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There are many depictions of sea-serpents and other similar creatures in a strange pose: The head (and often the tail too) is held above the water, with the body being arched in such a way that some of the body is out of the water. How and why would an animal hold itself in this way?

Diagram

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    $\begingroup$ comment because its a bit short for an answer. They don't. its an exaggeration of how snakes actually swim. $\endgroup$
    – IT Alex
    Commented Jul 13, 2021 at 13:41
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    $\begingroup$ @IT Alex - snakes swim with lateral movements. Mammals swim with vertical movements. $\endgroup$
    – Willk
    Commented Jul 13, 2021 at 16:21
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    $\begingroup$ This is not an answer, rather an explanation. These beasts were invented in medieval bestiaries. Often, the sea serpent is huge and has a dragon like head. Why would it be multi-arched ? well.. if you invent a spectacular water creature like this, you may as well show it off. When its body would be under water, it would look like any other Nessie. A medieval bestiary was intended for attribution of species. So my answer would be: it is multi-arched for aestetic reasons.. and to show all of it. $\endgroup$
    – Goodies
    Commented Jul 13, 2021 at 21:44
  • $\begingroup$ Not an answer to the worldbuilding question, but the reason why they are drawn that way might very well just be that this was the easiest way to fit a very long snake on a single page... $\endgroup$
    – sh4dow
    Commented Jul 14, 2021 at 6:23

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A few Thoughts:

There are a variety of reasons you can see sea serpents who look like this, so here are a few reasons:

  • Sexual display: Most sea serpents don't normally swim this way. In fact, you rarely see them at all. But they were once land animals, and reproduction is highly conserved. So males come out and perform this as a display for females, trying to impress them with how strong, buoyant and flexible they are.
  • optical illusion: From the shore or a boat, looking down, the serpent appears to be thrusting out of the water (and maybe they occasionally do, slightly) but in actuality, they are just doing the standard serpentine swim pattern common to all snakes. It just LOOKS like they are rising above the surface.
  • exotic swimming pattern: Perhaps the serpents have evolved a highly exotic swim stroke. Maybe they are really corkscrewing through the water. Perhaps their swimming style involves pushing internal air bubbles thought their boy, constantly changing location. but they end up thrusting out of the water.
  • exotic lung arrangement: These serpents are actually a kind of worm, and have multiple lungs along the length of their body. They roll which part of their body is out of the water so they can breathe, and rise up high to prevent waves from splashing into their lungs.
  • It's not really a serpent: These structures are not really from a serpent, but instead are something else. The creature actually has fins that APPEAR to be serpent segments thrust out of the water. Or these creatures are really using the buoyancy of trees to support them (especially when resting) like a swimmer with a float. Or these are really multiple different swimming creatures, each one which crests the water separately in a school.
  • Hunting Lure: These sea creatures scare away prey, so if they hold segments out of the water, prey mistakes them for many smaller fish and are less on guard. Or sea serpents prey on large predators (like sharks) and the multiple segments in water look like many prey animals, luring the predators close enough to kill.
  • Cleaning birds: A certain species of sea gull eats parasites off the sea serpent. When the sea serpent is infested with skin parasites/lamprey, they perform this display to attract the sea gulls, who gleefully flock to the happy serpent and pick the parasites clean.
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    $\begingroup$ sexual display is a good one, whenever you find something weird in biology start with the assumption it has something to do with sex. $\endgroup$
    – John
    Commented Jul 13, 2021 at 18:41
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Just change how they swim.

If they swim with vertical undulation instead of lateral, that is exactly the pattern you would expect to see while swimming on the surface. Although the loops would not come completely clear of the water like that but they would be on the surface.

snakes often hold the head clear of the water while swimming to improve vision or to improve the sense of smell.

this is even kind of ironic since the stories of sea serpents are probably just pods of whales seen at a distance, and whales swim with vertical undulation.

so more like this

enter image description here

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  • $\begingroup$ because you get greater drag (and therefore traction) on the surface of the water, this sort of vertical undulation would outperform a fully submerged one, but underperform a horizontal undulation on the surface so you'd probably want to explain the vertical preference. Perhaps there was some pressure to reduce the amount of them visible from above or below (possibly due to predation by even larger creatures, or maybe overheating due to exposure to the sun) $\endgroup$
    – Tristan
    Commented Jul 14, 2021 at 9:43
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Not a serpent.

