Purring is caused by a simple airflow over connective tissue masses embedded in the vocal fold of the larynx

Cat purr is produced by simple airflow through the larynx
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Cat purring is like people talking with a vocal fry.

Researchers of a new study – read on.

There have been lots of theories about how domestic cats purr. At one time some experts thought that it was caused by turbulent blood! And Dr. Desmond Morris gets close to understanding how the purr sound is produced with his report on the ‘false vocal cord theory’ (in Cat World). He says the air passes over vestibular folds or false vocal cords. And when that happens, in combination with the air flow being interrupted by the contraction of the laryngeal muscles about 30 times a second, a purr sound is produced.

The last bit is wrong according to a new study which I believe is peer reviewed but the authors state that further work is required. The researchers found that it doesn’t take the contractions of the laryngeal muscles to produce the purring sound. This was established in a simple study which involved excising the larynx and then passing air through it. Note: ‘excising’ means in this instance surgically removing the larynx from dead cats. The cats had been euthanised because of illness we are told. I hope this is true. Update: it is true – see below. Owners consented.

They simply pushed the air through the larynx. Within the larynx there are connective tissue masses up to 4 mm in diameter embedded in the vocal fold. And it is this connective tissue which produces the purring sound when air is passed over it. That’s my understanding of this study.

They concluded that cats can produce the purring sound with frequencies of 25-30 Hz “without input or muscle contraction”. In other words, there is no input from the cat’s brain or autonomic nervous system sending signals through the nerves to the larynx. It is a simple airflow over the larynx.

And when a cat purrs that airflow is created when breathing in and breathing out. I don’t have any more on this at present. An interesting factor is that the kitten purrs when drinking their mother’s milk. They can do the two tasks simultaneously. I find that interesting and I would like it to be explained in light of the findings of this study.

When humans drink water we stop breathing. Certainly, when kittens drink their mother’s milk, they don’t stop breathing. Humans can’t drink water and breathe at the same time. If they did the water would pass into their lungs. There is a flap of tissue that closes the windpipe to prevent the water going into the lungs. At the moment I’m not sure how cats deal with this. It has not been explained in this study but I only have access to the summary.

Rhys Blakely, The Times journalist has some nice information in his article (9th Oct 2023) which I will, with respect to him, use. He starts off his article about this research into the cat purr by referring to the American author, Roger Caras who said that “a purring cat is a form of high praise”. It serves to reinforce the thought that humans are good company. Comment: I guess we are to our cats provided we look after them well.

And Mr Blakely states that the purr is a bit like the human “vocal fry”. I’ve not heard of that phrase before but it means a croaky speaking voice which is sometimes used by reality TV stars.

The frequency of the purr is around 20-30 Hz. And this kind of deep vocalisation normally comes from larger animals such as elephants which have naturally got much larger vocal cords. And as mentioned further up in this article, the long-held theory was that the brain sends signals to the larynx to constrict it 30 times a second which was part of the purring process.

But as mentioned according to this research this isn’t the case. An airflow over the specialist tissues of the cat within the larynx produces the sound.

Blakely writes that the research “suggests that purring might be produced in a similar way to the vocal fry register in humans”.

Interestingly, he also states that the cats’ owners gave consent to the research. They had been euthanised after illnesses and therefore the foundation for this research was carried out ethically which pleases me greatly.

Blakely says that the scientists “pinched the animals’ vocal cords” before the air was passed over them. He suggests that the mass of fibrous tissues embedded in the cat’s larynx might cause the vocal folds to vibrate more slowly and thus be able to produce the low frequencies of the cat purr.

We know, by the way, that cats don’t just purr when they are content. They can purr when they are frightened as they do sometimes when they are at a veterinary clinic on the consultation table being handled by a vet or even when about to be euthanised. I have an article on that and Dr. Desmond Morris usefully explains when cats purr. He says that it is more a request for friendship or a recognition of friendship than a pure expression of contentment.

The leader of the above-mentioned study, Christian Herbst, a voice scientist at the University of Vienna, said that the results of the study could shed some light on animal and human vocal communication. He added:

It appears that there is a very successful shared physical mechanism that allows vertebrates to produce sound: by converting aerodynamic energy (i.e. breathing) into acoustic energy via the passive, cyclical oscillation [a movement back and forth in a regular rhythm] of tissue. Across different species, this mechanism evolved in several organs: in humans and other mammals it’s the larynx with its vocal folds; in toothed whales, such as dolphins, it’s their nose; and in birds it’s the syrinx. So, this is a deep question of evolutionary biology. For 50 years, cat purring was believed to be produced by a physiological mechanism that is an exception to the rule. Our study shows that this exception might perhaps not be strictly required.”

Pages on purring:

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