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Goosebumps are a leftover fur reflex
Cold and fur
When cold makes bumps rise across the skin, tiny muscles attached to the hair follicles are contracting. On a furry animal, standing the hair up traps a thicker layer of insulating air next to the skin and helps the animal stay warm.
Fright response
Fright triggers the same reflex. Raised hair makes a frightened animal look larger and potentially more intimidating to a predator.
A reflex without fur
Humans kept the automatic response and lost most of the hair, so it accomplishes little. A scary movie, a cold breeze, or a powerful piece of music still sets it off, an ancient mammalian survival reflex firing on a body that no longer has much use for it.
What makes the hair stand up?
Each hair follicle has a tiny smooth muscle attached to it, called the arrector pili. The muscle is controlled by the sympathetic nervous system, so no one can raise it on purpose. It sits at an angle to the follicle, so when it tightens it pulls the hair upright and dimples the skin around it. That dimpling is what the eye sees as a bump.
Why does music give some people chills?
Researchers call the chill that comes with music frisson. In a 2011 study in Nature Neuroscience, Valorie Salimpoor and colleagues at McGill University scanned listeners while they heard music that gave them chills. They found a release of dopamine in the brain's reward circuitry at the moment of the peak, and in anticipation just before it. The same brain chemistry is involved in food and other rewards.
Which animals still use raised hair well?
Cats, dogs, and chimpanzees all raise their fur when threatened, which makes their outline look larger. A porcupine goes further: its quills are modified hairs, and muscles beneath the skin lift them into a defensive spread. Many birds fluff their feathers in the cold for the same insulating reason that a furry mammal's raised coat traps warm air. In humans the same muscles remain in the skin of the arms, legs, and neck.
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