I’ve noticed that when I’m really cold or sometimes when I’m scared, I get these tiny bumps all over my skin, which I think are called goosebumps. Why does this happen? What’s the biological reason behind it?
In: Biology
Your body is covered in hair. Every hair has a muscle (the arrector pilli) around the base of the follicle. [Each “goosebump” is the muscle around the base of one hair contracting, which makes that hair stand up](https://www.researchgate.net/publication/343051431/figure/fig2/AS:915950454988801@1595391014202/Sympathetically-innervated-arrector-pili-muscle-contraction-causes-goosebumps-a-visible.png) rather than laying flat like normal.
This is a leftover reflex from when our ancestors had much more body hair everywhere. If your body still had hair like that, [“goosebumps” would look like this.](https://www.shutterstock.com/shutterstock/photos/268930307/display_1500/stock-photo-the-cat-s-hair-bristled-up-when-it-saw-the-dog-268930307.jpg)
When you’re cold, the puffed up hair traps a layer of air against the skin, providing an insulating barrier layer to help keep warmer.
When you’re scared/threatened, being puffed up makes you look bigger and hopefully wards off whatever is threatening you. It also signals “hey, I see you and I’m prepared to fight”.
You can still see [cold animals puffing up their fur](https://s3.us-east-2.amazonaws.com/journeynorth.org/images/graphics/robin/randyindish/winter2_full.jpg) and cats and [dogs using their goosebump-produced raised hair](https://www.silentconversations.com/wp-content/uploads/2023/03/silentconversations_piloerection_comp2.jpg) today. Modern humans don’t have much body hair, so you see the muscle bumps and not the raised hair and it doesn’t actually do much. Now it’s just a reminder that we’re a bunch of naked apes, no so far removed from other animals.
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