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...

恒温性（endothermy）的演化是脊椎动物演化史上的重大过渡事件，同时也是鸟类与哺乳动物多样性形成的关键驱动因子。尽管恒温生活方式具备多项优势，但这些类群中恒温演化的速率与模式，仍是古生物学与演化生理学领域最具争议的议题之一。本研究将热传递模型与兽脚类（theropod）系统发育中的体型估算数据相结合，以重建鸟类茎干谱系沿程的代谢率演化历程。该模型表明，从祖先兽脚类恐龙到鸟类的体型持续缩小，是恒温性演化的阻力最小演化路径：此举可最大化热生态位的扩张空间，同时规避了能量需求提升对种群规模的负面影响。在此情景下，代谢率将随着以侏罗纪早中期（约180至170……）为主的加速体型小型化过程稳步提升。