A Purr-fect Way to Build Up Bones

by Vijaya Khisty Bodach

Why do cats purr? We have wondered about that soft and soothing sound since cats became our companions over 5,000 years ago.

You’re right if you think that cats purr because they are happy and content. Cats choose to purr, just like you choose to laugh. Purring is part of a cat’s communication. It signals a friendly social mood. But did you know that cats also purr when they are frightened, hurt, or even dying? Such observations have led scientists to ask if purring is involved in healing.

Elizabeth von Muggenthaler is a scientist who has always been interested in animal communication and the sounds animals make. She and her team from Fauna Communications Research Institute in North Carolina recorded the purrs of several different types of cats: the common house cat, the puma, the ocelot, the serval, and the cheetah. (Larger cats, such as tigers and jaguars, can roar. But they are not known to purr.)

Von Muggenthaler and her group discovered that all these cats’ purrs had a very specific sound. What exactly is sound? Vibrations. If you pluck a rubber band, it vibrates or moves back and forth, creating sound. The frequency of the vibrations can be measured in hertz (Hz): the number of vibrations per second.

A cat’s purr is created by the movement of the diaphragm and the voice box. The twitching of these muscles causes the vocal cords to rapidly narrow and widen, which in turn causes the air molecules around them to vibrate at the same rate, or frequency. Amazingly, cats that have had their voice boxes removed due to disease can also purr. This means that the vibrations of the diaphragm alone can initiate the purr. A cat can purr while breathing in or out or with its mouth completely closed. A kitten can purr while it nurses.

When your cat is lying on your lap, you can feel the vibrations of its purr on your lap and under your hand as you pet it. But how do you hear the cat purring? Again, it has to do with vibration. The vibrating air molecules bump into neighboring molecules, which start to vibrate at the same frequency, and so on and so forth, until vibrating air molecules enter your ear and bump against the eardrum. Now your eardrum vibrates. That vibration is converted to an electrical signal in the inner ear. The auditory nerve sends it to the brain for processing. And you hear the happy sounds of your purring cat. Humans can hear frequencies of 20 to 20,000 Hz; cats can hear up to 100,000 Hz!

My two cats sound like motors when they purr. And guess what? The hum of a diesel engine has the same range of frequencies as a cat’s purr. So do the lowest notes on a piano. But they differ in two important ways: their intensity (loudness) and their quality.

Think of playing a key on a piano and listening as the string vibrates. You hear the note that string produces. But different parts of the string are vibrating at higher rates, and these higher frequencies, called overtones, define the quality of the sound so you can tell you’re listening to a piano and not a guitar or a violin ... or a purring cat.

What the Fauna Communications Research team found is that, just like the vibrating piano string, a purring cat produces a number of different frequencies with portions of the purr registering at 25, 50 and 100 Hz. As the frequency increases, so does the pitch. (Think of how each key on a piano produces a higher pitched sound than the key just below it.) A cat’s purr can go as high as 250 Hz. All the vibrations are in perfect harmony. No wonder we love to hear a purring cat. It calms us. But what else is so special about these frequencies?

They are the same ones that help bones to heal and grow!

Several years ago, Dr. Clinton Rubin and his team from the State University of New York found that exposure to low-intensity, low-frequency vibrations increased bone density. They placed one set of sheep on a gently vibrating plate for twenty minutes, five days a week. The other set of sheep, known as the control group, remained in the pasture. After a year, Dr. Rubin found that the vibrated sheep had stronger bones. He got the same results with turkeys and rats.

Other scientists also found that low-level vibrations help bone growth and fracture repair. Physical therapists have long used vibrations to strengthen muscles, ligaments, and tendons, and to lessen pain and swelling in people who are hurt.

The studies on healing frequencies, bone strength, and purring cats have led some scientists to hypothesize, or tentatively suggest, that purring is a natural healing mechanism. As any cat owner can tell you, cats spend a lot of time lounging around. Regular exercise is the best way to keep bones and muscles strong, but if a cat exercises only now and again, purring while resting would be good. It would stimulate bone growth, increase muscle and ligament strength, and maintain good health. And if a cat were wounded, purring would help to heal and comfort it. Many veterinarians have observed that bone and muscle diseases are rare in cats. Cats are remarkably resilient and recover quickly from injuries. Maybe purring is the secret of their “nine lives.”

Our bodies are constantly dissolving old and damaged bones and replacing them with new bone. Walking, jumping, weightlifting, and many other kinds of physical activities help keep our bones and muscles strong. But what about elderly women, children using wheelchairs, or astronauts in the weightlessness of space – all of whom are especially prone to osteoporosis, or “porous bones”? In his research, Dr. Rubin found that gentle vibration at low frequencies (within the same range as those of a purring cat) not only help to maintain healthy bones, they even reverse bone loss. This is good news. Dr. Rubin and other scientists think that an exercise program that includes a way to vibrate humans could be used to promote strong bones in people unable to exercise. Gentle vibration of children that use wheelchairs for just ten minutes a day can help to keep their bones healthy and prevent fractures.

So the next time you have a purring cat on your lap, think of how it is becoming stronger. And who knows? Perhaps it’s helping your bones to become stronger, too!

This article was published in the June 2006 issue of Cricket magazine and the Oct. 2006 of Odyssey magazine.  Reprinted here with permission from Carus Publishing.