Have you ever noticed "goose bumps" or the hair on your arms "standing up" ? When does it happen and why?
If you pay close attention, you would see that we get goose bumps when it is chilly. Therefore, this has to be a mechanism to regulate our body temperature and warm us up. Does it happen only in humans? Not at all! This mechanism of keeping warm is common to many mammals.
The principle behind the mechanism is that tiny muscles at the base of each hair, known as arrector pili muscles, contract and pull the hair erect. As the hair "stand up" a layer of insulation is created trapping in the heat and not letting it escape, warming us up.
Sometime in the distant past, this mechanism was extremely useful to humans since we used to have a lot more hair and a lot less clothes. The "goose bump" mechanism also applies to stressful or "fight or flight" situations
where the sympathetic nervous system is activated and adrenaline is released.
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