Why the cat's purr is one of zoology's greatest mysteriesS

It's probably one of the most comforting sounds in the world if you like cats. A purr feels like the essence of contentment, but it also sounds downright weird. How do cats make this perfect vibration sound for minutes on end? After decades of study, nobody knows for sure. But there are a few things we do know about purring.

Photo via Robert Eklund.

Purring as Emotion

Most of what we know about cat purring comes from observation, and most of those observations come from people who have cats as pets. And you probably know what that means: Our observers are doing a lot of petting their kitties, which pretty much unfailingly causes purring. As a result, the most typical explanation for purring is that it's an expression of happiness and contentment. But cats also purr for many other reasons. UC Davis professor of veterinary medicine Leslie Lyons writes:

Cats often purr while under duress, such as during a visit to the veterinarian or when recovering from injury. Thus, not all purring cats appear to be content or pleased with their current circumstances.

The fact is, we simply don't know for sure why cats purr, but it seems clear that the sound serves multiple purposes. Famous animal behaviorist Paul Leyhausen, who studied cats for several decades, suggested that purring is one way cats communicate with each other, signaling that they don't wish to fight. A recent study found that cats emit a special kind of purr when they want humans to feed them (cats often purr while eating, too).

Purring Cats and Roaring Cats

What we do know about purring is how it sounds — and, to a certain extent, how it works. Swedish linguist Robert Eklund is fascinated by the kinds of sounds that animals (including humans) make when they inhale. These sounds are called "ingressive," and Eklund and his colleagues have proposed that we define purring as "continuous sound production must alternate between pulmonic egressive and ingressive airstream (and usually go on for minutes)." In other words, to count as a purr, the cat must make the sound as it exhales and inhales. Eklund has recorded countless cat purrs, and found that domestic cats purr at between 20.94 and 27.21 Hz while exhaling and between 23.0 and 26.09 Hz while inhaling. Purring also produces strong harmonics.

Here you can see Eklund recording a cheetah's purr, which is louder as the big cat breathes out. Many big cats, like lions and tigers, do not purr. This led early researchers to divide cats into "purring cats" and "roaring cats." It's possible that roaring cats can't purr because of an anatomical difference from domestic cats, cheetahs, and other purring cats. (You can hear many kinds of cat purrs on Eklund's site, Purring.org.)

Frustratingly, nobody is really quite sure what causes the purring noise in cats. There is no "purring organ," or specialized part of the cat throat that's responsible for this irresistible noise. Some veterinary researchers have suggested that purring is created by the muscles of the larynx, which could be dilating and constricting the cat's vocal chords. But nobody knows for sure.

Healing Hypothesis

Elizabeth von Muggenthaler, a bioacoustics researcher at Fauna Communications, has suggested that cats may purr in order to heal themselves. She writes:

Frequencies between 20-140 Hz are therapeutic for bone growth/fracture healing, pain relief/swelling reduction, wound healing, muscle growth and repair/tendon repair, mobility of joints and the relief of dyspnea . . . Fauna Communications has recorded many cats' purrs, at a non-profit facility and the Cincinnati Zoo , including the cheetah, puma, serval, ocelot and the domestic house cat. After analysis of the data, we discovered that cat purrs create frequencies that fall directly in the range that is anabolic for bone growth.

While this might explain the evolutionary benefits of purring, it is just a hypothesis. Many researchers find it plausible, but we have yet to see any study that offers solid evidence that cat bones heal faster because of purring.

So we know when cats purr, and what those purrs sound like. But we still aren't sure why cats keep purring and what allows them to make such an unusual sound. It may be that when we finally have the answers, pet owners won't want to hear them. After all, we've been calling the purr the ultimate sound of snuggly happiness for centuries. In the end, it may turn out that kitty isn't purring because he's happy, but just because he's trying to comfort himself while the monkey people pat his head ineptly for the fifty thousandth time.