This is for a book I’m writing where mermaids and sirens are more of a natural plausible predators instead of magic, so i more want to know the movement of a animal having a certain tail aswell as the possibility of a mammal having it.
In: Biology
So in real life – fish and sharks are vertebrates (have spines) which evolved in the sea. Never evolved legs. Just like a snake slithering side to side the motion of the whole body made sense so the tails that move side to side – so vertical tails.
Sea mammals (whales, dolphins, walruses, otters etc….) come from walking land animals that went back to the sea for food. Ended up swimming so much they they re-evolved tails. Just like it is easier for you to kick up and down when you swim instead of side to side they evolved an up-down motion to move thru water – thus a vertical tail.
If your fantasy world has evolution, and the mermaids evolved from humans, then they should probably have sideways tails. However, the would also then not have scales, as their is no reason for mammals with hair to re-evolve scales (mammals already have great water-shedding hair, check out the otters).
If your fantasy world is where insane wizards combined a fish and human to magically make a mermaid -then the tail with be vertical, as that is how a fishes spine would line up with a humans when you patch them together.
Typically, fish have vertical tail fins, and mammals have horizontal ones. I don’t think there’s any specific reason to prefer one over the other, it’s just it would be hard for evolution to make the switch.
There are other possibilities, eg you’ll see sea slugs move by rippling their side “fins”, or squid (and, I suppose, jellyfish) propelling water directly backwardss
Swimming fast with a vertical tail means rocking the body from side to side. A horizontal tail means swinging it up and down. Mammals find it easier to flex their spines forwards and backwards (ie, up and down) rather than side to side.
Basically, fish have vertical tails. Long ago, they started to move onto land and evolved feet over time and eventually into mammals. Then, some of those mammals with feet went back into the water, where over a long time, the feet evolved into horizontal tails.
So, in regards to your mermaids, it stands to believe they are some form of mammal and would have horizontal tails, even though they typically are depicted with fish like scales.
Ok, so I’m going to give it a go, fish have evolved a laterally/horizontally flexible spine, that later translated to amphibians and later still to reptiles, it makes sense fish have vertical tail fins because that way it’s perpendicular to the direction of the motion of the tail, of the spine, if you look at modern legged reptiles, they all have legs splayed out to the side of their bodies for the same reason, their spines are especially flexible laterally, if you look at a lizard running you can see how it moves it’s body side to side,
Now mammals on the other hand have evolved vertically flexible spines, mammals generally have better range of motion and stronger movements of the spine on the vertical axis, you can also see that mammals have legs underneath their bodies and subsequently (since all marine mammals have evolved from land mammals) marine mammals retain that vertical flexibility hence evolved horizontal tail fins also perpendicular to the direction of the motion of the spine.
P.S.: there have been exceptions: dinosaurs, genetically far removed from modern reptiles, yes, but still technically reptiles, all(or vast majority) had legs under the body and vertically flexible.
Crocodilians, non alive today but there have been many crocodilians that have evolved to have more vertical spine flexibility, land crocs really, see: Kaprosuchus.
Cheers!
One intuition is that fish tails are really tails, like dog tails or cat tails, so they have tail-style muscles attached to them that move them in tail-style ways. But whale tails aren’t really tails, they’re legs-and-feet that are acting like tails ‘cuz the evolutionary path was easier that way.
Think of a tail like a rudder on a boat. It helps steer you in the right direction! Animals with horizontal tails (like fish) use them to move side-to-side, darting through coral reefs or chasing schools of smaller fish. The tail helps them change direction quickly and makes them super agile.
On the other hand, animals with vertical tails (like sharks) use theirs more like a big propeller. They swing it back and forth to push themselves forward, kind of like how a surfer uses their board to catch waves.
Having a horizontal or vertical tail also affects how well an animal can maneuver in different environments. For example, fish with horizontal tails are great at navigating through tight spaces, while sharks with vertical tails might be better suited for open ocean swimming.
As for mammals having these types of tails… it’s definitely possible! Some animals, like the platypus and echidna, have a kind of “duck-billed” tail that helps them swim. But if we’re talking about mermaids or sirens being natural predators, they might evolve to develop more fish-like features, including a horizontal or vertical tail!
Think airplanes – a big whale dont need to move very fast. So they can just geently move their big – but heavy, horizontal fins slowly and that gives them forward motion for a certain amount of energy. A fighter jet (shark) needs speed – so they have as little resistance as possible, but also need to exert a lot more energy to get to those speeds.
I believe that sea mammals have horizontal fins due to the shape of the bones from ancestor land mammals, i.e., sea mammals developed from land mammals that went marine.
Latest Answers