Re Godzillaesque type creatures what are the real world size limits for terrestrial animals?

There are fossil recordsof dinosaurs with projected body lengths of 200 feet and 135 tons.

Is this the absolute limit relative to how big a complex, mobile living organism can get in earth’s gravity or is there room to grow?

Regarding the physical stuff, Science of Godzilla talks about scaling issues, bone density, joint structure, heat dissipation, etc. that become increasingly problematic as organisms get gigantic. A lot of this can be helped by being an aquatic animal (blue whales, etc.).

Evolutionarily, ecosystem considerations may outweigh the physical/gravitational constraints. The Wikipedia article on Megafauna describes some of the issues with having fewer and larger individuals, namely that they can’t recover from population decimations as quickly (whether it’s from human hunting, carrying capacity collapse, global extinction events, etc.).

In nature, species are always a balance of various factors and maximizing any one trait at the expense of others doesn’t necessarily yield successful creatures over evolutionary time. In other words, an animal isn’t just a stack of meat and muscle cells that you can mathematically scale up to arbitrary dimensions in a given gravitational field; its behavior and ecosystem must be considered too. Those, not necessarily physical scaling laws, might be your limiting factor…

More interesting reading:

There’s this old academic paper I remember reading that suggests that animals as we understand them could probably not exceed 1,000,000 kg under any circumstances, although you could most likely have one that exceeded 100,000 kg. It would not be moving all that quickly, though.

The Square-Cube law probably comes into it, where the weight of extra muscle increases cubiclly whereas the extra strength from it only increases quadratically

As an absolute upper limit, regardless of Godzilla’s structural components – say he’s got bones of unobtanium and nuclear-steam-engine powered carbon-nanotube muscles – I thought of considering the problem of getting rid of his internal heat.

So…assuming a cylindrical Godzilla of height h and aspect ratio f (= width/height), and giving him an average density equal to p, we can calculate his mass and his surface area. Apparently Kleiber’s Law (’s_law) suggests the metabolism of a creature grows with the 3/4 power of its mass, so we can calculate Godzilla’s heat production by dividing his mass by the mass of a man m0, taking the 3/4 power, and multiplying by the heat production of a man Q0. Then we can use Newton’s Law of Cooling ( with a heat convection transfer coefficient of k to calculate the degrees dT by which Godzilla’s skin must exceed ambient temperature to get rid of all the heat.

My horrible final result is:

dT = [(pi p f^2 h^3/m0)^(3/4) Q0]/[k pi f h^2 (1 + f/2)]

Putting in:

p = 1000 kg/m^3 (density of flesh = the density of water)
f = 0.2 (aspect ratio)
m0 = 75 kg (weight of man)
Q0 = 200 W (metabolism of active angry man trying to kick down Tokyo)
k = 10 W/(m^2 oC)

All the numerical factors work out to about 15, so the result is:

dT = 15 h^(1/4)

As a sanity check, if we put in 1.8m, the height of a man, we get dT = 17 oC, which is in the ballpark, e.g. if the air is 15oC skin temperature must be 32oC, which is 90oF.

What is the maximum possible temperature of Godzilla’s skin? If we put in the boiling point of water, 100oC, i.e. dT = 85oC (again assuming ambient temperature is 15oC), then h = 1000m, meaning Godzilla can be a full kilometer tall.

That must be what the fins are on Godzilla’s back- cooling fins. To make them really efficient perhaps they should be able to flap about.

This reminds me of an old Physics joke, updated: Assume a spherical Godzilla.

What if it isn’t 'spherical"? I recall the “Strata Beasts” from James White’s *Major Operation. *They were continent sized creatures - but flat. And very slow moving, naturally. So long as such a creature isn’t too thick it wouldn’t be crushed under its own weight, I’d think.

Come to think of it, don’t Godzilla’s fins glow red-hot when he’s exerting himself more than usual?

I remember them glowing, too. Also my brother had a model Godzilla, and the fins glowed in the dark.

You do know Godzilla is a reptile, right?

Well, he looks like one, but that’s not solid evidence. Has anyone ever sequenced his genome to see? And even if he is, he may still be warm-blooded, and even if he’s not, he still has to get rid of the heat when active. I suppose we could assume that if he’s not upset and the Science Patrol has stood down, he can minimize his heat output by remaining perfectly still and lowering his basal metabolism.

I thought of that, but since it’s a worst-case scenario for cooling, it would be reasonable for Godzilla to complain of the calculation being rigged. A cylindrical Godzilla probably isn’t a whole lot better, though.

The real surprise (to me) is that the cooling (in this crude approximation at least) isn’t really much of a problem. His structural issues are clearly much more important. Which says that truly huge creatures could exist in the ocean, or on planets with much lower gravity, or in a Niven smoke ring, provided in each case they can solve the problem of food gathering, perhaps by not moving very much or sponsoring photosynthetic symbiotes.

He’s a dinosaur, actually. A mutated Godzillasaurus. (Or Gorgosaurus in earlier films.)

His fins glow not from exertion, but when he’s performing a particular attack, specifically his atomic breath.

Long before you get to the size of Godzilla, the difference between “warm-blooded” and “cold-blooded” becomes irrelevant. Even lizards and fish produce some heat internally, and when you’ve got that much mass with that little (relatively) surface area, it builds up. It’s similar to how large office buildings need air conditioning, not heating, even in the winter.

He lowers his body temperature by exhausting excess heat through his fiery breath, so overheating is not a concern.

The boundaries of ‘warm’ and ‘cold’ blooded can get kind of hazy. Mole rats don’t really thermoregulate much (they don’t need to), and leatherback turtles do thermoregulate to some extent (they apparently maintain substantially higher temperatures than their environment).

I can’t find anything that backs up what you said, although I must admit I didn’t try for more than a few minutes. And dinosaurs are a type of reptile, even if they’re quite different from modern ones.

But still, assuming Godzilla generates heat at a similar rate to a human is unreasonable.

Eh, you get that general scaling (to within an order of magnitude or so) just from the fact that it’s a certain size, and moving itself and its limbs about in a certain gravitational field. The internal metabolism doesn’t matter as much as you’d think.