Purring – Some Pearls of Wisdom on a Perplexing Phenomenon

Is there a more satisfying sound on earth than the rumble of a good purr? It’s enough to keep us at times rooted in our seats, as if pinned in place by a happy little tractor with soft fur and kneading paws. An intimate communication, audible only to the lucky few close enough to hear – perhaps the pillow talk of cats!

As much as we know and love the sound of purring, it holds more mystery and magic than you may realise, and science is still far from understanding it completely. While generally associated with feelings of contentment and happiness, cats can also purr when they are very ill; in pain or stressed. Most of the time when you hear a cat purring they are in a positive emotional state, but not always, and context is relevant.

For example, many cats purr during visits to the vet, and while we like to tell ourselves it’s only because they love spending time with us, in this case it’s more likely a type of coping mechanism. Purring is now thought to be an evolutionary technique of self-soothing, similar to how some people twirl their hair when they feel anxious, or have a cigarette, or take a few deep breaths. Your cat’s purr releases endorphins that help to relieve pain or improve their emotional state(2). Everybody copes with scary situations differently, sometimes resorting to panic or aggression; so you can think of your cat purring as an admirable and peaceful way of trying to manage difficult circumstances.

The sound of purring is generated by muscles relaxing and constricting an area of the larynx, which causes vibration of air(1) in both inhalation and exhalation. Interestingly, purring and roaring are mutually exclusive, with the cat family divided loosely into those that can purr and those that can roar, with no real cross over in between. This difference comes from the vocal cords as well as part of the anatomy within the larynx – in roarers the laryngeal apparatus can stretch to produce an impressive roar; while in purrers it is rigid, and so vibrates to create a purr.

Roaring cats include lions, tigers, leopards and jaguars(3); and purring cats include cheetahs, wildcats and domestic cats. Snow leopards have slightly different vocal cords which lack a fatty, elastic layer, and so they are unable to truly roar or purr(3), which seems a bit unfair really.

This is where things get interesting! There are many different theories about why cats purr, and no definitive answer. Besides being a means of self-expression and friendly communication, as well as a tool for self-soothing, purring offers other potential benefits as well.

One theory is that purring evolved as a way for kittens to locate their mothers to nurse. Because kittens are born with their eyes and ear canals closed, their senses of sight and hearing are initially very limited. In the dark and quiet world they find themselves in after birth, purring offers a form of communication through vibration; and helps kittens to find their way to their next meal. The purring of mother cats during nursing may also have evolved as a way to camouflage the mewling sounds of their kittens, at a stage when they could be vulnerable to predators(4), kind of like an internal white noise machine. Smart!

Kittens start purring from a few days old(1), most probably as a care soliciting signal. Some cats take this a step further and change the pitch of their purring according to context, with hungry cats emitting a purr of higher frequency than that of contented cats. This makes the frequency of the hungry purr similar to a that of a human baby distress cry(1). It’s possible that this alteration in pitch is a learnt behaviour, being rewarded with meals from sympathetic owners. (Confession: During my research for this blog I may or may not have started recording my cat, Gorbi, every time he purrs, to try and decipher if he has adapted this clever form of feline manipulation. I’m not convinced that he has, but it has made for some interesting voice notes to my husband!)

Perhaps the most intriguing theory about why cats purr is that it plays a role in bone and soft tissue healing. Domestic cats purr at a frequency that promotes tissue regeneration and increases bone density(1,5). Cats in the wild spend long periods of time resting between hunts, and as we all know, our domestic companions spend much of the day lounging - they are in fact experts at relaxing, sleeping on average between 15 and 20 hours per day! These extended periods of rest could put bones at risk for becoming weak and brittle, and so purring may have evolved as a means to keep bones healthy; ready to withstand the high impact of hunting - or jumping off your kitchen counter. It seems kind of typical of cats to have evolved a way of reaping the benefits of exercise without having to leave the couch. Fetch, apparently, is for losers.

The good news for us is that purring may benefit cat owners as well. Purring frequencies correlate to those established for therapeutic treatments in humans(1), so may promote bone and soft tissue health in us too. Besides the possible physiological benefits, purring also has a calming effect and positive psychological impact. After all, owning a cat can decrease the risk of stroke and heart disease by as much as 30 - 40% (1). So the next time you risk running a bit late because you’re pinned to your couch by a purring little tractor with soft fur and kneading paws, don’t feel too guilty. An apple a day may keep the doctor away, but so may a cuddle with your cat!

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References

1. Dowling, S. (2018, July 25). The complicated truth about a cat’s purr. BBC. Retrieved from: https://www.bbc.com/future/article/20180724-the-compplicated-truth-about-a-cats-purr

2. Laliberte, M. (2020, March 21). Why do cats purr? The reasons behind it. Reader’s Digest. Retrieved from: https://www.rd.com/article/why-do-cats-purr

3. Mills, S. Why can only big cats roar? Discover Wildlife. Retrieved from: https://www.discoverwildlife.com/animal-facts/mammals/why-can-only-big-cats-roar

4. Stewart, D. (1995, April 1). Do lions purr? And why are there no green mammals? National Wildlife Federation. Retrieved from: https://www.nwf.org/Magazines/National-Wildlife/1995/Questions-and-Answers-About-Wildlife

5. Venton, D. (2015, May 8). Why do cats purr? It’s not just because they’re happy. Wired. Retrieved from: https://www.wired.com/2015/05/why-do-cats-purr