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Simply put is there an upper bound for the largest possible living creature in a scientifically rational world without incorporating anti-gravity, magic, the force, or other physical concepts we are not aware of?

In order for me to consider it living, it must respire (though not necessarily breathe), repair itself, grow/have grown in some way, and be able to react to stimuli in a way inanimate objects do not. If you have other suggestions as to what belongs there, I am willing to hear them, but I want to permit exotic creatures.

If a portion of the body does not have properties I would attribute to life, it does not count towards the size of the creature. For example, your hair does not count towards your size even if you have a REALLY big afro.

Intentionally I am not putting many restrictions. I do not care if it is terrestrial, mobile, reproductive, or sapient. All I care is that it is as big as possible but reasonably possible knowing what we do about physics, chemistry, and biology.

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  • $\begingroup$ Here's a problem, I see no reason why something (many times) larger than the observable universe couldn't be a living creature. We just can't fathom a creature like that. You may want to edit your question to cut out that possibility. $\endgroup$ – overactor Oct 9 '14 at 13:48
  • $\begingroup$ your overall parameters would not exclude a robot that could repair itself and add parts or "grow" $\endgroup$ – bowlturner Oct 9 '14 at 13:48
  • $\begingroup$ Compare with largest marine animal. Can we assume this question is not restricted to animals (can include plants, fungi, etc as long as it reproduces as a whole)? $\endgroup$ – trichoplax Oct 9 '14 at 13:49
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    $\begingroup$ It makes a big difference whether the life form is to evolve naturally from scratch, or be built for a purpose. $\endgroup$ – trichoplax Oct 9 '14 at 14:31
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    $\begingroup$ Please note though, i think it is important that it is one creature, not an unconnected nebula called one with a few repair droids in it. $\endgroup$ – kaine Oct 9 '14 at 14:35
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The thing is, gravity is actually quite a weak force. You need a lot of mass for it to start to be significant — and it falls off with the square of distance so the effects of the mass also fall off fast.

Imagine a life-form shaped like a giant net. It gathers space dust into itself to grow, using light from stars both to power its growth and for propulsion as a massive light-sail.

The strands of the net are far enough apart that gravity falls off faster than it gathers and the strength of the strands is more than enough to keep its shape.

These creatures could grow to theoretically unlimited size, just constrained by raw materials and solar energy. You could well see them sweeping into star systems and raiding the rings around planets, asteroids and even small moons for raw material — cleaning them out then moving on growing all the time.

They could reproduce by firing off spores — or more likely just by splitting in two once their size became too large for them to sustain with available resources.

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  • $\begingroup$ This is closer to what I was hoping for. Do you have any information about what dimensions it would have? If you could confirm boundless size, what density could it have? $\endgroup$ – kaine Oct 10 '14 at 15:23
  • $\begingroup$ Boundless size assuming it can find the raw materials to grow with. The tentacles/vines/whatever could have whatever density you like, the critical thing is that the total density per cubic area be low enough. For exact figures on maximum density needed we might need to take this to physics.SE $\endgroup$ – Tim B Oct 10 '14 at 15:26
  • $\begingroup$ I wonder how such an organisms evolutionary line would begin, perhaps the building blocks for life come together in space on an asteroid instead of on a planet $\endgroup$ – TrEs-2b Oct 14 '16 at 20:28
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Your best bet for large organisms (with earth-based biology and understanding) would be either fungi or plants. We can consider something like the giant sequoia to be rather large with heights reaching over 300 ft or trunks 100 ft thick these are very larger structures.

However, those are fairly small when compared to the quaking aspen grove located in Utah(tree): This single genetic creature is a large grove of trees that are all derived from a common large root system. Estimated to weigh over 6000 short tons.

In terms of coverage area though, some fungi can claim to be largest, covering several square milesthrough a network of mycelium that is located inside trees and underground.

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    $\begingroup$ I agree that a plant-like structure would be the best way to get a large terrestrial one. Especially if cultivated and put on a planet with unnaturally perfect conditions for it, how big could we realistically imagine something like that could get? $\endgroup$ – kaine Oct 9 '14 at 16:19
  • $\begingroup$ Well, if you could coat earth 100% VERY deep at the density of wood which i don't know if you could do would result in a mass of 10^23 grams. $\endgroup$ – kaine Oct 9 '14 at 18:14
  • $\begingroup$ Some of the speculation regarding both of the examples above point to no/low competition and no good natural predators, so in ideal conditions I would say the scale is probably boundless $\endgroup$ – Culyx Oct 9 '14 at 18:24
  • $\begingroup$ The point of this question is to figure out what that, probably planetary scale, bound would be. It isn't boundless or it would collapse under its own gravity. $\endgroup$ – kaine Oct 9 '14 at 18:26
  • $\begingroup$ @kaine I remembered this xkcd what if article, sort of relevant since they discuss a "planet" out of a mole of moles but in general it would seem to suggest that the size is dependent on a number of factors like how much pressure can crush/injure your organism (how deep into the core of a large structure could it go?) what-if.xkcd.com/4 $\endgroup$ – Culyx Oct 9 '14 at 19:04
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For some more details, see my answer over at "How do I prevent my turtle from collapsing under its own gravity?" linked on the right.

