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So I am finishing up a functional concept of an alien I have been working on (a large semi-aquatic amphibian) and I am at the final piece of the puzzle. Their sails.

I wanted these creatures to evolve spinal sails but didn't have a use for them yet (which I later came up with a way to incorperate it into how they use wasted energy and food). But I am curious if they would have a use for two more sails on its side, close to the center of the spine then going out diagonally but I am having trouble as to a possible use for said sails. If used for display it would probably use the larger, central sail for that so I'm curious to see other possibilities for this evolution... if possible.

Edit: Thank you everyone for all the suggestion. Some pointed out the sails could be for heat, some for movement, and others for just sexual attraction. But in the end I decided to turn their sub sails into a super respiration system. Thank you all once again.

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    $\begingroup$ You should tell us what the creature looks like. It's habits and habitat. Does it swim extensively? Things like that. Be more specific about the details of the sails. I can't think of a reason for three sails if I don't know more about your creature. I'm imagining an over sized newt right now. $\endgroup$ – Lonha Jan 28 at 6:46
  • $\begingroup$ @Lonha it used to swim exclusively but evolved to become semi aquatic. The main sail has multiple spines while the two others gave a single large spike similat to kentrosaur. en.wikipedia.org/wiki/Embolomeri this is probably the closest animal to it $\endgroup$ – SentiCarter Jan 28 at 14:25
  • $\begingroup$ Note that in the Animal Kingdom, it's common for one gender to evolve useless flair to communicate their fitness, to attract mates. That's why male birds have bright colors, male peacocks have their tails, etc. $\endgroup$ – Mooing Duck Jan 29 at 1:16
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Gills

You know what else takes lots of surface area? Respiration. The alveoli in land animal lungs have a huge surface area to allow the exchange of oxygen and CO2 with the blood (there is no active pumping of these gases, it's just driven by diffusion). Fish gills also have a relatively large surface area for the same reason.

What would lead to an aquatic animal evolving super-gills? Well, the same thing that led to the evolution of lungs mostly. If they are large animals, the cube-square law dictates that you need relatively larger respiration area to maintain the same metabolism rate. And maybe they are evolved to have higher-than-average metabolism for any number of reasons like maintaining efficient homeostasis in extreme temperatures, or traveling long distances at high speed, or maybe powering an oversized brain.

As a bonus, if the sailgills are meant to be exposed to air, the available oxygen content is higher, which could allow even greater respiration. But you don't want your gill membranes to completely dry out, so you have to rotate the exposed surface back into the water, which gives you the spiral pattern. If this also provides propulsion, you could have a species of slowly spinning creatures, idly sailing across the oceans while pondering the mysteries of the cosmos with their overpowered brains.

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  • $\begingroup$ Great answer, with some great imagery. $\endgroup$ – barbecue Jan 28 at 19:04
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In real-world biology, Sails are all about adding to surface-area.

They're usually for either catching the wind or for temperature control.

Aquatic animals with sails include the Portuguese Man of War, a Jellyfish which tends to float on the surface and uses a large frilly dorsal fin as a sail to navigate and propel itself. More lizardlike, the Spinosaurus used its large spinal sail primarily for temperature control.

If you've got multiple sails, it's because your creature needs either more surface area to control its temperature with, or it uses the additional sails to steer while the main one is the larger one that does most of the wind-catching.

In addition to those, large areas of the body can often serve as a signalling organ, either for Mating Displays or for communication over long distances. In that regard, having the central sail be brightly coloured with dramatic patterns, and then having the two secondary sails able to close up around it to conceal or reveal the pattern would serve to provide camouflage when not displaying and could be flashed as part of a Mating display.
See for example, brightly coloured butterflies that can fold their wings together to conceal themselves from predators.

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    $\begingroup$ Small correction: a Portuguese Man o' war is not a true jellyfish, but rather a colonial organism existing of many smaller organisms. $\endgroup$ – Nzall Jan 28 at 12:10
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    $\begingroup$ "More lizardlike, the Spinosaurus used its large spinal sail primarily for temperature control." That's one out of at least 4 possibilities and last I checked they weren't sure yet which it was primarily. The other possibilities include that it's for attracting partners (like a peacock's tail), that the sail was actually more of a hump (like a camel) or that it may have something to do with navigation (since a Spinosaurus can swim). $\endgroup$ – Mast Jan 28 at 14:10
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    $\begingroup$ @Mast my understanding, though I can't seem to figure out where I learned this at the moment, is that the base of the spines (about the first third of there height) show signs of muscle attachment sites. This would indicate the possibility of a muscular ridge topped with a sale. Use cases of this ridge would include assisting the animal in pulling some of the very large fish in the environment from the water. Ridges akin to this are also known in other large theropods, such as acrocanthosaurus. I am not a paleontologist though, so of course feel free to take this with a grain of salt. $\endgroup$ – Myrdden Wyllt Jan 28 at 17:35
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An unsual option predicated on more-surface-area-is-better: water collection.

In the Namib Desert on the coast of southern Africa, precipitation is very rare - but a fog rolls in from the coast for a few hours most mornings. There is a species of darkling beetle that uses its shell as a condensation surface to capture a drink.

Many coastal areas are subject to wet/dry cycles (from tides or other mechanisms), so this wouldn't have to be unique to a desert. Any area with periodically humid air but little rainfall would be plausible.

Being amphibians, your species would have a greater need for water than most. Perhaps they can use the morning fog to make enough of a puddle to last through the rest of the day - and more surface area for sails means more water.

Having sails be foldable in the heat of the day would be desirable to prevent drying. When the air turns cooler near dusk, perhaps the sails are raised again to attract potential mates to a creature's particular puddle, and again here larger / more colorful sails would help visibility. In fact one of our world's most terrestrial fish, the leaping blenny, has a "sail" that it uses for this purpose.

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    $\begingroup$ Hi Bear, awesome answer! Welcome to Worldbuilding :) $\endgroup$ – Dubukay Jan 28 at 16:40
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A few options come to mind:

  1. Its an amphibian, so it could use the fins to steer itself while swimming.

  2. Sexual attraction, like a peacock.

  3. Elephants use the surface area of their ears to lose heat. These creatures could use use their fins in a similar fashion.

  4. Any combination of the above, which is most likely to be the case.

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  • $\begingroup$ I like the fins idea. Finlike sails can probably be folded up. More surface area to push against the water could help an anthropoid creature that swam with lateral wriggles like a fish. Although I cannot think of a fish that has fins in that configuration... $\endgroup$ – Willk Jan 28 at 13:38
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I got 2 ideas

First is, the sails have use similar of a Dimetrodon's Sail, which is to capture sunlight to heat himself up, because he is warmblooded. Another one is, its a leftover product of a evolution, you may say that your creature evolved from a Sailed fish, then your creature will evolve on something else.

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In addition to the answers given, why not let the creature use its sails as ..sails?

It could be an extremely efficient way to travel while consuming relatively little energy.
They could use this mode of transportation

  • to find suitable mates (possibly as solitary creatures);
  • to migrate by being able to traverse very long distances (like the sea turtle or albatross);
  • to semi-passively feed on krill, or other small sea creatures (provided these (fictional) creatures have a high enough density at the sea/ocean surface).
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Underwater breathing.

The sails are actually exotically-shapped gills. They absorb oxygen from the water as the creature swims, helping it to extend its time underwater before needing to go outside to breathe. More sails would add more surface area for oxygen collection.

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