Size & Osmoregulation

Based off discussions in this thread: The Nucleus Paradox
With some important connections to this thread: Surface Area, Volume, and Ratios

We’ve been discussing the possibility of revamping the nucleus and implementing more detailed mechanics related to the impact of surface area, volume, and size on evolutionary strategies. Various cool concepts and conclusions have been derived from these discussions, but they all inherently revolve around an underlying mechanic: the relationship between osmoregulation and size.

We currently are thinking about implementing a non-linear relationship between mass and osmoregulation cost - osmoregulation costs will increase at a more rapid pace than size does. This will serve to represent the square-cube law, which essentially states that volume increases at a faster rate than does surface area as size increases, meaning energy costs can rapidly outpace nutrient/resource input. This will serve several functions…

  1. Limits massive cells. Players won’t just be able to stumble into making huge Discord-chungus units by adding more and more parts; they’d have to adopt proper morphological and behavioural changes to support such a build.
  2. Introduces a strategic layer to the editor. Players will have to decide whether or not additional size is worth the additional metabolic demands. Once the other concepts related to metabolic size are implemented, there will be rewards for engaging deeply with the editor as the player attempts to subvert the challenge of scaling up.
  3. Realism. The square-cube law is a very important aspect of biology that will serve immense roles in future stages as well.

With that said, we will need to answer several questions…

  1. What should we name this new size-related cost? Reading up a bit more, increasing osmoregulation costs wouldn’t necessarily be an accurate representation of the effects of the square-cube law.
  2. How big should the average prokaryote be? This will help us determine how size-related costs will scale for non-nucleus bearing organisms.
  3. How big should the average eukaryote be? This will help us determine how the nucleus should be balanced.
  4. How big should we allow cells of various types to get? This will let us standardize size to better allow for balancing and a more consistent gaming experience, and also prevent players from not really considering the square-cube law until they are ginormous.

In the cell stage, the only significant impact square cube would have on game mechanics is probably just on energy balance and mass. In the event that it effects energy production, we would have to explain to the players some way that the effectiveness of their energy production is being altered by the size/shape of their cell. This might be hard, especially when players have an aversion to any text beyond a sentence.
We would have to add a modifier to the tool tips of each process or something to display the impact.
On the other hand, if it just effects energy cost, we could have a energy drain adjacent to osmoregulation in the balance bar that is explained to be impacted by size/shape.

Either way, the exact term is not something I am sure about, maybe just surface-area?

Probably not very big. Even now they are grossly out of proportion compared to eukaryotes but that’s gonna have to stay for sake of visibility and such.

They are already 50% smaller than eukaryotes so it can already be somewhat challenging to reach comparable sizes. This relative size is probably here to stay, so outside of the size modifier, I think it best we just treat prokaryotes just like eukaryotes with square-cube.

There’s quite a few parts that eukaryotes require to advance, and more that help. The nucleus alone being 10 hexes in size guarantees that eukaryotes will easily be larger than most prokaryotes with little effort. This combined with most of their parts being 2 hexes in size means I would say that 35-40 hexes should be the point where a eukaryote begins to push it’s limits in most cases.

All but the most productive cells (Such as photosynthesizers) will probably be pretty consistent in size as the resources they need are not in such limitless supply.

And now I’ll pose a question of my own;

Cells will be double in size when they are ready to reproduce, how are we going to handle that? This isn’t normally an issue right now as energy production scales appropriately. But if we make energy production begin to slow down as size increases, this can actually make a player begin to starve as they grow, with no clear warning as to why.

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Very good point to bring this up just so this isn’t forgotten. Any final design / implementation will need to address this.

Well I guess that’s pretty fair, I’d have said something like 50 is starting to push it. Just because I think a lot of players naturally end up really close to that anyway so putting the “limit” even lower might feel too restrictive. I guess we could also design how this new feature interacts with the game difficulty levels so expert players can have more of a challenge where they can’t really waste any organelle space on unneeded organelles.


I think that 40-50 range is a good range where size starts to become a notable factor among eukaryotes. I think size would start having an effect before that point, but not enough to significantly dissuade the player from adding new parts. We can nail this down with balancing thankfully.

