Why do cats fur stick up along their backs?

When a cat's fur stands on end, it is a sign of fear or aggression. This is known as piloerection, and it is caused by the arrector pili muscles contracting. These muscles are attached to the hair follicles, and when they contract, they pull the hair shafts up, making the cat's fur look fluffy.

Piloerection is a natural defense mechanism that helps cats to make themselves look bigger and more intimidating. When a cat is threatened, it will often arch its back and fluff up its fur in order to appear more threatening. This can help to deter predators and make the cat more likely to escape danger.

In addition to being a defense mechanism, piloerection can also be a sign of excitement or pleasure. For example, a cat that is being petted or played with may have its fur stand on end because it is enjoying the experience.

If you see your cat's fur standing on end, it is important to pay attention to the context in order to determine what is causing it. If the cat is in a threatening situation, it is important to try to remove the source of the threat and calm the cat down. If the cat is simply excited or enjoying itself, there is no need to worry.