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The answer may give you goosebumps.

Brian May (Queen), Robert Plant (Led Zeppelin), Joe Elloitt (Def Leppard) are rockstars famous for their long locks. Contemporary country musicians like Keith Urban sport extended manes, and departed legends of music, Michael Jackson and Kurt Cobain, also had hair down to their shoulders. How many bald men that are pop superstars can you name? Not many, right? The reason may give you goosebumps.

I said in my TEDx talk that the brain, our organic computer,

evolved for movement, and therefore, as upright humans developed larger brains due to their ability to walk or run long distances, we lost fur to cool down our bodies. After all, computers need cooling systems. We didn’t lose hair because hair helps us to detect insects.

A few years ago, a study from the University of Sheffield, published in the Royal Society’s Biology Letters, set out to investigate the biology of blood-sucking insects. They wanted to see if bodily hair played any role in intercepting insects. It turned out hairs are our insect detecting cutaneous antennae. In a study on 29 people recruited off Facebook, researchers planted bed bugs on arms and monitored biting activity. It became clear that insects on hairy arms were sensed quicker than on shaven arms, with hairs serving as motion detectors. Further, insects on bushy arms took longer to partake of a human blood meal as hairy limbs hindered their ability to find a decent spot to bite into!

In our ancient societies hairy men would be more likely to host less parasites on their skin and were perhaps more desirable as mates. Men tend to be more hirsute than women on arms and legs and studies confirm that males get bitten by insects less than women.

Even though we humans have therefore evolved to be less hairy, either due to our evolutionary histories or societal habits, our hair follicles still contain tiny muscles—arrector pili—that pull hair upright giving us goosebumps especially when we are cold. This is similar to the standing up of hairs on a dog when it raises hackles at an intruder to appear larger and more threatening. Other mammals use these hair follicle muscles when cold to raise their fur, thereby trapping a layer of warm air just above their skin. But humans have lost this function, and therefore cannot voluntarily produce goosebumps on demand. So why do we get goosebumps? It turns out is it not purely due to these hair follicle muscles. It is a story of inspired evolutionary biology and organic symbiosis that make muscle-anchored nerves form connections with hair follicle stem cells resulting not only in the standing up of in response to cold, but also the generation of new hairs to keep us warm.

The standing up of hairs, technically called piloerection, is controlled by sympathetic nerves that supply these arrector pili muscles that contract to raise hair follicles during a “fight or flight response” when an animal is in danger. Hair follicles are a reservoir of stem cells. As I wrote in my book, Skin, a Biography (4th Estate, 2013), the ‘Sonic Hedgehog gene’ is one of the early genes expressed by placodes, which are essentially pre-buds of tissue. The Sonic Hedgehog gene—named by gaming-geek-scientists—has a crucial role in instructing dermal cells to form hair follicles.

If you are wondering how this gene got its name, Sonic Hedgehog, the gaming character, is essentially a humanoid hedgehog who curls up to form a ball and attacks enemies at supersonic speeds. In fact, in 2008 The Telegraph newspaper reported that revealed Sonic Hedgehog to be the most popular video game character in the world—ahead of Lara Croft (played by Angelina Jolie) of Tomb Raider. When drosophila, the fruit fly was genetically modified to lack this Sonic Hedgehog gene, it developed spines like a hedgehog.

It turns out that one needs all three components—sympathetic nerves, hair follicles and their interconnections—to work in unison to produce goosebumps. The hair follicle, modulated by the Sonic Hedgehog gene links the arrector pili muscle unit with sympathetic nerves. This muscle unit maintains the nerve supply to the hair follicle stem cells. Therefore, sympathetic nerves can modulate stem cells through synapse-like connections using neurotransmitters such as adrenaline (epinephrine) and noradrenaline (norepinephrine).

Adrenaline, which in humans is produced in tiny bean-shaped glands that sit atop the kidneys, not only causes the contraction of skin muscles (and thereby goosebumps) but also influences many other body reactions. If adrenaline (epinephrine) is the fight-or-flight hormone, noradrenaline is the arousal hormone and a cousin of adrenaline. It is also fast acting and makes us hyperalert and vigilant. Noradrenaline is secreted when we think we sense a burglar and hence such situations can also give us goosebumps. When a drug addict withdraws from opiates, the bodily stress induces sympathetic nerve stimulation that leads to goosebumps, hence the expression “cold turkey.” Certain neurological disorders such as temporal lobe epilepsy can also cause goosebumps.

It turns out that the more musical you are, the more prone you are to goosebumps. Researchers at the University of North Carolina found in a study that folks who get goosebumps from music tend to be arty and more open to new experiences. Or you may be a high-octane heavy metal rocker with a stress response that is thrilled with the idea of living on the edge unleashing the muscles in your hair follicles.

Whether it is exuberant performances in front of rapturous crowds, or a recording studio scene, in the heat of musical passion, goosebumps are made.

Research published in the prestigious Cell journal reveals a new role for goosebumps: the muscles and nerve cells involved in this response to cold trigger new hair growth by activating stem cells. Therefore, the more prone you are to goosebumps—like rockstars and musicians—the more hairs you grow. Goosebumps—call them by their medical name cutis anserina or horripilation—are here to ensure a hairy end to a chilling or thrilling event.

Axl Rose (real name William Bruce Rose) the lead singer of Guns N Roses is another rockstar famous for his long hair. In fact, Vogue magazine even reviewed “The Best Axl Rose Hair Moments of All Time” in an article which stated: By the early ’90s, Rose’s strands had evolved into a sleek, salon-worthy blowout with an aura of hard-partying insouciance. Of course, it wasn’t just the hair itself that was so seductive, either.”

Goosebumps in other animals raise the body’s hair and would have made our human ancestors appear larger to scare off predators. But for humans this is a vestigial reflex but has led to hours of artful entertainment. But the newfound knowledge of the link between cold, goosebumps, and hair growth is leading researchers down new avenues into treatments for hair loss and baldness.

Slash, also from Guns N Roses, and famed for his long hair, has been quoted as saying about the opening riff of “Sweet Child O Mine”, one of Guns N Roses’ biggest hits, “It was an interesting pattern, and it was really melodic… I really just thought of it as a joke, but lo and behold, Axl was upstairs in his bedroom, and he heard it.” It was basically Slash fooling around, playing what he later called a “circus melody” and “guitar exercise” while he and some of the band members were jamming in the group’s communal home in Griffith Park, Los Angeles. Slash and Axl Rose are natural harmonizers of music, and we now know from research that those with a natural penchant for melodies are also goosebump generators, and more likely to have long hair.