I’m a cat person. 

Don’t get me wrong; I like dogs.  But I like cats more.  (It’s an odd thing: although many dog people absolutely hate cats, I’ve seldom met a cat person who hates dogs.  But that’s probably a topic for another column.)

One of the nicest features of a cat is its purr.  Holding a contented cat and feeling that rumbling vibration is one of life’s truly nice moments.

Considering that cats have lived with humans for thousands of years, it’s surprising to discover that we don’t really know for sure how–or even why–cats purr.

When I was a kid, a read a book that claimed purring results from turbulence in the bloodstream of the vena cava, the main vein returning blood to the heart from the body.  The turbulence supposedly arises because the vena cava is constricted where it passes the liver and diaphragm, and when the cat arches its back, as the book claimed cats inevitably do when you stroke them, the blood forms eddies in this bottleneck, which sets up vibrations in the chest that are passed up via the windpipe to resonate in the sinus cavities.

This is so obviously ludicrous–cats don’t always arch their backs when you stroke them and they can purr in any position, back arched or not–that it’s stuck with me for 30 years; I think it was one of the first times I realized that just because something was in a book, it wasn’t necessarily true.

To be fair, an updated version of this theory does away with the arched-back component and claims it’s the emotional state of the cat that causes the turbulence in the bloodstream–which seems equally ludicrous considering cats can be so relaxed they’re on the edge of sleep, hardly in a heightened emotional state, and purring up a storm.  And besides that, studies of cats’ brain activity have shown that to some extent they choose to purr–it’s a conscious act, not a reflex action.

Today, the most-accepted explanation for purring is that it is caused by the vibration of two folds of membrane, called the false vocal cords, behind the true vocal cords in the larynx.

Let’s go with that explanation.  That still leaves the question, “Why do cats purr?”

Everyone knows cats purr when they’re contented, but they also purr when they’re in great pain– when giving birth, for instance.  Ttheories abound.  Maybe purring increases the efficiency of the circulatory system.  Maybe it’s a form of communication–a calming signal from mother to kitten, and vice versa, or a way for a dominant cat to reassure a subservient cat that it won’t be attacked, or a way for a threatened cat to convince its attacker that its harmless.  Maybe cats in distress purr to reassure themselves, like a human whistling in the dark.

Or maybe purring helps gives cats nine lives.

Researchers at the Fauna Communications Research Institute in North Carolina have analyzed the frequencies at which cats purr and discovered that purrs fall into the a range of frequencies that have been shown to improve bone density in humans and other animals.

House cats purr at frequencies between 27 and 44 hertz, while various types of wild cats–cougars and cheetahs, for instance–average 20 to 50 hertz.  (Almost all cats purr except tigers.)

In one study, rabbits exposed to sound between 27 and 50 hertz had stronger bones whose fractures healed more quickly.  In another, chickens placed on vibrating every day for 20 minutes grew stronger bone. Scientists are so intrigued that research is now underway to see if low-frequency sounds can halt osteoporosis and even renew bone growth in post-menopausal women.  NASA is also interested, since astronauts tend to lose bone density in orbit.

Veterinarians have long known that cats have strong, fast-healing bones, compared to dogs and other animals.  There’s an old veterinary adage to the effect that, if you put a cat and bunch of broken bones in the same room, the bones will heal.  A recent study in the Journal of the American Veterinary Medical Association found that out of 132 cats that fell an average of 5.5 stories, 90 percent survived.  One fell 45 stories and survived!

So it could just be that when your cat purrs in your arms, she’s not only telling you how happy she is to be there, she’s also keeping her bones strong.

Whatever she’s doing, and however she’s doing it–it’s nice.


Permanent link to this article: https://edwardwillett.com/2001/04/purring/

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