Cat purring is like people talking with a vocal fry

I'll try and explain the title. The human vocal fry is vocalising in a deep, creaky, breathy voice. I think you hear it quite a lot in advertisements when a man speaks in a vocal fry register to compel people to be attracted to a product.

And it may surprise people to know that a recent study which I believe has been peer-reviewed but which requires follow-up studies, indicates that when a cat purrs it's a simple case of an airflow passing over connective tissue masses within the larynx of the cat. Human vocal fry requires a shortening of the vocal cords.

The study researchers compressed the larynx slightly and then passed air through it and they produced a purring sound. How did they achieve this? They received the consent of owners of cats who had been euthanised because of chronic illness. And they surgically removed the larynxes of the disease cats. They were then able to pass an airflow through the larynx having, I believe, restricted it slightly.

The point is this: it was a very simple test. It only required an airflow going over these tissue masses inside the larynx to create the sound. This is a very low rumbling sound as we know made by animals much larger than the domestic cat. It is therefore a very specialist sound.

It registers with a frequency in between 20-30 Hz as I recall. The study is called: "Domestic cat larynges can produce purring frequencies without neural input."

The referencing the title to "neural input" means that the brain doesn't have to send signals to the throat to constricted and then release it many times a minute which is what the experts originally thought was the process of creating the purr. It was more or less universally thought that in order to create the purring sound the throat had to be constricted and then released rapidly through a nervous system signal. We now know that that isn't the case.

Although, the people who conducted the study say follow-up work would be useful, the lead researcher in the study, Christian Herbst, believes that it should shed some light on animal and human vocal communication. It shows that the cat converts aerodynamic energy which is the flow of air caused by breathing into acoustic energy via the "passive, cyclical oscillation of tissue".

In the early days, many experts were mystified by the domestic cat purr and some proposed that it was created through the flow of blood through the cat's main veins into the heart. When it was increased it caused turbulence and the sound of the turbulence were transmitted to the animal's chest and amplified by the diaphragm. A very complicated and entirely incorrect hypothesis as it happens.

An interesting aspect of the study is that we know that kittens purr while they are drinking their mother's milk. So, cats can purr and drink at the same time. I would like that to be explained to me. There must be a simple explanation for the ability of a cat to do that.

The last point to make is that cats purr when they breathe out and in, which creates this continuous sound.

Why do cats purr?

Cats don't just purr when they are contented. Therefore, you need to find an answer to the question which covers all circumstances under which domestic as purr. Dr Desmond Morris, the celebrated zoologist and animal behaviourist came up with the right answer in 1986. I'm a bit surprised that is not more widely cited so I will mention it here. In fact, I will quote him verbatim if I may:

"A more precise explanation, which fits all cases, is that purring signals a friendly social mood, and it can be given as a signal to, say, a vet from an injured cat indicating the need for friendship, or as a signal to an owner, saying thank you for friendship given". 

I don't think I can explain it any better than that. The point that he is stressing is that domestic cats purr when they are in a veterinarian's clinic, on the consulting table where they are clearly nervous and stressed. And sometimes they purr when they are about to be euthanised by a veterinarian. Clearly under these circumstances domestic cats are not happy and contented. This is why Dr Desmond Morris was obliged to come up with an all-encompassing reason for the production of this sound. He did that. 

There is another answer to the question, "Why do cats purr?" and it is this: there are different reasons for purring depending on the circumstances. It is a flexible sound that means contendness when being petted or when the cat is on a lap and it means a need for companionship when about to be euthanised.

People sometimes ask whether the wild cat species purr. Some do and some don't. The bobcat and lynx can purr and it is a very impressive sound. The big cats can't but they can roar instead. The small wild cat species can purr but when they do so it is a slightly different sound to the gentle and melodious domestic cat purr.