AUDIE CORNISH, HOST:
Watch a snake for a while and you'll see what it means to be cold-blooded, not a lot of action going on there. Watch the monkeys though and it's like they've had one too many cappuccinos in part because they are warm-blooded. Most animals are one or the other, but once upon a time the Earth's dominant animals may have been a bit of both. Were talking about Dinosaurs here. NPR's Christopher Joyce has the story.
CHRISTOPHER JOYCE, BYLINE: Scientists once thought dinosaurs were sluggish cold-blooded creatures. Tail draggers they called them. Like Sunbathing lizards that absorb most of their heat from outside their bodies. Then scientific opinion swung the other way, maybe dinosaurs were quicker more high-energy, more like warm-blooded mammals and birds whose bodies generate and maintain their own heat. John Grady, now offers a third way.
JOHN GRADY: What I'm suggesting is neither, rather they took a middle way. Kind of like Goldilocks and it seemed to work out very well for them.
JOYCE: Grady's an ecologist at the University of New Mexico. He thinks the not too hot or too cold lifestyle was a useful adaptation by dinosaurs. After all they evolved into a world already populated with big slow cold-blooded reptiles.
GRADY: You know if you are a little bit warmer blooded than a reptile essentially your muscles fire faster, your nerves fire faster, your kind of a more dangerous predator.
JOYCE: At the same time a bit of cold bloodedness has its charms. You burn energy more slowly so you don't have to eat as much to grow. Think of a crocodile or a snake that can live for a month on one meal.
GRADY: And that means they could maybe get a lot bigger than a mammal could be. Which wouldn't be able to eat enough if it was the size of a Tyrannosaurus rex.
JOYCE: A mammal the size of a T-Rex would have to eat 24/7 to feed its supercharged metabolism. So Grady suggests that dinosaurs probably could generate body heat but not maintain a constant temperature. That meant they could be fast and big. Writing in the journal "Science" Grady explains how his team figured all this out. They relied on this fact, warm-blooded animals grow fast and cold-blooded ones slowly. And the dinosaur bones show that they grew faster than reptiles but not quite as fast as mammals. Grady determined that from reading growth lines in dinosaur bones. They're kind of like tree rings. Paleontologist Greg Erickson at Flordia State University pioneered a way to read those rings.
GREG ERICKSON: I liken what we do to forensic science. You have what little remains you come across to work with.
JOYCE: What Grady's group did was compare those remains, dinosaur bones, with bones of modern animals.
ERICKSON: The present's the key to the past. If we can garner an understanding of how living animals work and it then we could get similar information from fossils then we can understand what happened in the past.
JOYCE: But what about the present? Are there animals now that combine warm-blooded and cold-blooded traits? Yes I few. The echidna, a mammal that looks like an ant eater and lays eggs, Some turtle and two of the biggest fastest predators in the ocean. The white shark and the tuna. Christopher Joyce NPR News.
CORNISH: You're listening to a ALL THINGS CONSIDERED from NPR News.
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