How do cats purr? New finding challenges long-held assumptions

One of the most delightful sounds to a cat lover is their feline friend’s rumbling noise when they get a little scritch behind the ears. Yet how cats produce their contented purrs has long been a mystery.

[science.org]

A new study may finally have the answer. Domestic cats possess “pads” embedded within their vocal cords, which add an extra layer of fatty tissue that allows them to vibrate at low frequencies, scientists report today in Current Biology. What’s more, the larynx of these animals doesn’t appear to need any input from the brain to produce such purring.

“Purring has historically had a complex, nonscientific explanation,” says Bonnie Beaver, a veterinary scientist at Texas A&M University who wasn’t involved in the study. Nonscientific, she says, because although scientists had devised various theories to resolve the mystery, few were ever tested. The new study, Beaver says, is a good step forward.

Domestic cats are small, with most weighing about 4.5 kilograms, and researchers had puzzled over how these animals manage to generate the low-frequency vocalizations—typically between 20 and 30 hertz (Hz)—involved in purring. Such frequencies are usually only observed in much larger animals, such as elephants, which have far longer vocal cords. And whereas big cats such as lions and tigers are capable of loud roars, domestic cats are only able to produce low-frequency purring.

Most mammal vocalizations, including other cat noises such as meowing and hissing, are produced in a similar way—a signal from the brain causes the vocal cords to press together, and the flow of air through the larynx causes the cords to knock against each other hundreds of times per second, producing sound. This process, known as flow-induced self-sustained oscillation, is a passive phenomenon: Once the vocal cords start to vibrate, no further neural input is required to keep them going.

But in the 1970s, scientists proposed that purring was different. The so-called active muscle contraction hypothesis holds that domestic cats actively contract and relax their laryngeal muscles about 30 times per second in order to purr. The idea, based on measurements of electrical activity in the laryngeal muscles in purring cats, caught on and has been a common explanation for cat purring ever since.

The new study challenges this. To conduct the work, scientists removed the larynges from eight domestic cats, all of which had been humanely euthanized because of terminal disease and were investigated with the full consent of their owners. The researchers pinched the vocal cords together and pumped warm, humidified air through them. By isolating the larynx this way, the scientists guaranteed that any sound produced was occurring without muscle contractions or any input from the brain.

The team was able to produce purring in all of the larynxes—a “great surprise,” says lead author Christian Herbst, a voice scientist who holds dual appointments at the University of Vienna and Shenandoah University. Without any active neural control, all eight larynges produced self-sustaining oscillations at frequencies between 25 and 30 Hz—suggesting purring doesn’t necessarily require active muscle contractions.

Looking at the anatomy more closely, Herbst and colleagues turned to unusual masses of fibrous tissue embedded in the cats’ vocal cords. Anatomists had noticed these masses before, but nobody knew what their function might be. It’s possible, Herbst says, that these “pads” increase the density of the vocal cords, causing them to vibrate more slowly and making it possible for cats to produce low-frequency sounds despite their relatively small size. Anatomically, he says, the process works similarly to “vocal fry”—a droning vibrato sometimes added to the ends of words—in human speech.

The new experiment instead suggests that purring, like meowing and hissing, is a passive phenomenon that plays out automatically after cats’ brains provide the initial signal to purr, the researchers conclude. That explanation “is much more in line with what we know about how vocalizations are produced in other vertebrates,” says Karen McComb, an expert in animal behavior and cognition at the University of Sussex who wasn’t involved in the study.

However, David Rice, a biomechanical engineer at Tulane University who has conducted research into the mechanics of cat purring, isn’t fully convinced. He says there’s no guarantee that living cats’ vocal cords behave the same way as the surgically removed cords from the study. Just looking at excised larynges, he says, is “akin to removing the mouthpiece from a wind instrument and analyzing its sounds in isolation.”

Herbst suspects that purring is probably driven by a combination of neuronal control and self-sustaining oscillation—but it will be difficult to ever know for certain. As he notes, a cat will usually only purr when it feels safe, comfortable, and content—something that wouldn’t be possible if the felines had uncomfortable probes inserted into their larynxes. Until scientists find a way around that conundrum, this particular cat will likely remain in the bag.