The short answer is that biological entities are dynamic structures which use energy to maintain structure and offset forces like gravity. That make it hard to calculate what a maximum size could be because you have to calculate not just gravity by all kind of mechanical forces, magnetic etc.

If we want to create a planetoid or planet sized life form with the goal to create the largest appearing organism, something that could be mistaken at a distance for a natural geologic object.

It will have to be able to harvest energy from space. Sunlight, solar wind, cosmic rays, etc are all good energy sources.

It will start small, perhaps a few kilometers in size. But, it will grow with the intention of reaching a maximum size. As a life form, it will need to move so it will produce some way, likely magnetic or solar sails

To offset gravity, it will begin spin as it grows and keep adding angular momentum as it grows spinning faster the larger it gets. It's goal is to balance gravitation with centrifugal force.

Planets cannot do this, their angular momentum is fixed when they form, even if they are altered by impacts. A planet can never so fast that it's gravity would be significantly offset. If so, it could never form in the first place.

The life form can though. In principle it could spin so fast overtime that parts of it would experience little gravitation contraction forces at all.

The problem would be keeping a spherical shape. In a spherical shape, the equator spins faster than the poles. In a gravity reducing spin, the equator would be nearly "weightless" while the poles would have their "weight" consummate with their mass. To avoid this block, the life form could grow into toroid to keep most of the mass towards the equator. A cylinder shape would be even better so that more of the mass is a fixed distance form the axis of rotation. Make the ends open and most interior very light weight like thin girders, perhaps even hollow.

Although I haven't run the numbers, not sure I can for such a shape, but I think you could get something with the apparent surface area of earth, using biologically plausible materials eg graphemes.

But... there is likely a biological limitation: cancer.

In a living organism with a vast, vast number of cells or cell like structures, each with the potential to reproduce on it's own, cancer would likely limit size. Cells naturally replicate themselves. Multicellular organism can only exist because of genetic mechanism that suppress that innate tendency. When those mechanisms fail, cancer results.

With a vast, vast life form, you would have equally vast number of cancers. Cancers actively evolve, under natural selection shedding more and more safeguards until they run wild. In a life form really to big to kill anytime soon, natural selection would shape the cancers into life forms that would occupy niches in the internal ecosystem. Over time, these would evolve into different species and then eventually symbiots. A vast organism would likely live millions of years in that time would gradually turn from a single organism to complex ecosystem.

So, the OP's original criteria of a "single organism" not an ecosystem, nebula or planet with life, might not be met, likely by anything any larger than a mountain.

If life lives long enough and grows large enough, it becomes many things.

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    $\begingroup$ Created a local account here, just to applaud your characterization of cancer as something that arises, migrates, and develops or even evolves! I would add that many large animals with long lifespans and more cells than a human manage not to get cancer somehow, or at least not as much as you would think they would from simple scaling. There's a tradeoff between efficiency of repair mechanisms and likelihood of cancer development. But even though it makes me shudder a bit, I love your image of bubbles of subclonal cancer cells developing into suborganisms! $\endgroup$ – Jenn D. Nov 20 '14 at 17:39
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This is only a partial answer (it only addresses land animals), but I don't know if anyone else is going to mention it, so I might as well.

Land animals rely on their bodies to support their weight. A lot of that support comes from their bones. Now, a bigger animal will have bigger bones, so the volume of one of them (I'll say the femur) must be larger. However, to support the added weight, the bone must also be thicker (i.e. greater in diameter). As animals get larger, the thickness of femurs keeps increasing until the thickness is unfeasibly large. Past this point, any animal could not have a femur of the correct size, and thus there is a weight limit (and thus size limit) to any land animal.

Note: This section was essentially a summary of Appendix A of my copy of Professor Walter Lewin's book, For the Love of Physics. It goes into more detail than I did here, but I think this paragraph should be sufficient to communicate this [minor, for the purposes of your question] point.

