Goosebumps are a survival reflex that makes animals look larger and more intimidating to predators.

The scientific name for goosebumps is cutis anserina, resulting from the contraction of arrector pili muscles located at the base of each hair follicle. This physiological response is controlled by the sympathetic nervous system, the same system responsible for the fight-or-flight response. When an animal perceives a threat, the adrenal glands release epinephrine, triggering these tiny muscles to contract and hair to stand on end.In mammals like porcupines and cats, this mechanism serves two purposes. It provides better thermal insulation by trapping a thicker layer of air near the skin and makes the animal appear significantly larger to potential predators. Charles Darwin discussed this phenomenon in his 1872 book, The Expression of the Emotions in Man and Animals, noting that it is a vestigial trait in humans who no longer have enough hair for it to be effective for warmth or defense.Modern research from Harvard University published in the journal Cell in 2020 suggests that the nerves triggering goosebumps are also linked to hair follicle stem cells. This study indicates the reflex might help stimulate long-term hair growth in response to cold environments. While the visual effect is largely lost on humans, the underlying biological mechanism remains intact from our evolutionary ancestors millions of years ago.