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Q&A

Why would merfolk have hair?

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A classic image of mermaids are their long flowing hair that swishes in the water. But in the scientifically realistic sense this makes none. The hair would be nothing more than a nuisance to the mermaid getting tangled on rocks and offering handles for potential predators, so why would mermaids evolve hair?

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This post was sourced from https://worldbuilding.stackexchange.com/q/42617. It is licensed under CC BY-SA 3.0.

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9 answers

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If the merpeople's babies are relatively helpless or start as weak swimmers, head hair could remain as a useful point for the young to cling to as the adult swims around, keeping its arms free. This would be analogous to other ape young which cling to their mother's body.

For the purposes of worldbuilding, you could run with this idea without needing to excessively dirty up their hair with ecosystems of symbiotes nor tying their lifestyles to humans in any way.

I can't claim it as my idea, nor can I find where I first heard it, but it was definitely in the context of the aquatic ape hypothesis.

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Ask yourself, why do humans have hair? Some ancestor of ours lost most of the hair on its body-- and men sometimes go bald. So why do humans have hair? It's probably because of sexual selection.

Many swimming and sea-going mammals such as beavers, sea otters and seals have maintained their fur. Maybe a mammalian mermaid would keep hair on its head, like modern humans have?

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This post was sourced from https://worldbuilding.stackexchange.com/a/42672. It is licensed under CC BY-SA 3.0.

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Though maybe not the best solution, Depending on the environment, the Hair might just not be enough of a menace to be evolved away. Humans have advanced to such a degree that even heavily disadvantaged, we can still thrive. If the reasons to not have hair are simply for Speed and such, then if they with hair already move faster than they ever need to, there is no reason why it would disappear.

Though I cant list off anything with certainty, Humans definitely have Disadvantageous Genes and traits which get passed on anyway, because a man born without legs can still get around and live a very fulfilling life using only his arms and a wheelchair. Technology and being the dominant species overcomes many of the minor evolutionary disadvantages to the point where they still exist even today. People with Dwarfism can still have kids who also have Dwarfism. Merfolk could be in the same situation.

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This post was sourced from https://worldbuilding.stackexchange.com/a/42675. It is licensed under CC BY-SA 3.0.

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It just looks like hair; those are actually tentacles. Merfolk are not descended from fish or mammals, but are a type of Cnidarians, who use stingers in their tentacles to stun and kill prey.

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This post was sourced from https://worldbuilding.stackexchange.com/a/42824. It is licensed under CC BY-SA 3.0.

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A mermaid that "realistically" evolved would look very different from the traditional mermaid in many ways. But let's assume that your mermaids do look like the classic type. Why do they have hair?

Option one: Sexual selection
Your mermaids evolved from a human-like species. The hair was so important to their mating process that it remained even as other body parts changed to adapt to the new environment.

Option two: Obfuscation
As other answers have elaborated, the "cloud of hair" might be able to hide the mermaids from predators.

Option three: Symbiosis
The mermaids cultivate some kind of ecosystem in their hair. Small fish hide in it, some plants grow in it. This could have a variety of uses:

  • small fish help drive away parasites
  • plants could improve the water quality around your mermaids
  • they could act as a food source - the small fish attract bigger fish, which the mermaids eat

Option four: Mimicry
Your mermaids are very specialized predators that try to attract human sailors and feed on the content of their ships. Or, more realistically, they are parasitic on humans in some other way -- because evolution takes a long time, and ships are a relatively new invention. Either way, selection favored those mermaids that look like attractive women, and that includes the hair. Maybe mermaid society has different classes, like you get with ants, and most remain underwater while only the "man hunter" class grow hair.

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This post was sourced from https://worldbuilding.stackexchange.com/a/42649. It is licensed under CC BY-SA 3.0.

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Other than for hiding,sexual selection ,make the merfolk look bigger or making people think they are humans

Filtring plankton is an option too. The hair could also mimic some plants to trap the fish who eats them. could also work as glands capable to produce substances that could be disturbing for predators.

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In a fictional world with merfolk, the hair could not be a part of their body at all, but some kind of algae or other organism that would look like hair to a human or humanoid observer from another species. The symbiote would simply prefer to attach to the merfolk's head and not other body parts due to one motive or another.

Bonus saddistic points if the symbiotes in question are coelenterata (i.e.: related to jellyfish). That mermaid looks so beautiful and gorgeous, let me fondle her hair... Seriously, though, this would add some level of protection against predators. Think of hermit crabs and anemones. Except the anemone looks like a wig and hermit crab is a half-mammal-half-fish creature.

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This post was sourced from https://worldbuilding.stackexchange.com/a/42819. It is licensed under CC BY-SA 3.0.

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Maybe the hair is vestigial? Depending on the evolution of the mermaids it could have served a purpose before.

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This post was sourced from https://worldbuilding.stackexchange.com/a/42660. It is licensed under CC BY-SA 3.0.

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Long hair is inherited from the common ancestors of humans and merpeople.

In-hair-itance

As whales evolved from an ancestor of the hippopotamus and manatees from an ancestor of the elephant, merpeople evolved from an ancestor of humans. Thus merpeople are a hominin species with aquatic adapted physiology and joined legs.

DJMethaneMan's answer to "Why would merfolk evolve arms?" suggests that a population of early hominins was forced to adapt to an aquatic lifestyle after being marooned on an island during deglaciation. This is reminiscent of the aquatic ape hypothesis promoted by Westenhöfer, Hardy, and Morgan, which proposes that humans' ancestors led a semiaquatic lifestyle. Humans have far less body hair than chimps, for example, and what hair they do have follows the flow of water over the body.

Taking the adaptation a step further are creatures in Scottish and Irish folklore known as selkies. On land, they appear human; in the sea, they wear sealskin swimsuits that men of the land tend to steal. Selkies have physiological adaptations akin to those of Michael Phelps and Gudlaugur Fridthorsson but can interbreed with humans. Merpeople have adaptations along similar lines but to an even greater extent, though like seals and dolphins, they'd still breathe air.

Occasionally humans are born with a limb difference called sirenomelia, in which both legs are fused into one hind limb. This limb has two femurs, four lower leg bones, and ten toes. In humans, it's associated with defects elsewhere, and few with with the condition survive infancy. (A photo of one survivor named Milagros Cerrón can be seen as a transitional form between humans and merpeople.) But among merpeople, sirenomelia is normal and beneficial, as it eliminates turbulence between the legs when performing a dolphin kick. So over the generations, having what amounts to one thick leg became fixed in that population.

Benefits

So if split legs were selected out of the population, why hasn't hair also been selected out? Other answers explain several reasons for retaining it, which I'll summarize:

  • Sexual selection: Mermen still find long hair attractive. In fact, if long hair is as maladaptive as some claim, it may invoke the handicap principle in the same way as a peacock's tail feathers.
  • Mimicry: Attracting human sailors in order to cause ships to wreck and then plunder the ship's supplies.
  • Obfuscation: Disguising themselves in the water by creating a "cloud of hair".
  • Symbiosis: Kelp hair decorations attract sources of protein.
  • Providing raw material for fishing lines, nets, and other tools, with more tensile strength than steel.
  • Shading the head and neck while hauled out on land.

Any of these could explain the sighting of a mermaid using a salvaged dinner fork as an improvised "dinglehopper," their term for a hair pick.

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