(Intro)

Michael Aranda: If you're a dinosaur fan, you might have noticed that the way we portray them has changed over the years.  I'm not just talking about the feathers.  We used to think of dinosaurs as sluggish, cold-blooded reptilian monsters, and at some point, they became the swift, active hunters of Jurassic Park, more like warm-blooded creatures.  But dinosaurs might not have been either cold-blooded or warm-blooded.  Instead, they were probably a little bit of both. 

Ectothermic, or cold-blooded organisms like reptiles and fish, rely on their environment to warm their bodies, which helps them conserve energy.  Endothermic, or warm-blooded creatures, like birds and mammals, hold their body temperatures steady.  In your case, that's a toasty 37 degrees Celsius.  There are advantages to being warm-blooded, since you can stay active even when it's cold out, but it also takes a lot of energy to maintain that high temperature, which means you need plenty of food. 

For decades, paleontologists have been debating which category the dinosaurs fall into.  Since warm-blooded birds are descended from dinosaurs, there must have been a transition to endothermy somewhere in the dinosaur family tree, but according to a study published in 2014 in the journal Science, that might have happened gradually, with dinosaur metabolism sort of in-between endotherms and ecotherms. 

The key was in their bones.  When bones grow, they lay down growth rings just like trees.  By looking at these rings, scientists can calculate how fast the animal grew, even if it's been dead a hundred million years, and they can use growth ring data to estimate how an animal's metabolism worked.  Animals with faster metabolisms use more energy, so they eat more food and grow faster. 

So, the team looked at the growth rings in dinosaur fossils and compared them to those of modern animals, both endotherms and ectotherms.  They pulled published data on hundreds of vertebrates and dozens of dinosaurs and crunched the numbers using statistical analysis.  They found that the dinosaurs had a metabolic rate somewhere between reptiles and birds, meaning they weren't endotherms or ectotherms, they were mesotherms, or middle-blooded. 

Mesothermy is an entirely different type of temperature regulation, in which animals generate their own heat, like endotherms, but don't hold their bodies to a specific temperature, like ectotherms.  They end up with a body temperature that's a few degrees higher than their surroundings, but they're still dependent on their environment to some extent.  Like modern methoserms, such as the great white shark or the echidna, dinosaurs might have relied on their muscles to produce heat, and their large size to retain it, without sticking to a particular temperature.  Evolutionarily, it would make sense, because the dinosaurs would get to conserve a bit of energy while still being more active than their cold blooded competition. 

However, some paleontologists had problems with this study.  For one thing, not everyone agrees on how to interpret the growth ring data.  Using other statistical methods, it might actually support much faster growth rates, giving us warm-blooded dinosaurs.  For another, not all dinosaurs were the same, maybe the more bird-like theropod had faster metabolic rates than the more lizardy sauropods. 

Lumping them all into one statistical group like the researchers did would hide these potential differences, but we might soon get a much more definitive answer, because there's a totally new way to study dinosaur fossils that's giving us a close look at the red blood cells.  That could help solve the dinosaur metabolism problem, because the size of red blood cells is related to metabolic rate.  Typically, the bigger the red blood cells, the warm-blooded the animal. 

For a long time, paleontologists thought that the soft stuff, like skin and feathers and blood and tissue were rarely preserved in fossils.  But living bones have lots of soft tissue inside them, and when a group of British researchers pulled apart 75 million year old bones to look for some of that soft tissue, they found some, including red blood cells.  It wasn't perfectly intact, but it was enough to make out what the tissue structure would have looked like when the dinosaur was alive, and the researchers found soft tissue in nearly all the bones they searched, even though they weren't all that well-preserved or anything, which means that lots of dinosaur fossils might still have squishy organic bits of dinosaur in them.  So future studies that look at soft tissue and more bones should be able to tell us if dinosaurs were really cold-blooded, warm-blooded, or something in between.

Thank you for watching this episode of SciShow, which was brought to you by our Patrons on Patreon.  If you wanna help support the show, you can go to Patreon.com/SciShow and if you wanna keep getting smarter with us, just go to YouTube.com/SciShow and subscribe.

(Endscreen)