Goosebumps are the result of piloerection, a temporary raising of the hairs on the surface of the skin that occurs when the piloerector muscles contract. These tiny muscles are attached to the individual follicles from which each hair arises. Piloerection is a voluntary response directed by the sympathetic nervous system (the one that triggers the "fight or flight" response), and is elicited by cold, fear or a startling experience. Goosebumps have two functions that serve little purpose to less hairy, modern-day humans. One is to keep us warm, which they don't do a good job at on humans because we're not furry, Roach, a physician from Canada says. For example, cold weather can trigger piloerection in mammals — as well as birds — causing their hair (or sometimes their feathers) to stand up and then reset. This action can create a layer of air underneath the animal's fur that helps insulate their bodies from the cold temperatures. Piloerection also occurs when animals perceive a threat is near. In this situation, when the piloerector muscles contract and cause the hair to rise, it creates a "fluffed up" appearance that makes the animal seem larger and may help deter an attack by other animals. As a result, goosebumps serve no real purpose in humans since we evolved to less hairy creatures.