Why do our hairs stand on end when we’re cold?
When we’re chilly, tiny muscles contract at the base of each hair to make them stand on end, distorting the skin to create goosebumps. All mammals share this hairraising trait, called piloerection, of using hair or fur to trap an insulating air layer. The process may have helped to keep our hairy ancestors warm, but today’s human body hair is too fine to be of much use. Shivering does a far better job of warming us up through rapid muscle contractions.