There is a belief among cat owners that their pets purr when they are happy. Cats often purr when people pet them, which may lead to a link between purring and pleasure. The cats’ behavior suggests that they may also be trying to encourage more interaction, as if signaling that they should continue to be petted. Research shows that cats may purr for a variety of reasons, using the soft rumble as a means of communication. Purring can also be a sign of stress.
Kittens are born blind and deaf and remain so until about two weeks old. However, after only a few days, they start purring, mainly to let their mother know where they are and to attract her attention during feeding time. This behavior can also be observed in adult cats. But this is just one of many different ways to use purring. A 2009 study found that when cats purr in search of food, the sounds sound more urgent and less pleasant, suggesting that they can manipulate their purrs.
Scientists have long wondered how small animals like domestic cats, with short vocal folds, sometimes called vocal cords, can produce such low sounds. We expect it to be found in larger animals that have longer vocal folds, such as elephants. For many years, it was thought that purring was produced by a unique mechanism that involved the periodic contraction and relaxation of the laryngeal muscles, requiring constant neural input from the brain.
In new research published in the journal Current Biology (DOI: 10.1016/j.cub.2023.09.014), scientists suggest that special structures found in cats’ vocal folds are responsible for their distinctive purr. Moreover, according to scientists, cats have no conscious control over purring and it is a negative behavior that continues automatically after the brain sends the signal to start.
The new research is behind a team of scientists led by Christian Herbst from the University of Vienna in Austria. This work challenges the common belief that purring results from active contractions of the laryngeal muscles. According to Herbst’s team, purring is caused by “cushions” in the vocal folds.
Domestic cats are small in size, most of them weigh around 4.5 kilograms. However, they can produce low-frequency sounds, typically 20 to 30 Hz, which are typically observed in larger animals with longer vocal folds. While big cats like lions and tigers can roar loudly, the low-frequency sounds made by domestic cats take the form of purrs.
Most sounds that come from the throat of mammals, including other feline sounds such as meows and hisses, are produced in a similar way. A signal from the brain causes the vocal folds to compress, and airflow through the larynx causes them to collide with each other hundreds of times per second, producing sound.
In the 1970s, scientists proposed an explanation for purring. According to this concept, domestic cats tense and relax their laryngeal muscles about 30 times per second, generating a purr. The idea, based on measuring the electrical activity of the laryngeal muscles in purring cats, caught on and has since become a popular explanation for cat purring.
New research challenges previous ideas. In their work, the researchers analyzed the excised larynxes of eight domestic cats. These cats were euthanized due to various incurable diseases, and their throats were removed with the consent of their owners.
The researchers compressed the vocal folds and pumped warm, humidified air through them. By isolating the larynx in this way, scientists ensured that any sound produced would be produced without muscle contractions. The team was able to induce purring in all larynxes. Without any active neural control, all eight larynxes produced self-sustained oscillations at frequencies between 25 and 30 Hz, indicating that purring does not require active muscle contractions.
By taking a closer look at the anatomy of the cat’s larynx, the researchers noticed unusual “cushions” of connective tissue embedded in cats’ vocal folds. These structures have been known for a long time, but until now no one knew what their function was. It’s possible that these “cushions” increase the density of the vocal folds, making them vibrate more slowly and allowing cats to make low-frequency sounds despite their relatively small size, Herbst says.
However, not everyone agrees with the proposed interpretation. As David Rice, a biomechanical engineer at Tulane University who has conducted research on the mechanisms of cat purring, acknowledged, “There is no guarantee that the vocal folds of a live cat behave in the same way as the vocal folds of a surgically removed larynx. Just looking at the larynx says that The ones that were removed are “like cutting out the trumpets of wind instruments and analyzing their sounds separately.”
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