The long-held idea that dinosaurs were lumbering, cold-blooded beasts with sluggish metabolisms was turned on its head following the publication of a controversial study that concluded that the animals were likely neither warm-blooded nor cold-blooded, but actually somewhere in between. Much like modern animals occupying this metabolic middle ground, such as echidnas and great white sharks, mesothermic dinosaurs would have been able to generate enough heat to keep their bodies warmer than their environment, but not enough to maintain a constant internal temperature.
Although these findings never truly settled the score, a new study has now shaken things up for paleontologists once again, suggesting that dinosaurs were probably, surprisingly, warm-blooded. This would mean that they would have experienced growth rates not unlike those observed in today’s mammals. But this investigation is not the result of novel data; rather, a scientist noticed flaws in the previous study that could have influenced the way the data was interpreted and the conclusions that were drawn. After taking these into account, he conducted his own analysis of the data, published in Science.
“The study that I re-analyzed was remarkable for its breadth—the authors compiled an unprecedented dataset on growth and metabolism from studies of hundreds of living animals,” Dr. D’Emic, bone microanatomy specialist at Stony Brook University, said in a statement. “Upon re-analysis, it was apparent that dinosaurs weren’t just somewhat like living mammals in their physiology—they fit right within our understanding of what it means to be a ‘warm-blooded’ animal.”
But before we delve into the deep bits, what exactly does “warm-blooded” mean? Warm-blooded is the common term used to describe endothermic organisms. These animals, which are primarily birds and mammals, are able to maintain a constant body temperature, independent of the environment, by producing heat from the metabolism of food. Ectotherms, or “cold-blooded” animals, on the other hand, rely on things in the environment to control their body temperature, like sunlight.
So how did D’Emic come to the vastly different conclusion that dinosaurs were likely endothermic? First off, he suggests that the researcher’s improperly converted annual time scales to daily, which would have led to an underestimation of the dinosaur’s growth rates. As D’Emic explains in a statement, the reason this conversion is problematic is because animals do not experience continual, even growth throughout the year; many slow or pause during stressful seasons, such as cold winters. This is supported by the presence of rings in their bones, much like the ones observed in trees.
Second, D’Emic asserts that, as living descendants of dinosaurs, birds should have been grouped with dinosaurs for the analysis. “Separating what we commonly think of as ‘dinosaurs’ from birds in a statistical analysis is generally inappropriate, because birds are dinosaurs—they’re just the dinosaurs that haven’t gone extinct,” he argues.
Bearing these flaws in mind, his re-analysis concluded that rather than falling into neither the endothermic nor ectothermic categories, dinosaurs were probably the former. Once again, this probably won’t lay to rest the debate, but perhaps it will spur further research into this intriguing area.
