High Octane Dinosaurs
The question of whether dinosaurs were endothermic has been a rich source of controversy for decades. Although they were originally portrayed as sluggish reptiles that crept their “cold-blooded” way through the Mesozoic, over time evidence has suggested that they may have actually had active and athletic lifestyles, with fast-running metabolisms to match. Everything from growth rates to diet to integument has been used as evidence that dinosaurs, if not as fully “warm-blooded” as mammals, at least ran on a higher octane than many modern ectotherms. (“Cold-blooded” and “warm-blooded” are misleading terms; some ectothermic reptiles, such as large marine turtles, maintain consistently high body temperatures through behavioral adaptations, and some endothermic species, such as hummingbirds and bats, have a wildly variable body temperature that periodically drops low enough to drop them into torpor. Poikilothermic = variable temperature, homeothermic = consistent temperature, and endo- or ectotherms can be either.)
There is a new paper in PLoS ONE today that jumps into the fray of the dinosaur energetics question. Pontzer and Hutchinson (2009) test the hypothesis that dinosaurs were endothermic, using biomechanical analysis to model the metabolic rate of 13 bipedal dinosaurs, in addition to an outgroup ornithodiran, Marasuchus. They calculated metabolic demands of both walking and running using locomotor anatomy (limb length and active muscle volume), and compared their results to the aerobic capacity of extant ectotherms and endotherms. These comparisons can give us clues where the dinosaurs might have fallen along this metabolic spectrum.
So, what did they find? The results showed strong evidence that dinosaurs had aerobic capacities that exceeded the maximum limits of extant ecotherms. In other words, they were most likely endothermic to at least some degree, otherwise they could not have afforded the amount of energy that it apparently cost them just to move around. It is suggested that development of endothermy could be a reason for the long and extensive reign of the dinosaurs, which continues to this day in the form of their avian descendents.
It is certain that the debate over dinosaur energetics is far from resolved, but this study definitely adds a fascinating piece to the puzzle.
Pontzer, H., Allen, V., & Hutchinson, J. (2009). Biomechanics of Running Indicates Endothermy in Bipedal Dinosaurs PLoS ONE, 4 (11) DOI: 10.1371/journal.pone.0007783