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Not too hot, not too cold: Mesothermy discovered in dinosaurs

Paleontologists have often debated whether dinosaurs were warm-blooded (endotherms) like birds and mammals or cold-blooded reptiles (ectotherms) like reptiles and amphibians.

Endotherms are animals that maintain a constant body temperature even when environmental temperatures fluctuate. In contrast, the environment regulates the body temperature of ectotherms and thus their metabolic rate tends to be lower than the metabolic rates of endotherms, which must maintain a high, constant body temperature.

In Friday’s issue of Science, John M. Grady and his colleagues at the University of New Mexico (UNM) have found evidence for “mesothermy” in dinosaurs. Using a novel metabolic scaling approach based on dinosaurs’ body mass (which could be estimated based on the length and size of their thighbones), the authors identified the metabolism rates for 21 species of dinosaurs and found that when the effects of size and temperature were considered, dinosaur metabolic rates were intermediate to those of endotherms and ectotherms.

Ectotherms have a 50% efficiency of energy conversion to biomass, whereas endotherms have an average energy conversion efficiency of 1.4%. Thus, in comparing mammals and reptiles of the same body mass, the energy required for maintenance in a mammal is nearly 10x-13x greater than for a reptiles.

Thus, the “mesothermy” found in dinosaurs likely allowed them to move quickly, given that they would not need to constantly eat in order to maintain their body temperature (as do endotherms). As well, the dinosaur’s mesothermic metabolic rate would have decreased the vulnerability of these species to extreme fluctuations in external temperature, allowing them to exert some control of body temperature via internal homeostatic mechanisms.

It then seems that dinosaur metabolism was just right.