What is the purpose of goose bumps?
Dear Straight Dope:
What is the biological function of goose bumps?
It's a skin level response, trying to make a sweater for you, sort of.
What we call goose bumps (the physiological response, not the R. L. Stine books) is the body's attempt to retain heat. Technically known as piloerection, the bumps are caused by a small muscular response, which raises the hair on one's arms or legs (most heat loss is here) and creates a mat of hair, which traps a layer of air next to the skin. This layer holds the heat next to the body, exactly the way a down vest does. Trouble is, human beings no longer have the thick coats of hair that their ancestors did, so the insulating response is no longer as effective as it once was. We still have the muscles that make this try to happen, but we no longer have enough fur to make it work. You can see this same response in animals, especially the common house cat. When a cat is cold or frightened, its fur will stand up due to the same muscular response. It is still a useful trait for them, as they still need to conserve heat, and by increasing their overall volume, they can often frighten away predators much larger than themselves.