Metabolic Rate System


I’ve been wondering a lot about the transition from microscopic to macroscopic, and how we will design gameplay as an early 3D organism. We have a very good understanding of what Microscopic organisms function like, but the concept for Macroscopic organisms is more murky. We have vague ideas like that we want the player to be able to place organs like hearts, lungs, stomachs, etcetera. But this raises so many questions:

  • What do these organs do?
  • How do they work together?
  • Can an organism evolve the highest level heart possible without ever evolving a lung?
  • And how do we make it possible for simple organisms to not even have a heart? Or not even have a stomach?
  • And how do we design this in such a way that it feels like a natural continuation from what you played through during microscopic gameplay?

I recently came up with an idea to address all of this called the Metabolic Rate System. I would usually keep this in my notes and save it for later when this topic comes up, but I feel like it actually ties into some gameplay features we have planned (such as the Diffusion Rate trait I’ve discussed before). So I want to bring it up, explain what I think makes it worthwhile, and how it would impact our future game design if accepted.


There will be a new trait called Metabolic Rate. This represents the total amount of energy that the organism can produce in any given second. Think of the organism like a power plant, and their Metabolic Rate is the total electricity they can produce per second. Metabolic Rate will be used to determine a creature’s top speed, strength, stamina, and many other variables. It will almost be like ATP in the microscopic stages.

However, for a creature to produce energy, they must consume food, water, and gas. So we will add in traits like Circulation Rate (the rate at which gas is circulated to the tissues), Hydration Rate (the rate at which water is absorbed by the creature) and Digestion Rate (already exists. The rate at which food is absorbed by the creature). ALL THREE of these rates will be needed to maintain a creature’s metabolism, therefore the Metabolic Rate will be calculated as the LOWEST of these three rates of a creature.

One extra layer: Digestion Rate and Circulation Rate will both depend on another rate themselves. Ingestion Rate will represent how quickly an organism can consume food, which is then passed over to digestion. Respiration Rate determines how quickly an organism can absorb gas, which is then passed over to the blood for circulation. Both Digestion Rate and Circulation Rate will be limited by Ingestion Rate and Respiration Rate, respectively.

Therefore, the final chart comes out to be:

If any of these three components of the Metabolic Rate ceases, an organism will begin to die. Therefore, a creature can die of dehydration (Hydration Rate at 0), starvation (Digestion Rate at 0), or suffocation (Circulation Rate at 0).

A creature can of course stock up on a certain resource so that they do not need to constantly produce it. A filter feeder may constantly be ingesting food, but a predator may have an irregular eating pattern. Instead it does not eat for a while, then catches a big prey which lets him consume a lot of food all at once and store it, slowly digesting it and feeding its metabolism until he can find his next prey.

This system is also very flexible to different metabolisms. The metabolism of a creature defines what rate it needs of each component. Some creatures may not need to eat at all, surviving purely off of water and gas. Some creatures may survive on a different fluid than water, but we will still refer to it as hydration. Different foods (plants, meat, inorganic material) will all fall under food that is consumed under the Digestion Rate component.


