Most people are familiar with the sound of a cat's purr, and the sound is often referenced as an indicator of contentment or well being. However, as most cat owners are aware, the purr is not restricted to periods of contentment, but will also manifest during times of stress or pain. The physiology and reasons for the noise are not completely understood, although there are several hypotheses as to the reasons for the feline purr.
Most cat species, large and small, purr. Zookeepers have recorded instances of big cats such as leopards purring, and even some unrelated species such as raccoons are also capable of making a distinctive purring noise. It is believed the sound is caused by the vibration of vocal cords, and is a voluntary act on the part of the animal. When purring, the muscles of the voice box can act as a valve, causing air traveling in both directions to vibrate, which creates the illusion of continuous sound.
As anyone who has held a purring cat knows, the whole body of the animal vibrates, and the noises come in varying intensities. The purr can also be combined with a meow, although the result is sometimes comic. The purr is generally considered to be a soothing sound, and cats are sometimes used as therapy animals for this reason.
Originally, it was thought the noise was simply an expression of an emotional or physical state of contentment, safety, and well being. It is true that happy cats will purr, and that when cats mutually groom or cooperate on a task they will frequently purr. While it was a pleasant theory, the clearly demonstrated fact that cats will make the sound when they are not happy suggested that there was more to it than that.
The vibrational frequency of the purr ranges from 25-130 Hertz. This happens to be a frequency which promotes healing of bones and eases muscle pain, suggesting that perhaps cats are actually healing themselves with the mechanism. This suggestion has been borne out by studies which show that cats tend to have faster healing bone fractures than other animals, and suffer less from osteoarthritis and other bone disease.
Cats will often purr when they are tired, which could be considered a waste of energy unless the action is serving a physiological function. Cats may be massaging themselves from within after a long day, easing muscle soreness, inflammation, shortness of breath, and exhaustion.