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How do cats purr, why and is it something other animals do as well?

They come in a range of shapes and sizes and seem to have their own individual personalities. But one thing cats share is their ability to purr.

Based on electromyography of the muscles of the larynx, scientists now think that the classic sound is produced by rhythmic contractions of the cat's larynx muscles and diaphragm, says Associate Professor Vanessa Barrs, feline medicine specialist at the Valentine Charlton Cat Centre with the University of Sydney.

The rhythmic contractions of the muscles and vocal chords open and close the glottis. As the cat breathes in and out, air hits the vibrating larynx muscles in the throat producing the purring sound.

"These contractions restrict the amount of air flowing through the larynx. The larynx relaxes again letting more air go though."

Each cat will purr with their own consistent pattern and at a consistent frequency usually between 25 and 150 Hz per second.

"The frequency is controlled by the autonomic nervous system, so it's sort of a muscular tremor rather than a human vocalisation," says Barrs.

"The nervous system also regulates the activity of the diaphragm during laryngeal constriction and relaxation, and thus contributes to the genesis of purring."

But despite the autonomic responses involved, Barr notes that cats do have a conscious control over purring.

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Why do cats purr?

When it comes to the reasons why cats purr, things get a lot more complicated. It's fair to say that ten different cats will have ten different reasons for purring, says Barrs.

We can be fairly sure that cats purr when they are happy.

"We know they purr when they're being patted by their owners. They can also purr when they're being fed or when they're nursing. In fact, that's probably the first time kittens purr."

But as many cat owners will have seen, cats also purr when they are anxious, feel threatened or are in pain, indicating that purring might be a comfort mechanism during times of stress, says Barrs.

"Some cats have even been known to purr when they're giving birth."

Recent research has also reported a type of purr that domestic cats use when they want to be fed — a soliciting purr. This variation on the usual purr contains a cry with a similar frequency to a human infant's.

Adding this high frequency cry into the usual low-pitched purr could subtly exploit humans, the UK researchers suggested, by tapping into an inherent 'mammalian sensitivity' to such a cry.

Solicitous purring is one means of communication, but Barr points out that cats also communicate by vocalising or meowing.

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All cats purr

People used to think only small members of the cat family (known as Felis), such as the domestic cat, could purr.

However, Barrs says bigger cats (known as Panthera) such as tigers, lions, leopards, panthers, cheetahs and jaguars also purr. But they only do so when breathing out, unlike domestic cats who purr when they inhale and when they exhale.

"Interestingly Charles Darwin observed that as well as domestic cats, pumas, cheetahs and ocelots also purred. But he thought that lions, jaguars and leopards didn't."

The term "purring" has been used fairly freely to include sounds made by civets, mongoose, genets, bears, badgers, hyenas, rabbits, squirrels, guinea pigs, tapirs, ring-tailed lemurs, elephants, raccoons, and even gorillas when they eat .

But Barrs says using the strict definition of purring, only members of the cat family and two species of genets, which are somewhat related to cats, really purr.

Associate Professor Vanessa Barrs, feline medicine specialist at the Valentine Charlton Cat Centre with the University of Sydney, was interviewed by Stuart Gary.