Why do cats purr?
We all think we know why cats purr. It’s because they’re happy, right? Well…it’s a bit more complicated than that. Let’s explore one of the best things about having a cat in your lap: the mesmerizing, soothing, vibrations we call the purr. My blood pressure dropped a few points just thinking about it.
In the animal kingdom, scientists classify all cats into the family “Felidae.” But they divide that group further into “felinae” and “pantherinae.” The felinae group includes our friendly house cats, and also bobcats, cheetahs, and pumas, to name a few. These cats are the purr-ers.
The pantherinae group includes most of the really big cats: lions, leopards, jaguars, and tigers. These are the roar-ers. They split off, evolutionarily speaking, from their felinae cousins some 11.5 to 10.8 million years ago. There are some physical differences between the two cat families that explain why one group can purr and the other can roar, which I’ll explain in minute.
How do cats purr?
Believe it or not, scientists are not 100% sure they know exactly how cats produce a purring sound. They used to think (and not all that long ago) that purring was the sound of blood rushing from the big vein, called the inferior vena cava, that carries blood from the bottom half of a cat’s body back to the heart. It does sort of sound like a cat’s whole body is purring, doesn’t it?
But they were completely wrong. Researchers Frazier-Sissom, Rice, and Peters studied purring in depth in 1991 and concluded a cat’s laryngeal muscles are responsible for purring. These muscles move the cat’s vocal cords, and when a cat is purring, it’s because the laryngeal muscles are causing the vocal cords to rapidly open and close. They know the laryngeal muscles are involved because when a cat experiences paralysis of the laryngeal muscles, she loses the ability to purr.
A structure in the cat’s brain, called the “neural oscillator,” seems to control the whole show. A signal from the neural oscillator gets the laryngeal muscles twitching, which in turn produces little bits of sound that occur at least 25 times each second. Those little sound bursts, all strung together, are the familiar hum or buzz we call the purr.
There’s actually another little body part in play that seems to be partly responsible for the purr and explains why our cats can purr, but lions can’t. But before I explain that, let me just point out that some big cats can make a sound that is similar to a purr, but probably isn’t.
When our cats purr, they do so as they both inhale and exhale. The purr is continuous throughout the whole breathing cycle. Some big cats make purring-like noises – probably better described as a rumbling growl – but they seem to do it only on the exhale. It’s possible that a whole different structure is responsible for the purr-like noises that lions and tigers and their buddies in the pantherinae group make. But no one knows for sure.
What is the anatomical difference between felinae and pantherinae that causes one group to purr and the other to roar? There’s a weird little bone suspended in the muscles of the neck (we have one, too) called the hyoid bone. Felinae members, like our house cat, have a very hard, bony hyoid. A very stiff hyoid kind of limits a cat to purring, a sound that averages a soft 25 decibels.
Pantherinae members, like lions, have a more flexible hyoid that connects to the base of their skulls by an elastic ligament. This whole stretchy mechanism (combined with a difference in the way pantherinae vocal cords fold), enables the big cats to produce a really big sound with less lung pressure. A lion can roar at 114 decibels, which is about 25 times louder than a gas lawnmower.
Do cats purr voluntarily?
We actually don’t know whether a cat decides to purr or not. It’s possible that the neural oscillator is on autopilot and a cat just purrs when his brain demands it. Dr. Leslie Lyons, associate professor at U.C. Davis School of Veterinary Medicine, told NPR that purring might be both voluntary and involuntary, much like blinking. Most of the time we blink without thinking about it, and we can’t really stop ourselves from blinking for long, even if we try hard. But we can also blink if we want to. It’s likely that the purr is more of a kind of a muscle twitch most of the time, than a conscious sound made by a cat.
When do cats purr?
Cats are born to purring. Kittens can start purring when they are only two days old. Their eyes aren’t even open, and already they are purring! It’s possible that kittens purr because it allows them to communicate with their mothers at a low intensity so they don’t attract predators, but we don’t know for sure.
We do know that mother cats also purr to their kittens. Kittens are born blind and deaf. Does the rumbling of the purr allow them to steer their kittens to warmth and safety with vibrations they can feel? We don’t know that either.
