Shapeshifters and mass... ever the bane of realistic science fiction.


Let's say someone is changing shape (for whatever reason, voluntary or involuntary, it doesn't matter for this purpose). Don't worry about what is causing this to happen. Let's also say that the start and end forms have non-trivially different size (mass).

Is there a credible way a shapeshifter could gain/lose body mass when changing forms? asks about some ways that this can happen. The unfortunate reality, as noted in some of the answers there, is that if we want to stick to the realm of biological plausibility (which I want to do), the sort of "Hollywood" shapeshifting that happens in a matter of seconds or minutes is simply not possible. Realistic shapeshifting takes months, or even years... right?

Rather than asking how it can happen, I'd like to ask how quickly it can happen. I expect "days" (at least as a single-digit number) is right out, but what is a plausible duration?


Let's also assume that the body of the "shifter" (victim? Er... let's just call her "Alice") isn't changing radically, so we're not worried about having to replace functioning organs with completely different ones. This gives us a nice cop-out for increasing mass; we can look at growth rates of real critters and use that as a reference, maybe with a little bit of fudge factor because we're bending the rules a bit as to how this is being triggered in the first place. What about decreasing mass?

I can think of three possibilities:

  • The body "eats" itself in a carefully controlled manner such that everything is being constantly put back together, only smaller. Mass that isn't needed is expelled through "usual" mechanisms (sweat, shedding, urination, defecation).
  • The body uses the resources of the outer parts to grow a new body inside of the old one, which is then cast off in a much squickier version of reptile shedding.
  • The body liquefies everything but the most critical bits and grows a new body from the resulting soup.

The last is both the most and least plausible; similar to how eggs and cocoons work (indeed, for my purposes, plenty of insects are shapeshifters to a greater degree than I need), but seems dubious without some sort of protective shell. The second seems like it would have issues with the skeleton. I lean toward the first for several reasons:

  • Alice is plausibly motile (if clumsy) for the entire duration.
  • Alice isn't starting from scratch as far as motor control; the skeleton, muscles and nerves are all in the same configuration. (Again, some clumsiness expected due to the size change, but this is basically the process we all go through growing up, only backwards.)
  • Lost mass is dealt with in a gradual manner via ordinary mechanisms, rather than all at once in a great big mess.

In addition:

  • Alice is able (but not required) to eat take in nourishment during this process at a rate of up to 3,000 calories / day, in addition to what her body can get from "eating itself". Anything extra is shed or excreted.
  • The transformation process is allowed to (and almost certainly must) "overdrive" Alice's normal healing ability, but not "magically" so (less "Wolverine"/"Deadpool", more "young child"). For the purposes of the transformation, cancer is a non-issue.
  • Aside from whatever triggered the transformation (which may be external!), Alice's biology is otherwise normal. Answers requiring exotic organs or the like must include the time to grow those organs in the first place. (I'll allow them to be "left over" at the end, but really, avoiding this sort of thing is preferred.)


So... is this at least somewhat plausible, and if so, how quickly could a process like this occur? (My goal is for Alice to lose about 60% of her initial mass in about a month, but answers that can be generalized/extrapolated are better for the community.) Again, I'm hand-waving why this is happening, but I'd like to keep rest of the mechanics fairly grounded in plausible biology.

  • $\begingroup$ What is Alice turning into? $\endgroup$
    – Halfthawed
    Commented May 14, 2020 at 16:46
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    $\begingroup$ Losing weight is really easy for humans (active humans), actually it's mechanically way harder to gain mass than losing it.... Fortunately alice is a shapeshifer, she's no human so she can be even faster. Just look at real life shapeshifter like eggs, butterflies and other wormy things like maggots. Some animals have special ways to lose instantly mass, crabs can amputate their own limbs, snails can cut themselves in half, worms too... And lizards can amputate their own tails. But I guess even a normal person could amputate her own limbs if she really needed to lose weight desperately. $\endgroup$
    – user75689
    Commented May 14, 2020 at 16:46
  • $\begingroup$ @Halfthawed, answers that don't need to know that preferred. That said, I did specify that "we're not worried about having to replace functioning organs with completely different ones." The best case would be "a younger version of herself", but I think it shouldn't matter as long as no major changes need to happen. $\endgroup$
    – Matthew
    Commented May 14, 2020 at 16:49
  • $\begingroup$ @Kyu, I'm not asking about just "losing weight", I'm asking about transforming, shapeshifter-style, from one form to another, smaller but still fully functional form. That means things like bones and organs (and limbs) need to get smaller, not just get lopped off. (At best, you're talking about my "option 2".) $\endgroup$
    – Matthew
    Commented May 14, 2020 at 16:53
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    $\begingroup$ @Matthew There was a similar question which left me with axolotls and whale calves on my mind for the speed of biological growth lol, if you can justify how waste leaves the body fast and safely and major changes could be done during sleep, then the speed of growing the smaller organs or shaping them down could be the biggest factor. $\endgroup$
    – user69935
    Commented May 14, 2020 at 17:28

2 Answers 2


This isn't an answer to the question as asked, but since I mentioned:

We can look at growth rates of real critters and use that as a reference, maybe with a little bit of fudge factor because we're bending the rules a bit as to how this is being triggered in the first place.

I thought I'd follow up on that. (Also, thanks to RandySavage for hinting to look at whales!)

Okay, so... gaining mass?

Well, cattle go from ~85 lbs to ~1200 lbs in 18 months (source); an increase of roughly 14 times birth weight. The growth rate of blue whales "is likely one of the fastest in the animal world", starting at almost 9,000 lbs and gaining up to 200 lbs per day (source).

