

People also think that cats purr when they're happy. It turns out, they purr in many other circumstances as well, including when they're nervous, frightened, severely injured, or giving birth. If your cat looks like it's not feeling well and it's purring, don't assume that because it's purring it's happy and therefore not sick. In fact, you should probably assume the opposite, that's it's purring precisely because it's not well.

People who've studied cats think that purring may help to relieve a cat's anxiety or pain, and maybe even help in healing injuries. Researchers have found that the sound frequency of cat purring helps improve bone density and promotes healing.

Some companies have assumed, hey, if it works for cats, why can't it work for people? Many music & gizmo companies now offer recordings of a cat purring, which they try to sell as a relaxation or healing tool.

But there might be something missing from those gizmos. Veterinarians have discovered that a cat's brain also releases endorphins while a cat is purring. So the cats are actually getting a little stoned. Maybe this is why they like cat nip so much...?

By the way, thanks for the link, a3dmofo!

Howstuffworks, "How does a cat's purring work? How do cats make the purring noise?"
Leslie A. Lyons, response to Why do cats purr? in Scientific, January 27, 2003, Tip 71 - Cat purring - Why do cats purr?
Wikipedia, Purr
Bruel & Kjaer, "Solving the Cat's Purr Mystery using Accelerometers," company magazine, No. 1, 2003.

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