  • It gives a function for the many different digestive, circulatory, and respiratory organs we want the player to be able to place in the Organism Editor.
  • In nature, the digestive, circulatory, and respiratory systems are highly linked. This is the only realistic way I can think of of tying all three systems together. A name I’ve seen mentioned is the “Gastrovascular System” to refer to these three systems working together.
  • It creates natural bottle-necks that means that you can’t evolve one part of your Gastrovascular System too far without making sure other parts are appropriately evolved. For example, a larger heart is useless if you cannot feed it with more gas, which may require you to first evolve gills before you grow your heart. Perhaps a small aquatic creature has no problems with hydration and circulation, so if he wants to increase his metabolism only digestive system mutations would be useful. This is actually very realistic, as when we look at nature we see that species have always had to evolve their Gastrovascular System in this stepwise process.
  • It ties in well with the Microbe Stage. Diffusion Rate is already a trait I’ve suggested before that should be added, AKA a variable rate at which a cell can absorb compounds from the environment. This would effectively be the Respiration Rate.
  • Because multicellular life starts simple and underwater, most of this chart would not apply at first. Hydration Rate would always be filled because the creature would be surrounded by water which will constantly flow into their organs via their skin (until they evolve thicker skin that cannot do this). They will have no circulatory system, so their respiratory rate will translate straight into metabolic rate. They will have no digestive system, so their ingestion rate will flow right into their metabolic rate. So it will just be 2 nodes feeding into 1 node, and progressively become a more complex tree as the creature evolves to become more complex.
  • It allows for simple life to never evolve some of these systems. A creature can ingest food via its skin, and respire via its skin. Therefore such a creature will never need to evolve a heart or a stomach or an intestine.
  • This will naturally produce a barrier for aquatic organisms to adapt to living on land, since living underwater will provide them with a constant flow of water and living on land will suddenly take that away. So creatures will have to evolve a means of finding and storing water on land if they want to survive there.
  • With this system in place, there are few other systems needed to implement all the remaining organ types. Reproductive organs are straightforward enough. Endocrinal organs will be a part of the Agent System. Integumentary Parts (skin, fur, scales, horns, spikes, shells) only confer bonuses to existing traits (physical defence, chemical defence, speed, etc.). Muscles and skeletons are also straightforward enough. It’s only really the Nervous organs and the Sensory organs that may need an extra system or two to give them purpose.


Here are examples of how the many planned organs (weird to say this for once instead of organelles) can have a use now with such a system:

  • Gill Slits: Increases ingestion rate of filter feeding while swimming by 50%. Increases respiration rate while swimming by 10%.
  • Gills: Increases respiration rate while swimming by 50%.
  • Blood: Enables Circulation Rate (base rate is 100). Increases Respiration Rate by 100%.
  • Aorta (Simple Heart): Increases Circulation Rate by 100%.
  • Gastrovascular Cavity (Simple Mouth/Stomach): Reduces Digestion Speed by 50%, but increases nutrients yielded by digestion by 100%.

Next Steps

  • If it’s agreed that such a system makes sense, I can document it on the wiki for future reference.
  • We can look at how the Microbe Stage should transition into this system. Perhaps we could make the traits in the Microbe Stage renamed to match these names to make it more of a seamless transition?

A great backbone to work with here, nice thinking! I believe this is just the right amount of complexity for managing an organism’s vital statistics without going overboard with the nitty gritty details. We just need to be careful about how we decide on implementing this, else many players may find themselves overwhelmed by it still. The key is in gradually introducing new elements, and allowing the player to run into them at their own pace. Luckily for us, this problem mostly solves itself.

Ingestion rate is self explanatory for the most part, and shouldn’t be hard for players to figure out. They can absorb microscopic prey and detritus as a basic filter feeder, or develop a proper mouth to eat more or larger prey.

Digestion is a stat players will already be familiar with from the microbe stage, so it shouldn’t really present any additional complexity, even when developing specialized organs for it.

Respiration will undoubtedly be the new osmoregulation that players will contend with as they grow bigger and more complex. They’ll now need to keep an eye on how much gases their processes require, and make sure they are able to meet the needs of their processes. We want to make sure that this is forgiving enough that players can comfortably experiment with straining their limits without going extinct immediately.

Circulation is a brand new concept that players will be presented with. I like to think of it as a progression gate like the nucleus as players will need to develop some form of it before they can become bigger or more complex. We need to make sure that players are easily able to understand what is required, and how to acquire it.

Hydration could be easily ignored, or really just packaged into respiration for the most part until the player begins to emerge from their aqueous origins. However; I recognize that the ability to separate salt from water is the defining difference between salt and freshwater species on Earth, so we need to decide on if we want to require players to develop these capabilities themselves and how in depth it will be, or just omit it for ease of understanding. I suppose we could package it as a salinity tolerance stat and leave it at that.

