$~~~~$Goose bumps are one of those fight-or-flight reactions of the sympathetic nervous system. When you experience cold or fear, a nerve reaction is sent to the muscles, which then contract involuntarily. This muscle contraction causes your body hair to stand erect. The skin with standing body hair 16 a goose’s skin after its feathers have been plucked—hence the name goose bumps.

$~~~~$But 17 is this helpful to us as a response to fear or cold? Back in the days when our ancestors may have had a lot more body hair, this might have helped to keep them warm or scare an oncoming predator. These days, however, goose bumps are 18 to human beings. Goose bumps, or the reflex that causes them, also occur in many other mammals. Many mammals fluff up their fur when 19 , to look bigger and thus more dangerous. For example, when a porcupine encounters a predator, its long sharp pointed hairs will stand up as a means of 20 . In animals with a thick hair coat, the rising of hair traps more air between the hairs, providing another layer of insulation to keep them warm.