How Do Cats Purr?

Up until now, it has not really been known how cats produce their purring sound, which is a sign of their contentment. However, scientists have finally discovered the answer to the question How do cats purr? Surprisingly, cats employ a technique similar to ‘vocal fry,’ the raspy voice used by singer Katy Perry and reality TV star Kim Kardashian. Vocal fry is where the voice drops to its lowest register, creating a deep, croaky voice.

Scientists have long been puzzled by the ability of small domestic cats to produce such deep resonating purrs, a vocalization typically associated with animals such as elephants that have much longer vocal cords. Since the 1970’s, they believed that purring required a unique mechanism involving the cyclic contraction and relaxation of muscles in the voice box, which would require continuous input from the brain.

However, it turns out that purring is much less demanding. Led by Dr. Christian Herbst at the University of Vienna, a team of researchers performed dissections on the vocal cords of eight cats. These cats had been put to sleep because of terminal illness, and were used in the study with the consent of their owners. The researchers pinched the cats’ vocal cords and directed warm, humidified air through them, mimicking the mechanism humans use to speak and produce vocal fry. This experiment resulted in self-sustained oscillations, or purrs, from all of the voice boxes, indicating that the sound does not require constant input from the brain.

Further investigation revealed the presence of fibrous pads of tissue embedded in the vocal cords, which increased their density. According to Herbst, “This may explain how such a small animal, weighing only a few kilograms, can regularly produce sounds at incredibly low frequencies (20-30 Hz, or cycles per second), even lower than the lowest bass sounds produced by human voices.” Herbst’s research, published in Current Biology, also noted similar structures in roaring cats like lions and tigers.

Previously, the prevailing theory suggested that cats relied on a different mechanism to produce a purring sound. It involved the brain sending rapid and continuous bursts of signals to the throat muscles, causing them to contract 30 times per second and producing a purr. This new study, however, suggests that purring, like hissing and meowing, is a passive activity that continues after the cat’s brain has initiated the signal to purr.

Want to know which cats purr the most? See our article on the friendliest cat breeds.