Respiration and hydration I feel are pretty easy to carry over from the simpler stages as we can easily tie it to the stats of cells; High compound absorption of tissues can tie into increased natural respiration and hydration intake, and be reduced by organism size. Meanwhile, the respiration requirement would scale with the amount of organelles that require gasses to function.

This gives new multicellular organisms a passive ability to survive as long as they stay under a certain size threshold. Thus species will need to develop proper respiratory and intake systems to increase the soft cap on organism scale, or otherwise maintain a slower respiration rate by relying less on aerobic processes.

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Was just thinking, such a system could actually be quite easy to display to the player. We could have a “Metabolism” tab in the Organism Editor, that shows what the organism depends on for survival. Then we could have little Metabolism Trees that show the different food, gasses, and fluids that the organism needs for different functions.

This was just a super quick mockup, but for example an oxygen breathing carnivore would have this as their tree. Eat meat, fat, and bones. Breathe oxygen. And drink water. And your body will slowly turn these into energy.

Then you can click on Growth to see what compounds your body uses to grow you (from childhood to adulthood). Typically these will be the exact same.

Other Metabolism trees could include Poison/Venom/Toxins, Electricity, Pheromones, etc. The game would basically add them in for whatever extra metabolic functions you have.

Based on what the cells of that organism evolved to eat in the Microbe Stage, these trees will reflect different combinations of compounds needed to eat, breathe, or drink.

Lastly, we could have an energy balance bar (perhaps at the top of this panel) to show how much energy is produced each second by this organism’s metabolism, and how much of that is being spent on maintaining different parts or activities. The Energy Balance Bar, like the ATP Balance Bar, assumes you have all raw materials available. Obviously, if one of these compounds runs out, your organism stops producing energy.

EDIT: I think visualizing it like this greatly helps to imagine how all these systems will work together and be presented to the player. I may go back and create a more detailed visual concept of what the Metabolism tab could look like to represent the Metabolic Rate System.

Okay, as I mentioned on Discord, here is my megapost to bring together all my concepts on the Metabolic Rate System, Organism Editor, and Organism Gameplay, and how they all tie together.


To recap, this all started because I was researching what organs evolve into what organs (like gills into lungs), and was assigning functions to some of these organs. I then realized it was hard to assign functions to any of the Circulatory, Digestive, or Respiratory organs because we have absolutely no idea how those systems will work, so here is a comprehensive concept.

I will go through the different aspects of the game, and explain how this system will look for each aspect.

Organism Editor

We’ll start with the Organism Editor.

Metabolism Tab

The Organism Editor will have a Metabolism tab. This tab will contain the organism’s Energy Balance Bar, and the organism’s Metabolism Trees.

Energy Balance Bar

The Energy Balance Bar will be identical to the ATP Balance Bar. It will show you your sources of energy and your sinks. Energy can be consumed by locomotion and other activities, but it can also be consumed by maintaining organs like a poison gland or gills or a complex brain. Having an Energy Balance Bar will maintain continuity with the ATP Balance Bar of the Microbe Stage and be one less feature the players will have to learn in this stage. I had not included a concept since this feature is self-explanatory.

Metabolic Processes

Below the Energy Balance Bar will be the Metabolic Processes section of the Metabolism Tab. These will show all of the metabolic processes the organism carries out.

Collapsed view of the Metabolic Processes

Every metabolic process will be classified as being for the purpose of Energy, Growth, Toxins, Pheromones, Electricity, or some other category. In the collapsed view above, we see all compounds your organism needs to produce that particular product (with ratios). In this case, the needed nutrients for this organism are meat, bone, fat, oxygen, and water. These are all used to produce energy. They are divided into solid, gaseous, and liquid compounds based on whether you must eat, breathe, or drink them (which ties into Hunger, Respiration, and Thirst, which I’ll discuss later).

Expanded view of the Metabolic Processes

In the expanded view, we see how many exact metabolic processes produce that product, and we see the exact ratios (with possible alternatives for any reagents). We even see the names of the metabolic processes, which carry over from the Microbe Stage. Note that the metabolic processes you evolve in the Microbe Stage will determine your metabolism once you are a macroscopic organism.

