New Research Suggests Why Mid-Sized Animals Are The Fastest

Here's a puzzle: a giraffe has much longer legs than a cheetah, but it runs only about half as fast.

It seems long legs don't make you the fastest in the land. Yale University biologist Walter Jetz says theoretically, they should. Animals with longer limbs and lots of muscle should be able to cover ground — or water or air — the fastest.

"I mean it's really cool actually," Jetz says, "to think about these really large birds or mammals or even large dinosaurs, in principle they could have been super super fast." 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 studied over 400 animals species and 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, or the falcon, for example.

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. This is where muscles are working hard and fast — anaerobically, or without much oxygen.

Animals only have so much of this 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. "In practice, they run out of energy in their acceleration before they can even reach that theoretical speed," Jetz says. They switch over the aerobic running, which is lower intensity and much slower.

Mid-sized animals, however, have a mix of energy and limb size and muscle that puts them in a speed "sweet spot." They don't burn out as fast. "The leopard or the jaguar have enough acceleration energy to make it all the way to nearly your theoretically maximum speed," says Jetz.

Jetz reports his findings in the journal Nature Ecology and Evolution. Once his research team proved their calculations worked on living animals, they turned their analysis to dinosaurs. They found that the giant Tyrannosaurus rex topped out at a mere 17 miles an hour.

That reassures Matthew Carrano, curator of dinosaurs at the Smithsonian's Museum of Natural History, who came to much the same conclusion from studying the T. rex skeleton. "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'."

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. "How wide-ranging was a triceratops?" Carrano wonders. "Was it like a bison? Some people think these animals migrated across the continent."

Or it might shed light on how the velociraptors of Jurassic Park fame actually hunted. According to the Jetz's research, the velociraptor would have been in that speedy sweet spot like mid-sized mammals, possibly running as fast as 33 miles an hour.

That's something to think about.

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