User blog comment:SolZen321/Could Godzilla actually exist/@comment-7622429-20140512024531

Well, the thing is, past iterations have always had a rather self-sustaining nuclear power system as the main function for their organs. When you build a creature as large as Godzilla, you simply can't have him be made of the same material as normal animals and expect him to be just fine. You gotta replace it. But slowly, and over several generations, with the aid of some random beneficial mutations from radiation, thus creating the "nuclear evolution cycle".

However, every once in awhile, the creatures will need a top-off, which is where them taking in radiation at all comes into play. Food becomes a non-issue, as does oxygen. Size is compensated for by replacing ordinary biological building blocks with stronger elements built up alongside general adaptations to bigger sizes in generational stages, along with rapid self-mediated "radioactive intake evolution".

The strengthening of the body to compensate for such large sizes impossible in normal evolution also comes with the added benefit of being able to tank much more heavy damage than an ordinary creature proportionally. The structures present, like gills, take on new uses as radiation-mediated self-evolution creates a new process known as the "atomic breath", thus requiring vast intakes of extra gases in the atmosphere and/or water to ionize and stabilize using nuclear energy into super-heated nuclear-enveloped plasma.

Speaking of bodily structures, since gills evolved in the first place, but became redundant for actually breathing in water, as I've stated before, why evolve completely new structures for a new process if ones already there work just fine?

Well, that's just my two cents for how it all works. I'm no physics or biology expert, so feel free to take what I've said with a grain of salt.