On the not-so-infrequent nights when I’m plagued by insomnia, no combination of melatonin, weighted blankets, and white noise will do. Just one cure for my affliction exists: my cat Calvin, lying atop my shoulder, lulling me to sleep with his purrs.
For veteran members of Club Purr, the reasons are clear. A purr is warm tea, a roaring fire, and fresh-out-of-the-oven cookies, all rolled into a fleece-lined hug; it is the auditory salve of a babbling brook; it is coffee brewing at dawn. It is emotional gratification incarnate—a sign that “we’ve made our pets happy,” which just feels darn good, says Wailani Sung, a veterinary behaviorist at the San Francisco SPCA.
But purrs—one of the most recognizable sounds in the animal kingdom—are also one of the most mysterious. “No one, still, knows how purring is actually done,” says Robert Eklund, a phonetician and linguist at Linköping University, in Sweden. Nor can experts say, exactly, what purring means. Cats purr when they’re happy—but also sometimes when they’re anxious or afraid, when they’re in labor, even when they’re about to die. Cats are perhaps the most inscrutable creatures humans welcome into our homes, and purring might be the most inscrutable sound they make.
There is, at least, some consensus on what purring is. In the strictest sense, the sound is a rhythmic, rumbly percolation that’s produced during both exhales—as is the case with most typical animal vocalizations—and inhales, with no interruptions between. Purrers also run their motor with their mouths entirely closed, like little feline ventriloquists; the sound simply springs out of the body at a frequency that roughly spans the range between 20 and 150 Hertz. Back in the 1960s, one scientist posited that purring was the product of blood percolating through the vena cava, a vessel that returns the body’s blood to the heart; that notion was later disproved. Now it’s generally understood that the source is the voice box: The brain pings electric signals to the vocal folds, prompting them to flutter open and shut like little muscular doors.
Lots of animals can imitate the sound of purring, among them bears and guinea pigs. But only a small number of creatures can cook up a bona fide version of the burbly noise: In addition to house cats, genets—little cat look-alikes native to Africa—can do it; so can lynx, ocelots, and dozens of other smaller members of the felid family. Eklund recounted for me how one captive cheetah, named Caine, emitted booming purrs from “the second he woke up to the second he fell asleep,” he told me. But lions, tigers, and jaguars can’t rouse the same rasps; scientists have not documented any cats that can both purr and roar. Scientists can’t say for certain what separates the purrs from the purr-nots. It may have something to do with the length, shape, or thickness of certain species’ voice box, or the tissue architecture that surrounds it; or perhaps it’s the squishiness of their hyoid, a U-shaped bone suspended in the throat. Or maybe not. Purring isn’t easy to study: Felines aren’t usually keen on producing the sound around researchers in labs.
Whatever its mechanical basis, purring seems hardwired into certain cats from birth. They start revving their little locomotor engines within days of exiting the womb, while still blind and deaf. Kittens and mothers seem to exchange the sounds as a form of early communication, swapping essential messages such as I’m hungry and Hey, here comes Mom, says Hazel Carney, a cat veterinarian and purr expert based in Idaho, where she also cares for her own three cats—Wyatt Earp, Calamity Jane, and Hi Ho Silver. Those early, positive associations might be part of why purring sticks around through adulthood, reappearing whenever cats get content—curling up with their favorite humans, say, or chowing down on an especially tasty snack. Zazie Todd, an animal-behavior expert and the author of the book Purr: The Science of Making Your Cat Happy, told me that one of her cats, Harley, will sometimes rumble the moment Todd walks into a room, which is “really lovely.” For other felines, Sung told me, mere eye contact with a beloved human may be all it takes to get that engine going.
But the gears of purring can also turn under some far less cheery circumstances. Mikel Delgado, a feline-behavior expert in California, told me she once had a cat that would purr at the vet. Sung has even heard the noise while inserting a catheter into a patient. Scientists can only speculate about what’s going on. Carney told me that in some animals, purring could be a sort of vocal tic, like nervous laughter; cats might also be trying to send out pleas for help or warning messages to anyone who might dare approach. Or maybe bad-times purrs are self-soothing, says Jill Caviness, a veterinarian and cat expert at the University of Wisconsin at Madison, and parent to a feline named Electron. They could even be a cat’s attempt to dupe its pain-racked body into a less stressed state.
In the early aughts, a researcher proposed that purring might even have palliative properties for cats—pinging out vibratory frequencies that could, for instance, speed the healing of wounds or broken bones. The thought isn’t totally bonkers, Eklund told me. Vibratory therapy has shown some promise in animals such as rabbits; even NASA once pursued it, hoping to stave off or even reverse bone loss in astronauts headed for long stints in space. Carney has had plenty of clients who “swear that the cats lying in bed, purring beside them while they were ill, kept them from passing away,” she told me. But alas: Although cats can purr at frequencies that overlap with those used in vibration therapy, none of the research on these treatments has actually involved felines. “I don’t think we have any studies that are like, I sat with a purring cat on my broken leg for 15 minutes a day; I healed more rapidly than someone else,” Caviness told me; the same goes for the effects of purrs on the purrer. Carney’s more open to the healing idea, though she, too, admits: If people feel better around their cats, that might be less about purring’s direct mechanical effects on human tissues, and more about the entire companion animal being a psychological balm.
With cat communication now undergoing a bit of a research boom, Eklund told me—new papers on the subject appear “basically every week”—purring is perhaps less perplexing than it’s ever been. But among its cat-vocalization cousins, its rumbles can still be unusually difficult to parse, not least because, across contexts, purrs just sound so similar. Meows can also be a bit cryptic, but they have more discernible logic: It’s not so difficult to parse Calvin’s Feed me; I am legitimately starving mewl from his Why am I in this cat carrier? yowl. Carney, who’s spent years listening to purrs of all sorts, told me that such differences may exist with purrs too: Contented rumbles tend to be more melodious and lower, while anxious revs trend higher and harsher. And one study, from a few years ago, suggested that humans could pick out their pet’s “solicitation” purrs—an urgent, pitchy sound that cats emit when seeking a meal—from other purrs that they made on the regular. But differences like those are very hard to pick out, especially in unfamiliar cats; even Caviness’s veterinary students can’t tell them apart in the clinic, she said.
And unlike many other cat noises, purrs stubbornly elude human imitation (though some people on YouTube might beg to differ). Humans can easily meow back at their cats; “it’s like a very rudimentary pidgin language,” Eklund said. But purring? Our brains and throats just aren’t set up for the stuff. Which, to me, is a soft tragedy: The rumbles of my two cats, Calvin and Hobbes, are missives of love, of joy, of bliss; they are tactile and auditory feedback to my touch. They are a token of affection I can receive, but cannot send back.
Certain devices and soundtracks can offer substitutes. Some vet clinics play cat music in exam rooms, with a calming purr bass track; Delgado mentioned that a shelter she used to work at purchased surrogate nursing machines for orphaned kittens, which could be outfitted with a synthetic purr. Purr enthusiasts can even put on a podcast of an orange cat from Ireland named Bilbo purring for 30 minutes straight.
Purring is a language barrier we have yet to surmount. Which, in some ways, is so, so cat. Humans have spent generations breeding dogs to emote in very people-esque ways, using their soulful eyes and slobbery, smiley mouths. Cats, though, continue to thrive on subtlety; their mugs aren’t evolutionarily set up for obvious expressions, defaulting instead to “resting cat face.” Even compared with other cat vocalizations, purring is subtle and intimate, a form of communication that hinges on proximity, on closeness, on understanding a cat’s wants and needs—and maybe, sometimes, on them understanding ours.
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