Doing some back-of-the-napkin math suggests that doubling in mass in a month is pretty plausible, even for normal animals under normal conditions. Keep in mind we're talking about functional mass here, not just fat reserves; that is, it seems plausible for a shapeshifter to realistically transform from a Golden Retriever (~70 lbs) to a Great Dane (~140 lbs) in a month, while going from Mountain Lion (~180 lbs) to East African Lion (~380 lbs) is getting near the edge of plausibility. (Obviously, this assumes an adequate food supply! Note also that I've chosen forms that are very similar aside from size; having to do more internal shifting-around is probably going to cut into how quickly a shifter can accomplish changes in mass.)

Being able to functionally incorporate mass faster than that (i.e. incorporate it into the skeleton, especially, as well as muscles and organs) probably starts getting into hand-waving territory, which we're trying to avoid.

  • $\begingroup$ Thats a good example using cattle to get your growth speed, are you asking if growth could be faster than this using known biological processes? $\endgroup$
    – user69935
    Commented May 14, 2020 at 18:46
  • $\begingroup$ Not really. My specific case is interested in decreasing mass, anyway, and that's the case that can't (as I show here) be trivially answered by looking at real growth rates. Since I mentioned in the question that giving a rate for increased mass can be done fairly easily by looking at actual animal growth rates, I figured I'd follow up by actually providing that answer rather than leaving it as an exercise for the reader. That said, answers explaining why the above is conservative are welcome in the hopes of helping others, but don't address my immediate issue. $\endgroup$
    – Matthew
    Commented May 14, 2020 at 18:55
  • $\begingroup$ I am wondering if a brain can be shrunk down to 40% size but still function the same? $\endgroup$
    – user69935
    Commented May 14, 2020 at 19:04
  • $\begingroup$ Well, for the case of an adult shifting to a child (ahem 😉), Willk says there's no issue. For other cases, there's worldbuilding.stackexchange.com/questions/3108. However, I consider that off-topic for this question. $\endgroup$
    – Matthew
    Commented May 14, 2020 at 19:18
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    $\begingroup$ I'm not sure how relevant this is but the immortal jellyfish is coming to mind especially when it reverts back to an immature stage by recycling its old cells "transdifferentiation" , allthatsinteresting.com/immortal-jellyfish $\endgroup$
    – user69935
    Commented May 14, 2020 at 19:39

Your shapeshifter is absorbing (reabsorbing perhaps) exotic weakly interacting particles that for the most part are always floating around him/her through a process that converts them into straight up baryons. (This isn't very plausible given the Standard Model though). These particles might not even have mass in the strict sense, until converted, and thus would be difficult to detect (but this isn't a requirement if there's only a few thousand pounds/tons of them in the immediate vicinity).

There would be an organ or organs located internally that does this, and it probably spits out wholly formed cells. This process is also exothermic, so that the shapeshifter's biology should be able to tolerate a large amount of heat being dumped into it. The cells don't need to be capable of division, so maybe they don't even have nuclei (like mammalian red blood cells).

Through a similar process, mass can be unconverted back into the weakly interacting particles at will. This might or might not use the same organs for that process.

To explain why this "dark matter" is always close to the shapeshifter, we either have to posit that it exists everywhere in more or less equal proportion, or that its attracted to them via some unknown physical force. Since we're already creating novel particles, that's only slightly more implausible than the rest.

This would allow for rapid shape-changing, something on par with the Incredible Hulk.

  • $\begingroup$ Uh... no. There's a reason I added reality-check. I'm only hand-waving why Alice's body is doing this. For the how, please try to stick to real physics/biology. When your answer even says things like "this isn't very plausible given the Standard Model though", that should be a tip-off that your answer probably isn't compatible with reality-check. $\endgroup$
    – Matthew
    Commented May 14, 2020 at 18:33
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    $\begingroup$ @Matthew reality is that nothing like a shapeshifter is possible. That's not a helpful answer though. It wasn't marked hard-science, my answer's good even if it doesn't appeal to you. $\endgroup$
    – John O
    Commented May 14, 2020 at 21:43
  • $\begingroup$ Depending on your definition, butterflies (in fact, many of insects) beg to differ. If we go loose enough, you are a shapeshifter; you have changed shape quite a bit from when you were born. RandySavage even mentioned an organism that does revert to an earlier life stage. Slow shapeshifting (months) is at least somewhat plausible. What isn't is shapeshifters like we see in media that can change form in seconds or minutes. The whole point is that I am explicitly not asking about that variety. Accordingly, answers that try to justify "Hollywood shapeshifting" aren't really on topic. $\endgroup$
    – Matthew
    Commented May 15, 2020 at 13:18
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    $\begingroup$ @Matthew Should I vote to close your question because you worded it poorly then? You're not asking for shapeshifters at all in any sense that anyone else would mean. You're just asking for organisms that develop into different shapes, which is pretty much every organism ever. $\endgroup$
    – John O
    Commented May 15, 2020 at 14:12
  • $\begingroup$ How is it worded poorly? I thought I clearly indicated that I am talking about "shifters" (note the quotes, also in the subject) that operate in a manner that is actually plausible. That is, in fact, the entire premise of the question. Maybe you are confused because I asked "If an organism changes shape in a realistic manner, how quickly can it do so?". You seem to have answered "If an organism changes shape in a quick manner, how can it realistically do so?". Okay, so the difference is "only" a couple words in different order, but in meaning those are very different. $\endgroup$
    – Matthew
    Commented May 15, 2020 at 14:59

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