It is possible that the association between being purring and being fed is one that continues into a cat’s adulthood. It may be why your cat purrs to convince you that it’s suppertime, or purrs when she eats.
Cats purr when they are happy
For all of you who raised a hand a little too quickly when I asked, “Why do cats purr?” you were right. Cats definitely purr when they are feeling contented. A contented cat may purr in the company of others or alone. You’ll know for sure if it’s a contented cat who is purring if the cat is also displaying other signs of relaxed happiness, like head bunting or kneading.
But contentment is not the only emotion or condition that will cause a cat to purr.
Cats purr to appease other cats
We do not know everything there is to know about how cats communicate with each other, but it does appear that cats use purring as a kind of appeasement behavior with other cats. If you have more than one cat, you may notice that one cat will purr when grooming another, as if to soothe and keep tensions low.
Sometimes these purrs are a somewhat higher pitched purr than a cat’s usual contented purr and may be presented with a stiffer body posture, than the relaxed on-the-lap purr.
Cats purr to get a response from you
There’s a particular kind of purr that may already know about, that seems designed to get you to do something for your cat. Dr. Karen McComb and a team of researchers at the University of Sussex in the United Kingdom, studied the “insistent purr.” Dr. McComb decided to conduct this research because she found her own cat’s insistent purr particularly annoying.
The insistent purr is a normal purr with an “overlay of a high-frequency cry-meow.” Humans tend to find this sound obnoxious and want to do something to make it stop. Dr. McCombs found that the “solicitation purr,” as she called it, tended to develop in quiet households where the cat has a one-on-one relationship with a person, probably because this is the kind of situation where an annoying sound will get noticed.
It’s possible that cats, smart as they are, have learned to tap into our primitive mammalian instinct to attend to a crying human infant. The solicitation purr is similar in frequency to baby’s cry (220-520 Hz, compared to a baby’s 300-600 Hz), and it has the dual effect of annoying us while simultaneously appealing to our impulse to nurture.
Cats purr when they are nervous, stressed, or sick
Pregnant cats may purr when they are in labor. Frightened cats may purr at the veterinarian’s office. Cats who are in pain often purr. And cats may purr as they die. Why?
It’s possible that purrs are self-soothing, like thumb-sucking for a child. We adults perform behaviors that serve to self-soothe, too, like fidgeting when we’re uncomfortable, or laughing when we’re nervous.
The really strange thing is the purrs designed to soothe may actually have the power to heal. Sports medicine doctors know that certain high-frequency vibrations have the power to repair and regenerate bones, joints, muscles, tendons, and ligaments, or to heal wounds. Purrs occur at a frequency of 25-100 Hz and we know that bone growth can occur in the presence of vibrations of 25-50 Hz and soft tissue at 100 Hz. High frequency vibrations can increase a body’s production of its own natural anti-inflammatories, and cause minute contractions and relaxations in muscle fibers which, in turn, stimulate the growth of bone cells.
Is it possible that cats are performing their own kind of physical therapy?
Humans benefit from a cat’s purrs, too
Purring is not only healing for cats, it can be healing to humans as well. Petting a cat has a calming effect. But you already knew that, right?
But did you know that this “calming” effect may also extend your life? Owning a pet has been shown to decrease blood pressure and cholesterol levels, and reduce feelings of loneliness. But a study by the American Heart Association in 2016 found that owning a cat (rather than a dog) was significantly associated with a reduced risk of dying from cardiovascular disease, especially stroke, in people who had no history of cardiovascular disease.
As if you needed a reason to pet that purring cat on your lap.
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 Ogechi I;Snook K;Davis BM;Hansen AR;Liu F;Zhang J; “Pet Ownership and the Risk of Dying from Cardiovascular Disease Among Adults Without Major Chronic Medical Conditions.” High Blood Pressure & Cardiovascular Prevention : the Official Journal of the Italian Society of Hypertension, U.S. National Library of Medicine, pubmed.ncbi.nlm.nih.gov/27174431/.