Why could a real Godzilla never stand up?

Galileo's oversized bone

In 1638, Galileo Galilei argued that it would be impossible to build "the bony structures of men, horses, or other animals" so that they held together "if these animals were to be increased enormously in height", unless the bones were made of a harder material or grew far thicker¹. He sketched "a bone whose natural length has been increased three times" and whose thickness had to be "multiplied" to do the same job. "From the figures here shown you can see how out of proportion the enlarged bone appears," he wrote¹.

The square-cube law

Make a body k times bigger in every direction and its weight grows with k × k × k, but its bone cross-section, which sets how much load the bones can carry, only with k × k. So every square inch of bone carries k times more. In 1926, Haldane worked this out for the giants Pope and Pagan from Pilgrim's Progress: ten times as high as the hero Christian, "so that their total weight was a thousand times his", yet "the cross sections of their bones were only a hundred times those of Christian"². And "as the human thigh-bone breaks under about ten times the human weight", they "would have broken their thighs every time they took a step"².

A monster 30 times your height

Picture a monster about 50 meters tall: roughly a 15-storey building (at about 3.3 meters per floor) and about 30 times a 1.7-meter person. (The Godzilla of Godzilla Minus One, from 2023, is officially 50.1 meters tall, according to Toho⁷.)

Now assume it is built like you. Its bones get 30 × 30 = 900 times your bone cross-section, but its weight goes up 30 × 30 × 30 = 27,000 times, about 1,900 tonnes if you weigh 70 kilograms. Each square inch of its bone would carry 30 times the load yours does. By Haldane's rough tenfold rule², its thigh-bones would be loaded about three times past breaking just standing still: on paper, it would collapse before its first step. Modern measurements leave little room either: in mammals running hard, peak limb-bone stress reaches about a quarter to half of the breaking stress, a safety factor of only 2 to 4^{3, 8}.

Ants, elephants and you

Going small flips the rule. "The smaller the body the greater its relative strength," Galileo noted, guessing that a small dog could carry two or three dogs its own size, while a horse "could not carry even one"¹. In a 2019 experiment, leaf-cutter ants could lift at most, on average, 8.78 times their own body mass⁴.

Haldane wrote that a gazelle grown large would break its bones unless its legs became "short and thick, like the rhinoceros", or long and slanted like a giraffe's². Biomechanist Andrew Biewener found that large mammals run with more upright limbs, which greatly reduces the forces on their bones³. Elephants, even at their fastest, never have all four feet off the ground at the same time⁵.