People who live with animals believe that they can understand what dogs and cats are thinking. We assume that when a dog wags his tail he is happy, when he rolls on the floor and shows you his belly he wants you to scratch it, and when he barks and bares his teeth at a visitor, he is suspicious. This is knowledge we’ve collected from centuries of inter-species togetherness, observation and anthropomorphism (attributing human characteristics to animals). It’s “common sense” and not likely to be questioned.
The purring of cats is somewhat more complicated, as Annalee Newitz points out in io9. If a cat curls up next to you, vibrating like a motor, you will assume that she is happy and contented. You take this as an invitation to rub her fur, which makes her purr even more. Aww, the cat likes you. In truth her purring may have very little to do with you, so don’t switch careers to cat whisperer just yet.
What is purring? Swedish linguist Robert Eklund defines it as “continuous sound production that alternates between pulmonic egressive and ingressive airstream and usually goes on for minutes.” In “mammal vocalization literature” (scientific papers about the sounds cats make), “pulmonic egressive” and “pulmonic ingressive” are the terms for “purring while exhaling” and “purring while inhaling”, respectively.
Apparently there used to be two divisions in the cat family based on differences in their hyoid bones (the bone in the neck that supports the tongue). There were the roaring cats: lions, tigers, jaguars and leopards; and the purring cats: everyone else. However, this classification became obsolete when it was proven that the hyoid does not affect a cat’s ability to purr or roar. I know this personally because when my oldest cat is in a foul mood she can roar like a lion in the Serengeti.
Meanwhile, scientists still don’t know exactly how cats purr, or why. They can land a rover on Mars and take cool pictures with it, but they can’t tell me why my middle cat is sitting on my keyboard doing an impression of a motor. It is true that a contented cat will purr while you rub his fur, but cats also purr when they’re stressed. Observe how your cats behave when you take them to the vet for their shots: they’re purring, but not for joy. (Whenever my youngest cat knows he’s going to the vet, he starts gasping and drooling.)
Behaviorists believe that purring is one of the ways cats communicate among themselves. Breastfeeding kittens purr to signal their mother that they’re getting enough milk; the mother purrs back to tell them everything is fine. I suspect cats purr when you stroke them because it reminds them of how their mother used to lick their fur. You’re their mother now.
Cats also purr when they’re injured. Bioacoustics researcher Elizabeth von Muggenthaler proposes that the purring of cats is a form of healing therapy. Cats purr at frequencies between 20 and 140 hertz—frequencies known to promote “bone growth and fracture healing, pain relief and swelling reduction, wound healing, muscle/tendon growth and repair, mobility of joints and the relief of dyspnea.”
No wonder they have that “nine lives” reputation: if this theory is correct, they can heal themselves. According to veterinary medicine professor Leslie Lyons, there may be practical human applications for this purring-healing therapy. Astronauts in zero gravity environments suffer from muscular atrophy and bone density loss. Purring is a low-energy way to stimulate muscles and bones. Astronauts can start purring, or we can send cats out in space ships. Then we can land rovers on other planets and take even more awesome pictures of cats.