What are the cost and restrictions or benefits of a creature being able to slowly morph from an animal to another while retaining memories?

The creature morphs to giant eagle then morphs into fish and then back to its original shape. While transformed, the creature retains all its mental abilities and memories.

Changing from one species to another is more like learning how to use a bike, can be difficult at first but then it gets easier.

Such creatures can also selectively morph only specific parts of their body,for example growing tails and wings or changing limb shape.

Question inspired by the Heteromorphic characters of Overlord

In YGGDRASIL, there existed a multitude of different races. They were generally categorized into three main races; namely the "Human" race, "Demi-Human" race, and "Heteromorphic" race. The following context below is a list of current races that were introduced so far in the Overlord series.

The shapeshift won't be instantaneous like in Overlord but might take a few days for things like changing the shape of ears and eyes or months for turning into a different animal.

  • $\begingroup$ Welcome to worldbuilding. Please take the tour and visit the help center to better understand this site and its policies. For your question, I think you might need to provide additional information: what type of environmental pressure exists on these species? Do they all share the same territory? $\endgroup$
    – L.Dutch
    Commented Sep 21, 2018 at 10:20
  • $\begingroup$ What's the human population? Can humans breed with these other species? How is the geographical distribution? $\endgroup$
    – Sasha
    Commented Sep 21, 2018 at 10:23
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    $\begingroup$ You can't have both earth-like (question is about planets that are almost, but not quite, entirely like Earth) and alternate-worlds (question is about planets that have very few similarities to Earth). Also, this question doesn't seem to be about the planet these creatures exist on, so I'm not sure either applies, but if you want either, then please choose one of them. $\endgroup$
    – user
    Commented Sep 27, 2018 at 7:09
  • 1
    $\begingroup$ We should rather ask why not, then you might find answers why it is not feasible given the evolutionary cost-benefit. Also assume that you couldn't just change your mass with shapeshifting, so that alone would make it restrictive. Also it would require high intelligence to use such an ability, which has its own evolutionary cost. And who says just intelligence isn't already better for the purpose of "changing your body" given the technology and on-the-fly adaptation it allows? And even with intelligence, it may turn out badly if species have too much conscious control over themselves. $\endgroup$
    – Battle
    Commented Sep 27, 2018 at 7:31
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    $\begingroup$ Possibly polymorphic rather than heteromorphic, poly is many, hetero just means different. $\endgroup$
    – Separatrix
    Commented Sep 27, 2018 at 10:39

4 Answers 4


flying fish is a logical predecessor: imagine

(a) a fish thag flies, like flying fish

(b) a bird that dives, like penguins

now, take ether example, first having part of the fish/bird's appendages to possess a foldable, yet large extension, for example, outer fin, or primary flight feathers. a fish tail and a bird tail is really not very different.

for the flying configuration, your creature extends their aerodynamic appendages, taking the eagle/puffin configuration, possibly right after leaping out of water, flapping them to take flight.

for the aquatic configuration, the creature retracts it's wings and can now swim, taking fish configuration, possibly before diving into the water.

this creature would be a inhabitant of the high seas: fish groups in very distant groups, so anything that hunts fish have to be able to: either) migrate for long distances, following shoals of fish or) being able to dive for a largh depth, therefore hving access to the more dispersed fish inhabitating the ocean bottoms and deep seas. and) being able to reproduce without vicinitu of land, unless a) is satisfied, and it can breed on shores.

this would require the creature to be able to move very far very fast, yet limiting the creature's energy requirement for daily activities, therefore, it hunts as a fish/penguin and moves as a bird/flying fish, it does not take the eagle/bird like configuration unless the available food soursees are scarce. bonus if the creature spawns in the water, which it must have at least one swiming phase, even if it have to hunt while flying.

the fish origin would be the standard: the gills of a fish can work, if properly configured, in a continuous airflow just as if it was a continuous waterflow, using a layer of mucus as the exchange medium, running the fish's body like a turbofan/piston propeller engine whild a bird's lungs probably wouldn't be able to take oxygen from water. it may be diving periodically for a long time, and surfaces to breathe regularily, while swimming to hint, just like penguins or cetaceans.


This is a very expensive ability to have. You need the genetic plan for multiple different forms. You need to be able to transition between those forms while still having every form in between viable (or use a cocoon type system). How is a giant cat with half grown wings instead of its front legs going to run and hunt? It certainly can't fly yet.

And yet you're right, these changes can't happen fast. Even weeks or months is remarkably quick for those sorts of transformations. A good model might be the transition of a tadpole into a frog, which you will note is viable at every stage of the cycle.

As to how this would arise? There would need to be an extremely pressing urge so I suggest hugely variable planetary conditions. Either it orbits a variable star, or has an eccentric orbit, or similar.

This would cause it to have massively different seasons and life to develop multiple ways to cope. Some would hibernate, some would leave seeds and die, and some would adapt to the seasons.

The cat form might hunt and feed itself up, it then hibernates for a few weeks to transform into a flying form which allows it to fly to another part of the world and transition into another form to hunt again. When the 10 year winter approaches as the sun cools it would transform into an aquatic form and dive below the ice.

It's a little far fetched but that sort of highly dynamic environment is the only thing I can think of that might drive something as drastic as you describe to evolve.

