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It turns out that a single strand of human hair measures 17 to 181 millionths of a meter wide. But anyone who creates or reinvents a fictional species of humanoid has the freedom to change the dimension of the head, including the size of a strand of hair (if they choose to still have hair.)

So in an alternate world, we share that world with another species of Homo, one that still has hair on its head, only that each strand now measures 1/2-1 inch in diameter. The question isn't what the hair would look like--I'm imagining a bunch of leaflike lobes--but what kinds of adaptive advantages such hairs can have for the wearers?

Oh, and by the way, the hair in the question is focused on the head, nowhere else.

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    $\begingroup$ It hair gets ridig (like porcupine spikes), it could be beneficial as protection. Flexible hair this thick could be dumping ground for toxic chemicals that the body cannot process $\endgroup$ – Bald Bear Apr 3 '18 at 16:27
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    $\begingroup$ Hair is made of the same stuff of your nails and one thick example can be a rhino horn $\endgroup$ – jean Apr 3 '18 at 20:28
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    $\begingroup$ At that point it becomes a horn. 1" diameter keratin will not bend. $\endgroup$ – Mazura Apr 4 '18 at 0:52
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    $\begingroup$ 1 inch wide, leaf-like lobes? I believe you are describing feathers. $\endgroup$ – Shane Apr 5 '18 at 3:39
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    $\begingroup$ In any case, leaves look like feathers. In the question, you said hair that looks like leaves. That's (basically) feathers. Hair is a single strand of keratin. If it is an inch thick, you'll get quills or horns like everyone else said. If you want this keratin to look like a leaf, it will be more feather like than horn like. $\endgroup$ – Shane Apr 5 '18 at 4:30
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Baby Porcupine

You would end up with a head full of horns or quills.

Human hair is made from keratin. It's the same with your fingernails and toenails. In fine form, hair is soft and flexible. With thicker amounts of keratin, it becomes stiff.

In the animal world, most horns have a bone core covered with a thin sheath of keratin. Since we lack boney protrusions on our heads, humans with thick hair might end up looking a bit like a porcupine or a hedgehog. with long hairs covered with keratine. Or if they started to clump together, our heads could end up looking like a rhinoceros horn, which is made of entirely of keratin.

With a head full of quills, humans have another form of defense. Ramming people with your head could cause painful injuries. Imagine tangling with a human sized porcupine.

If it formed like a rhinoceros horn, it would act like an helmet, further protecting the skull. It would become a defensive and offensive weapon.

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    $\begingroup$ that picture got my vote $\endgroup$ – Reed Apr 4 '18 at 13:33
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    $\begingroup$ "In the animal world, most horns have a bone core covered with a thin sheath of keratin" - you really need to discuss horns vs antlers here. Horns (as in rhinos) are bone; antlers (as in deer) are keratin. $\endgroup$ – Rand al'Thor Apr 4 '18 at 14:19
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    $\begingroup$ @Randal'Thor a quick look at wikipedia suggests you've got that completely wrong! : ) $\endgroup$ – Grimm The Opiner Apr 4 '18 at 15:34
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    $\begingroup$ Antlers are made of bone. @JohnWDailey was asking specifically about hair, not bones. So I addressed how I feel human hair would behave if it thickened to a diameter of 1 inch. From personal experience, I found that when humans have hairs that grow that are much thicker (10mm), they tend to act as a small horn. $\endgroup$ – gwally Apr 4 '18 at 17:09
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    $\begingroup$ "In fine form, hair is soft and flexible." - And yet, still stabby $\endgroup$ – Izkata Apr 4 '18 at 22:05
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I see some serious disadvantages:

Such hairs would fold and crease versus bending, leading to hair snapping off and/or major split ends much more readily.

Such hairs would, of structural necessity, have much thicker walls, leading to a massive per-hair weight; as a result, hair follicle depth would need to be far larger, leading to a monstrously deep dermis & epidermis.

Deeper hair follicles would demand a far thicker skin total on the head, leading to possibility of actual injury from hair pulls, yanks etc.

If such hairs were not basically circular in cross-section (OP's leaflike lobes) they would be quite weak across the shorter sectional angle.