In this case we can see the organism consumes meat for a medium amount of energy, and fat for a large amount of energy. We can also see he has evolved to consume bones as well as a substitute for eating meat, which means his body can use either compound for “Protein Catabolism”.

Also note that the Eat/Breathe/Drink distinction allows for many different feeding types. Underwater creatures don’t need to drink. Plants mostly only need to breathe. Some animals may need to eat a lot and some very little. Some will need to eat extra compounds to produce energy because they have unique metabolisms.

So to summarize, the player should be able to fully understand how much energy they produce, and what they need to eat to get that energy, from looking at the Metabolism tab. They can also get extra information like what they need to eat to grow, produce poison, etc. Basically they now know what to eat and how much energy and growth and other results it brings them.

Organism Mode

Now on to gameplay.

Nutrient Panel

Organisms will have a Nutrient Panel just like how cells have the Compound Panel. It’s basically identical. Nutrient Panel is just a more accurate and less technical sounding word that players will be able to relate to better. By definition, anything that you eat for some metabolic process is a nutrient, so even iron and hydrogen sulphide can be nutrients.

Note that there are also separate Hunger, Respiration, and Thirst Bars, discussed below. These “Instinct Bars” are meant to show you an overview of how many nutrients you have left simply organized by solid, liquid, or gas, whereas the Nutrient Panel breaks it down to show how much of each specific nutrient compound you have stored.

Instinct Bars

The Instinct Bars refers to the bars in the bottom right corner of the screen that display the health and condition of your organism. In the Microbe Stage these were just Health and ATP. Here, they are different.

Health Bar

Here you see the overall health of your organism.

We could make it so that as an organism ages past adulthood, their maximum health slowly decreases (it would not count as damage).


Health can be reduced by Damage. Damage comes in several different types. Some already exist in the game, like Physical and Chemical:

  • Internal Damage: I was originally going to name this as Physiological Damage, but it was too similar to Physical Damage. Perhaps we could rename Physical Damage to Concussive Damage? Anyways, this refers to damage that you take from a lack of food, air, or fluids (AKA starvation, suffocation, or dehydration). The way a lack of any of these three would cause damage is that they would reduce or eliminate your energy production. And just like how in the Microbe Stage, a deficit in ATP production causes your health to slowly reduce, so too will an energy deficit here cause gradual damage. We could expand it a bit more if we wanted to, and make small energy deficits at first not damage you but just make you less efficient at doing things.
  • Physical Damage: Already exists in the Microbe Stage. In the Macroscopic Stages this will refer to damage from Slash (claws, talons), Blunt (clubs, tail swings, punches, kicks), Pierce (horns, teeth, tusks), or Grapple/Crush (jaws, arms/legs, tentacles). This can be discussed in different threads.
  • Chemical Damage: Can be discussed more in a different thread.
  • Electrical Damage: Can be discussed more in a different thread.
  • Environmental Damage: Damage from extreme Acidity, Temperature, Pressure, or Salinity. Can be discussed further in a different thread.

Energy Bar

When the bar is full, it means your organism is producing 100% of the energy it needs. Anything below that shows a deficiency, and you will receive penalties (and your health will decrease over time).

I suggest that we make organisms only produce as much energy as they need. So if their Energy Balance Bar has a surplus, they just naturally scale down their energy production to match their need. If we don’t do that (and keep it as it is currently in the Microbe Stage), we could simply have a full bar represent 100% or more energy production (compared to need).

As discussed above, if energy falls below 100% of the need, we could have it apply different negative modifiers, or slowly damage you, or some combination of effects.

Hunger Bar

The hunger bar shows how much food you have stored out of your total storage capacity. If you have fat reserves, a notch in the bar shows where your regular food storage ends and where your fat reserves begin. If you eat above that amount you start producing fat, instead of having the food go into storage (stomach or tissues).

