A purring cat appears to have achieved a state of total contentment; however, a cat's purr can mean other things as well.
Although there are many theories, until recently the mechanics of purring has not been completely understood.
Recent research suggests that purring begins in the brain, where a neural oscillator sends rhythmic, repetitive signals to the laryngeal muscles, causing them to vibrate at a rate of 25 to 100 times per second. This causes the vocal cords to separate during both inspiration and expiration, which elicits the unique vibrato of purring. Because the purr is of such a low pitch, we are able to hear it as well as feel it.
Purring is not a trait seen in domestic cats only. Some wild felines and closely related species like civets, genets, and mongooses also are capable of purring. Big cats like mountain lions and pumas can also purr, but are unable to roar; and big cats like lions and tigers can roar, but are unable to purr. This has to do with the density of the hyoid bone in the different species of felines.
It is thought that big cats evolved the ability to roar to protect the pride. Roaring, along with urine marking, is very important in defining their territory.
So what is the evolutionary purpose of purring? As pet owners, we perceive purring as a sign of pleasure and contentment. Yet, cats will also purr when frightened and/or threatened. Kelly Morgan, a veterinarian at the University of Illinois College of Veterinary Medicine, equates purring with smiling: "People smile when they are nervous, when they want something, and when they are happy, so perhaps the purr can also be an appeasing gesture."
Often times cats add a high frequency cry-meow into the purr. It has been suggested that cats learn this more annoying purr to get their owner's attention. This so-called solicitation purr, which most owners perceive as obnoxious, works very well for the cat in terms of getting his wishes seen to in a timely fashion. As recent adopters of a kitten, we can say that this "skill" was attained in a matter of days.
In addition to contentment, cats also purr when nursing, when in pain, when injured and sick, when in labor, and when near death, so purring seems to have evolved for other purposes as well. Perhaps it causes the release of endorphins.
Elizabeth von Muggenthaler at the Fauna Communications Research Institute, whose area of interest is bioacoustics (the study of frequency, pitch, loudness, and duration of animal sounds), theorizes that cats get beneficial healing effects from purring. The 25 hertz frequency of a cat's purr may be a kind of built-in form of physical therapy. Interestingly, work is being done in which this frequency is being used to promote healing in human patients.
Cats are now the most popular pet in the United States. One possible reason may be that our cats have been shown to have a significant effect on decreasing stress levels and blood pressure, and purring is thought to be the contributing factor.
East Bare Hill Road resident Fred Karotkin is a retired veterinarian.