Cats may be secret creatures at initial, but their ability to purr has long confused scientists. How can such a small animal make such deep sounds?
Now scientists could be about to solve this enigmatic pet puzzle. Cats, they say, have pads on their vocal cords that can help produce the low-frequency vocalizations involved in purring, according to a new paper published last week in the journal Current Biology.
Large animals such as elephants have longer vocal cords than small animals, which allows them to produce weaker sounds. The same rule applies to musical instruments: a large double bass, for example, can produce lower notes than a small violin.
“As a general rule, the larger the animal, the longer the vocal cords and, consequently, the lower the frequency of the sound produced,” explains Jason Arunn Murugesu, co-author of The study Christian Herbst, vocal scientist at The University of Vienna.
However, domestic cats with their relatively short vocal cords seem to be an exception to this rule. Although they usually weigh about ten weights, when they purr, they can produce low-frequency sounds in the range of 20 to 30 Hertz, which is lower than the deepest bass emitted by the average human voice.
To explain this phenomenon, the researchers examined eight domestic cats that had already been euthanized due to a terminal illness. With the consent of the cat owners, the scientists removed the animals’ larynx from their bodies, and then pushed heat air through them to simulate the sounds of cats.
Using this method, the researchers were able to create purrs at frequencies in the middle of 25 and 30 Hertz without the cat’s brain adding anything to it and without muscle contraction. The vocal cords vibrated in a way similar to the “vocal rumble” in humans, or the high-pitched, deep sound that some people make when they speak.
Other vertebrates produce sounds in a similar way through a passive process known as flow-induced self-preserved oscillation. When this happens, the brain sends a Signal to the vocal cords that causes them to contract. When the air circulates in the vocal cords, they begin to vibrate, and from there, Physiology takes over and the brain is no longer involved.
But for about 50 years, scientists have been thinking that purring does not happen this way. Instead, they hypothesized that cats actively contract and relax the muscles of the larynx up to 30 times per second, being constantly fed by their brain, according to Phie Jacobs from Science. However, the results of the new study suggest that the long-standing theory deserves a second look.
The researchers analyzed the vocal cords of the expired cats and discovered masses of tissue that they believe could be the key to purring. These structures, which they called “Pads”, could slow down the vibrations of the vocal cords by making them denser, allowing the animals to emit low-frequency sounds despite their small size.