Food is intaken via passive absorption or via active ingestion. For example, early life will be able to just swim through water and absorb the nutrients out of the water through their skin. However, more complex life will need to find a way to more actively predate, or to produce food themselves (plants). Food is stored in the tissues or in a specialized digestive tract.

There do not need to be any additional penalties to your Hunger/Food bar reaching zero, since it is already represented by your energy production falling/halting if you run out of food.

One last thing, this and the following two bars could perhaps be moved to a different part of the screen, because otherwise we would have five different bars in the bottom right corner.

Respiration Bar

The respiration bar shows how much gas you have remaining. The bar is full when your gas storage (or lungs) are full. Unlike the Microbe Stage, gas needs to be acquired, stored, and consumed.

Gasses are intaken via passive breathing, or via active breathing. For example, some underwater organisms need to swim to get air to flow through their gills and inhale it, which means they’ll eventually suffocate if they stop moving. Gas is stored in the tissues or in a specialized respiratory tract.

This bar will rarely drop. It will only drop if you are an aquatic organism taken out to land, or a terrestrial organism thrown underwater. It also reduces if you perform a lot of strenous activity at once and use up all of the gas you have in storage faster than you can inhale more. Therefore, respiration (as well as hunger and thirst) are all different components of how much stamina you have (how long you can maintain energy expensive activity).

Thirst Bar

Still not sure whether this bar should come third or second in the UI.

The Thirst bar is full when your fluid storage is full. This will mean it’ll almost always be full for Aquatic organisms, and only deplete for terrestrial or aerial organisms.

Fluids are intaken via passive absorption, or via drinking. It is stored in the tissues or in a specialized digestive tract.

Other Factors

Environmental factors like Temperature, Acidity, Pressure, Light/Radiation, Salinity could also be represented by bars, but I think that would clutter the UI far too much. Instead, I think these are better represented by notifications. If you are in a normal state for any of these, you receive no notifications. If you are trending towards an extreme for any of these, or are currently in an extreme state of any of these (for example a temperature outside of your tolerable range), you receive a notification on the side of the screen.


So what are the effects of choosing such a system for the Macroscopic Stages?


  • Digestive system organs now have a purpose: This now enables the organs of the digestive system to have useful effects. They can store foods that have been ingested but need to be digested. They can increase the amount digested per second, or increase the efficiency of digestion (so no nutrients are lost). Some organs can specifically be evolved to improve digestion of a specific compound (like the foregut for ruminants to eat grass).
  • Respiratory system organs now have a purpose: This now enables the organs of the respiratory system to have useful effects. They can increase the rate at which gasses are inhaled/exhaled. They can allow gasses to be stored up. They can increase the efficiency at which gas is extracted from your breath. They can enable passive or active respiration. They can enable breathing underwater or above water or both. They can enable holding your breath or breathing in low gas environments.
  • Circulatory system organs now have a purpose: This now enables the organs of the circulatory system to have useful effects. The whole system acts as a second bottleneck after respiration. It improves the efficiency of respiration and allows for larger organisms to get gasses from their skin/mouth deep into their tissues where they need it. It also means the circulatory and respiratory systems need to evolve in tandem (since circulation depends on respiration).
  • Organisms don’t need to evolve Digestive, Respiratory, or Circulatory systems, but it greatly helps them to do so (if they can manage the tradeoffs). This allows for both simple and complex life to exist.
  • Models many physiological states with one system: This system models hunger, thirst, respiration, fatigue from a lack of any of them, and death from a severe lack of any of them.
  • Easy to understand: I know I wrote a lot, but at the end of the day this is actually a relatively simple system. You have three bars to track; Hunger, Respiration, and Thirst. All three bars are very intuitive as these are instincts we have ourselves. There is a clear Metabolism tab in the Editor that shows you what you need to eat. And a deficit of any of these three bars causes you to have less energy and potentially die.

Next Steps

If we accept such a system, I can start updating the wiki with these concepts. There is not much more to do beyond that.

It may also prompt us to consider how the Microbe Stage can be designed, going forward, to have as much continuity as possible with this system.

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