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The Evolution of Purring

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By Victoria Heuer    July 15, 2009 at 08:26AM / (1) comments

In an article published this month in the science journal Current Biology, a team of researchers from the University of Sussex in Britain are positing the hypothesis that cats have developed an effective tonal frequency designed to press humans to react more urgently to their (the cat's) needs.

 

Sly little creatures that they are, cats have learned over time to make proper use of one of the very sound effects that cat fanciers find most charmingly disarming in their feline companions: the purr. The Sussex team assembled a group of volunteers, who were told to record their cat’s purrs, both when the cats were happy and content, and when they were "soliciting" something, such as a meal. The results of the study found that cats are able to embed a high frequency cry, similar to the frequency of a baby's cry, within the tonal vibration of the purr -- thus creating a sound that is not as pleasant as the more soothing low frequency purr and prompting the human caretaker to action.

 

One of the lead researchers of the purr study, Dr. Karen McComb, told the BBC that it was her cat, Pepo, that inspired the study. Pepo, like other cats, wakes his master each morning with what Dr. McComb describes as a "rather annoying" sound that is a mixture of purring and whining. Humans are able to distinguish this higher tonal frequency purr apart from the lower, non-whining frequency as one that is more insistent, and indeed, demanding. Even people who have not lived with cats are able to distinguish the higher pitched purr/whine as an urgent communication, leading researchers to conclude that humans are responding in much the same way one would respond to a crying baby.

 

We are all highly sensitive to the cry of a human infant, an evolutionary behavior that ensures survival of the species, and it appears that cats have also learned this survival technique through successful repetition, even exaggerating it to achieve a faster response.

 

Curiously, this is not the first study to conclude the evolutionary benefits of purring. Of course cats purr when they are content, but cats have also been found to purr when they are severely injured, giving birth, and even when they are dying. Because it takes energy to purr, researchers sought to find the answer for why cats expend physical energy on purring when they apparently need all of their energy for the physical task at hand, whether it is birthing, or dealing with pain from a trauma.

 

Elizabeth von Muggenthaler, of the Fauna Communications Research Institute, has made a strong argument for purring as an evolutionary technique for self-healing. Veterinarians can tell you that cats are better at healing from broken bones, infections and other various life-threatening injuries, and on average need much less post surgery care than dogs, but no one knows why this is. There is an old adage that goes, "If you put a cat and a bunch of broken bones in the same room, the bones will heal," and there appears to be some anecdotal evidence to support the conclusion that the vibrational frequency of purring helps cats heal more quickly than other animals.

 

Muggenthaler, who specializes in bioacoustics, compared the vibrational frequencies of purring with the known healing effects of vibration therapy for humans. Frequencies between 20 and 140 hertz have been shown to encourage faster healing of bone and tendon injuries, healing of wounds, relief of pain and swelling, and to increase breathing capability for symptoms of dyspnea. Cats, on average, purr at a frequency of 50 and 150 hertz, which Muggenthaler found is the best frequency for bone growth and healing of fractures. Indeed, research has found that cats that are affected with dyspnea are able to breathe unaided when purring, lending some proof to the idea of vibrational self-healing.

 

Muggenthaler concluded from her study that cats have evolved this physical trait as a self healing mechanism, which may explain why cats purr when they are under duress. Her study has led others in the animal therapy business to conclude that having a purring cat nearby will encourage the healing of sick or injured humans as well. Many cat owners can remember times when they have been ill or laid-up with an injury and their cat will lay close, even atop their bodies, purring loudly and steadily until the danger has passed.

 

While science still looks to explain why vibrations are beneficial and how cats are even able to purr -- the mechanism behind the purr is still largely hypothetical -- what we do know is that purring is good for them and for us. So the next time you hear your cat whining for his breakfast, add an extra little treat and hold him close. He may be helping you more than you’re helping him.

 

Image: QuinnDombrowski / via Flickr

 

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