Why Do Cats Purr?

As a veterinarian, I am commonly asked “Why do cats purr?”Most people believe cats purr when they are content or happy. While cats do purr when they are content, researchers attempting to uncover the answer to this 3,000-year-old mystery are finding the answer more complicated than previously thought. All domestic cats purr, as well as many wild cats, like pumas, ocelots, lions and cheetahs. Purring can occur in a variety of situations. When cats purr in the presence of other unknown cats or kittens, the behavior may serve to convey submissiveness or a friendly greeting. While it is true that cats purr contentedly while on their pet parent’s lap, they also purr when they give birth, when they are frightened, severely injured and even while dying. Because kitties clearly cannot be content in all these situations, contentment or friendliness cannot be the only reason they purr.

So why else would they purr?
Perhaps it is helpful to look at purring in the context of natural selection. Natural selection tells us that a particular behavior or trait will persist from generation to generation only if it is beneficial to an animal’s survival. For purring to exist in both domestic and wild cats, there must be something vital about the behavior. Purring is created by the vibration of the cat’s larynx and diaphragm, and therefore requires an expense of energy. If a kitty is sick, they would not use precious energy to purr unless there was a very good benefit.

Researchers have found certain types of purrs are meant to communicate with their people. In 2009, researchers discovered a high-pitched cry, similar to that of a human infant, embedded in the purrs of cats soliciting food. They were using the purr to signal their human caretakers that they needed something. Those sneaky kitties!

I’m sure you have heard the expression that “cats have nine lives”. Similarly, veterinarians have an old saying that if you put a cat who has broken bones in a room with other cats, the breaks will heal. In fact, cats are amazing self-healers: they have fewer post-operative complications than dogs, have a lower incidence of bone and joint disease than dogs, and 90% of cats survive high-rise falls – I’m talking falls from 5 story buildings! (Robinson, et. al 1976) What could possibly account for these facts?

One theory is that the purr has healing properties. Researchers have found that vibrations in the frequency range between 25 and 50 hertz promote bone strength, stimulate healing of fractures, provide pain relief, and help heal tendons and muscles. In 2001, National Geographic reported a study where chickens grew stronger bones after been placed on a vibrating plate for 20 minutes daily.

Bioacoustic researchers have recently studied purring in 47 cats, both wild and domestic. They studied the frequency, pitch, loudness and duration of purring in relation to the cat’s behavior, and guess what they found? The domestic house cat purrs in the range of 25 and 50 Hz: the exact range associated with healing properties such as increased bone density.

Maybe this has something to do with a cat’s uncanny ability to “heal by association”. Perhaps purring is part of the reason why, when we fall ill, having a cat sit on our laps can actually make us feel better. Whether it is simply the comfort of having a friend nearby, or whether it’s the vibrational frequencies of your kitty’s rumble, the joy of a cat purring on your lap is priceless.

Whatever the reason, I encourage you to take care of your cat. Keep him happy and purring and you’ll likely both lead healthier and happier lives.

Thank you for all you do to make the world a better place for your dear companions.

Dr. Jane Bicks, DVM

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