A Nerdy Dissertation on Purring (part one)

Given the stubborn independence of today’s domesticated feline, my guess is that they’ve been behaving precisely the same way since they first accepted a position in our homes. Our relationship with cats has lasted for thousands of years, yet it often seems like we’re still no closer to understanding exactly how they work. Purring is a fine example. Humans, unable to accomplish it themselves, are fascinated by it. How is it possible that the mechanics of purring are still unknown?

An early theory on purring had it arising from turbulence of blood flow in the vena cava, which is the largest diameter vein in the body and feeds directly into the heart. To any modern medical professional, this idea sounds preposterous. Producing a noise (audible to the unaided human ear!) would seem to require an enormous amount of chaos inside this large, important blood vessel—something like shaking up a can of soda. How could one survive this amount of disturbance inside their bloodstream? The vena cava was eventually discarded in favor of the larynx, or voicebox, as the source of these loud vibrations.

A few things are known: felines of all sizes and shapes are similar in structure; however, those that can roar are unable to purr (and vice versa). The feline vocalization community found its feet all the way back in 1834, when Richard Owen postulated that the ability to roar (or purr) was centered in the rigidity of the bones in the larynx. Some species have a more pliable larynx, and this was thought to be the reason they can roar but not purr. It wasn’t until 150 years later that the stunning snow leopard bounded onto the scene, easily capable of purring despite its roarer-style larynx. Back to square one.

In 1972, a different research group entered the chat, proposing that the purr is really a neurologic phenomenon. Their hypothesis was “active muscle contraction”—several independent muscles located in the chest and throat, working in tandem to produce an organized sound. This school of thought stated that the vibration was caused by individual nerves firing 20-30 discrete times per second (i.e., 20-30 Hertz), tensing and relaxing the musculature over and over again, for the duration of the purring session. This theory, if correct, might help to explain the mystery of how such a low-pitched sound can be emitted by such a small animal. You can easily know the comparative size of a dog by the sound of his bark alone, because a toy poodle is equipped with a much higher-pitched voice than a big beast like a Rottweiler. The domesticated cat purr somehow rumbles along at a frequency of 20-30 Hz, which is much more typical of massive animals like giraffes, elephants, and even whales. Perhaps those aforementioned nerves are intervening to slow the motion of the feline larynx all the way down to elephant speed.

That sounds pretty plausible; however, we’re still laboring to explain this seemingly simple, everyday event. Some important, contradictory data was just published about a year ago, but the picture still isn’t clear. Will this popular feline pastime ever be fully understood?

Dr M.S. Regan