However, those considerations aside, were these super-thick hairs quite short and numerous, they might have enough resilience (due to the space inside the shafts) to act in aggregate as a sort of natural helmet, absorbing the first portion of mechanical shocks to the head. The caveat to that would be that should the shock exceed a certain velocity and power density, the follicles in the deepened head skin would tear, leading to haematoma, bleeding, risk of infection etc, though even that might still be preferable to the alternative: sub-dural haematoma and TBIs which significant head traumas almost always risk.

enter image description here

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    $\begingroup$ "hair follicle depth would nee to be far larger" perhaps leaving less space for the brain pan if he wants to retain a similar head size & dimensions to "normal" humans, half an inch thick follicles, that'll have a hell of a root to it :) $\endgroup$ – Pelinore Apr 3 '18 at 19:27
  • $\begingroup$ Well more to the point, the total depth of the dermis-epidermis layers needed to support a hair that big would be 6"+ I think - see the appended image on my answer now that I've edited it! $\endgroup$ – GerardFalla Apr 3 '18 at 19:36
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Ah! Fun question.

As mentioned above, hair has a wall to it. If you look at magnified hair pictures, you can see that it looks fairly scaly. As hair grows longer, the hair kinda flakes off in tiny bits and the hair gets thinner. If you had wide follicles, you could imagine you might get something like more flexible fingernails that hang off the head. Having the bed of the hair be angled like fingernails are, you add strength without making the dermis deeper.

As the hair grows long, it will likely split and break in ways similar to how nails do. Different grains may evolve to allow splits to happen more laterally, but it will likely not be even or pretty when it does happen. If not cut, the hair may have frayed looking ends as if you took a weed wacker to a leafy bush. There are many ways this hair could evolve. It may be easier to identify the conditions of it's evolution and work forward. I imagine a species that lived in sandstorm conditions and needed hair that could both protect and not tangle. A species like that may develop an oil that helps protect the hair and keep it stuck together more. In that case it may look more like scales than hair unless they just got out of the shower.

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As @GerardFalla mentioned, the walls of the hair would need to be pretty thick. So, think of circular fingernails growing out of the head. What could be the benefits and issues of such?

More than a eighth of an inch long (.25 cm) and it becomes pretty brittle unless there is a supporting, living medium inside. If they are dense enough, they would support each other and operating like overlapping armor. The scalp would have to be at least a quarter of an inch thick (probably closer to half an inch) for the follicle of the hair. If the hairs were not dense enough to support each other, then to get any kind of length, there would have to be living tissue inside the hair. That would mean that any hair that breaks off too far from the tip would bleed (like trimming a dogs claws too far).

All that thick hair would be very heavy and would require thicker neck muscles.

Also, one of the main reasons for hair is to provide evaporation cooling. The brain is one of the big heat generators of the body. Unless the people lived in a cold environment, they would cook their brains without some other means of cooling their heads. I suppose you can use active cooling by running blood vessels up the hairs and use them as radiators but hair breaks would cause much more bleeding.

So you could bet the benefit of armor and/or spines but at a pretty high cost.

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  • $\begingroup$ Head hair, unlike shorter hair elsewhere on the body, does not facilitate evaporation cooling. Ask anyone who's recently shaved their head! The large quantities of hair on our heads mainly provide protection from the sun and a certain amount of trauma, and overall act as insulation, not radiators. $\endgroup$ – Logan Pickup Apr 4 '18 at 9:51
  • $\begingroup$ @LoganPickup, it depends on the type of hair. True, straight hair blocks the wind and acts as an insulator. However, very curly hair offers lots of wind channels for cooling sweat that wicks up the hair. If the very large hair has spacing, it would provide the same function. $\endgroup$ – ShadoCat Apr 6 '18 at 0:06
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I picture this along the line of large fish scales, which are keratinous like hair.

If they were leaflike or roughly oblong in shape and say, 2-3mm thick, they would have a pretty significant armoring effect on the head especially if the skin underneath had some fat and muscle layers...could they make it e.g. stand up for emotional signaling? Mating displays?

I'm also guessing it would hurt like hell if you scuffed your head and the hairs/scales were pulled straight out away from your skin or bent back on themselves.

That large a mass of keratin would require a lot of nutrients to produce, you'd need to provide some kind of evolutionary benefit. Heavy metal sequestration?

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The most practical way of implementing this is to make it hollow.

If it's not hollow, then it would be very heavy and inflexible; it would end up being more like a horn or a claw than a hair. It would also require much stronger structures at the base to keep it fixed to the head.

Making it hollow gets around these issues. It would still be fairly rigid, but not totally inflexible. The end result would be something similar to a porcupine quill or the central shaft of a feather.

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