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On a planet completely covered by water could grow a massive alga, all around the globe. It respires as a normal plant does, and exchanges nourishments with plankton and other aquatic creatures.

This type of organism could be very large because you can imagine a rock core surrounded by very deep water, and the giant alga living near the surface, where pressure is reasonable and light is sufficient for its metabolic processes.

An even larger version of this could take place on a gas giant, where the "alga" is floating in the dense gas at mid altitude, respiring gases present in the atmosphere, using light for photosynthesis and taking nourishment from flying spores, bacteria and similar beings.

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  • $\begingroup$ This seems to be on the right track, but it doesn't answer the question yet - respiration aside, you're describing a planet, not an actual organism. Also, by your description, the creature would be breathing itself. If you elaborate further on how this could work, besides the respiration, it could be a good answer. $\endgroup$ – mechalynx Oct 10 '14 at 15:49
  • $\begingroup$ Sorry but I'm not sure to understand your comment. Why you say the algae would breath itself? It can "breath" as a real terrestrial algae does, producing oxygen during photosyntesis. $\endgroup$ – think01 Oct 14 '14 at 10:15
  • $\begingroup$ Yeah but if an organism is composed of algae (it's body is made of them) and they're breathing an atmosphere or sea that belongs to the organism as well, then the algae would be breathing the organism. In other words, if you had bacteria on your skin and they eat skin, they're eating you. $\endgroup$ – mechalynx Oct 14 '14 at 11:01
  • $\begingroup$ Maybe now I've got it. I never wanted to say that the "gigantic living being" is the planet itself. The alga (singular, not plural) is the enormous creature. $\endgroup$ – think01 Oct 14 '14 at 13:38
  • $\begingroup$ That makes more sense, but you should edit your answer a bit to make it more clear. You should also clarify where the atmosphere or sea comes from (does the creature carry it? does it collect it from somewhere?). Otherwise, nice idea, just needs some clarifications. $\endgroup$ – mechalynx Oct 14 '14 at 13:43
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In general, cube-square laws largely govern the scale of land animals, because volume and mass grow roughly as the cube of the longest dimension of the animal, while strength generally grows roughly as the square of it. There is also an issue of how you define size (weight, volume, surface area, longest dimension, etc.).

The other big factor that has historically (over the length of life on Earth) driven animal size is the percentage of oxygen in the air. More oxygen leads to bigger fauna, less oxygen leads to smaller fauna. It isn't entirely clear if this is mostly due to animal respiration or mostly due to availability of food.

Size limitations are less demanding in water (where gravity is effectively reduced) and would also be where gravity is low, because the need for a structure to support the body from collapsing would not be present.

In both land and sea examples historically, the largest animals and many of the larger animals tend to be herbivores rather than carnivores. The largest sea creatures are filter feeding whales, and the longest sea animal (a kind of filter feeding jelly fish) can have tentacles with a spread up to four times as long as a blue whale (but weighs much less than a blue whale). Among the large herbivores on land are the brontosaurus, the buffalo, the cow, the hippo, the elephant and mammoth, the rhino and wooly rhino, the giraffe and the panda.

It isn't clear if social animals like bees and ants with specialized individuals within a colony who are the only ones who can reproduce count as one animal or many animals (there is good reason to think of them as a single organism since no one individual in the colony is complete even over an entire life cycle). Ants, for example, can make up as much as 25% of terrestrial biomass in their territories (and are also often herbivores who farm their own food). One could easily imagine a variant of a large terrestrial herd herbivore that had a colonial bee or ant colony like structure (indeed, many farming operations already have some herd animals like studs who are specialized for reproduction and others who are specialized for food production). Is the fact that the intellectual action centers are decentralized in colonial animals relevant?

A similar issue to the question of colonial animals presents itself in Issac Asimov's 1989 novel "Nemesis" which involves an organism with many bodies but a unified mind connected by radio waves. A distributed computer network presents similar issues and one of the side plots in the science fiction novel "Blindsight" by Peter Watts' was that he wife was in charge of dealing with massive complex computer networks that developed consciousness. "Blindsight" also considers humans with "hive minds" that again raises the colonial animal question, and an alien organism for which the boundaries between the organism and the things created and used by the organism are vague. If the control systems of the largest skyscraper in the world became self-aware would it be an organism? If something can grow and repair itself but not actually reproduce, does it count?

