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article imageDinosaurs really were warm-blooded, study suggests

By Megan Hamilton     Jun 1, 2015 in Science
Dinosaurs grew quickly and were warm-blooded just like modern mammals, says a scientist who researched the metabolism of these ancient creatures.
Paleontologist Michael D'Emic did this by studying the body mass and growth rates deduced from species including Tyrannosaurus rex, Al-Jazeera reports.
He wanted to re-think the conclusion of another study last year that concluded dinosaurs weren't cold-blooded or warm-blooded, but instead had a metabolism that was somewhere in-between.
In that study, led by Dr. John Grady, of the University of New Mexico, and his team of researchers, coined the term "mesothermic," to explain this intermediate metabolism,Sci-News.com reports.
Because of his knowledge about how dinosaurs grew, D'Emic, of Stony Brook University, in New York, analyzed the study, and this led him to a completely different conclusion — dinosaurs were much more like mammals than reptiles in their growth and metabolism.
"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," he said. "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' mammal."
In re-analyzing the 2014 study, he looked at two different aspects. First of all, the study had scaled yearly growth rates to daily ones as a way of standardizing comparisons, Sci-News.com reports.
"This is problematic, because many animals do not grow continuously throughout the year, generally slowing or pausing growth during colder, drier, or otherwise more stressful seasons," D'Emic said. "Therefore, the previous study underestimated dinosaur growth rates by failing to account for their uneven growth. Like most animals, dinosaurs slowed or paused their growth annually, as shown by rings in their bones analogous to tree rings."
"The growth rates were especially underestimated for larger animals and animals that live in very stressful or seasonal environments – both of which characterize dinosaurs."
What is the second aspect of the re-analysis of the study? It takes into account that dinosaurs should be statistically analyzed within the same group as living birds, which are known to be warm-blooded and are descendants of dinosaurs.
"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 said.
"Re-analyzing the data with birds as dinosaurs lends more support that dinosaurs were warm-blooded, not occupants of a special, intermediate metabolic category."
What do the terms "warm-blooded" and "cold-blooded" really mean?
DinoBuzz notes that the terms aren't really as cut-and-dried as they sound.
On a scientific level, there is no such thing as "cold-blooded" or "warm-blooded," (DinoBuzz uses the term "hot-blooded.") The familiar term "hot-blooded" actually means having an average body temperature that's higher than the surroundings. "Cold-blooded" means the opposite. Scientists have several terms that they prefer to use:
Endothermic: Generating internal heat in order to moderate body temperature. This category includes modern birds and mammals.
Ectothermic: Relying on the environment and behavior to regulate body temperatures. Reptiles fit this category.
Homeothermic: Maintaining a constant internal body temperature. This category also includes modern mammals and birds.
Poikilothermic: Having an internal body temperature that fluctuates depending upon nearby environmental conditions. Reptiles are in this category, as are actinopterygiian fish.
For the last several decades, there's been a great deal of back-and-forth among scientists as to whether dinosaurs were warm- or cold- blooded, About.com reports.
The controversy took flight within the first 100 years after fossils of these ancient creatures were being dug up. Paleontologists and evolutionary biologists believed that dinosaurs were cold-blooded. There seems to be three lines of reasoning regarding this assumption:
• Some dinos were huge, and this led researchers to believe the metabolisms of these creatures worked very slowly, because it would take enormous amounts of energy to fuel a hundred-ton herbivore well enough to maintain a high body temperature.
• These dinosaurs were also thought to have extremely small brains for such big bodies, and this contributed to the image of them as slow, lumbering, stupid creatures—kind of like a big Galapagos turtle rather than a speedy Velociraptor.
• Because modern reptiles are cold-blooded, it made sense to think that “lizard-like” creatures such as dinosaurs must have also been cold-blooded.
The evidence for warm-blooded dinosaurs:
• Evidence shows that there were at least some dinosaurs that were active, intelligent and fast. About.com notes that some dinosaurs exhibited “mammalian" behavior, and this entails a level of energy that, most likely, can only be maintained by a warm-blooded metabolism.
• As previously mentioned, dinosaur bones show evidence that supports an endothermic metabolism. Microscopic analysis shows that the bones of some dinos grew at a similar rate to modern mammals. These bones also have more features in common with the bones of mammals and birds than they do with modern reptiles.
• There have been plenty of dinosaur fossils found at high latitudes — places where cold-blooded creatures, like reptiles, aren’t likely to be. They are far more likely to evolve in warm regions, where they can use the environment to keep their body temperatures constant.
• Since birds are endotherms, that means dinos must have been as well. Many biologists think of birds as “living dinosaurs,” and as such, reason that this is evidence for their ancestors having warm-blooded metabolism.
The huge sauropod dinosaur Giraffatitan.
The huge sauropod dinosaur Giraffatitan.
Dmitry Bogdanov [Public domain], via Wikimedia Commons
• The circulatory systems of most dinosaurs demanded a warm-blooded metabolism. If a huge sauropod like Brachiosaurus kept its head in a vertical position, in the same way as a giraffe does, that would have placed enormous demands on the giant animal’s heart, meaning that only an endothermic metabolism could fuel its circulatory system.
As can be expected, some skeptics have poked holes in this and are in the "cold-blooded dinosaur camp," About.com reports.
The skeptics have different opinions about these huge sauropod dinosaurs and say they are too large to be endothermic. Some experts say that a 100-ton sauropod with a warm-blooded metabolism would have overheated and died, whereas a cold-blooded dinosaur of the same weight could have been an "inertial homeotherm," meaning that it warmed up slowly and cooled down slowly, helping it to keep a relatively constant body temperature.
They also say the bone evidence is overrated, and even though some dinosaurs may have grown at a faster rate than previously thought, this still may not be evidence in favor of a warm-blooded metabolism. One experiment has shown that cold-blooded reptiles can generate bone quickly under the right conditions.
Another fly in the proverbial ointment is that dinosaurs (at least the ones that have been discovered) seemed to lack respiratory turbinates. In order to keep their metabolism up to snuff, warm-blooded creatures breathe about five times as often as reptiles. Warm-blooded creatures that live on land have these structures in their skulls and they help keep moisture in while the creature is breathing. So far, no one has found conclusive evidence of these structures in dinosaur fossils, so the skeptics say that dinosaurs must have been cold-blooded.
About.com notes that the majority of scientists believe that dinosaurs were warm-blooded, but new evidence can always sway this one way or the other.
Stay tuned.
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