Organ Systems of the Late-Multicellular/Macroscopic Stages

Deus · June 28, 2025, 1:03am

Organ Systems, Constraints, Scaling, And The Macroscopic Stage

This started as a write up on how we can handle players editing scaling in the Macroscopic Stage, but then turned into a breakdown of how organ systems can work in the Macroscopic. This concept relies on the constraints I brought up in the Macroscopic Editor Thread (Macroscopic Editor, Progression, and Principles - #41 by Deus), but otherwise, isn’t tied to the external editor mechanics I propose.

I would like to clarify that allometry as a field is an extremely advanced science, one that I am not very familiar with. So if there is anyone who can offer more direct feedback or clarity on the topic, I’d really appreciate it.

I will also note that a lot of this is inherently simplified, especially when it comes to energy efficiency. Someone can spend a lifetime researching biomechanics and allometry and still know so little.

Size in the Macroscopic

We want to place controls on how quickly players scale up or down. But we also want to make it so that players can become whatever size they want to become.

“Scale” works pretty smoothly within the macroscopic editor, and the actual action of scaling up an organism isn’t that complicated. It just takes your existing body plan and makes it bigger or smaller.
Scaling is limited by the effect size has on your energy costs. Scaling up dramatically alters constraints such as surface area and mass, corresponding to the square-cube law. These in turn have dramatic effects on energy, and requires alteration of organ systems, appendages, and skeletal structure to counteract energy inefficiencies.
Scaling within a certain amount doesn’t require an immense restructuring, though it does alter movement or combat-related stats and has some impact on energy. More intense changes happen across larger differences in scale.

Scaling Interaction with Constraints

We can present the challenges of scaling via constraints.

Mass

As an organism scales up, the mass of its parts increases by the cube of the scaling factor (Mass X Scaling Factor ^ 3.) So if I increased the size of a 20 gram claw by 2 without changing its dimensions, the new mass would be 160 grams (20 (grams) X 2(scaling factor)^3 = 160.)

Surface-Area to Volume Ratio

As an organism scales up, the surface-area to volume ratio is divided by the scaling factor. So if the original ratio of a part is 6:1 and the part scales up to be twice its original size, you get a new ratio of 3:1.

Guess what? The above two describe the square cube law. And the below writing explains how to integrate it within the Macroscopic Stage in what can be a very engaging way.

How Constraints Affect Energy

Mass will be the most influential constraint in determining your basal metabolic needs.

Each part, organ, and structure in your organism has its own individual mass.
The energy requirements of that part is determined by that mass, times certain coefficients based on the nature of the part/organ. More intensive parts, such as a brain, advanced organ systems, or explosive extremities have a more strict coefficient.
The cumulative sum of all your parts’ mass results in your total mass. And the cumulative sum of all these energy demands results in your Basal Metabolic Rate - equivalent to Osmoregulation in the Microbe Stage, the energy you need to stay alive.

Remember that part mass changes with size in the ways I mentioned in the prior section. Those rules apply to all appendages, organs, mandibles, extremities, and more, affecting surface area, mass, and other constraints. So the function and nature of different parts will vary dramatically - what was a good way to increase surface area early in the game won’t be later in the game, and so on.

METABOLISM AND THE INTERNAL ORGAN EDITOR

Metabolic Process - The “Revenue” Side

The metabolic process will be the way your organism gets energy from the food it eats. I mentioned it in a prior post (Metabolic Rate System - #5 by Deus), but broken down simply, it goes like this…

You eat something. The amount of organic matter you receive depends on your ingestion efficiency influenced by your mouth, with a higher ingestion rate towards a certain material resulting in more organic material to eat. This is the Macroscopic equivalent of “Engulfed Matter”.
Whatever organic material you ingest turns into nutrients depending on your digestive efficiency, influenced by your digestion system. Your nutrients are the Macroscopic equivalent of “Glucose/Iron/Sulfur/etc.” in the microscopic stages.
Your respiratory efficiency then breaks down how efficiently those nutrients turn into ATP/energy, based on your respiratory system; this results in the Macroscopic equivalent of the ATP bar.

