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Here's a puzzle. A giraffe has much longer legs than a cheetah, but it runs only about half as fast. So if long legs don't make you the fastest, what does? NPR's Christopher Joyce reports on a new explanation.
CHRISTOPHER JOYCE, BYLINE: Yale University Biologist Walter Jetz says theoretically, the biggest animals should have the highest top speeds. They have longer limbs and more muscle.
WALTER JETZ: I mean it's really cool actually to think about these really large birds or mammals or even large dinosaurs. And in principle, they could have been super, super fast.
JOYCE: For example, based on muscle mass and body size, an elephant in theory could run 60 miles an hour. But Jetz and his fellow researchers found that the biggest animals fall short of their potential. It's actually mid-sized animals that run almost as fast as their bodies theoretically can go - the cheetah, the marlin, the falcon. It turns out that reaching your top speed potential depends on your fuel tank.
Acceleration is all about burning the body's high-octane fuel to get those fast-twitch muscles going. Animals only have so much of that kind of fuel. What happens is the biggest animals take more time to accelerate, and they run out of it before they reach their top speed.
JETZ: In practice, they run out of energy in their acceleration before they can even reach that theoretical speed.
JOYCE: Mid-sized animals don't burn out as fast.
JETZ: A leopard or a jaguar have enough acceleration energy to make it all the way to nearly your theoretical possible maximum speed.
JOYCE: Jetz reports his findings in the journal Nature Ecology & Evolution. Once they proved their calculations worked on living animals, the researchers turned their analysis to dinosaurs. They found that the giant Tyrannosaurus rex topped out at a mere 17 miles an hour, which reassures Matthew Carrano, curator of dinosaurs at the Smithsonian's Museum of Natural History. He came to much the same conclusion from studying the T. rex skeleton.
MATTHEW CARRANO: You know, this is one of these things. I had a professor who said, I don't know if this is true, but it deserves to be true.
JOYCE: Carrano says accurately deducing a dinosaur's movement from its size is a handy tool. It could provide clues to how dinosaurs traveled, like the horned triceratops...
CARRANO: How wide ranging was the triceratops? Was it like a bison? Some people think these animals migrated across the continent.
JOYCE: ...Or how the velociraptors of "Jurassic Park" fame actually hunted. According to Jetz's research, the velociraptor could reach about 33 miles an hour. That's something to think about.
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ARIANA RICHARDS: (As Lex, screaming).
JOYCE: Christopher Joyce, NPR News.
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