Image of the Day: Shrinking Dinosaurs
Image of the Day: Shrinking Dinosaurs

Image of the Day: Shrinking Dinosaurs

As dinosaurs got smaller, their metabolism increased, paving the way for bird evolution.

Emily Makowski
Jan 3, 2020

ABOVE: An artist’s depiction of a heatmap showing smaller dinosaurs’ increased ability to generate heat

How did dinosaurs evolve into warm-blooded birds? It may be related to an increase in metabolic rates as dinos’ body size decreased, according to a study published yesterday (January 1) in Science Advances

Birds are endothermic, or warm-blooded, animals—they can regulate their own body temperature. Dinosaurs, on the other hand, didn’t have as much of an ability to control their body temperature as modern-day birds and mammals do. While they weren’t fully ectothermic, or cold-blooded, they still had to partially rely on the environment by sunning or shading themselves to adjust their internal temperature.

Enrico Rezende, an ecologist at the Pontifical Catholic University of Chile, and colleagues wanted to learn more about the transition to endothermy. The team calculated the metabolic rates of theropods, a group of dinosaurs that evolved into birds, by combining body size estimates with a heat transfer model. They found that throughout most of the Early to Middle Jurassic period (180–170 million years ago), the dinos’ metabolic rates increased as they got smaller.

Endothermy is energetically costly—an endotherm needs the same amount of energy as an ectotherm eight times larger—and shrinking body sizes allowed the animals to require less of it. The results “suggest that a reduction in size constitutes the path of least resistance for endothermy to evolve,” the authors write in the paper. 

E.L. Rezende et al., “Shrinking dinosaurs and the evolution of endothermy in birds,” Sci Adv, doi:10.1126/sciadv.aaw4486, 2019.

Emily Makowski is an intern at The Scientist. Email her at