Were dinosaurs endothermic (warm-blooded) like present-day mammals and birds or ectothermic (cold-blooded) like present-day lizards? Reporting in PLoS ONE, Herman Pontzer at Washington University in St Louis and colleagues sought to answer this simple-sounding yet important question by determining whether a variety of dinosaurs and closely-related extinct animals were ectothermic or endothermic and when, where, and how often in the dinosaur family tree this important trait may have evolved, using a combination of simple measurements, rigorous computer modelling techniques and their knowledge of physiology in present-day animals.
The question of whether dinosaurs were endothermic or ectothermic has important implications: if dinosaurs were endothermic, they would have had the potential for athletic abilities rivalling those of present day birds and mammals. They could have survived in colder habitats that would kill ectotherms, such as high mountain ranges and the polar regions, allowing them to cover the entire Mesozoic landscape. These advantages come at a price, however; endothermic animals require much more food than their ectothermic counterparts because their rapid metabolisms fatally malfunction if they cool down too much, and so a constant supply of fuel is required.
Studies of present-day animals have shown that endothermic animals can sustain much higher rates of energy use (that is, they have a higher "VO2max") than ectothermic animals. Following this observation, Pontzer and colleagues Vivian Allen and John Hutchinson at The Royal Veterinary College, UK, reasoned that if the energy cost of walking and running could be estimated in dinosaurs, the results might show whether they were warm- or cold-blooded. If walking and running burned more energy than a cold-blooded physiology can supply, these dinosaurs were probably warm-blooded.
But metabolism and energy use are complex biological processes, and all that remains of extinct dinosaurs are their bones. A recent work of Pontzer's showed that the energy cost of walking and running is strongly associated with leg length – so much so that hip height (the distance from the hip joint to the ground) can predict the observed cost of locomotion with 98% accuracy for a wide variety of land animals. As hip height can be simply estimated from the length of fossilized leg bones, Pontzer and colleagues were able to use this to obtain simple but reliable estimates of locomotor cost for dinosaurs.
To back up these estimates, the researchers also used a more complex method, which estimates the actual volume of leg muscle dinosaurs would have had to activate in order to move. Working this out in extinct animals involved using basic principles of locomotion: determining how large the forces required from the legs would have to be to move the animal and how much muscle would be needed to supply these forces.
The team then applied these principles to examine recent anatomical models of 14 extinct dinosaur species, using detailed measurements of the fossilised bony levers that limb muscles attached to. From this, the authors were able to reconstruct the mechanical advantage of the limb muscles and calculate the active muscle volume required for each dinosaur to walk or run at different speeds. The cost of activating this muscle was then compared to similar costs in present-day endothermic and ectothermic animals.
The results of both methods suggest that based on the energy they consumed when moving, many dinosaurs were probably endothermic, athletic animals because their energy requirements during walking and running were too high for cold-blooded animals to produce.
Interestingly, when the results for each dinosaur were arranged into an evolutionary family tree, the authors found that endothermy might be the ancestral condition for all dinosaurs. This pushes the evolution of endothermy further back into the ancient past than many researchers expected, suggesting that dinosaurs were athletic, endothermic animals throughout the Mesozoic era. This early adoption of high metabolic rates may be one of the key factors in the massive evolutionary success that dinosaurs enjoyed during the Triassic, Jurassic and Cretaceous periods, and continue to enjoy now in feathery, flying form.
The debate over dinosaur physiology will no doubt continue to evolve, and while the physiology of long-extinct species will always remain somewhat speculative, the authors hope that the methods developed in this study will provide a new tool for researchers in the field.
Competing Interests: The authors have declared that no competing interests exist.
Funding: Washington University, USA supported HP for this project. The National Science Foundation, USA funded JRH from 2001-2003, which supported development of this work, as did the Department of Veterinary Basic Sciences at The Royal Veterinary College (2003-2009). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
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Citation: Pontzer H, Allen V, Hutchinson JR (2009) Biomechanics of Running Indicates Endothermy in Bipedal Dinosaurs. PLoS ONE 4(11): e7783. doi:10.1371/journal.pone.0007783
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Related image for press use: http://www.plos.org/press/pone-04-11-pontzer.tif Caption: "Schematic diagram to show how the mechanical advantage and active muscle volume in the dinosaur hind limb were reconstructed." Credit: This image forms Figure 1 of the published paper; under the terms of the Creative Commons Attribution License, any reuse should cite the authors and journal.
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