Why do cats purr?

October 1st, 2012

There???s more to that evocative, soothing sound than you might think.

Cat purrCats always keep you guessing. Are they happy to see you? Cross with the world? They rub against your legs when you???d forgotten they were in the room, purr in your ear in the middle of the night??? And what???s with that purr, anyway? Like so many things about cats, their purr has its own mystique and meanings. No question that when your purring furball is curled up on your lap, those rhythmic rumbles are saying they???re perfectly content, thank you very much.

But cats may also purr when life isn???t so hunky-dory: when they???re in pain or frightened. A friend tells how when accompanying her fast-fading moggie to the vet, she was surprised to hear her purring: ???I???d previously associated her purrs with happiness. It made me wonder whether she was purring to comfort herself.???

This is plausible, according to psychologist Dr Stanley Coren: ???It may be that purring is usually such a positive signal that cats gain reassurance from hearing their own purring sounds, much like a frightened child who whistles as he walks past a graveyard on a dark night. The cheery noises he is making give him encouragement that everything will turn out well.???

As to how cats actually purr, there is still some mystery. ???The current idea is that a cat???s purr originates in the larynx and is generated by oscillation of airway structures that result in turbulent airflow during respiration,??? explains veterinary lecturer Carolyn McDaniel from Cornell University Feline Health Center in the US. ???We can do an MRI on a cat, treat its cancer with chemotherapy, control its asthma with inhalant therapy ??? but we can???t explain its purring mechanism.???

We do know purring starts early in a cat???s life. All domestic cats and most wild felines are born able to purr. Kittens purr before they can even open their eyes, probably to reassure their mother that they are alive and well ??? and mum purrs back to let them know she???s friendly and ready to nurse them. Aside from mutual reassurance, the vibrations from the mother???s purrs allow the young kittens to find her more easily.

Purring requires energy and, scientists argue, an injured animal will generally not expend precious energy on an activity not directly connected with survival. Since the purr has lasted through hundreds of cat generations, it follows that there must be a survival mechanism behind it, and many scientists believe it is a natural healing tool.

The Fauna Communications Research Institute in the US has recorded the purrs of different types of cat and found that they have a frequency between 25 and 150Hz. Sound frequencies in this range can improve bone strength, mobility and healing ??? physical therapists have long used vibrations to strengthen muscles, ligaments and tendons, and to lessen pain and swelling.

Regular exercise is the best way to keep bones and muscles strong, but if a cat exercises only every now and again ??? and most cats certainly spend a lot of time lounging around ??? purring while resting would stimulate bone growth, increase muscle and ligament strength, and maintain good health. Veterinary professionals have observed that bone and muscle diseases are rare in cats and that cats are remarkably resilient and recover quickly from injury.

There may be even more to a cat???s purr than meets the casual listening ear: according to research published last year, cats??? purrs can in fact be an ingenious acoustic ruse to persuade their owners to feed them. The study found that our feline friends will modify their signature purr when seeking food, adding a higher-frequency element (300 to 600Hz) that is remarkably similar to that of a baby???s cry. The resulting mixed signal taps into a natural human nurturing instinct and is extremely difficult to ignore, according to animal behaviourist Dr Karen McComb of Sussex University. When the research team digitally remastered the purr to remove the high-pitched section, humans no longer felt the sound indicated any urgency.

Dr McComb was originally inspired to do the research by experiences with her own cat, which often wakes her in the morning with an insistent purr. ???The embedding of a cry within a call that we normally associate with contentment is quite a subtle means of eliciting a response,??? she says. ???Solicitation purring is probably more acceptable than overt meowing, which is likely to get a cat ejected from the bedroom!???

Do you have your own stories about your cat???s purr? Let us know by commenting below???

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