You’ve got a chimpanzee lurking inside of you. This might come as a surprise, I know. But you might as well embrace it because whether you like it or not, your inner chimp will always find a way to reveal himself.

Under certain conditions, your sympathetic nervous system launches a cascade of seemingly useless hormonal and muscular reactions that betray your evolutionary history. Technically, it’s called piloerection, and if you read this blog regularly you likely know what this means. Others may know it as goosebumps.

Humans tend to get goosebumps under two conditions: when we are emotionally excited, most often due to fear or surprise, and when we are cold. Why does this happen? Our chimpanzee cousins have the answer.

Many animals have an automatic response to fear that involves making themselves look bigger than they really are – think about a dog raising her hackles, a cat arching her back, or a cobra unveiling its hood. Chimpanzees respond to danger similarly by unconsciously contracting tiny muscles around their hair follicles, thereby making their hand stand on end. Piloerection can turn a small, goofy chimpanzee like Burrito into a fearsome and formidable presence.

Piloerection, therefore, is the body’s way of protecting itself through intimidation. So what does it have to do with cold weather? In addition to making chimps appear larger, piloerection can also help to insulate them by trapping air between the hair and skin. It’s like changing out your sweater for a nice puffy parka.

In humans, piloerection is a vestigial trait. Since we split from our shared ancestor millions of years ago, we’ve lost much of our body hair but retained the biological response of raising what little hair we have left when our adrenaline gets pumping. But for chimpanzees, it remains a useful way to protect themselves from both cold temperatures and the dangers of physical conflict. So the next time you get goosebumps – whether it’s because you heard something go bump in the night or because you left your jacket at home – just think of it as your inner chimp trying to get out.

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