There would be no difficulty in guiding a warm-blooded system with an alligator-sized brain stem. The link between being warm-blooded and having big brains must be, at best, an indirect one. Brain tissue is vulnerable to changes in temperature. (life size dinosaur)Human brains addle when heated to 108°F even for a few minutes, and higher cerebral functions become erratic when the brain is chilled below 90°F. So warm-bloodedness and a constant body temperature are prerequisites for a large brain. It may well be that warm-bloodedness evolved first, and the evolution of large brains followed. Warm-bloodedness would have been an advantage even to an animal with an alligator’s brain power, because a constant high body temperature speeds growth, accelerates reproduction, optimizes muscular output, and increases digestive efficiency. And once acquired, it might have launched an evolutionary tendency toward larger brains.
More importantly, a survey of brain sizes in today’s creatures leads me to conclude that large brains aren’t essential for having a high metabolism. Humans have the biggest brains ever evolved for our weight class.(walking dinosaur costume) But we don’t possess a higher metabolism than a German shepherd, which has a brain one seventh the size of ours. Ostriches have tiny heads for their bodies, and ostrich brains are only one fortieth the size of that of a human of the same weight. Does an ostrich therefore have a metabolism one fortieth that of a human? On the contrary, its metabolism is higher, pound for pound, than ours. It is thus very difficult for me to believe that metabolism and brain size evolved in a kind of evolutionary lock step.