Until now, scientists were split between those who thought that dinosaurs roamed the earth in the cold-blooded way of reptiles and those who favored a hot-blooded mammalian core temperature.
But a study just published in Science concludes that both camps were kinda right and dead wrong. When it comes to whether dinosaurs were cold-blooded ectotherms or hot-blooded endotherms, researchers studied the layers of vertebrate fossils and genetic material from 21 extinct species to conclude that "when the effects of size and temperature are considered, dinosaur metabolic rates were intermediate to those of endotherms and ectotherms and closest to those of extant mesotherms."
In other words, dinosaurs had lukewarm blood. John Grady, a professor of ecology at the University of New Mexico and the study's lead author, told NPR:
"They took a middle way, kind of like Goldilocks. And it seemed to work out very well for them."This study goes for all sizes of dinosaurs, from the mighty carnivore Tyrannosaurus rex to species more closely resembling birds like the Troodan. Researcher Brian Enquist, a professor of evolutionary biology at the University of Arizona, said that an analysis of metabolic and growth rates concluded that dinosaurs didn't act quite like any species currently calling the earth home.
"They did not act like mammals or birds, nor did they act like reptiles or fish," Enquist told Reuters. "Instead, they had growth rates and metabolisms intermediate to warm-bodies and cold-blooded organisms of today. In short, they had physiologies that are not common in today's world."
This lukewarm-blooded animal is called a mesotherm. It's this characteristic, Grady presumed, that might have allowed dinosaurs to grow to such monumental sizes, since they didn't need to consume as much food as today's mammals.
"It is doubtful that a lion the size of T. rex could eat enough to survive," said Grady.
What surprised Grady and Co. the most was how feathered dinosaurs turned out to be mesotherms too. All of these creatures, Grady told National Geographic, had mild metabolisms and because of that grew into adulthood very slowly: "This thing that was feathered like a bird wasn't that much different to these non-feathered dinosaurs in how fast it grew," taking Arthaeopteryx about two years to grown into adulthood. "Its energy use was much lower than modern birds, but it was covered in feathers. Maybe it was an endotherm with a low metabolic rate, or something like the echidna. The jury's still out."
What this tells some experts is that modern species like birds evolved later into warmer-blooded species and that modern reptiles evolved to have colder blood... that species today now have cold or hot blood but they used to have something like a combination. Cool.
[Images courtesy of Bing]