Glossary

Piloerection

The upright bristling of a cat's fur, most often a sign of fear or arousal, which makes the cat look larger.

Also called: puffed-up fur, raised hackles, bottle-brush tail, fluffed fur

Piloerection is the involuntary bristling of the fur when tiny muscles at the base of each hair contract. In a frightened or angry cat the fur along the back and tail stands up, and the tail swells into what people call a bottle brush. Seen from the side, the cat looks bigger than it is, which is meant to put off a rival. It often goes together with an arched back, sideways stance, flattened ears and hissing.

This reaction belongs to the autonomic nervous system, so the cat does not choose it. Fur can also rise when a cat is cold, since raising the coat traps a layer of warm air, or in the middle of excited play.

When you see it, read the whole animal. A cat with raised fur and a tense, crouched posture is scared and may lash out if cornered or touched. Leave it alone, remove the trigger if you can and let it calm down in a quiet space. Do not pick it up. A cat whose fur rises for no obvious reason, or that seems to ripple along the back and twitch the skin, should be seen by your vet.

Last reviewed November 7, 2026

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Related terms

Tail language What the position and movement of a cat's tail reveal about its mood, from a friendly upright tail to a lashing one. Hissing A sharp, breathy warning sound a frightened or angry cat makes to tell another animal or person to back off. Redirected aggression A sudden attack on a nearby person or pet by a cat that is highly aroused by something it cannot reach. Body language How a cat shows mood and intent through its tail, ears, eyes, whiskers, posture and fur.

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How to read a cat's body language How to read a cat's ears, eyes, tail, whiskers, posture and sounds together, spot fear and pain early, and respond in a way the cat understands. Whiskers, ears and tails: what they tell you How to read a cat's whiskers, ears, tail and eyes together, and which changes in those signals mean stress, pain or illness.