Hair Today; Gone Tomorrow

Matt in Phoenix said: I'd like to know why head hair grows to where you have to cut it and body hair does not? Why don't our eyebrow and eyelash hairs grow?

Thank you for the question, Matt. Hair appears only on mammals, and comes in many forms: hair on the heads of humans, whiskers on cats, bristles in a lion's mane and even quills on a porcupine. It contributes to many functions, including insulation, camouflage, sexual signaling, protection, and sensing the world around us.

In humans, ear and nose hair protects from dust; eyebrows and eyelashes protect against small particles and sunlight; body hair provides insulation. These hairs do not need to grow beyond a certain length in order to execute their function.

Human head hair is different. Hair provides a great deal of insulation (90% of body heat loss occurs through the head); so, up to a point, longer hair equals better insulation. But researchers also believe head hair plays a role in sexual attraction. Long, thick hair is a visible sign of health in the animal, which could explain why longer head hair is often viewed as being sexy.