What is it that limits the size of creatures?

One reason is the square-cube law. Say you double the height, length, and width of an animal. The bones will become four--two squared--times as strong, but the animal will weigh eight--two cubed--times as much. This is why elephants and rhinos have such thick legs in proportion to their body size.

And think of the food supply needed for super sized animals. Where could they find the food necessary to live?

Gravity is also a factor. Here's an interesting thread on the topic:

Gravity limits organism size

Size of an animal is the reason we have specialised respiratory systems and specialist digestive systems. In single celled organisms, the surface area to volume ratio is such that nutrients and gases can diffuses into the cell and all parts of it get sufficient to survive. As the cells clump together, the SA:vol ratio changes to be less efficient. Eventually, you reach a point where cells in the centre die and leave a gap... this gap increases the SA:Vol ratio and becomes a tube. As time goes on this tube gets more specialised and evolves into a bronchus or a oesophagus (or an intenstine).

Bascially, things like lungs, gills, stomata, etc are ways of increasing the surface area of an organism to allow it to get bigger without reducing the SA:Vol ratio. However, this has limits. You can't have giant insects because insects have a respiratory system which cannot get much larger. Mammals can't get much bigger than a whale because lungs have reached their limit (and even then they have to live in water because of the aforementioned gravity issue).

To get larger animals you would have to think of another method to allow gases and nutrients to travel.

There's also the evolution factor (see giant flightless birds, giant sloths etc).

Creatures tend to evolve a larger offshoot when they are isolated away from predators, allowing them to be larger, slower and less agile without getting eaten, and where food is abundant.

LOG said:

So, I've heard that super-sized creatures cannot exist (on Earth).

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Insects with 2-3 foot wingspans and allosaurs that could step on an elephant did exist, so obviously it is possible.

LOG said:

Like King Kong should turn into a pile of mush as soon as he moves.
What is it that makes something like King Kong impossible?

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King Kong's parents and his environment are the limiting factors.

The dinosaurs probably had metabolic pathways and tissue histology that modern creatures lack, enabling them to move larger bulk on less nutrition compared to modern examples. In short, they were different enough that our modern, "rules," don't apply. Simply taking a modern creature and giving it giantism is not the same as having a creature that evolved to be larger by today's standards.

On the environmental front, altering the ambient Oxygen by just a couple percentage points can have a huge impact on a creature's ability. Look at any COPD patient on Oxygen via nasal canula. Without a mask, just using the nasal prongs, you can really only raise the adjusted O2 by 2-3%. That can translate into a difference of whether grandma can get to the bathroom by herself.

A couple of key points of clarification,

The square-cube law generally governs the maximum size that animals can attain largely due to structural strength of their bones. You need to factor in sheer forces in essentially tubular-shaped bones when calculating when a volume of mass exceeds the structural capacity of its bones to support it.

Kleiber's Law governs an organism's metabolic rate as it increases in size scaling as a factor of 3/4 of the size increase. A mouse 100 times the size of a normal mouse would not have a metabolism 100 times as great as the normal mouse's.

Insect respiratory systems are something on the order of 200 thousand and 10 thousand times more efficient than human respiratory systems in the exchange of O2 and CO2, respectively, than mammalian respiratory systems are. Current O2 levels in our atmosphere do, indeed, limit the maximum size of insects currently, but it is nowhere near their current size. Studies have shown that larger insects are certainly possible, as much as 30-35% larger on average before problems begin to occur. CO2 levels, because the exchange rate of getting rid of the CO2 is much lower than the exchange rate of O2, are much more critical.

The real limiting factor in insects is their internal body mass to respiratory system volume ratios. As the body mass increases insects acquire "constriction points" in their internal anatomy that prevent additional expansion of their respiratory systems to transport O2/CO2 to/from the extremities. The limitation is somewhere in to 25-35% increased size area, right about the point where sheer stresses start to play an important point in how much larger they can become. In addition, and maybe not coincidentally, this is about the estimated oxygen concentrations for periods when giant insects historically occurred.

I researched this subject extensively for my book Hatchings and more than a few entomology PhDs found the science in the book sound.

Also, big animals run more of a risk of injury if they trip and fall. Think how badly it hurts when you face-plant on the sidewalk. Now imagine what kind of damage you'd do to yourself if you weighed half a ton.

There's also a lower limit on the size of warm-blooded animals. Small animals lose heat much more quickly than large ones, so they have to eat a lot of food (relative to their body weight) or high energy food.

Bone sheer strength is simple physics. Volume to mass is simple physics. Ratio of the volume of respiratory systems to the volume they have to pass through is simple physics. O2/CO2 exchange ratios are simple physics.

It's all simple physics.

What's not so simple is combining them all in such a way that you can make realistic and scientific estimates of how big things can really get.

If you want a really simply, quick and dirty methodology, use the Square-Cube Law, but remember that it does not cover everything that needs to be taken into consideration.

See thing is, the limits imposed by physics are dependent on factors.

For example, you could not have a 12-foot-tall human being, because basic laws of physics that govern bones and muscle are such that the human skeleton is not shaped in a way that would allow it to scale up to that level and support itself.

But if you changed that structure a little, you could. You could have a giant rabbit, for example.

Massive animals can exist if they evolve to be that way, in an environment that supports them. Huge dinosaurs managed to exist and move without their own bodies collapsing on them.

What you can't necessarily do is take a modern, dinky rabbit, run its genetic code through a science maguffin-o-matic and pull out a twenty foot tall version of that, with the same exact structure, relative size of body parts and method of motion.

Bunnymaz, Brilliant and faultless, as far as I can tell, explanation. Great job.

Aw thank you! *blush*

FOTSGreg said:

Bone sheer strength is simple physics. Volume to mass is simple physics. Ratio of the volume of respiratory systems to the volume they have to pass through is simple physics. O2/CO2 exchange ratios are simple physics.

It's all simple physics.

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Not really. The, "rules," regarding that have as their first assumption that dinosaurs had the same metabolism as us. The very fact that we know that they did exist and that they're gone and we can't handle being anything close to their size means that that first premise is obviously wrong. There was something very basic in their bone structure and metabolism that we lack. Our, "rules," don't apply to them. To understand them, it's really better to think of dinosaurs from the realm of xenobiology rather than comparative evolution.

FOTSGreg said:

If you want a really simple, quick and dirty methodology, use the Square-Cube Law, but remember that it does not cover everything that needs to be taken into consideration.

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Exactly.