The Explanatory Power of Energetic Fitness Models Across Living Amniotes and Extinct Non-Avian Dinosaurs

Vertebrates have varied body size distributions. Among these, non-avian dinosaurs have uniquely large minimum and maximum sizes, but the reasons remain unknown. Energetic fitness models provide a potential explanation, by describing how physiological scaling relationships should structure body size distributions under selection to maximize the conversion of energy into offspring. We show that the body size distributions of birds, mammals, and turtles fit the expectations of energetic fitness models, but snakes, lizards, and crocodylians do not. Moreover, no reasonable combination of physiological scaling relationships explains the up-shifted minimum body size of non-avian dinosaurs or flightless birds. We propose that energetic fitness models perform well when body size distributions are governed primarily by intrinsic energetic factors. However, extrinsic constraints imposed by ecological factors such as competition, predation, or ecological niche may also be needed to explain why species body size distributions differ from the model expectations in some groups. The abrupt macroevolutionary shift from non-avian dinosaurs with minimum adult body sizes of 400 g, to birds with modal body size of 30 g cannot be explained by changes in intrinsic physiological factors, and may instead have been caused by an abrupt release of ecological constraints, potentially related to the origin of flight.