How do cats purr? I mean how do domestic cats physically make the purring sound? I present three theories. The first one that I describe has far more merit than the second.
False Vocal Cord Theory
As for other feline vocalisations, the purr is created in the cat’s voice-box or larynx. The cat possesses ‘false vocal cords’ aka ‘vestibular folds’. It is this anatomical structure which allows the domestic cat to purr, a sound said to heal the cat and us and to signify that he/she is contented with a friend or in need of friendship1.
We know domestic cats can produce the sound for long periods without opening their mouths. It is the defining sound of our feline companions.
In this theory the purr is in effect noisy breathing akin to the human snore. When the cat both inhales and exhales air passes over the false vocal cords which makes the noise of the purr. The larynx is contracted by the muscles in the larynx about 30 times per second which interrupts the passage of the air.
The Turbulent Blood Theory – Early Outdated Theory
In this theory the cats voice-box is not involved in creating the purr.
When the blood flow in the cat’s veins into her heart is increased there is turbulence. The highest level of turbulence occurs when the main vein returns blood from the body to the heart at a point where it is constricted by the chest. It is the turbulent blood flow which causes the purr. The sound of the turbulence passes up the windpipe and into the sinus cavities in the head. It is amplified by the animal’s diaphragm.
The increased blood flow is caused by the cat’s emotional changes. Some experts say it is the arching of the back which increases the turbulence but I can’t see this being true.
Comment: when a cat purrs you can feel the vocal cords vibrating. Therefore the first theory is the better. I am not sure why it can’t be the vocal cords themselves rather than the false ones which produce the sound. This leads to the third.
Laryngeal Muscles Twitch
A ‘repetitive neural oscillator’ sends signals to the muscles of the larynx causing them to ‘twitch at a rate of 25 to 150 vibrations per second’. As a consequence the vocal cords are activated when air is passed over them when inhaling and exhaling (source: mother nature network). Dr Bradshaw in states that special feline muscles vibrate the vocal cords banging them together. This is probably the correct theory.
I also believe that the reason why cats purr is to signify contentment. Dr Morris says it is signifies ‘contentment with a friend or in need of friendship’ in order to explain why cats purr when anxious such as when they are on the consulting room at the vets about to be euthanised.
I am not sure I agree with Dr Morris’s assessment. I prefer the theory that the cat’s purr has more than one purpose. Dr Morris decided to try and find a single universal purpose. Might it be the case that cats purr when frightened at the vets because they use their purr to calm themselves much like a fearful soldier might whistle to calm himself. The human whistle has more than one use too.
1. Theory of Dr Desmond Morris.
The source other than stated is Cat World by Desmond Morris.