As such, we can derive the general purpose of the digestive and circulatory systems off this breakdown.

Players interact with the Digestive System to get nutrients from engulfed food. Changes to the Digestive System change what food sources you get nutrients from, and how efficiently you get nutrients from these food choices.
Players interact with the Respiratory System to get ATP from nutrients. Changing the Respiratory System changes how efficiently nutrients get transformed into ATP.

Process Efficiency v. Process Rate

Overall, processes are governed by two aspects…

Rate - The speed at which the process works, often influenced by how much volume it can churn.
Efficiency - How efficiently the process does what it’s supposed to do, given a standard amount of volume. What I discussed in the section above.

So if we look at a respiratory system, the respiratory rate is how quickly oxygen transfers throughout the body, and the respiratory efficiency is how much oxygen is able to be taken in at a given breath. I discussed efficiency already, but digestive rate essentially determines how quickly something goes from engulfed matter to nutrients, and respiratory rate determines how fast nutrients turn into ATP.

If you have a fast rate, you’ll generally go through your food bar more quickly. This is beneficial if you have adaptations which require a lot of energy, like striking, sprinting, flying, etc. but obviously requires a much more rapid food intake.

Digestive Efficiency X Digestive Rate = Nutrients Generated Per Time Unit. If you digest things too quickly - as in, you have a full nutrient bar and you’re still digesting stuff - you lose out on valuable nutrients. If you digest things too slowly, your respiratory system won’t have any nutrients to break down, meaning less energy.
Respiratory Efficiency X Respiratory Rate = ATP Generated Per Time Unit. If your ATP generated per time unit is less than your Basal Metabolic Rate per time unit, your organism takes damage, similar to the Microbe Stage. This proximates overexertion, suffocation, starvation, and more.

Representing process rates presents a very interesting design question, which can reflect evolutionary strategies. Having a faster rate means more immediate access to energy, but means you need a higher food intake. This means that organisms which rely on explosive displays of energy, such as animals which run fast, fly rapidly, or otherwise use demanding abilities need to eat more. Meanwhile, having a slower rate means less room for explosive movements, but allows you to go longer without food - useful for animals living in extreme situations.

This system also provides part of the answer to the “why would any animal not have the most advanced/efficient organ system” paradox: if your digestive system outpaces your respiratory system, then you waste a lot of nutrients. If your respiratory system outpaces your digestive system, then you burn through your nutrients too quickly. There’s a fun balancing act there.

The other part of answering this paradox comes in the following sections.

Constraint Interactions

In general, as an organism gets larger, process rates decrease, but process efficiency increases (largely due to organ system improvements, as far as I’m aware). So in Thrive, process rates decrease as mass increases, and process efficiency increases as your organism gets more advanced and gets more room within itself to add more dedicated parts.

ORGAN SYSTEMS

The Respiratory System, Digestive System, & Constraints

Organs in these systems will have their own surface area measurements, influencing their process rates and efficiency. Along with the inherent stats of the organ, surface area will play an important role in these processes, moderating efficiency.

Less advanced digestive/respiratory systems tend to have less inherent surface area. This means that they scale poorly, requiring changes to the organ systems as you evolve. This will influence certain adaptations as well - for example, amphibians, which rely heavily on their skin for respiration, will face challenges at larger scales that less surface-area dependent organisms evade due to their reducing ratio. Book lungs, as seen in arthropods, scale up poorly compared to vertebrate lungs. And external gills, like those in certain amphibians, fish, and primitive organisms, don’t scale very well.

On the other end however, certain adaptations larger animals utilize to increase surface area. For example, additional folding in larger lungs or digestive tracts, or in our brains for example - just don’t have the same marginal benefit when scaled down. In many ways, that’s because of volume constraints - you just can’t fold something enough times within a smaller area to maximize surface area.

