What stands hairs on end causing piloerection?

Piloerection, or the standing of hairs on end, is primarily caused by the contraction of tiny muscles called arrector pili located at the base of hair follicles. This reaction is typically triggered by cold temperatures, which help to trap heat, or by strong emotions such as fear or excitement, which can activate the body's fight-or-flight response. The sympathetic nervous system stimulates these muscles, causing the hairs to stand upright. This reflex is a vestigial response inherited from our animal ancestors, where it would make them appear larger to predators or to retain warmth.