Many of us can state, with certainty, that dinosaurs of all shapes and sizes were cold-blooded, or ectothermic, a fact that has been ingrained in our minds since we were children.
Scientific consensus, however, is not that certain, and has been shifting in the opposite direction towards the endothermic, or warm-blooded, hypothesis. Contributing to this debate is Herman Pontzer, a professor of anthropology at Washington University in St. Louis.
In an article recently published in PLoS ONE, Pontzer and his colleagues utilized two biomechanical methods to predict the metabolic demands of 14 different species of dinosaurs. These species were chosen based on the completeness of their fossil record, its accessibility and the species' ability to represent the evolutionary lineage from dinosaurs to present-day birds.
In addition, these dinosaurs were also bipedal, walking on two legs instead of four, making it possible to base their energy demands off of those of modern-day bipedal animals. The researchers predicted the amount of energy that would be needed for different walking and running speeds for species of all sizes.
Their results strongly support the warm-blooded hypothesis, as even the least demanding of physical activities in the larger dinosaurs, such as a slow walk, demand much more energy than a cold-blooded physiology can provide.
Cold-blooded animals rely primarily on heat from the sun to control their body temperatures, and thus maintain a relatively low metabolic rate. On the other hand, warm-blooded organisms adjust their metabolism to warm themselves up or cool themselves down, similar to adjusting the thermostat when heating or cooling a room.
"Our results suggest these species would need a warm-blooded physiology, able to deliver oxygen (and therefore energy) at a higher rate," Pontzer said in an e-mail to The News-Letter.
Initially, scientists suggested that dinosaurs were cold-blooded, linking them to the reptiles from which the name dinosaur, meaning "terrible lizard," originated. Later discoveries and continually mounting evidence support the reverse.
"Ectothermic systems are limited in the rate of energy that they can provide. Endothermic physiology can provide energy at a rate that is 10-times higher than that of an ectothermic system," Pontzer said. From an evolutionary standpoint, scientists still have yet to determine whether warm-bloodedness arose from the need for self-regulation of the dinosaurs' internal temperatures or from the energy demands of their physical activities.
The study did not focus on the relation of size to the energy demands of the dinosaur species. "Size had nothing to do with our analyses per se . . . but it did turn out that the clearest results happened to be for species larger than about 20 kg," Pontzer said. "Still, our methods suggested the small species were likely warm blooded as well."
Furthermore, this conclusion was based on the assumption that the cold-blooded metabolism of a dinosaur functions in the same way and has the same energy requirements as the modern ectothermic species that were used for comparison.
There is a possibility that a very different metabolic process could have allowed for movement in large dinosaurs.
Results from the study show that only the smallest of the dinosaurs examined, Archaeopteryx, was capable of having an ectothermic physiology that could fulfill its energy demands for all but its fastest speeds.
There is still much more to this debate, with researchers producing findings that support and refute both hypotheses.
"Both cold-blooded and warm-blooded hypotheses are favored, by different researchers," Pontzer said. "Recently, consensus seems to be shifting toward warm-blooded."
Please note All comments are eligible for publication in The News-Letter.