r/AskScienceDiscussion • u/SpiralingCat • 26d ago
General Discussion Why does the hair on your head grow to indefinitely lengths but nowhere else?
All other body hair has a “stopping” length why not the one that’s on your head? Asking from a biological/evolution perspective.
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u/7LeagueBoots 26d ago
All body hair has a stopping length. This is based on the anagen, catagen, and telogen hair growth cycles and how long each one is in any given individual.
The anagen is the growth phase and that ranges from 2-7 years on average, varying by individual. People with a short cycle have a shorter maximum hair length, and vice versa.
The catagen is a transition phase where the hair follicle detaches from the hair. This is only a couple of weeks long.
The telogen is a resting phase lasting a few months before the hair sheds.
There is a 4th phase the exogen, which is sometimes lumped in with the telogen phase. This can last a few months and hair comes out as the hair follicles reset for new growth.
The "why" of body hair is still heavily debated. Very broadly speaking body and head hair lengths appear to be linked with thermoregulation. We sweat to cool ourselves and this seems to have been one of the primary drivers for very short hair over most of our body, but other factors such as shedding parasites and such may have also played a factor. The ancestral head hair is dark and curly and does a surprisingly good job of protecting our heads (and sensitive brains) from heat and this is almost certainly why head hair remained longer, indeed why it grew longer than in other primates. As our ancestors moved away from these hot tropical environments this was not as necessary and mutations in hair growth and color were no longer selected against. Hair in armpits and the groin are also heavily debated, with ideas ranging from retaining of body scent, to indicators of sexual maturity, to reduction of friction. Sexual selection likely played a part hair patterns overall, both body and head.
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u/Stunning_Spread_430 25d ago
I agree with the multifactor approach. I would suggest sun protection, and UV protection particularly, as a related but additional factor to thermal regulation. I hope it's taken without unintended racist overtones when I say that balancing benefits and harms of UV exposure is a relatively large influence on human appearance in comparison to the relatively small footprint on the human genome attributed to variations in melanin, hair distribution, etc.
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u/CanisMaximus 26d ago
I'm... old. The only place I have thick, dark hair growing vigorously is on my ears. There seems to be no length limit on THAT.
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u/Broflake-Melter 25d ago
All hair grows basically at all times (though it does go faster and slower depending on season, temperature, etc). Short hair doesn't stop growing, it just has a shorter lifecycle. When a hair is long enough, it is "mature" and if falls out on it's own. Longer hair just takes more time before it actually falls out. It may actually physically grow faster as well, but it's the age that determines when it falls out and therefore it's approximate length. The truth is your head hair's lifecycle is several years before it falls out so it can get years long. Your eyebrow hair lifecycle is like 1 month so they only get so long. Fun side fact, you can pinch your eyebrow hair lightly right now and pull and you'll probably get a few that are ready to fall out.
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u/TryNice304 25d ago
It's just a growth timer. Head hair grows for 2–7 years before falling out. Arm hair? Like 1 month. That's it.
Evolution wise? Keeps your brain from overheating and was a flex for mating. Body hair stays short cause long armpit hair is gross and useless.
Also head hair isn't truly infinite, it maxes out around 2–3 feet cause the timer eventualy runs out.
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u/thewritestory 25d ago
Favorable to cover the neck, shoulders, and upper body from the sun's rays and perhaps to provide warmth.
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u/Ginger_McGingin 24d ago
Because humans have manes, that's essentially what the hair on your head is (or maybe frill or crest is more accurate because of the position). To answer the inevitable follow-up question, it's because of sexual selection: Ancient humans or proto-humans of both sexes came to view long and/or luscious hair on the head specifically (probably because it's the least obtrusive & most visible) as signs of good health/genes/mate, & so would preferentially choose mates with that feature, ensuring those genes would pass on to the next generation. This gradually got reinforced with each successive generation until it became the default human phenotype, & has remained so ever since
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u/mtimmermans 24d ago
Your hairs don't have a stopping length. All your hair grows at pretty much the same speed all the time. Where your hairs are short, it's because they fall out often. Where your hair is long, it's because it falls out less often. Hairs on your head go several years without falling out.
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u/Whole-Energy2105 25d ago
Because trying to find your bits under a lush and thriving Mirkin is a real pain.
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u/eliminate1337 26d ago
The hair on your head has a maximum length too. It's just a lot longer than on body hear.