Shrinking dinosaurs and the evolution of endothermy in birds

The evolution of endothermy represents a major transition in vertebrate history and a major factor underlying the diversity of birds and mammals. Despite the several advantages of an endothermic lifestyle, the tempo and mode of the evolution of endothermy in these lineages remains one of the most controversial subjects in paleontology and evolutionary physiology. Here, we combine a heat transfer model with body size estimates in the theropod phylogeny to reconstruct the evolution of metabolic rates along the bird stem lineage. This model suggests that the continuous reduction in size from ancestral theropod dinosaurs to birds constitutes the evolutionary path of least resistance for endothermy to evolve, because it maximizes thermal niche expansion while obviating the im- pact of elevated energy requirements on population size. In this scenario, metabolic rates would have increased steadily with the accelerated miniaturization observed primarily in the Early-Middle Jurassic (~180 to 170...