How physics stops animals getting really really big in 285 words

Consider a small sauropod of length x, as shown on the left above. Its mass is proportional to x cubed, it stands on leg bones whose cross-sectional area is x squared, and it ingests food through a gullet whose cross-sectional area is x squared. Now consider a larger sauropod of length 2x, as shown on the right above. Its mass is proportional to 2x cubed = 8x, it stands on leg bones whose cross-sectional area is 2x squared = 4x, and it ingests food through a gullet whose cross-sectional area is 2x squared = 4x. The bigger sauropod has to carry proportionally twice as much mass on its leg bones, and ingest proportionally twice as much food through its gullet. (Similarly, a 104-foot tall gorilla, 20 times as tall as a real one, is only 400 times as strong but 8000 times as heavy — which is why we can't have Skull Island.)

In practice, big animals tend to have adaptations such as thicker limb bones that mean the numbers aren't quite as bad as this, but the principal holds: the bigger an animal gets, the worse the problems imposed by scaling. It's not possible to "solve" this problem because so many biological properties scale this way. Something is always the limiting factor. Suppose it were leg-bones or gullet. If somehow a hypothetical ultra-sauropod evolved extra thick leg-bones and gullet, scaling of respiration would suffocate it, or scaling of digestion would starve it, or scaling of heat-loss through the skin would boil it. The fundamental reason that you can't just scale an animal up is that some parts of its function scale with volume while most — respiration, digestion, etc. — scale with surface area.