Cat purring in contentment. “He was purring the entire ride home” – Photo: Reddit.com

At one time, in the long distant past, there was a theory that the cat’s voicebox had nothing whatsoever to do with purring. Experts argued that it was to do with the cat’s blood flowing through her veins into the heart creating turbulence. The diaphragm amplified the vibrations it was thought. The theory seems fanciful today. Another theory put forward by Dr Desmond Morris in his cat encyclopaedia called Cat World refers to “The False Vocal Chord Theory“.

Dr Morris’s Theory on the Cat Purr

This theory describes purring as originating in the cat’s voicebox otherwise known as the larynx. We are told that the cat possesses a second pair of vocal chords in addition to the ordinary vocal cords. These are referred to as ‘false vocal cords’ or vestibular folds.

This second pair of chords is the secret behind the purring mechanism. The theory suggests that purring is no more than noisy breathing a bit like snoring when asleep. When the cat inhales and exhales air passes over the false vocal cords making the well-known purring sound. Dr Morris says, “air has to be interrupted by the contraction of the laryngeal muscles about 30 times a second”. See also the link below the article. Cats purr towards each other as well as towards humans.

Dr Bradshaw’s Theory on the Cat Purr

Dr John Bradshaw describes how cats purr in his book Cat Sense. This book was written much more recently and therefore relies on better knowledge. He says that the rumbling sound is produced as the vocal cords are vibrated by a special set of feline muscles. The sound is not like a hum produced by air passing over the cords but rather by the vocal cords banging together, “like an old-fashioned football rattle”. He says cats sometimes hum at the same time which reinforces the purring sound. And sometimes cats add a meow-like sound to the purr to create an added urgency to it; a high pitched variant (play the audio file below). This happens when cats are looking to be fed.

The domestic cat purrs instinctively. Therefore it is not a rational decision to purr at a certain moment. It is a natural, reflex, instinctive action as a consequence of the circumstances under which a cat finds herself. In that regard, perhaps it is a bit like a person humming to himself when he’s content or perhaps whistling. Although, we all know by now that purring does not only take place when a cat is content. For example, it can happen when a cat is about to be euthanised at a veterinarian’s clinic. I discussed the reason behind purring on another page.

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