Piloerection refers to a reaction of the sympathetic nervous system that causes certain muscles to contract and hair follicles to protrude outwards from the skin. Commonly called goosebumps or gooseflesh, it is a physiological response to cold air and intense emotions, especially fear. It is most common to see goosebumps on the forearms, though they can also appear on the legs, buttocks, chest, and neck on some people. Piloerection is a defense mechanism in many mammals to threats or cold weather, but there are no known benefits of this response in humans.
Another medical term for piloerection is cutis anserina. Goosebumps appear when the sympathetic nervous system causes the arrectores pilorum muscles under the skin to contract, creating small depressions in the skin. The depressions cause the surrounding skin and hair follicles to protrude, resulting in a pimply, rough appearance.
Medical evidence suggests that adrenaline rushes initially cause the sympathetic nervous system to react the way it does. It is believed that adrenaline is an important component in humans' inherent fight or flight response. Goosebumps are one of many physiological signs that a person's body is feeling intense stress or sudden fear, along with an increase in heart rate, focused vision, and improved reflexes. When rushes of adrenaline and other hormones put the body in a hypersensitive state, a person can utilize strength to fight off danger or speed to flee the scene.
As part of the fight or flight response, goosebumps do not offer any real security to people since human hair is relatively sparse. Many other mammals, however, rely on involuntary piloerection to protect themselves from danger. A cat's body, for example, initiates piloerection to make the hairs on the back and tail stand on end. This process makes the cat appear larger than it actually is, possibly enough of a physical change to make predators think twice before attacking. Along with other fight or flight responses, goose bumps are an important part of a lone cat's survival.
Piloerection is also beneficial for animals when they are exposed to extremely cold weather. Goosebumps extend the hairs on a cold animal, creating a thicker coat that offers more protection. Erected hairs provide extra insulation as they trap heat more effectively than a fur coat in its normal state.