  • $\begingroup$ Genetic plans are cheap. Some plants carry around 100 times more genetic material than they really need. See this article $\endgroup$
    – Peter Shor
    Commented Sep 27, 2018 at 12:02
  • $\begingroup$ @PeterShor But if one of those plans goes wrong the plant doesn't try to transform into flying mode and discover it forgot to plumb in the heart and die. $\endgroup$
    – Tim B
    Commented Sep 27, 2018 at 12:27
  • $\begingroup$ Not a problem. Humans have 100s of genetic plans, one for each type of cell (nerve, muscle, liver, blood, fat, skin, bone ...). If just one of these plans goes wrong, we can die. But this doesn't happen very often. However, you're right about the transitions being really expensive. (Although I think weeks rather than months is the right timescale—evolution is good at figuring out how to do things efficiently.) $\endgroup$
    – Peter Shor
    Commented Sep 27, 2018 at 16:43
  • $\begingroup$ And let me say that, aside from the nitpick about genetic plans being expensive, this is a great answer. $\endgroup$
    – Peter Shor
    Commented Sep 27, 2018 at 22:08
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    $\begingroup$ This is probably the best answer as a general matter. Shape-shifting from an evolutionary perspective (at least, the reversible sort) is only viable if the environment changes drastically on a regular basis. An eccentric orbit would do nicely if chosen properly, since it would produce huge seasonal swings even near the equator; a land with Russian winters and Saharan summers would certainly favor animals that could adapt to wild extremes. Multiple answers are possible, but they would form a spectrum; shape-shifting would fall under "expensive but highly effective", making it a viable option. $\endgroup$
    – Palarran
    Commented Sep 29, 2018 at 1:38

For evolution, what you need to start with is the ability to change between two somewhat different forms.

Suppose a life form lives in a jungle where there is a wet season where everything is underwater, and a dry season where the water is concentrated in a few rivers. (Parts of the Amazon are somewhat like this.) It would be advantageous to be able to switch between a form adapted to the dry season and a form adapted to the wet season. Maybe it could start with something as simple as growing webs between their fingers and toes.

These forms wouldn't have to be that different at first. Evolution would make them diverge more. And then, after descendants of this life form move out of the jungle, evolution would retain this switching ability, but the descendants would switch between forms optimized for different purposes.

With respect to evolution, one of these polymorphic creatures is going to be outcompeted by a single-body-type animal unless it has some good reason for needing to change from one body type to another. So I would guess most creatures on the planet would be monomorphic.


You can't without magic.

You simply can't change shape that drastically that fast, your are talking about months of transition time, you can only restructure bones and tissue so quickly. Worse you have a large creature changing breathing mechanism, digestive system, and metabolism so months will kill your creature, it will starve. There is a reason there are no animals of significant size that undergo metamorphosis.

you are going to have to either handwave it or invoke magic.

Once you do that, then the reason is easy, adaptability, human are a perfect example the more adaptable an organisms is the wider it will spread. humans invented things like clothing an shelter so they could survive in environments they never could without them, your creature is just doing it to their body instead of with technology. Your earliest shapeshifters would only be able to alter small things and it would evolve gradually from there. You may also want to go the "thing" route and have it sample said creatures to copy them, that will make it slightly more believable otherwise they are going to need massive brains to work out each transformation through trial and error.

  • $\begingroup$ Who said anything about restructuring bones?—maybe the same bones can be used (in different configurations) for the eagle and the fish. Or maybe these creatures don't have any. And what about caterpillars and butterflies? Tadpoles and toads? Those change their digestive systems and their breathing mechanisms. Magic (unless the change takes place in seconds rather than days) is completely unnecessary. $\endgroup$
    – Peter Shor
    Commented Sep 27, 2018 at 16:38
  • $\begingroup$ If you want to go from a fish to a bird you need to restructure bones, the bird skeleton is one of the most unique things about them, flight requires some very specialized and strong supports. caterpillars use a cocoon (they all but break down inot a mass of cells) and are tiny, scale them up and you are still talking about several weeks if not more (and you can't eat while doing it). tadpoles and toads don't change their digestive system and it still takes months. All of these are doable, even reversibility, just not on the timeframe you want. $\endgroup$
    – John
    Commented Sep 27, 2018 at 21:03
  • $\begingroup$ Bats don't have hollow bones, and they fly. And there's no reason that fish couldn't have hollow bones. They just don't. $\endgroup$
    – Peter Shor
    Commented Sep 27, 2018 at 22:05
  • $\begingroup$ bats also aren't birds or fish, and don't fly in the same way as birds. There is a big reason fish can't have hollow bones, hollow bones require air sacs which are attached to lungs. they also make the fish lighter which is pretty deadly to a fish. Sorry you just cant make a flying animal out of fish bones without seriously restructuring the bones. form is important to function. $\endgroup$
    – John
    Commented Sep 27, 2018 at 22:12
  • $\begingroup$ You could have a creature whose bones were hollow, but filled with liquid in its fish form, and with air in its bird form. And cormorants' bones are hollow, and filled with air, but they still manage to maneuver very well under water. $\endgroup$
    – Peter Shor
    Commented Sep 28, 2018 at 12:23

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