Erectores pilorum (singular Erector pili) are tiny
muscle fibers attached to each
hair follicle, which contract to make the hairs stand on end, causing
goose bumps. They exist in most
mammals including
humans.Erectores pilorum are
smooth muscle, not
skeletal muscle, which explains why humans cannot voluntarily give themselves
goose bumps. In other animals with more hair than humans, they serve an important function — they raise the hairs so air gets trapped between them, providing a layer of insulation to keep the animal warm. Some animals also contract their erectores pilorum when they are cornered, in order to appear larger and more threatening. A prominent example of this function is the
porcupine which uses erectores pilorum to raise its quills when threatened. Although humans' erectores pilorum also contract in response to cold or arousal, they are
vestigial because humans do not have enough hair to make them effective.
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