Of the many mysteries surrounding the life history of dinosaurs, one of the more enduring is how such gigantic organisms--some reaching 42 feet tall and weighing 90 tons--regulated their body temperature. In a new study published in PLoS Biology, James Gillooly, Andrew Allen, and Eric Charnov revisit--and resolve--this debate.
Some scientists had assumed that dinosaurs, which evolved from reptiles, were cold blooded (ectotherms), whereas others thought that dinosaurs, like mammals and birds, might have been warm blooded (endotherms). Still others argued that while most dinosaurs had a metabolism similar to contemporary reptiles, the large dinosaurs managed a higher, more-constant body temperature through thermal inertia, which is how modern alligators, Galapagos tortoises, and Komodo dragons retain heat. Thermal inertia allows the body to approach homeothermy, or constant body temperature, when the ratio of body mass to surface area is high enough. If this "inertial homeothermy" hypothesis is correct, dinosaur body temperature should increase with body size.
The researchers used a model that provided estimates of dinosaur body temperature based on developmental growth trajectories inferred from juvenile and adult fossil bones of the same species. The model predicts that dinosaur body temperature did increase with body mass, and that large dinosaurs had body temperatures similar to those of modern birds and mammals, while smaller dinosaurs' temperatures were more like contemporary reptiles. These results suggest that the largest dinosaurs (but not the smaller ones) had relatively constant body temperatures maintained through thermal inertia. Gillooly et al. then compiled data from eight dinosaur species from the early Jurassic and late Cretaceous periods that ranged in size from 30 pounds to 28 tons. The growth trajectories, taken from the published research papers, were determined by using bone histology (microscopic study) and body s
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