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.

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.

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.

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.

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.

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.

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).

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.