A primer on purring

Posted: Wednesday, August 17, 2005

The mama cat lay on the rug with four nursing kittens docked side-by-side. Surrounding them was a cloud of sound like a thousand tiny bees. I petted the mother and placed my hand on each of her offspring in turn. Each was vibrating; they were purring, all of them.

A kitten can purr in its second day of life. It's thought that purring serves as feedback to the mother cat, letting her know that each kitten is feeding and doing well. A kitten can purr while nursing, as purring is done with the mouth closed - or full, as in this case.

Domestic cats are one of very few animals that purr. Most of the big cats do not. Curiously, Mother Nature seems to have determined that a cat can either purr or roar, but not both. Lions, tigers, leopards and jaguars can't purr. Tigers sometimes try, but the best they can manage is a sort of sputter as they exhale. Domestic cats purr steadily as they inhale and exhale. Cougars purr, as does the completely unrelated raccoon.

Cats purr when they are feeling secure and contented, but they also purr at the opposite end of the comfort scale. Females purr while giving birth. Frightened or injured cats purr, as do those recovering from surgery. Cats often purr when they are near death. Noting that cats purr under stress, some animal behaviorists suggest that purring may be reassuring or comforting, kitty's equivalent to our "whistling in the dark."

Be that as it may, it is well documented that purring is a healing mechanism much like ultrasound. Cats purr at a frequency between 25 and 150 Hertz. Exposure to that frequency improves bone density, stimulates muscles and promotes healing in humans. It may be why cats heal very quickly and recover from such serious injuries that they are said to have nine lives. Purring may keep cats from developing hip dysplasia and other bone and joint problems commonly found in dogs, but rarely in cats.

Cat owners, especially older ones, have consistently lower blood pressure than their catless counterparts. Cats serve as therapy animals in nursing homes. Here in Juneau, they purr their healing mantras in laps and on beds at Wildflower Court and the Pioneers' Home.

Purring is voluntary. The individual cat chooses when to turn it on and off. Some purr little; some a lot.

The most recent studies say purring is triggered by the release of endorphins in the cat's brain. That makes sense because endorphins are released when an individual feels either pleasure or pain. Endorphins activate the system in the brain that translates thought into movement. So what moves?

The vocal cords vibrate. The diaphragm pushes air in and out past the vibrating cords. The result is the musical humming we call a purr.

Purrs vary from cat to cat and do not correspond to the cat's size or shape. Both of my cats, who are medium-small, sound like idling diesels. By contrast, the 23-pounder of my childhood purred silently. He just vibrated when you petted him.

Then there was Baby Roo, a "pocket kitten" who had been rescued in infancy and hand-raised by well-meaning humans. Unfortunately, it never occurred to them that she might benefit from contact with other cats. She became a beautiful, healthy cat with some very strange ways.

Roo didn't meow until she was two and spent a long weekend boarding with other cats at the Kitty Motel. She went through life with her left side impeccably groomed, but never got the hang of grooming on the right. As a result, her hair always looked like she'd just gotten out of bed. When purring, she'd take a deep breath, purr a few bars, then exhale with an audible whoosh. She couldn't figure out how to breathe and purr at the same time.

• Linda Daniel has spent her life in the company of cats, most of whom simply showed up at her door. She volunteers at the Gastineau Humane Society.

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