In a previous piece, we learned some basic facts about the feline purr. As usual, there was too much fascinating stuff to fit into a single blog segment. This commonplace, soothing sound actually has a pretty complicated origin.

A curious question is why this is such a rare and unique skill among the world’s animals. One study of epileptic seizures from 1936 accidentally revealed the existence of a “purr center” inside the feline brain, where focused stimulation would produce genuine purring, even in an unconscious cat. This tiny area is located adjacent to the posterior pituitary gland, which is the release point for oxytocin. That’s pretty interesting: oxytocin plays a crucial role in the initial bonding between mother and offspring and fortifies the bond between mates, both high-purr situations. It’s famous for flooding the system when two people hug one another. It’s been shown to increase pain tolerance, while diminishing depression and anxiety. Studies of this hormone have actually demonstrated significant overlap between the actions of oxytocin and those of marijuana. You might think of oxytocin as the “elixir of chill”, so maybe it’s not so surprising that its source is right next door to the purring center.

A pretty sophisticated study from 1972 delved deep into the mechanics of purring. It revealed that this distinctive sound comes not from the mouth, but from muscles in the larynx (top of the chest) and diaphragm (bottom of the chest) that tense up for just an instant, a few hundredths of a second. The two muscles take turns doing this, over and over, never overlapping, in a meticulously orchestrated sequence that’s composed in the brain. The result is actually a microscopic grunt that repeats so rapidly—a minimum of 20 times per second—that it resembles a flutter, and that’s the origin of the soothing sound. This is faster than a heartbeat or eye blink, even faster than the shivering and tremors caused by cold, epilepsy, or Parkinson’s disease. Very few animals are physically capable of this feat. Each individual cat uses a slightly different vibration frequency, and each cat purrs precisely at its own frequency without fail.

Obviously cats purr when they are affectionate and content, but researchers have puzzled over why they also do it in situations of anxiety and pain—even as they are dying, sometimes. Purring requires about twice as much effort as breathing quietly, so doing this when you’re weak and hurting would seem to be a waste of precious energy. As it happens, unrelated research has demonstrated that vibrations in the house cat frequency range can speed up fracture repair and wound healing. Are painful cats reaching for some ancient tool to try mending their own broken bodies? (And could this be why cats sometimes seem so attentive to injured humans?) Are they reaching for one final sip of chill elixir? Either scenario seems possible. We might never know for sure, but I’ll be watching for more research and ensuring that my cat inventory stays high enough to address any unexpected injuries at home.

Dr. M.S. Regan