How do cats purr?

Happiness and contentment would make humans smile and laugh…make dogs wag their tails and make cats purr. Yes, in cats, contentment is personified by purring. How and why cats purr has stirred up many controversies among scientists and cat lovers. There are several theories that answer the question how cats purr. One theory believes that the purring is caused by the rushing of de-oxygenated blood in the inferior vena cava. However, recent studies have pointed to the laryngeal muscles, the voice box and a neural oscillator as the creators of the purring sound. This school of thought was supported by the fact that cats with laryngeal paralysis cannot purr. Cats are believed to have a special “wiring” that is connected from the brain to the voice box. Cats can purr whenever they want to. This connection causes the muscles to vibrate thereby producing the purring sound. The rapid opening and closing of the voice box muscles as the cat inhales and exhales causes the vibration that creates the purring sound.

In lower forms of animal life, sounds are produced by the structure called glottis. This structure is what makes snakes hiss, frogs croak, birds chirp…and cats purr. Purring is a distinct vocal feature of domestic cats. Cats produce this unique sound in spite of the fact that they do not have a distinct anatomical feature that will make them generate the purring sound. Cats can make the purring sound during the entire respiratory cycle. Other animals make sounds at the expiration of breath. Cats cannot meow and yowl while they are breathing. However, cats can certainly purr while inhaling and exhaling.

The hows and whys of purring have puzzled scientists. It was once believed that the rushing of blood in the inferior vena cava, the large vein where de-oxygenated blood from the lower half of the body would pass to get to the right atrium of the heart creates the purring sound. This theory however, was disputed by the fact that only domestic cats can purr. As de-oxygenated blood would be carried by the inferior vena cava in other animals, these animals should be able to produce the purring sound as well. Studies have been conducted and the most accepted theory is that purring sounds are created when the brain signals the laryngeal muscles to vibrate. The laryngeal muscles cause the opening and closing of the space between the vocal cords (glottis). The vibration of the vocal chords and the piston pump-like action of the diaphragm causes the air to be pushed in and out of the vocal chords. This action produces the purring sound that is not unlike a musical hum. The vibration is strong enough so that anyone cuddling a purring cat would hear the sound and feel the vibration that occurs throughout its body. A purring cat can be likened to an idling engine. Scientists that have conducted studies on this unique ability of cats believe that purring is a voluntary act that is triggered by a command from the brain. Cats would purr only when they want to.

Purring is an innate ability of cats. Kittens would start purring a day or two after birth. Purring is believed to be a form of communication. While it is commonly believed that purring is a sign of happiness and contentment, cats would also purr when they are sick, stressed or frightened. If the frequency of a purr will be measured it would fall between 25 and 150 cycles per second. This kind of frequency is believed to stimulate healing. Recent studies have found out that by purring, cats would release endorphins. This form of natural analgesic promotes the healing process as it reduces pain. Because of the purring cats have become therapy animals.

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