Have you ever seen a picture of a cat with an arched back or seen your cat do it and wondered why? Cats have incredibly flexible spines (in fact, that's why, if given enough time, they can right themselves when falling and land on their feet). So a cat is able to achieve a much tighter arch in their back than a dog. There are several reasons cats engage in back-arching behavior.
Cats love to stretch. They're predators, so staying nimble and keeping their joints and muscles supple is critical. That's one of the reasons they spend a lot of time scratching things and also why they stretch a lot, including arching their backs.
The classic Halloween cat picture of a kitty with a severely arched back, wild eyes, and puffed up fur is the result of this feline behavior. When a cat is afraid of something and feels the need to act defensive, he will often arch his back, and his hair will stand on end. This has the effect of making him look bigger and fiercer to a potential attacker.
When a cat is arched defensively, his other body language usually lets you know he's afraid and might be fearfully aggressive if approached. His muscles are probably tense. His eyes are likely to be intent and staring, and his pupils might be dilated. He could be growling or hissing.
When a cat, especially a kitten, is playing, he will often arch his back and then hop around. It's super cute, and it's probably practice for when he might need to arch his back more defensively, as described above.
You can tell your cat is arching his back for play instead of aggression because of the rest of his body language. His fur won't be puffed up and standing on end, he'll probably be hopping around playfully, and he won't be growling, spitting, or hissing.
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