What do cats do that dogs don’t?
Oh, you mean aside from climb trees, scratch your eyes out and resolutely ignore you except at mealtimes? Yes, we’re talking about PURRING.
People are fascinated by things they can’t do themselves, and feel the need to explain the hows, whens and whys of many natural phenomena. As a result of solving many of these questions, we can travel very fast on land, under the ocean, fly in the air and into space. Yet how the little domestic cat purrs is still stumping scientists! As for the why, humans can only surmise. Like treating the common cold - not every question we ponder gives up its mysteries easily.
Scientists first thought that cats might possess a special "purring organ." However, none has ever been found. Another early theory ** suggested that ’purring is the direct result of turbulence in the bloodstream of the vena cava, the main vein returning blood to the heart from the body. This constricts as it passes the liver and diaphragm, and the theory suggests that when the cat arches its back the blood forms eddies in this bottleneck. This in turn sets up vibrations in the the thorax that are passed up via the windpipe to resonate in the sinus cavities of the skull." **(THE BOOK OF CAT) Summit Books, New York.
These earlier theories of how cats purr have been discarded and the modern facts of the purr are now reasonably well-documented. The sound is due to air vibrating at about 25 Hz (= 25 pulses per second), with different decibels (loudness) depending on the individual. The resonance is felt as vibrations all over the cat’s body. Cats can purr even when their diaphragm (the muscle which divides the chest from the abdomen and whose movement provides some of the ‘pull’ to bring air into the lungs) doesn’t work, but the diaphragm gives the air more pressure and movement so the purr is more distinct.
How?
It was thought that the sound came from vibrations of the hyoid bone (which connects the base of the tongue to the skull) in the cat’s throat. However, pulsations of the vocal chords turn out to be the answer to the how. The vocal chords in a cat’s larynx or ‘voice box’ have extra folds (called false vocal chords) in them, and it is the rhythmic movement of these that makes the air vibrate as it passes between them to create the air disturbance we call purring. The folds contract to stop air going through (like a valve), then relax so a little pressure wave of air goes through. It is these little pressure puffs of air on our eardrums that we hear as purring.
There is a ‘purr centre’ * in the cat’s brain which sends cyclical pulses down the nerves to the vocal fold, and it was discovered in 1936. It has taken till now to define the mechanism!
Domestic cats can move their false vocal chords when they breathe in (inhale) and also when they breathe out (exhale). The structure of the hyoid is bony and rigid in the domestic and other ‘small cats’ and this gives support to the vocal folds so they can vibrate.
The larynx of the ‘big cats’ is attached to the skull by cartilage, which is flexible, thereby allowing them to roar. Only the domestic cat can purr on inhalation and exhalation, the other animals who can purr make their sound only when air flows in one or other direction.
The little Felis sylvestris catus is truly the purring specialist!
Who?
The who of purring is also an interesting tale. It is said that the cats who roar (the Pantherine lineage), cannot purr as they have a more flexible attachment of their hyoid bone to their skull. However, there are references to purring in Serengeti lions, so it may be a matter of interpretation of a sound. More commonly quoted ‘purrers’ in the feline family are the Ocelot, Cheetah, Bobcat, Serval, Lynx, Cougar,and Caracal. Other non-felid animals who can purr are Raccoons and Civets. Their mechanisms seem not to have been studied yet.
Why?
The why will remain mysterious until we can truly interpret feline motivations. Although there is a Japanese company which has produced the ‘dog language interpreter’ to give human words to dog vocalisations, and has now come up with the ‘cat language’ version, we may not really ever know.
After all, cats and dogs use urine and body language rather than email and shouting to tell each other things, so their view of the world is much different and vocalisations are only a small component of their communication repertoire. We really have no idea what it is like to have our world dominated by scents, and for all we know, purring is linked with other vibrations or smells to give it further context and meaning!
However, we do know when cats purr, and it is pretty much whenever they feel something profoundly - contentment (because purring is first associated with feeding from their mother), relaxation, but also fear and distress. Cats will purr in company they like, and also to reduce attack when they are in the company of a cat they don’t like. They purr around the time of mating, and when giving birth.
