PURRING

Music to a Cat Lover's Ears

by Joelle Steele

There is one sound that is music to the ears of all of us cat lovers -- the sound of our beloved kitties purring softly beside us. This motor-like sound has fascinated people for centuries. In her book The Cats, Muriel Beadle relates this myth about how cats got their purr: There was a damsel in distress who was given the task of spinning ten thousand skeins of linen thread within thirty days in order to save her true love from death. Since it was impossible for her to do this all by herself, she enlisted the help of her three cats. They all worked day and night and finished the job within the thirty days. As a reward, the cats were given the ability to purr, a reminder of the whirring sound of the spinning wheel.

Well, we can be pretty sure that's not really how cats first began to purr but, on the other hand, we modern-day people still don't know all that much about how or why cats purr. We do know that kittens begin to purr instinctively as soon as they begin to nurse and that the first kitten to suckle uses the sound to call others to begin feeding too. When they are very young, kittens purr in a monotone, but as they get older, they begin to include variations in speed, rhythm, pitch, and volume in combinations which produce many different varieties of purring sounds.

Feline behavior experts believe that purring is a communication device. Mother cats purr while nursing, possibly providing a measure of reassurance to their kittens which reminds them that their mother is there to watch over and protect them. In addition to purring when they are content, cats also purr when they are afraid, sick, in pain, or even dying. In fact, sometimes a dying cat will purr constantly for several hours while in the final stages of illness. With that in mind, purring could be a means of communicating strong sentiments, whether good or bad.

Some cats purr more than others, not unlike some people who talk more than others. And, some cats don't purr at all. The big cats, those wild and distant cousins of our domestic housecats such as lions, tigers, leopards, and panthers, do not purr. Tigers are known to make a vocal greeting called "chuffing" which is a purr-like sound, but in the wild, it is only the medium-sized cats, such as mountain lions and cheetahs, who purr.

When a cat purrs, it feels like his entire body is softly vibrating, particularly the throat and chest. Over the years there has been much speculation about where the purring mechanism is located and how it functions. There have been several theories about the physiological origin of the purring sound. One theory purported that purring was created by turbulence in a major blood vessel of the heart with the sound being transmitted through the upper air passages during inhalation and exhalation. Other theories suggested that the sound was produced when the soft palate vibrated, when the epiglottis opened and closed repetitively, or when the diaphragm muscle vibrated.

The most popular theory was that purring sounds were generated by the false (superior) vocal cords, two membranous folds located in the larynx behind the true (inferior) vocal cords. When a cat inhaled, this was believed to produce the coarser purring sounds while exhaling supposedly caused softer, smoother sounds. But, the theory which is probably the most accurate is the one which suggests that purring comes directly from the cat's voice box. This theory states that the purring sound is produced by the tensing of the vocal cords which vibrate during inhalation and exhalation.

David Rice, a biomedical engineer at Tulane University in New Orleans, studied purring along with an international team of researchers from Tulane and from the King Alexander Museum in Germany. The outcome of that research was noted in an August 1991 issue of Discover magazine: cats use only their vocal cords to purr.

Rice was quoted by Discover as saying that the vibration of the feline vocal cords by the muscles in the cat's larynx, "creates sound when the air flows through them," whether the cat is inhaling or exhaling. "The muscles create the purring vibration -- one twitch per vibration."

Why are Rice and his fellow researchers so sure that they have the right answer to the purring question? Unlike earlier researchers of purring, Rice and his team did not use restrained animals with instruments stuck inside of them. Instead, they studied unrestrained cats by using a microphone that was sensitive to low-frequency purring sounds. They moved the microphone around the cats' bodies until they determined that only the larynx was the source of the purring. They did not detect any purring activity from any other part of the body.

So, there you have it. Whether Rice and his colleagues prove to be correct about the source of purring can only be proven or refuted by future studies. But, where the sound comes from is not as important as how we cat lovers feel when we sit in our favorite chair with our feline friends curled up in our laps, motors going strong. As for the question of why cats purr, maybe they do it to comfort their human companions. That's a nice thought, isn't it?