Just the thought of something really scary is enough to make the hair on the back of your neck stand up… literally. The erector pili are tiny muscles at the base of hair follicles that give us “goose bumps.” The reflex goes back millions of years to our furry ancestors. When the muscles contract, hairs stand up, making the coat thicker and warmer. The effect also suggests a larger appearance that can potentially impress competitors or scare off predators. We don’t have thick fur like our ancestors, and our strategy for tens of thousands years has been to use the fur of other animals. It’s ironic that an animal, sensing danger, would puff up to look scarier. The human hunter would see the puffier coat as a warm prize, leaving the thinner-haired, weaker-looking animals alone. Of course, some body hair is helpful to humans; hair on the scalp offers protection and heat retention, while eyebrows keep sweat out of the eyes and can signal moods and emotions. Hair in the armpits and pubic areas provides a warm and moist environment, perfect for the fermentation of odors that can at once attract, and repel, our companions. Facial and body hair influence one’s choice of sexual partner, one way or another. Mammalian hair is analogous to the feathers and scales of reptiles, and serves some of the same functions: protection, temperature control, camouflage, and social signaling. Consider the messages broadcast by the wide range of popular hairstyles: from ponytails and big hair to flattops, buzz-cuts, Mohawks and punk spikes.