While driving to Paris Junior College a few days back, I had a mixtape playing that my wife burned for me many years ago. It has classic ’80s rock, ’90s rap, old country music, and even a touch of Whitney Houston and Madonna. Whitney’s version of “I Will Always Love You” was on at the time and when she hit those high notes, I got goose bumps.
Just a few days later I found myself covering the cause of goose bumps in class while discussing the integumentary system. What are they, and what are they good for? Why do they seem to appear anytime you are scared, excited or cold?
Piloerection, the fancy name for goose bumps, is an involuntary response controlled by your sympathetic nervous system. It is a fight or flight mechanism and a quite common one in nature. Electrochemical signaling and the release of adrenaline via a sympathetic nervous pathway will cause a very tiny muscle to flex and make body hair stand on end. You have no control over this as it is fully involuntary.
The muscle involved is a small, smooth muscle known as the arrector pili muscle. It attaches to the base of your hair follicles and surrounding connective tissue in the dermis of your skin. When this muscle gets the signal, it shortens and up pops your hair.
So what is the purpose of this? In other species that have these same muscles, they serve two main functions. The fluffing of hair can be used to create a layer of insulation when it is cold outside or the fluffing can be used as a display mechanism, often a warning of some type. It is easy enough to see their action if you have ever scared your cat or seen two male cats in their pre-fight display mode. This is also how a porcupine raises its quills in defense or how the hair on the back of an aggressive dog stands on end. The great apes still use their goose bumps to display aggression, excitement and to insulate.
What makes these tiny muscles a vestigial organ in our species is the fact that they serve no useful purpose, well, any more that is. We are mammals and are still absolutely covered in body hair, around 5 million hair follicles. That peach fuzz we call body hair still fluffs when we get cold, scared or excited. However, this fluffing no longer warms you or frightens off your predators.
These tiny muscles are simply a leftover or vestige of the past. Vestigial organs and behaviors are quite pervasive. Mexican blind cavefish have eye sockets but no eyeball, a great many species living in the darkness of caves have lost the eyes their ancestors once had. There are also a great many species from insects to birds that have vestigial wings. Our pili muscles are like the eye sockets in blind fish, obviously present, yet functionally pointless.
Simply put, they are a reflection of an ancestral past. SciShow has a great four-minute video on YouTube titled “Why Body Hair?” if you get the time to watch it.