Changing the severity of the osmoregulation penalty across difficulties sounds good, but we should be wary of extending this effect to the AI too much - we don’t want harder difficulties to have generally smaller AI threats. Perhaps we can look into bolstering the surface area and volume ratios concept for harder difficulties to make strategy that much more important there.

As to the terminology, I wonder if “size-related costs” would be a fine enough blanket term. Surface area will be its own thing if we implement the SA:V ratio concepts.

Good point on the doubling of size issue. Is it possible to “freeze” the “size-related costs” stat upon exiting the editor, or would that muck things up? By that, I mean having it so that players exit the editor with their size-related costs stuck and unaffected by whatever happens on screen. So they’d keep their original costs and not have their cells dramatically suffocate/starve for no good reason once replication occurs.

I suppose that is technically possible. It just makes the microbe design more complicated with a single stat being only calculated on spawn whereas other stats will dynamically change as a microbe divides more organelles.

Regarding the opened issue (Implement Non-Linear Osmoregulation/Size-Related Costs · Issue #4432 · Revolutionary-Games/Thrive · GitHub) it says:

Note that this cost would probably be represented as its own thing in the “Organism Statistics” panel in the editor rather than just being a tweak of osmoregulation, as osmoregulation is its own thing.

This feels a bit wrong to be for two reasons:

  • We already have a non-linear size cost implemented in auto-evo, so adding an extra non-linear cost on top of osmoregulation feels like auto-evo gets even farther from the gameplay reality, instead of being brought closer back together
  • I kind of dislike having two systems that basically do the same thing. Why shouldn’t we make osmoregulation non-linear? Though, I guess for max realism we would make osmoregulation diminish with size (thanks to less surface to volume ratio) and come up with another name for the non-linear size penalty.

Not sure if I understand this 100%, but wouldn’t it be making Thrive more reflective of auto-evo if we add some sort non-linear size cost since that currently isn’t a gameplay mechanic players grapple with? Or is it a mismatch because osmoregulation currently is non-linear in auto-evo?

Yeah, that’s the main reason. I was using osmoregulation as a catch-all term for any sort of cost related to the costs of increasing mass/size, but that isn’t very accurate since osmoregulation is specifically a component of osmosis and based on characteristics of the membrane. Obviously osmoregulation is effected by size, but mass-related phenomena, such as the square-cube law, aren’t directly effecting osmoregulation.

Yes it would. But only if the osmoregulation uses the same formula as in auto-evo. If I remember right it is something like size^{2.3}.

Would we be able to shift most size-related costs to, well, a new “Size-Related Costs” metric in auto-evo and have osmoregulation be its own cost that acts a bit differently? So we could have the auto-evo reflect the in-game non-linear energy scaling through “Size-Related Costs” (size^2.3 or whatever balancing measure we decide), which allows us to treat osmoregulation separately since we would ideally want some separation between the two as they’re not synonymous.

Increased osmoregulation costs also are how we are planning to penalize being outside of your environmental tolerance range, so it would be nice to have that separation. We would of course have to balance the two costs since they would both be acting on the organism.

You’ll have to come up with a full suggestion on what to do… I’m still not convinced that having 2 separate stats that do basically the same thing is not going to do that much good. And I look forward to all the questions on the community discord from people being confused what the two separate things mean. But yeah, feel free to come up with some overall design proposal that makes sense.

Thinking about it more I guess I see your point in not needing to have two different factors. In my head, I was thinking of having osmoregulation add up with environmental tolerance effects, size, and surface-area/volume ratio related effects to add up to the total cost amount, but that isn’t really necessary. We could just have a single number represent the cumulative effect of these various factors, though we still might want to reconsider naming “Osmoregulation” into a more general term at that point if we want to be picky scientifically: maybe into “Organism Costs” or something. Still, we don’t need to divvy things up at that point.

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Yeah, sleeping on this I realized the core of the issue was that I already considered osmoregulation just a nice scientific term we picked for the “staying alive base energy use” of a cell. So to me splitting that up into multiple factors that basically do the same thing for no other benefit than to make the game more complex for the player and maybe slightly more scientific accuracy. We already have a bunch of handwaving the science for more streamlined gameplay in Thrive.

Edit: I’ll put on the wiki far future features something pretty generic as “Non-linear size / osmoregulation cost for cells (and matching gameplay with auto-evo energy cost)”.

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