On the not-so-infrequent nights when I’m plagued by insomnia, no combination of melatonin, weighted blankets, and white noise will do. My only cure is Calvin my cat, who lies on my shoulder and lulls me to sleep by his purrs.
For Club Purr veterans, these are the obvious reasons. 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. Robert Eklund is a Swedish phonetician and linguist from Linkoping University. He says that no one knows exactly how purring works. Nor can experts say, exactly, what purring means. When they are happy, cats purr, but also when they feel anxious, afraid or in labor. Purring is perhaps one of the most mysterious sounds cats make, making them the best pets.
There is, at least, some consensus on what purring is. The purr is, in the strictest definition, an erratic, rhythmic percolation produced by both exhales and inhales. There are no interruptions. 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. It is now generally accepted that purring comes from the :voice box. The brain sends electrical signals to the vocal folds. This causes them to open and close like tiny muscular doors.
Many animals can imitate the purring sound, including bears and Guinea pigs. Only a few creatures are capable of making a genuine burbly sound. Genets (smaller felids that look like African cats) can mimic the sounds of purring. 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. It’s not easy to understand purring: Females aren’t inclined to make the sounds around lab researchers.
Whatever its mechanical basis, purring seems hardwired into certain cats from birth. Even though they are still deaf and blind, their locomotors start to rev up within days after leaving the womb. As a way of communicating early, mothers and kittens seem to swap sounds, such as I am hungry or Hey Mom ,,” Hazel Carney is a veterinarian and purr expert who lives in Idaho. She also takes care of her three cats, Wyatt Earp and Calamity Jane. These positive association might explain why cats purr through their adult lives. They may be content with cuddling up to their favourite humans or enjoying a 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 engine of purring may turn even under less joyful circumstances. Mikel Delgado is a California feline behavior expert. She told me that she had once seen a cat who would purr when the vet was treating her. Sung even heard it while inserting catheters into patients. Scientists can only speculate about what’s going on. Carney explained to me that purring in certain animals could indicate a vocal tic or nervous laughter. Cats might be trying to communicate their need for help with anyone they come across. Jill Caviness is a vet and expert on cats at the University of Wisconsin at Madison and mother to Electron, a cat named Electron. These purrs could be the cat’s way of easing its stress-ridden body.
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. Unfortunately, although cats are able to purr in frequencies similar to vibration therapy, no research has been done on feline patients. “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. yowl. 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. These differences are hard for humans to distinguish, particularly in untrained cats. Caviness said that even her veterinary students couldn’t identify them in the clinic.
And unlike many other cat noises, purrs stubbornly elude human imitation (though some people on YouTube might beg to differ). Eklund stated that humans can meow at cats easily; it’s “like a very basic pidgin language.” 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. These cats are tokens of my affection that I can give but not receive. Soundtracks and certified devices can be 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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