The evolution of endothermy in birds (1 Viewer)

Enrico L. Rezende, Leonardo D. Bacigalupe, Roberto F. Nespolo & Francisco Bozinovic, 2020

Shrinking dinosaurs and the evolution of endothermy in birds

Science Advances. 6 (1): eaaw4486
doi:10.1126/sciadv.aaw4486

Abstract: https://advances.sciencemag.org/content/6/1/eaaw4486

The evolution of endothermy represents a major transition in vertebrate history, yet how and why endothermy evolved in birds and mammals remains controversial. Here, we combine a heat transfer model with theropod body size data to reconstruct the evolution of metabolic rates along the bird stem lineage. Results suggest that a reduction in size constitutes the path of least resistance for endothermy to evolve, maximizing thermal niche expansion while obviating the costs of elevated energy requirements. In this scenario, metabolism would have increased with the miniaturization observed in the Early-Middle Jurassic (~180 to 170 million years ago), resulting in a gradient of metabolic levels in the theropod phylogeny. Whereas basal theropods would exhibit lower metabolic rates, more recent nonavian lineages were likely decent thermoregulators with elevated metabolism. These analyses provide a tentative temporal sequence of the key evolutionary transitions that resulted in the emergence of small, endothermic, feathered flying dinosaurs.

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Fred

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Fred

Reconstruction of metabolic levels and thermal niche of theropods. (A) Theropod phylogeny (15) with branches color-coded according to reconstructed metabolic levels. (B) Scaling of metabolic rate versus body mass (12) for ectotherms (MR = 0.68mass0.75) and endotherms (MR = 3.4mass0.75) and the predicted trajectory of the bird stem lineage during the transition from ectothermy to endothermy. Dashed lines show fold differences between ectotherms and endotherms (1× to 5×); open and closed symbols depict reconstructed values for the bird stem lineage and the tips of the phylogeny, respectively (see Methods). (C) Scaling of thermal conductance C and body mass (13) for ectotherms (C = 2.5mass0.5) and endotherms (C = 1.0mass0.5), fold differences from 2.5× to 1×. (D) Thermal gradient and fold differences calculated with Eq. 1 and values in (B) and (C). The log-log linear trajectories connecting MR and C of the ectothermic ancestor and the endothermic descendant, as well as the resulting trajectory in thermal gradient, are shown with the continuous lines.

Fred