Why do cats arch their backs?
The main reason that cats arch their backs is because they can. A fascinating study of the kitty spine showed that cats have naturally curvy spines, even when they’re just standing around, and that the design of the spine makes cats remarkably flexible.
Having extra backbones helps, too. Compared to humans, cats have a few additional vertebrae in the middle and lower portions of their backs. Those extra backbones give cats really mobile spines, especially compared to us clunky humans.
When you’re an animal, like a cat, who communicates primarily with body language, having a really flexible body makes it easier to say more things.
Now, what is a cat trying to say when she arches her back?
What is the meaning of an arched back?
An arched back can mean more than one thing to cats.
The problem with us humans is that we focus on sound when we communicate with others. We speak with words, with the occasional eye roll or furrowed brow thrown in.
Cats mostly speak to each other, and even to us, with body language. To other cats, all arched backs do not look alike. Cats can read all the other more-subtle clues that explain what this arched back means.
To better understand our cats, we have to become more attentive to all the other signals that our cats are giving us. In order to understand why your cat is arching his back, you have to look at context. What is going on in your cat’s world when he arched his back? Was there a clap of thunder at that moment? Did a feral cat just saunter past your cat’s favorite window?
Is your cat a kitten, and are her other litter members playfighting at the moment the arched back occurs? Are you petting your cat, and did you just touch a sensitive spot?
Answers to questions like these will help you interpret your cat’s arched back.
While you work on your ability to speak cat, here are some things an arched back can mean:
Your cat has an arched back because he is afraid
Many animal species try to make themselves appear larger when threatened. It’s often a last-ditch attempt to defend themselves against a perceived danger. It may be too late run away, and the only tool in the toolbox left is a warning that says, “look how ginormous and scary I am.” The warning, if not heeded, is usually followed by a defensive attack.
A puff adder is called what it’s called because it literally inflates itself to appear larger than it really is. Humans and other animals who stick around after that may end up with a venomous bite.
Many owls have the ability to fluff up their feathers and fan out their wings so that they look enormous. It’s a surprisingly effective deterrent to eagles and other would-be owl eaters.
It should come as no surprise, then, that cats do it, too. When a cat arches his back for this reason, it’s called “fear aggression,” meaning that your cat is scared, but feels backed into a corner with no choice but to fight. She may growl, hiss, spit, and show her teeth, too.
The arched back is a warning, like it is for the puff adder. The next step may be lashing out.
Piloerection is also a sign of fear-aggression
The other part of this performance is called piloerection. Piloerection refers to a cat’s hair standing on end. Tail fur puffs out, too, bottle-brush style. The arched back and the puffed-out fur work together to make a cat look larger and more threatening than she really is.
Actually, I said that wrong. She isn’t “trying” to make herself look larger. This behavior is believed to be an involuntary response. Piloerection and the arched back occur due to a rush of adrenaline. Adrenaline is the “fight or flight” hormone, and when it’s released, the tiny muscles around each little strand of hair contract, making the hair stand erect. It’s kind of like goosebumps in humans, but we mostly-naked creatures don’t look dramatically different when we piloerect.
So, a cat isn’t really thinking to himself, “I need to look bigger right now.” Thousands of years of evolution have made the arched back/piloerection response automatic.
A cat who is arching his back because he is afraid will have an increased heart rate and dilated pupils, too. When the adrenaline is eventually metabolized (processed by the body) and the stressful thing goes away, the hair will settle down, and so will the arched back.
What should I do if a cat is arching her back out of fear?
Don’t pet a cat in who is arching his back out of fear. Back away slowly, and give the cat his space.
Your cat has an arched back because he is feeling playful
Play is an essential part of development for many young animals, including, obviously, humans. Many animals, including cats, learn social etiquette and practice motor skills through play. Play is often a dress rehearsal for real adult behaviors, like hunting, and a way to promote bonding in a group, or relieve tension. Many cats continue play behavior well into adulthood.
It’s no surprise, then, that playful cats and kittens will display a behavior in play that they will later use in real-life situations, including the arched back.
One poster on a cat advice forum on Reddit refers to the playful form of the arched-back behavior as “doing big cat” and related that his youngest cat performed this little show before pouncing on his older brother. I love this description of the fun form of the arched back. That’s exactly what a kitten with an arched back is probably doing: playing “big cat.”