Most household, human-oriented cats treat humans as being rather unusual other cats -sometimes a giant mother cat, sometimes a giant kitten, and so will purr when we are around.
Puurrhaps it shows imagination... Cats certainly use more vocalisations to communicate with humans than they do cat-to-cat.
Then there is another question: What’s the use of purring for humans?
This little story has been floating around for a while, and may have some interesting implications. Scientists, from the Fauna Communications Research Institute in North Carolina, found that between 27 and 44 hertz was the dominant frequency of purring for a house cat, and 20-50Hz for the puma, ocelot, serval, cheetah and caracal. Previous studies have shown that exposure to frequencies of 20-50Hz strengthens human and chicken bones and helps them to grow and improve bone density. Bone density loss and muscle atrophy is a serious concern for astronauts during extended periods at zero gravity. Their musculo-skeletal systems do not experience the normal stresses of physical activity, including routine standing or sitting, which requires muscle and bone strength for posture control. Also, in the higher frequency ranges, the production of the body's own natural anti-inflammatory compounds increases thereby reducing joint pain and swelling. There is further evidence of muscle, tendon and ligament repair within these frequency ranges as well, which has led to popular use of vibrational therapy in sports medicine and gyms around the world, especially in the former Soviet Union where so much of this research was conducted.
Other scientific teams are researching whether "sound treatment" could be used to halt osteoporosis and even renew bone growth in post-menopausal women. Dr David Purdie, from Hull University's centre for metabolic bone disease, said that the human skeleton needs stimulation or it begins to lose calcium and weaken. He said that it was difficult to devise physical exercises for old people suffering from osteoporosis and speculated that it might be possible to create a mechanism to use cats' purring to help strengthen elderly bones. This could lead to a proliferation of cats in nursing homes supplementing their role as communication facilitators as promoted by the PAT and Delta programmes, and may even be medically prescribed, as well as becoming ‘Catastronaughts’! Purring may be the ultimate ‘energy efficient’ exercise - lying in the one
spot vibrating to keep your bones and muscles fit. Trust the cat! However, cats are prone to arthritis and other bony problems (contrary to some of the information out there), so they are still going to have to get up to go to the food bowl...
There has long been debate about whether humans domesticated cats, or whether cats domesticated humans in order to have regular access to food and warmth! It was, after all, the cat’s first job to kill the rats and mice who came to eat the grains stored by the Egyptians, in what was human beings’ earliest attempts at becoming civilised. This meant that cats had to start living closer than the 1 mile that separated the original desert cat ancestors, and develop a visual as well as scent-based inter cat communications,because they now had to share territory. It is also thought that the little domestic cat has changed some of its other methods of communication in order to fit more easily into crowded, human-dominated environments. We humans came to recognise the meaning of Tail Up, Head Rubs and Purring as we became more adept at cat-to-human communication. And just imagine, if purring cat on a lap makes us feel good, then it must make a cat feel fabulous!
*For the technophile -the feline "purr center" is located in an area of the brain near the hypothalamus called the infundibulum. This infundibulum, as it is called, is connected to the hypothalamus. One function of the hypothalamus is to feel and interpret emotions.It determines if a certain sensory stimulus is pleasant, like being petted, or painful. The interesting thing about the hypothalamus is that it releases endorphins, which are morphine-like substances, that stimulate the "purr center" causing that wonderful,vibrating purr that we are all so familiar with. A cat's body, like humans, is an intricate and beautifully designed series of checks and balances. Pain stimulates the hypothalamus to release it's endorphins as a natural-like endogenous morphine that helps block the pain. The release of these endorphins also stimulate the "purr center" in the cats brain and cause it to purr. This is why a cat that is injured or sick, or a queen in labor, may also purr. Most felid species produce a "purr-like" vocalization. In domestic cats, purring is most noticeable when an animal is nursing her kittens or when humans provide social contact via petting, stroking or feeding.
Prof Leslie A. Lyons, assistant professor at the School of Veterinary Medicine at the University of California, Davis.