So ultimately, this means…

To pace organ progression, we can make it so that certain organs work better at certain masses/scales.
If an organism is too small, the additional energy coefficient of more advanced organ tissues could be more costly than whatever surface area benefits a really advanced organ system offers.

If balanced properly, this should result in a solid progression system.

The Circulatory System

Process efficiency is well-handled via manipulation to the organ system itself, but the circulatory system will be the most important tool in manipulating the Process Rate of your organ systems.

At the beginning of the game, process rates will operate quickly enough to not require dedicated circulation system handling due to smaller mass - again, smaller organisms inherently have faster process rates. However, as you increase in size, your process rates can slow to the point that your processes don’t operate quickly enough to keep your organism energized. Therefore, the circulatory system will be a necessity for any advanced functions and scale.

This reflects real life evolutionary biology - smaller organisms generally don’t need a dedicated circulatory system, instead relying on hydrostatic forces to distribute resources efficiently. With scale however, it gets much harder for things to get where they are supposed to via unfocused channeling, hence the rise of a circulatory system.

As such, the circulatory system will become a really important measure of progression, needed to get energetic organisms at large scales. More advanced circulatory organ systems have a steeper energy coefficient attached to their mass, so that provides some scaling to ensure you can’t just spam blood vessels throughout your body, or in situations that aren’t necessary.

CONCLUSIONS

There’s a lot going on here, and it sounds really complicated to present to a player. But here’s the jist of it…

Mass and Surface Area to Volume Ratios are really important in influencing metabolism. Mass determines your metabolic costs, and surface area helps with generating energy.
Mass increases quickly if you scale up, and surface area to volume reduces. This presents challenges.
To generate energy, you must gather enough food, turn enough of that food into nutrients, and turn nutrients into energy quickly and efficiently enough to not die.
Different parts of your organism correspond to different parts of this metabolic process. You need to develop these parts in a way that balances the other out; you can’t just invest in one.

I think these fundamentals give us an exceptional base to make the constraints and Internal Organ Editor really engaging. Intertwining these with constraints…

Seamlessly represents various advanced biological phenomena in an approachable way. Different organs work better at different scales. Different sized organisms have different strategies, influenced by their metabolic needs.
Seamlessly represents size-related adaptations. Larger animals trying to maximize surface area for various reasons - heat tolerance, for example - have much more dedicated structures than smaller organisms, like sails, the ears of elephants, etc. On the other hand, smaller organisms which must stave off the cold must more intently adapt their body to minimize surface area.
Presents a solid pacing to progression. Scaling up or down requires intentional changes to your organ systems and bone structure. Organ systems cannot be developed independently, and must be handled in a way that doesn’t neglect one aspect of the editor.

One thing to address is how movement costs scale up with size. I think it will ultimately look like “your mass influences your movement cost, and your skeletal system influences both your mass and how it correlates to your movement cost.” Ultimately, your skeletal system can be very important for movement costs. So it’s something I think can be figured out in a dynamic way as well.

It’s also important to design the organ system editor in a way that makes things engaging for the player. Tweaking the circulatory system should reflect how important it is to ensure blood gets to wherever it needs to get in your organ system. Various customization options should be derived from player experimentation with the organ system.

Considering auto-evo is also important. Similar to how we treat unlocks for auto-evo in the Microbe Stage, auto-evo in the Macroscopic Stage should be much more free to create the organ system needed for a certain scaled organism than the player is. Just as we didn’t want unlocks to slow down CPU microbes, we don’t want an auto-evo that is really struggling to keep up with the player because it keeps tripping over its internal organ system, something that the player doesn’t even really see during active gameplay.

Beyond that, I think the importance and value of a constraint system has been reinforced - once again, basing the Macroscopic Editor on overcoming consistent parameters the player must deal with results in intuitive and engaging gameplay mechanics. The editor mechanics I propose in my prior Macroscopic Stage might be open for interpretation, but I do think constraints should be cherished in whatever Macroscopic Stage mechanics we go with.