The sideways run, the crabwalk, or sidewinding
How can you tell the difference between a cat who is playfully arching her back and a cat who is fearfully arching her back? For one, there will be no growling, hissing, spitting, or showing of teeth.
There may be hopping or pouncing behaviors directed toward another cat, a toy, or a person who is familiar to the cat.
But my favorite sign that the arched back is probably meant to be playful is the sideways run, crabwalk, or sidewinding motion. It’s often a sign of excitement and fun. We’ll never know for sure, but I suspect that a young cat with an arched back and a sideways run is trying to simultaneously entice a playmate to chase him, while also watching the other cat to see her reaction.
Notice that I said “probably” and “often”? Every cat is an individual and not every cat who runs sideways is in a playful mood.
Your cat has an arched back because she is stretching
I’ve often seen cats who get up from a nap and do a quick “scary-cat” arched-back stretch.
Stretching is a great way to prime the muscles for movement again after resting. Stretching gets blood flowing to the muscles and brain, and flushes out toxins that might have accumulated during periods of sleep or relaxation.
Cats enjoy a few entertaining back-stretching positions in addition to the arched back. Cats may perform a doggie-style play bow, in which the front end of the cat is down and the back end is up. Cats may stretch with their hind legs way out behind them, giving each leg its own private stretch.
How do you know if your cat is just arching her back to stretch? This is where context is important. Is your cat just waking from a long nap? Chances are, a newly woken cat with an arched back is just warming up her muscles.
Consider the rest of your cat’s body language, too. Is there piloerection? What are his eyes telling you? Are they relaxed from slumber, or are the pupils wide with fright? What about the rest of his demeanor? Is your cat playfully hopping and bouncing, or just languidly working the kinks out of her spine?
When an arched back is not really an arched back
A cat may have a bend in the back that's not a true arch. The arch is an extreme and usually temporary contraction.
A cat that is walking in a hunched position may actually be in pain. Cats with arthritis in the spine or other back pain may maintain a hunched position in an attempt to relieve their discomfort.
If your cat is hunching from pain there will usually be other signs. Your cat may be grumpier than usual, may not allow you to pick up or handle them, and may show signs of depression or lethargy. Your cat may also start having accidents outside the litter box.
If you suspect your cat may hunching from pain, seek veterinary care immediately.
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*Thanks to Paula Scappatura for the delightful photo of her girl on a windowsill, arching her back while stretching after a nap.
 Macpherson, J. M., and Y. Ye. “The Cat Vertebral Column: Stance Configuration and Range of Motion.” Experimental Brain Research, Springer-Verlag, https://link.springer.com/article/10.1007/s002210050348.
 Harlan, Ron. “10 Weird Self-Inflating Animal Species.” Listverse, 21 June 2014, https://listverse.com/2014/01/03/10-weird-self-inflating-animal-species/.
 “Living Large: Nine Outstanding Expanding Animals!” WebEcoist, 8 Nov. 2016, https://webecoist.momtastic.com/2010/01/05/living-large-nine-outstanding-expanding-animals/.
 Crowell-Davis DVM, Sharon. “Why Does My Cat Arch Her Back?” Vetstreet, http://www.vetstreet.com/our-pet-experts/why-does-my-cat-arch-her-back.
 Sandoval, Adri. “Ask a Vet: Why Does My Cat Arch His Back When He Is Afraid?” IHeartCats.com, 18 June 2016, https://iheartcats.com/ask-a-vet-why-does-my-cat-arch-his-back-when-he-is-afraid/.
 Lents, Nathan H. “Why Play Is Important | Psychology Today.” Why Play Is Important. Animal Behavior Helps Illuminate Why We Play., Psychology Today, https://www.psychologytoday.com/us/blog/beastly-behavior/201705/why-play-is-important.
 “R/Catadvice - Arched Back When Playing??” Reddit, https://www.reddit.com/r/CatAdvice/comments/g9z2fj/arched_back_when_playing/.
 Geggel, Laura. “Why Do Cats Stretch so Much?” LiveScience, Purch, 21 Apr. 2016, https://www.livescience.com/54480-why-do-cats-stretch.html.