Temperature regulation limits energy use
The large size may also reflect temperature control. The bigger the body the more heat it retains and the more constant the temperature will remain for the animal. Evidence for warm-bloodedness is that the chest cavities are large enough to hold huge hearts, like birds do today. The dinosaurs were known to migrate, and both their northerly and southerly limits to these migration routes would have not been possible for a cold blooded animal. The bone histology of dinosaurs (particularly the more advanced thecodonts) suggest that they may have regulated their temperatures the way birds and mammals do today. Specialized structures such as the parallel rows of plates on Stegosaurus have been interpreted as additional temperature-control mechanisms. These plates, although made of bone, are spongy and probably carried many blood vessels which could either dissipate excess heat or absorb heat from the environment.
Anatomical analyses of many dinosaurs suggested that they were active, fast-moving animals, and therefore probably possessing endothermic metabolisms. Finally the ratio of predator-prey ratios of fossilized dinosaurs do not correspond to the expected ratios assuming them to be ectothermic but does more closely resemble those of endothermic mammals. It is recognized that endothermy may take several forms and that some dinosaurs may have fell short of fully fledged endothermy. It has even been speculated that Tyrannosaurus rex underwent three vastly different growth stages and may have been equipped with a variable metabolism. A 2 metre juvenile would have been very active, capable of scampering around like some groundbirds do today. By contrast, mid-sized individuals, averaging 3.5 to 4.5 metres were probably less agile, and may have travelled in packs. A fully grown 12 metre adult weighing 8 tons would not have been agile, and may have reverted to a solitary life-style scavenging on carcasses. Further, all, but a few highly specialized endotherms have some kind of heat insulation in the form of hair, fat or feathers. Without it, the demands on energy are so extravagant, that it is difficult for such an animal to survive. However, the only fossil impressions of a dinosaur skin discovered suggests that their hides were not furry or leathery, but scaly and covered with bony bumps. It has even been suggested that the large herbivorousdinosaurs (sauropods) would have required hundreds of kilos of vegetation a day to sustain their enormous bulk and that they had a unique endothermic metabolism fueled by the heat given off by non-stop digestion.