Scientists have cracked the mystery of how cats produce the purring sound that signals their approval. It turns out they use a strikingly similar technique to “vocal fry” – the croaky voice used by the singer Katy Perry and reality television star Kim Kardashian.
Researchers have long puzzled over how an animal as small as a domestic cat can produce the deep resonance of a purr, when such vocalisations are usually only produced by animals with far longer vocal cords, such as elephants. For many years, they believed purrs were produced using a unique mechanism that involved the cyclical contraction and relaxation of muscles in the voice box – something that would require constant neural input from the brain.
However, it turns out that purring requires far less effort. Researchers led by Dr Christian Herbst at the University of Vienna in Austria, dissected the voice boxes of eight cats that had been put down because of terminal disease – having first obtained the cats’ owners’ consent. They then pinched the animals’ vocal cords and pumped humid air through them – roughly resembling the mechanism that humans use to speak and produce vocal fry. Doing so produced self-sustained oscillations or purrs from all of the voice boxes, suggesting the noise does not require constant input from the brain.
Further investigation revealed the presence of masses of fibrous tissue embedded in the vocal cords. “This may explain how such a small animal, weighing only a few kilograms, can regularly produce sounds at those incredibly low frequencies (20-30 Hz, or cycles a second) – far below even than lowest bass sounds produced by human voices,” said Herbst, whose research was published in Current Biology.
Similar structures have been found in roaring cats, such as lions and tigers.
The previously dominant theory suggested cats relied on a mechanism that seemed to be more or less unique in the animal kingdom, where the animal’s brain sent quick and continuous bursts of signals to muscles in its throat, causing them to contract 30 times a second and producing a purr.
Vocal fry – which the new study puts forward as an alternative explanation – is the lowest tone that our voices can produce, creating the effect of croaky speech.
