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Why do cats purr?

Q: Why do cats purr?

A: Your question has perplexed philosophers and cat lovers for centuries.

Cats purr when they are content and relaxed, often kneading with their front paws at the same time. Paradoxically, cats also purr when their bodies are tense, and they appear anxious or frightened—and even when they're injured. So, purring may be, in part, a self-soothing behavior.

Females purr during labor, and mama cats purr before and during nursing. Since kittens are blind and deaf for the first two weeks of life, the mother's purr vibrations may help them find their way to her milk and nurse more efficiently.

Kittens begin purring at 2 days old. By 3 weeks of age, they purr to litter mates, perhaps to alert them that Mom is available for nursing. And in adults, purring tells others that a cat poses no threat, which appeases a dominant cat and prevents an attack.

Purring occurs during both inhalation and exhalation when air moves through the glottis, the slit-like opening between the vocal folds, called vocal cords in humans. The sound oscillations are controlled by the brain, diaphragm, throat and nerves that connect them.

Cats purr at a frequency of about 25 hertz, or 25 vibrations per second, with harmonic overtones at 50 and 100 hertz. Vibrations of 18 to 35 hertz improve joint mobility. In many animal species, 20- to 50-hertz vibrations increase bone density, stimulate bone fracture repair and promote healing of injured muscles and tendons.

Frequencies of 50 to 100 hertz decrease pain in humans and may do the same for cats. Furthermore, 100-hertz vibration helps ease breathing in humans with chronic respiratory disease.

So, purring likely evolved because it gave cats a survival advantage. This magical healing system that has long perplexed us may even be the secret behind cats' nine lives.

Do cat purrs have healing qualities? Dr. Lee discusses the power of purrs on human healthin this blog.

Lee Pickett, V.M.D. practices companion animal medicine in North Carolina. Contact her at

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