First time poster, let me know if this is not about world building.

I am not asking about the function of respiration nor the chemical effect of oxygen and stuff. I am asking if there is any good reasons to force my creatures "to repeatedly perform an action for their whole lives in order to maintain their lives, while such action does not require any extra input and is essentially free and involuntary".

While these continuous life supporting actions include also eating, drinking, resting and excreting, all these require extra work or effort (e.g. hunting, sheltering, cleaning), only breathing is basically free. Air is provided, no harvesting is needed. Supply is virtually infinite, no over consumption issue, hence no war, no fighting because of resource depletion.

So why would I create creatures in my world requiring them to perform this seemingly unnecessary action to consume an infinite resource provided for free?

It seems to me this is just a mean to separate the territories of my creatures, e.g. terrestrial vs aquatic creatures. While the resource is infinite, the tools capable to consume the resource can be different, some specialised for one, some both. Another reason I could think of is to limit the movement boundaries of my creatures by setting up a space that does not have this resource, so that my creatures would hold captive their entire lives within an enclosed place.

Those are just my thoughts though. But then again, as civilisation advances, my creatures would overcome this limitation and start colonising in other territories or even planets, so it seems this is only there to delay this process? I hope there can be some important reasons behind to justify doing this that I overlooked?

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    $\begingroup$ Welcome to Worldbuilding. Are you looking for an answer to a concrete problem or just for some philosophical discussion? Because here we do the first, and I don't see trace of a concrete problem in your question. Also, I would challenge that breathing costs no effort or that it is free and available everywhere. Last time I dove to catch some fish I had to come to surface from time to time... $\endgroup$ – L.Dutch - Reinstate Monica Feb 26 '18 at 12:52
  • $\begingroup$ "Air is provided, no harvesting is needed " : why do YOU breathe then ? $\endgroup$ – GlorfSf Feb 26 '18 at 12:57
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    $\begingroup$ I don't get the question. Are you asking why your animals should have to breathe? Well, if you don't see a reason, don't have them breathe. There is no rule that says: Do whatever you wish in fiction, go faster than light, travel through a black hole, have a dog play soccer, just don't have animals that don't breathe. $\endgroup$ – Raditz_35 Feb 26 '18 at 13:13
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    $\begingroup$ Are you asking how to design an animal that doesn't have to breathe? If so, state so $\endgroup$ – Raditz_35 Feb 26 '18 at 13:24
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    $\begingroup$ Then this is sadly off-topic here, see the first comment: "Are you looking for an answer to a concrete problem or just for some philosophical discussion? Because here we do the first". I believe the question as I have asked it would be on-topic, but I of course might be biased $\endgroup$ – Raditz_35 Feb 26 '18 at 15:43

There are many creatures in our world that do not have to move their gills/branchea/trachea/lungs in order to breath. Earthworms, flatworms, sponges and microbes, for example. They just let the oxygen diffuse into their bodies.

Notice that while the oxygen supply in our atmosphere is (for practical purposes only and from the point of view of an animal) practically infinite, the speed at which you absorb it is not. There is just so much you can take in at a time. This flux limitation is also a threshold for how intense your activities may be. Many animals have evolved specializations that help them take in larger amounts of oxygen, and this is usually associated with animals that rely on intense bursts of speed or endurance in one way or another.

For example, sharks and tuna increase their amount of oxygen intake (related to similar sized fish) by ram-breathing. If they stay in one spot without moving they may suffocate.

Flying birds have their chest muscles attached to their sternum. This means that when flying, each flap of the wings causes a full intake/outtake cycle of air to happen. This is how humming birds are capable of flapping their wings so fast, and it also helps other species during long flights, or when they need to either hunt or escape from a predator.

In your world where creatures just let oxygen diffuse through their bodies, whatever species develops a way to increase the flow of air through them will become much "faster", possibly stronger (more oxygen for the muscles) than their competitors, and may have an evolutionary edge depending on the environment where they live.

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    $\begingroup$ Insects do that, but without an air pump it's very limited and one of the reasons insects are so small $\endgroup$ – Kilisi Feb 26 '18 at 13:40
  • $\begingroup$ Thanks. So you are suggesting even though the resource is free, the rate of consuming this life supporting resource can be a mean to control the evolution rate of different creatures? $\endgroup$ – user1589188 Feb 26 '18 at 14:04
  • $\begingroup$ @user1589188 no, I had not thought about controlling evolution. My point is that for two creatures of similar shape, mass, habits etc. A creature that is able to use harness more energy (for hunting/escaping, for example) will have tremendous advantage over their slower, weaker counterpart. On a very high level that is why animals that cannot rely on speed have to rely on hard shells/thick hides or camouflage to survive. At the same time a usual top predator feature in any ecossystem is being strong and fast, which implies large lungs or ram-breathing. $\endgroup$ – Renan Feb 26 '18 at 14:11
  • $\begingroup$ @user1589188 we could make an analogy with computers. Electricity from a wall outlet is practically unlimited for the purposes of feeding a computer. If you use a machine just to play solitaire and minefield, you don't need an expensive NVidia/Radeon chipset, and therefore you don't need a powerful UPS. But if you are going to play the latest FPS games you are going to need a graphic chipset that will probably require a 600+ Watts UPS in order to work. The UPS is then, to the computer, what a lung is to an animal. $\endgroup$ – Renan Feb 26 '18 at 14:26
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    $\begingroup$ @user1589188 nature does not have wants. Nature does not even have needs. Gills and branchea evolved underwater, and lungs and trachea above water, because each confered to their owners significant advantages. $\endgroup$ – Renan Feb 26 '18 at 14:37

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