Contraction of these muscles causes the hairs to stand on end,[2] known colloquially as goose bumps (piloerection).
[3] Each arrector pili is composed of a bundle of smooth muscle fibres which attach to several follicles (a follicular unit).
[4] Contraction of arrector pili muscles have a principal function in the majority of mammals of providing thermal insulation.
Many animals experience contraction of the arrector pili muscle in response to a perceived threat.
[5][6] Skin conditions such as leprosy can damage arrector pili muscles, preventing their contraction.