On the How of the Purr

June 1st, 2009 | Meera

I don’t even have to touch her, sometimes, before it begins; on my approach, her ears unfold to my footfalls, each one a triangular flag hoisted by a tiny sailor. Chin lifts from crossed paws, emerald eyes widen, and I couldn’t stop it now if I tried—for even before my thumb has arrived at her furry head bearing its promised caress, the whole thing is underway. I can feel a tremor in her, radiating outward from a secret, central source. It hums gently through her spine as I lay my other hand on her back: a bubbling, burbling thing. A purr.

She has curled up on my lap now, become a mottled grey egg with no edges, and it seems as if the whole of her body vibrates as one well-oiled machine: but two fingers carefully tucked under her chin find the strongest buzz. Squinting, I can actually see it, just—pushing and pulling at her skin, tickling the hairs into a movement like the whisper of a summer breeze through tall grasses. The purr is a force that is insistent without being urgent, and I love it. I think, in fact, that I may be addicted to it. If worry clamps around my chest, look for me on the sofa, waking the cat from sweet sleep into indignation so I can set her in motion.

blissed out

Most forms of vocalization, including human speech and cats’ meows, are created by the vibration of the vocal cords as a column of air is pushed past them. The vocal cords are diaphanous membranes that stretch across the hollow organ—how richly it deserves that appellation—known as the larynx. The nature of the sounds we produce depends on how tightly the vocal cords are pulled by the muscles of the larynx; like a guitar string, the greater the tension, the higher the pitch. The whole apparatus is capable of bringing forth an astonishing cacophony of peals and murmurs, and we can learn to manipulate them with a high degree of control. The cat in our house, for instance, is a stylish songstress who expresses herself with all sorts of vocal experimentation: chirps, meows, yowls, growls. Her commentary on the world varies widely in pitch and form, and she is not above running through every pattern in her power in order to make herself understood.

Purring is different. Purring has almost the regularity of a metronome. In most domestic cats (Felis catus), it is generated by vibrations that take place at an incredibly consistent frequency—somewhere between 20 and 200Hz, or vibrations per second, according to data collected by animal behaviorist Elizabeth von Muggenthaler. (In order to get these results, Muggenthaler took tiny accelerometers, devices that are more commonly used to do things like detect a falling laptop and trigger shock control measures, and glued them temporarily to the skin of cats. Then, in what must be one of the most relaxing experimental setups ever conceived, she had her test subjects lie on blankets, where they were occasionally stroked.)

The soft buzzing coming from the bundle in my lap hardly ever jerks or hiccups—doesn’t much speed up, doesn’t much slow down. Even more miraculously, the hum of a cat continues and continues and continues, pausing only for brief, almost imperceptible moments as the air inside its lungs changes direction. Speech, no matter how soothing the words, is messy; ragged; broken by breath. Anxiety can breach its gaps. A purr, on the other hand— a purr stitches together inhalation and exhalation. Breathing in and breathing out become a single expanse of vibration. It’s the reason I enjoy folding over my cat, letting her whirring breath wrap around me like a smooth blanket of sound.

I find the comforting reliability of the purr marvelously contradictory in the light of how it is produced. In a way, what biologists believe is its basic mechanism (there is still some debate on the issue) relies on discontinuity. When a cat purrs, its vocal cords aren’t just being stretched. They’re actually being alternately pulled open and shut across the larynx, constantly interrupting the flow of air as it passes in both directions. First they let air through, then prevent its passage, like a gate whose doors swing back and forth at incredibly regular intervals. Each time this happens, a small, pressurized puff of air builds up, whose sudden release produces an audible sound.


The reason the intervals between the gate’s opening and shutting are so regular, it seems, is that, like a heartbeat, they are governed by a timer. Somewhere in the brain of every cat is a pacemaker of sorts known as a neural oscillator. This is a neuron, or perhaps a small network of neurons, that—once activated—fires in a repetitive, periodic manner that is utterly predictable. In the case of the oscillator that controls purring, the purpose of this continued relay of synapses is to send signals to the laryngeal muscles, instructing them to tighten and relax in turn. As they do so, the tiny gate they control swings open and closed. The result of all these mechanisms working together is the beautiful, steady putt-putt-putt that brings me so much pleasure.

I wonder if you’ll understand when I tell you this: Knowing that the production of my cat’s purr relies on an automatic neurological process makes it even more dear to me. A cat is a creature of caprice, unpredictable and strange—my cat perhaps more so than any. If she herself shaped the rhythm of her purr, how could I rely on it for solace as I do?

Instead I imagine the purr’s musical hum existing in some deep, essential way that is not limited to her small frame—like the movement of the earth, like the tides. And as I hold her I think of this rhythm flowing through her into me, willing the timer in her brain to somehow speak to the muscles in my own throat.

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