<P> The arrector pili muscles are small muscles attached to hair follicles in mammals . Contraction of these muscles causes the hairs to stand on end, known colloquially as goose bumps . </P> <P> Each arrector pili is composed of a bundle of smooth muscle fibres which attach to several follicles (a follicular unit), and is innervated by the sympathetic branch of the autonomic nervous system . The contraction of the muscle is then involuntary--stresses such as cold, fear etc. may stimulate the sympathetic nervous system, and thus cause contraction . </P> <P> Contraction of the muscles has a number of different purposes . Its principal 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 to be forced along the hair follicle towards the surface, protecting the hair . </P> <Ul> <Li> <P> Insertion of sebaceous glands into hair shaft </P> </Li> <Li> <P> Cross-section of all skin layers </P> </Li> </Ul>

What is one major function of arrector pili muscle contraction