What Are Goose Bumps?

Feroche Esthetics Newsletter April 2010 Vol. #1, Issue #3 Excerpt

I thought this was very interesting when I first learned about it. The arrectores pilorum (singular arrector pili) are small muscles which connect the hair follicles to the dermis. Contractions of these muscles cause the hairs to stand on end a.k.a. goose bumps. In addition, contraction of this muscle is involuntary. Stresses such as cold, fear, etc. may stimulate the sympathetic nervous system and thus cause the contraction, but the muscle is not under conscious control.

Furthermore, contraction of the muscles has a number of different purposes. Its principle function in the majority of mammals is to provide insulation: air becomes trapped between the erect hairs, helping the animal retain heat. Erection of the porcupine’s long, thick hairs causes the animal to become more intimidating, scaring predators. Pressure exerted by the muscle may cause sebum (oil) to be forced along the hair follicle towards the surface, protecting the hair.

The effectiveness of the muscles' action in humans has been questioned, as humans have relatively little body hair to allow for thermal insulation.