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Mermaids are once believed to only exist in mythology or in literature but today they numbered over 9000! They are found thriving in many parts of the ocean especially the warmer water, they have head of a human except for serrated teeth which can tear seaweeds and fresh from animal easily. Then all mermaid grow hairs on their body and just like us human the hair is thick and long on the top and back of the head... What for since they almost never leave the water?

Edit: The mermaids have gills to breathe in water and also similar to a lungfish, it's powerful biological adaptation allows it to draw oxygen straight from the air and into the lungs!!!

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    $\begingroup$ Do mermaids breathe as humans do? Because you sort of have to leave the water (at least your head does) to do that. $\endgroup$
    – Cadence
    Commented Jan 15 at 8:58
  • $\begingroup$ What is the setting? Is this a hard sci-fi, soft sci-fi, fantasy, etc? $\endgroup$
    – Aos Sidhe
    Commented Jan 15 at 22:33
  • $\begingroup$ since we have no idea what the anatomy or evolutionary history of your mermaid is, nor anything about the setting, this is pure opinion. $\endgroup$
    – John
    Commented Jan 16 at 1:59

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An easy explanation is that it's a vestigial organ, memory of when their ancestors were still land-dwelling animals.

More or less as our own body hair is a memory of when our ancestors had a thicker fur.

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Generally when an animal has traits that don't make a lot of sense it's because of sexual selection. So many reptiles have frilly bits, some birds have feathers which actually are detrimental to getting around efficiently etc,. Often there are features that don't have a use except that they attract mates.

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  • $\begingroup$ while yes sometimes traits factor into sexual selection that are actually detrimental, usually the attraction to that trait stems from it being originally beneficial (or linked to a beneficial trait) and have just lost their usefullness over time either through changing environement or through the trait surpassing its "point of maximum benefit" $\endgroup$
    – datacube
    Commented Jan 15 at 15:58
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    $\begingroup$ @datacube a lot of the time it is because being able to (biologically) afford to spend energy on useless luxories shows that you are doing well. Many years ago being fat was considered good, because it shows you could afford to eat a lot. Animals are similar. $\endgroup$
    – SeriousBri
    Commented Jan 15 at 21:47
  • $\begingroup$ @SeriousBri fat being attractive in times where food was sparse is a excellent example of "linked to a beneficial trait". In this case the beneficial trait is "being able to acquire a lot of food". $\endgroup$
    – datacube
    Commented Jan 16 at 7:27
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Could be a lot of things.

Really long hair could serve as natural camouflage, allowing a mermaid to hide among kelp and seaweed more easily.

If your mermaids hunt humans the way the mythical ones do it's possible they developed a more human-like upper body and hair because it allows them to mimic their prey better. Long hair especially would help hide their features until a human got within reach.

Hair also might make it easier to sense currents in the water around them, even if it was on the shorter side.

The 'hair' could be something more like jellyfish tentacles, stunning animals that come in contact with it.

Or it could be a defensive adaptation, making them look bigger than they really are and/or detatching when a predator gets ahold of it like a lizard droppong it's tail.

Or it could have no purpose. Genes don't have to be useful to get passed on, they just have to not hurt your chances of reproducing too much. Assuming your mermaids weren't created by gods or wizards or something.

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Maybe the mermaids had quite inefficient body form with regards to swimming, with head and shoulders, and the quick evolutionary adaptation is that they have long hair which make their swimming significantly more hydrodynamically efficient. They form a smooth surface between head and shoulders, like a sea lion will have.

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Perhaps the hair is actually their gills?

Oxygen is not very soluble in water. One liter of water can dissolve about 22 micro-moles of oxygen gas. For comparison, if I've prodded WolframAlpha correctly, one liter of air contains about 9200 micro-moles of oxygen. That's over 400 times as much!

Due to this, I'm not entirely sure that it's possible for a set of gills that fits within a human chest cavity to extract enough oxygen from water in order to keep a human body and a human brain from asphyxiating. I don't have hard science to back up that hunch, and I could well be completely wrong about it if I've grossly overestimated how much oxygen a human body actually needs and how much water can be pumped through a set of gills, but... it's enough to make me doubt that water-breathing mermaids with internal gills could be viable.

However, there is a workaround: External gills! Mermaids' "hair" could actually be a veritable forest of external gills that could pass for voluminous hair at a glance. They'd likely live either in rivers or in tidal estuaries, where they can cling to the bottom and let the current wash fresh, oxygenated water over their gills.

Those gills would produce a lot of drag when the mermaids swim- they would not be able to swim very efficiently, and would have a hard time swimming fast. They might even drown if caught in the open ocean- swimming under their own power may consume more oxygen than it allows their gills to gather.

Perhaps they might be able to flatten their gills against their bodies, making themselves more streamlined at the expense of not being able to breathe. Then they could dart from one place to another when they've consumed all the oxygen in their immediate vicinity. This, plus being able to dramatically slow down their metabolism when sleeping, could allow them to survive in the open ocean.

I suspect these mermaids would have a hard time competing against, say, sharks or dolphins. Or, for that matter, against fully-mammalian air-breathing mermaids whose hair is, as other answers have suggested, a vestigial remnant of when they lived on land.

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Either a case of simple sexual selection, especially if they reproduce cleptogenicly by mating with male human sailors or it's part of the 'lure' (they're bodies from the waist up) used to entrap their favourite prey, again, male humans.

In the later case both male and female mermaids all look like female humans from the waist up, but aren't all female.

[If you really want to be freaky it can be both, they're all female, they mate with them then eat them]

Either case can also explain the mammalian breasts, or what appear to be mammalian breasts.

We really need more detail about your mermaids though.

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