How do cats purr? Current theory.

Over the years, there have been several theories as to how cats purr. Of the wild cats, the puma and the clouded leopard can purr, for example, but the big cats can’t. In 1996, Dr. Desmond Morris, in his book Cat World, suggested two theories as to how a cat purrs. One of them in 2022 is considered to be fanciful. This is the turbulent blood theory which states that the cat’s voice box has nothing whatsoever to do with purring. It was suggested that the cats blood-flow through its main veins into the heart increased causing turbulence. It was the turbulence that caused the purring sound. This is now discredited.

How do cats purr?
Contendly purring. Photo: in public domain.
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He also suggested the “false vocal cord theory”. This states that the purring originates in the cat’s voice box or larynx. Dr. Morris suggested that the presence of the second pair of chords called vestibular falls or false vocal cords are behind the purring mechanism. The theory regards purring as no more than noisy breathing. The kind of noise that humans make when they are snoring. With every inhalation and exhalation air passes over the false vocal cords and makes the purring noise. To produce the sounds the air has to be interrupted by the contraction of the laryngeal muscles about 30 times a second according to Dr. Desmond Morris.

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

I’m able to refer to a scientific study published before Dr. Desmond Morris’s book was published. The study is called How cats purr. The abstract i.e. the summary to the study tells me that “two-channel acoustic measurements confirm that the primary mechanism for sound and vibration production is a centrally driven laryngeal modulation of respiratory flow. The diaphragm and other muscles appear to be unnecessary for purring other than to drive respiration”.

This appears to be repeating what Dr. Morris states in his book. Dr. Bonnie Beaver, DVM, MS, DACVB, a Texan veterinarian writing on the Internet states that the purring sound comes from the larynx i.e. the voice box just like other sounds that the domestic cat and other purring cats make.

Lynx purring
Lynx purring. Photo in public domain.

She states that the “size of the airflow opening narrows, creating turbulence. This explains why the purr can be loud if there is a great deal of constriction or soft if the opening is only slightly constricted”. Her reference is: Remmers, J.E. and Gautier, H. (1972): Neural and mechanical mechanisms of feline purring. Respir. Physiol. 16: 351-361.

We know that kittens purr when they are suckling at their mother’s breast. They can drink and purr at the same time. And as mentioned, cats purr when they inhale and exhale. People say that it has healing powers for the cat. They even say that it is one reason why they purr. You can read more about why cats purr by clicking on the following link:

RELATED: The real reason why cats purr

And:

Why do domestic cats purr and meow while wild ones don’t?

A cat’s purr is very special, very noticeable and relaxing. This page is about how they create the sound not why they do. It is unique to the cat and the humble domestic cat is better at it than the mighty big cats. Tigers can produce a rather spluttered one-way (exhalation) purr but that is it. Domestic cats, as we know, can purr with exhaled and inhaled air, with their mouth shut and do it for a long time sometimes.

Clouded leopard purring
Clouded leopard purring. Screenshot.


 
It makes sense that purring has to be due to the air in the cat’s breath passing over a piece of anatomy that constricts and relaxes rapidly. The Cat Owner’s Home Veterinary Handbook by Drs Carlson and Giffin explain it like this. When a cat breathes in and out the cat alternately tenses and relaxes the muscles of the voice box and diaphragm. The diaphragm is a sheet of muscle that extends across the cat’s body at the base of the lungs. It separates the thoracic cavity (the space containing the lungs) from the abdominal cavity (the space where the stomach and viscera is).

This relaxing and tensing of these muscles results in turbulent air flowing through the trachea. The trachea is a tube running from the larynx (which is below the back of the throat) to the lungs. These rapid cyclical changes in air pressure are “superimposed” on normal breathing to produce the characteristic purring.

Purring is entirely instinctive and a kitten will purr from 2 days old. Purring does not always indicate that the cat is content. Cat purring can happen when the cat is in pain, hungry and/or upset. Cats have been known to purr just before dying.

Some more (overlap): Cat Purr.

Sources:

  • As stated in the text and:
  • http://animals.howstuffworks.com
  • Wikipedia (anatomy)
  • Cat Watching by Desmond Morris

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