Another issue related to the colonial animal one involves interdependent symbiont systems. For example, suppose you have a large filter feeder which has a moss-like growth all over it from which it sucks nutrition, or a parasite that can infect a large animal or plant or organism colony, and control it to some extent. At what point is it fair to call an entire ecology a single organism as under the Gaia hypothesis. https://en.wikipedia.org/wiki/Gaia_hypothesis

The hugest animal of all one could imagine would probably be a filter feeding in an oxygen and nutrition rich environment with little or no gravity. I could imagine, for example, a huge, lighter than the atmosphere, jellyfish-like or whale-like or airship-like shaped creature which would float around through and feed on the chemicals in the atmosphere of a large gas giant like Jupiter or Saturn, perhaps supplemented by an ability to absorb heat from its surroundings.

On the plant side there are larger organisms and the colonial issue presents itself again:

The largest single-stem tree by wood volume and mass is the giant sequoia (Sequoiadendron giganteum), native to Sierra Nevada and California; it grows to an average height of 70–85 m (230–280 ft) and 5–7 m (16–23 ft) in diameter. Multiple-stem trees such as banyan can be enormous. Thimmamma Marrimanu in India spreads over 1.0 ha (2.5 acres). The largest organism in the world, according to size, is the aspen tree whose colonies of clones can grow up to five miles long. . . .

The largest living fungus may be a honey fungus of the species Armillaria ostoyae. A mushroom of this type in the Malheur National Forest in the Blue Mountains of eastern Oregon, U.S. was found to be the largest fungal colony in the world, spanning 8.9 km2 (2,200 acres) of area. This organism is estimated to be 2,400 years old. The fungus was written about in the April 2003 issue of the Canadian Journal of Forest Research. While an accurate estimate has not been made, the total weight of the colony may be as much as 605 tons. If this colony is considered a single organism, then it is the largest known organism in the world by area, and rivals the aspen grove "Pando" as the known organism with the highest living biomass. It is not known, however, whether it is a single organism with all parts of the mycelium connected.

But, while your definition could certainly include a sessile organism, the requirement that it "be able to react to stimuli in a way inanimate objects do not" would seem to rule out many plant-like organisms. Surely a honey fungus colony or a large common rooted aspen tree wouldn't qualify under your definition. And, when it comes to reacting to stimuli, there is the question of how fast and how automatically.

Would a Venus Fly trap qualify? What about a trumpet vine that opens every day when it is light and closes when it is dark? What about something like Tolkein's Ents if it took them a full day to have a few sentence conversation? (The Portia spider which is a remarkably intelligent but very slow thinking animal is a less extreme example).

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  • $\begingroup$ Yes a venus fly trap and most plants count. For instance, they close their stoma to prevent excess evaporation when ir gets too hot/dry. They grow toward light. Etc $\endgroup$ – kaine Oct 15 '16 at 1:20
  • $\begingroup$ Another question concerned a Dyson Sphere made out of living material. So that blows away everything you are considering, which is limited to the surface of a planet. $\endgroup$ – JDługosz Oct 15 '16 at 1:56
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With no magic, and with only little twist on evolution, you may get planet-size living intelligent ocean as in Solaris, which is my favorite alien species, very alien and hard to compete with.

All you need if a planet with water, and all (remaining) life in water cooperates instead of competes. Then, it can be as big as the amount of water in all oceans.

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There is a theory that the universe itself is a living organism or its a part of living organism (even though there is no way to prove it). I think that's how big it can get. The universe grows reacts to changes and is composed of repeating structures like stars, black holes etc.. Living things are essentially network of repeated nodes (cells) which can change and respond to external and internal stimuli and can sense and control their surrounding to some extent so by that definition it is true.

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Theoretical height limit for plants in Earth's gravity:

For California redwoods (Sequoia sempervirens), the tug of gravity and the friction between the water and the vessels through which it flows mean that fluid cannot be dragged any higher than 122-130 metres, the researchers conclude in this week's Nature1. –Height limit predicted for tallest trees, nature.com


The Paradox of Large Dinosaurs, dinosaurtheory.com

In a column of a fluid the pressure increases during the descent from the top of the fluid to a lower level according to the relationship P = g D h, where P is the pressure, g is the acceleration due to gravity, D is the density, and h is the distance below the surface. Because of this, a pump and the tubing at the bottom of a column of fluid must be strong to withstand fluid pressure near the bottom of the column.

So, for animals, you're limited by how robust their cardiovascular system is, and I guess there is no upper limit other than what the underlying structure can support, to which I'd defer to the other answers that mention the square-cube law and whatnot.

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  • $\begingroup$ This is probably mentioned in ohwilleke's wall of text, but I'd rather have the short answer. $\endgroup$ – Mazura Oct 15 '16 at 2:59

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