|MadSci Network: Anatomy|
Good question! First, why do we get goosebumps? Well, goosebumps are a response to the cold. In the distant past when our ancestors had a lot more hair then we do, one response to being cold was to raise the hairs on their bodies. This formed a network of hairs over the body. The air between the hairs could not move freely and so it became warmed by the body heat and formed an insulating layer around the body. This response is still used by hairy animals today to keep warm.
Nowadays we are (mostly) less hairy than our ancestors. We still respond to the cold by trying to raise the hair on our bodies though, and a goosebump is just the place on the skin out of which the hair grows-it becomes swollen when the hair rises, so we can see it.
Now, why do we not get goosebumps on our faces? There are probably two main reasons. First, the hair on our faces is different to that on arms or legs, it is shorter and rougher and of course in women, there is a lot less of it. So there are fewer places where the hair comes out of the skin and it doesn't rise up in the same way. This makes the goosebump much less noticeable.
Second, we have a very strong blood supply to the face and head. If you have ever cut part of your face, you've probably noticed that even a very small cut here can bleed a lot. This blood supply helps to keep the face warm even when outside it is quite cold. One response to the cold is to reduce the blood supply to the skin surface in other parts of the body, but you know that when we are cold, our cheeks are often bright red. It seems that we have evolved to have a good supply of blood to the head at all times, even when cold, because this is the location of our most important organ-the brain. That way, we can think about ways to get warm again!
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