In addition dinosaurs may also have had four-chambered hearts suggestive of endothermy. Bird Ancestry Living birds are endotherms. Also, problems with fossil preservation and factors determining predator-prey ratios. Body Size Body size strongly influences metabolism. Because of the surface area to volume relationship, smaller animals have more surface area relative to volume than larger animals.
Small animals can heat and cool themselves more rapidly than large animals.
Very large dinosaurs would have overheated if they were endothermic. It has been argued that in an endotherm oxygen isotope ratios should vary little between the limb bones and the bones of the core of the body because the body temperature is nearly the same at both locations. A consistency of oxygen isotope ratios and thus temperature has been demonstrated in the extremities and core of the skeleton of Tyrannosaurus rex, suggestive of endothermy. However, studies of living mammals and birds show that temperature varies considerably between their cores and extremities, at least as much as in living alligators. The Origin of Birds Birds are feathered vertebrates with skeletons highly modified to be lightweight and rigid to enable sustained powered flight.