You're feeling a bit uptight. Nervous. You've had a harrowing day and you're trying to unwind. Suddenly, your cat leaps onto your lap and begins to purr. She's making a low, rumbling sound resembling an idling diesel. Soon, you're almost as calm as the feline ball of fur lying across your legs.
What is it about the "purr" that is so soothing? How do cats do that? And why?
Since the days when Egyptians worshiped the cat, philosophers and cat-lovers have wondered why cats purr. Scientists have probed the purring of the species, wild and domestic, in search of an answer.
Researchers have found that kittens learn to purr within the first few days of birth, suggesting a bonding mechanism between the newborn and its mother. It might be a way of communicating to each other that all is well. A sign of contentment.
According to some scientific findings, however, there might be a bit more to it than that. A closer look indicates that cats also purr when they're severely injured, frightened, and giving birth. They're also known to emit such sounds during stressful situations like visits to the veterinarian's office. That same scientific evidence suggests there's a practical reason behind the purr.
Purring causes vibrations that are known to be therapeutic.
Purr frequencies have been measured between 20 Hz and 200 Hz, which correspond to vibrations that promote pain relief and healing as well as reduce inflammation. This leads researchers to believe that self-healing is the survival mechanism behind why cats purr.
Among human beings, vibrations stimulation has been shown to relieve suffering in 82% of people suffering from acute and chronic pain. It's also been known to generate new tissue growth, inhibit bacterial growth, reduce swelling, and improve circulation and oxygenation. Purring, according to research, can keep a cat's muscles and ligaments in prime condition and less prone to injury.
Such results might be the basis for such feline mythology as the legend that cats are able to reassemble their bones when placed in the same room with their parts. But in the real world, the American Veterinary Medical Association notes that out of 132 cats that fell an average of five-and-a-half stories, 90% survived, including one that fell 45 stories.
Though all this evidence seems to have most experts in agreement as to why the cat purrs, they're far from one mind on how the sound is accomplished. There are two prevailing theories of how your pet produces that purring sound. 1) The Turbulent Blood Theory, and 2) The False Vocal Cord Theory.
First, it's believed that turbulence is created when blood flows through your cat's main veins into its heart. According to this theory, the constricted blood causes the purring sound as it swirls through the animal's chest. The problem with this theory, however, is that the cat purrs when it's relaxed, a state in which blood turbulence would likely subside rather than increase.
The second theory is based on the fact that your cat has two sets of vocal cords: the usual, or ordinary, vocal cords that allow her to communicate with other creatures by, for example, hissing, screeching, of meowing. Then there are the so-called "false" vocal cords, also known as vestibular folds. Some researchers believe the cat uses this part of he anatomy to produce what we call the purr. Scientists describe it as "little more that heavy breathing" and compare it to a sleeping person's shoring.
Theory #2 is seen by some as the simplest and most obvious explanation. It explains, the researchers say, the presence of the second set of cords, which would seem to have no other reason to exist. It would also explain how a cat can produce the sound at will.
Purring, it appears, is unique among domestic cats along with certain species knows as "big cats." Non-domestic cats that purr include the cheetah, the puma, the Eurasian lynx, and the wild cat.
The jungle cats that do not exhibit true purring include lions, leopards, jaguars, tigers, and snow leopards. The tiger, however, can voice a friendly noise described by some as a one-way purr" or "sort of a splutter." The domestic cat, on the other hand, makes that comforting whirring noise with each inward breath as well as when she exhales. This rhythm of our domestic companion happens even with her mouth firmly shut and when nursing at her mother's breast.
This seems to put our family cat one up on her jungle-dwelling cousin.
However, the big guys have a feature that would appear to make up for it - they can roar.