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The arrector pili muscles are small muscles attached to hair follicles in mammals. Contraction of these muscles causes the hairs to stand on end (i.e. "goose bumps"). Arrector pili muscles are composed of small bundles of smooth muscle fibres. These tiny muscle bundles are innervated by the sympathetic branch of the autonomic nervous system. Thus, the contraction of the muscle is involuntary. Factors such as cold or fear often stimulate the sympathetic nervous system thereby causing contraction of the arrector pili bundles. The primary function of contraction (of the arrector pili muscles) in mammals is to provide insulation. That is, the air trapped between erect hairs helps animals retain heat. [The function of the same process in humans is not understood as humans have comparatively little hair.] In some animals - like the porcupine (its quills are modified hairs), erection of the hair causes the animal to become appear larger - and thus more intimidating towards predators. Finally, contraction of the arrector pili muscles causes sebum (or oil) to be forced along the hair follicle outwards thereby protecting the hair.