We usually think of purrs as a sign of contentment and general well-being, and they often are. (For instance, as I sit at my computer, I have a cat purring away next to me, occasionally head bumping my arm — which I’ll conveniently use as an excuse for any typos that I might make.) But I’ve also heard cats purr when they are extremely ill or suffering from terrible injuries.
So, why would cats purr when they are happy as well as when they are in pain?
Here’s my as of yet untested theory: A cat’s purr is a complex emotional signal that is designed to communicate with other cats. Yes, of course, they use it to communicate with us as well, but cats really are not all that domesticated. Most of their behaviors are pretty unadulterated by human contact. I would be willing to bet that there are subtleties to a cat’s purr that we thick-headed humans are simply incapable of deciphering.
The science behind the purr is as follows. Purrs occur on both the inhale and exhale at frequencies between 25 and 150 Hz (meaning that the vocal cords are vibrating this many times per second). Good enough, but that doesn’t really help answer the "why" question.
Let’s look at a human example. We can describe all smiles in simple anatomical and physiological terms, but individually they can indicate many different emotional states: happiness, anxiety, condescension, a non verbal "hello," even a grimace of pain can look an awful lot like a smile. An alien species probably would not be able to tell the difference between these subtle variations of a smile, but other human beings certainly can. I’m sure cats can tell the difference between a purr of contentment and a purr of misery.
As an interesting aside, scientists have found that vibrations in the 25 to 150 Hz range have healing properties and can improve bone density (in humans). Perhaps cats purr when they are sick as a form of therapy. I know that being in proximity to a purring cat, at least one that I assume is purring out of happiness, sure improves my mood. Maybe some of those "good vibrations" can actually making us healthier too.
Dr. Jennifer Coates