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What could bring more joy than a purring kitty curled up in your lap? And there are so many different kinds of purrs. Some cats purr like a lawn mower engine, and some are so quiet you could barely hear them with your ear on their chest. Purring is a uniquely feline behavior. We associate it with happy kitties, but cats actually purr under a lot of different circumstances. This rather odd sound must serve a purpose besides being adorable. So why do cats purr?
Anatomy of the Purr
First things first: what even is purring? As someone who used to try it for myself as a child, I promise it doesn’t come naturally if you aren’t a cat. There was a lot of debate about how cats purred for a long time. One prominent theory was that the purring sound came from a change in the cat’s blood flow [1, 4, 6]. Now, biologists are generally agreed that cats purr using the muscles of their larynx and diaphragm [1, 3-5, 7]. By vibrating their larynx, which in turn vibrates their hyoid bone [5], a cat causes the air in their throat to vibrate as well, creating the sound we know as purring [1, 6].
Most vocalizations can only be made when exhaling. It just makes sense. You need air in your lungs to push through your larynx to talk, sing, laugh, etc. The same is true of other animals. However, cats can purr on the exhale and the inhale, which is what enables them to purr constantly [1-5, 7]. This is due to the continual vibration of the larynx as they breathe in and out through it [1].
Cats Purr to Communicate
Cats first start to purr by the time they are a week old [1, 2, 4]. The purring of her newborns may signal to a queen their location and wellbeing [1, 2, 4]. It is thought that each kitten has a unique purr their mother can recognize so that she can tell which of her babies is communicating with her [2].
Queens will also purr for their kittens [5, 6]. One reason may be to provide a tactile cue to help blind and deaf newborn kittens locate the milk supply [6]. Even though very young kittens can’t hear the purring yet, they can feel the vibrations because they are, ideally, never far away. Alternatively, the mother’s purring may disguise the soft mews of the kittens from the ears of predators [5]. Whatever the evolutionary reason, purring cements the bond between queen and kittens [5].
Some adult cats purr when feeding, possibly a throwback to that kittenhood behavior [1]. Cats may also purr to inform their person that they want their food, now please [1]! The purr of solicitation sounds different from the snuggly purr [1]. What’s the difference? It’s hard to say for sure. Our lack of knowledge about how cats communicate with each other makes it harder to understand the nuances of how they communicate with us.
One thing that’s certain is cats in the wild purr when they groom each other [1]. Grooming is an important part of a cat’s daily life and a positive social interaction between cats who have a bond. Perhaps this comes closest to the contended purring of a cat that is lying in a warm lap or being petted.
Healing Vibrations
Cats don’t just purr for happy reasons. They sometimes purr when they’re stressed, scared, injured, or dying [1, 3-5, 7, 8]. Cats have sometimes been seen lying alongside another, injured cat, both purring [7]. This is called “purr therapy” because the purring is believed to be a way of comforting the injured cat [7]. In stressful situations, purring can serve as a method of self-soothing [7]. However, it may be an even more powerful tool than that.
Cats purr at a frequency of 20-150 Hertz [1, 3]. These low-frequency vibrations have demonstrated healing benefits [1, 3, 4, 7, 8]. 25 and 50 Hz frequencies promote bone growth and fracture healing [8]. 100 Hz has therapeutic use for pain, wound healing, swelling, and difficulty breathing [8]. It may seem far-fetched, but bones do respond to pressure by hardening [1, 7]. And there is a good evolutionary reason why cats might vibrate their way to good health.
Cats are ambush predators. This means they spend a lot of their time resting and waiting in-between hunts. Purring could have evolved as a low-energy way to stimulate their muscles and bones while stationary [3, 7]. Then their bones wouldn’t weaken or become brittle over time from lack of use [7]. Interestingly, researchers have proposed just such a fix for astronauts. Muscle atrophy and bone density loss are a big problem on long space missions where the lack of gravity prevents normal exercise. Vibrating foot plates have been suggested to stimulate astronauts’ bones and prevent them from losing density [7]. It seems the cats had that figured out a long time ago.
Do All Cats Purr?
Probably not, but it kind of depends upon how you define purring, apparently. Once upon a time in the early 1800s, biologists believed that cats could either roar or purr, but it was impossible to do both [7]. This is how cats came to be divided into the “big cats,” or subfamily Pantherinae, and the “small cats,” subfamily Felinae [6, 7]. Although most big cats are larger than most small cats, those names are actually a misnomer, since the smallest of the “big cats,” the clouded leopard, is smaller than the biggest of the small cats, the puma [7]. The basis of the division on roaring vs. purring is also probably inaccurate.
The Pantherinae cats have a rope of tough, flexible cartilage where the Felinae have only the hyoid bone [5, 6]. This gives the larynx greater flexibility so that roaring is possible, although lions are the only ones to make regular use of this ability [5, 6]. Does this mean they can’t purr, though? These days, biologists tend to believe than most cats can purr, or at least something like it [3, 7]. Some call it a purr-like vocalization or a hum rather than a true purr [3, 4, 6].
Even granting that most cats can actually purr, some of the Pantherinae are usually believed to be exceptions: lions, leopards, jaguars, tigers, snow leopards, and clouded leopards [7]. But that may yet prove untrue. The anthropologist and animal behaviorist Elizabeth Marshall Thomas suggests that big cats may reserve purring for their cubs and that is why people think they don’t purr at all [6]. We simply aren’t around when they do.
Works Cited
- Dowling, S. (2018, July 25). The complicated truth about a cat’s purr. BBC Future. https://www.bbc.com/future/article/20180724-the-complicated-truth-about-a-cats-purr
- Edwards, A. (2006). The ultimate encyclopedia of cat, cat breeds, and cat care. Hermes House: London.
- Lyons, L. A. (2006, April 3). Why do cats purr? Scientific American. https://www.scientificamerican.com/article/why-do-cats-purr/
- Science Reference Section. (2019, November 19). Why and how do cats purr? Library of Congress. https://www.loc.gov/everyday-mysteries/item/why-and-how-do-cats-purr/
- Stewart, D. (1995, April 1). Do lions purr? And why are there no green mammals? The National Wildlife Federation. https://www.nwf.org/Magazines/National-Wildlife/1995/Questions-and-Answers-About-Wildlife
- Thomas, E. M. (1994). The tribe of tiger: Cats and their culture. Simon & Schuster: New York.
- Venton, D. (2015, May 8). Why do cats purr? It’s not just because they’re happy. Wired. https://www.wired.com/2015/05/why-do-cats-purr/
- von Muggenthaler, E. (2001). The felid purr: A healing mechanism? The Journal of the Acoustical Society of America 110(5), 2666. https://doi.org/10.1121/1.4777098
Published February 7, 2021
Updated June 20, 2022
