Wednesday 30 March 2011

Why Do Cats Purr?

Here is the definitive answer to the question, "Why do cats purr?" The answer that is usually said to be the obvious one - that the purring cat is showing his or her contentment - is not true.

This is because cats in great pain, those who are injured, in labour and even dying or even sometimes at the vets being euthanised, purr loud and long (or it can last a second3). Kittens purr when they drink mother's milk purr and the mother purrs back. They also purr when being groomed by mother or by siblings. Kittens start purring when a few days old1. All domestic cats purr. Sometimes cats purr when hungry. Body contact between the sender of the purr and the receiver may be an important factor in instigating it3.

Cats purr when contented also but it does not only happen when the cat is content. How do we find a reason for cat purring that encompasses all the circumstances under which it might happen?

Dr Desmond Morris in his famous book, Cat Watching, which is still probably the master work on cat behavior despite being first published in 1986, says the following.

"Purring signals a friendly social mood." It can act as a signal to ask ("the need") for friendship or say thank you for friendship received. Some cat behaviorists believe cat purring is a signal that the cat is not a threat2. I prefer Dr. Desmond Morris's all encompassing explanation. Purring is classified by scientists as a "close-range felid vocalisation". It is used most commonly between mother and kitten3. This, then, is the answer to to, why do cats purr?

When purring is used by a kitten in the first days of his or her life it is a signal to the mother, when suckling, that the kitten is receiving milk. This reassures mother. Her return purr acknowledges the message and tells her kittens that all is well with her too. Kittens and mother usually purr continuously while nursing1. Kittens aid the flow of milk by threading the abdomen (we know this as kneading, which frequently happens if our cat is on our lap). Cats often purr at the same time.

Calico cat - Photo by Crane1989

Purring can take place while the cat is drinking, therefore. It is a two way action - the sound being produced on inhaling and exhaling.

The big wildcats cannot purr. They make a sound similar to the purr but it is a one way (exhalation) process. The four big cats can, however, roar (see tiger roar). The cheetah is not classified as a big cat and it can purr2. Its purr is much louder than the domestic cat but is it as loud as Smokey's purr (see below)? The Eurasian lynx also purrs. It is a vocalisation used by mothers nursing their kittens3.

Purring is classified as a "murmur pattern"1. This is a sound that is produced with the mouth closed. The trill/chirrup is also made with the mouth closed.

Purring is produced in the cat's throat. It is not absolutely clear how it is produced. It is thought to be produced "through (the) alternating activation"1 of the muscles of the larynx and diaphragm. There is a build up of and release of air in the glottis. When we feel the purr by placing our hand gently against the neck we feel the vibrations of the air and muscles. The sound is louder and rougher when produced on breathing in.

Cats purr in the "frequency range of 25 to 150 Hz2. This is a low amplitude murmuring sound3. An interesting theory is that cats might be instinctively trying to heal themselves at the "cellular level". This is because at the frequency range of the purr, sound is thought to be "beneficial to healing"2.

It is said that a cat's purr is unique to the individual cat2. It can usually be heard three meters away. However, one famous cat named "Smokey", who is described as a 12 year old British Shorthair (this means a random bred cat with short hair) has a purr that is so loud it is more noisy than a vacuum cleaner! His owner describes the sound as if "Smokey had a dove caught in her throat". She has had eight different homes. Are the two connected? I think so. The purr has been recorded at 73 decibels (dB). Guinness World Records are considering making it a record but there are no competitors. It would be a new classification.

Read about cat behavior.

Read about cat sounds.


1. The Cat: Its Behavior, Nutrition and Health

2. Cat Owner's Home Veterinary Handbook - page 296 - 297.

3. Wild Cats of the World - pages 10, 422.

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