It is improbable that an elongated creature would move like that. There are two standard explanations.

1: Many smaller creatures.

1:  Many smaller creatures.

https://baleinesendirect.org/en/what-do-we-know-about-harbour-porpoises/

Depicted: porpoises. They move in groups and they can move fast. Seeing a clump of multiple humps (porpoise backs) emerging from the water can easily be interpreted as one single creature swimming along.

If you have bigger creatures swimming in this manner your eye could combine them into a very big creature. Depicted: whales.

whales https://whalesanddolphinsbc.com/education-parent/humpback-whales-megaptera-novaeangliae/

2: One big creature with humpy back.

white sturgeons

source

The sturgeon is usually the creature invoked for this explanation especially as regards lake monsters. They can get huge, they don't need to come up for air, they have bumps on their back, and they can go out to sea. A big sturgeon cruising along under the surface might let its back hang out into the air and so would look like a series of moving bumps. The bumps dont move up and down of course because like all fish the sturgeon is propelling itself with lateral movements of its tail.

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Being gigantic, one of these fierce creatures only weaknesses is the tendency to overheat, particularly in the thermoactive seas it prefers to hunt in.

In order to keep cool following a hunt or during long trips, the serpents have evolved to extend alternating portions of their bodies out of the water to promote cooling in the ever present trade winds.

Some sub-species of these serpents have even evolved "sails" like those of the Spinosaurus to promote rapid cooling in the sea breeze with the most evolved of these specialists able to assist their great migrations using these sails as actual sails to harness the wind

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    $\begingroup$ With the thermal characteristics of water vs air it would make more sense if they cool down inside the water. $\endgroup$
    – Trioxidane
    Commented Jul 13, 2021 at 14:22
  • $\begingroup$ @Trioxidane Potentially, I'm not an expert in thermodynamics. I thought though that with large wet areas to promote cooling via evaporation the air might be a better cooler. I also hedged with that reference to the water being potentially heated. $\endgroup$
    – JonSG
    Commented Jul 13, 2021 at 14:27
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    $\begingroup$ in the ocean the water is better at cooling than the air, if anything the parts out in the air will be slightly warmer, especially in sunlight. warm blooded animals in the ocean have the opposite problem, staying warm, since the ocean is so good at absorbing heat. $\endgroup$
    – John
    Commented Jul 13, 2021 at 18:45
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Effort through the water

A giant snake will undoubtedly get a lot of friction with the water. To get a good speed with the least effort, it'll move much if it's body above the water line for the lower friction. This way it can "cruise" for longer, instead of requiring to swim constantly. The how is simple. Just with muscle power and buoyancy.

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  • $\begingroup$ drag from the water's surface is much stronger than the drag below the surface, so raising some of the body out of the water would actually increase the amount of drag the serpent experiences (this may be worthwhile if the serpent depends on that drag for propulsion, but would be disadvantageous for cruising) $\endgroup$
    – Tristan
    Commented Jul 14, 2021 at 9:47
  • $\begingroup$ @Tristan a valid concern, but at these sizes I think it'll not matter as much. The air resistance is so much lower than the water that it should alleviate this concern. Older submarines used to surface for maximum speed. As far as I know, this is to reduce the drag of the water. That seems enough to say the same for the snakes, unless there is something crucial I'm overlooking. $\endgroup$
    – Trioxidane
    Commented Jul 14, 2021 at 10:24
  • $\begingroup$ it's going to depend on how much of their cross-section they can get out of the water for a given length at the waterline. As this undulating shape actually introduces new cross-sections (each bend has to cut through the water, and even though there'll be a certain amount of drafting off the one in front, that's still extra drag) I can't see how this could perform better than lying flat partially submerged $\endgroup$
    – Tristan
    Commented Jul 14, 2021 at 10:29
  • $\begingroup$ @Tristan I cannot see advantages compared to just partially submerged either, but that's not the point of the answer. The point if the answer is explaining why a snake would be bend like this partially out of the water. Not the most efficient method. $\endgroup$
    – Trioxidane
    Commented Jul 14, 2021 at 11:25
  • $\begingroup$ @Trioxidane, the additional drag they'll experience vastly, vastly, exceeds any theoretical benefit they'll have moving part of their body through a less dense medium. $\endgroup$ Commented Jul 14, 2021 at 20:01

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