T. rex may not have been able to run — but it was still pretty fast

Plenty of people have wondered just how fast a Tyrannosaurus rex could run. But that might be the wrong question. People should have been asking if this fearsome dinosaur was able to run at all. In fact, a new analysis suggests, the dino’s leg bones would not have stood up to the stress of a full-on run. Yet this creature could obviously outrace its prey. And scientists now say that if it were alive today, T. rex probably could catch most people, too.

William Sellers is a vertebrate paleontologist at the University of Manchester in England. His team described its new findings online July 17 in PeerJ.

Past studies have used several methods to try to estimate the speed of a T. rex, Sellers notes. Some accounted for the creature’s size, weight and muscle bulk. (Researchers have to guess at the size of a dinosaur’s muscles, since they don’t survive in fossils.) Others have looked at fossil footprints, taking detailed measurements of their size and spacing. Some researchers, including those on Sellers’ team, have even used computers to model, or simulate, the dino’s gait.

Such studies gave mixed results. Their estimates for a T. rex’s top speed ranged from 5 to 20 meters per second. That’s about 18 to 72 kilometers per hour (or 11 to 45 miles per hour). So T. rex topped out somewhere between a car inching its way through a school zone and one barreling down a highway.

Now Sellers and his team have added a new dimension to their computer model. Such models almost always include the dino’s overall size, how its bones were arranged and how strong its muscles likely were. But they have to simplify the dino in digital form. In the model used by Seller’s team, for instance, the T. rex was made of 15 pieces. It had a body, two legs with four parts each and two forelimbs, each with three parts. “Our T. rex model is the most complicated we’ve built, but it’s still pretty simple,” Sellers admits.

His team also included one more aspect that previous studies did not. They added data about the presumed strength of the dino’s leg bones.

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Teaching a digital dino to run

The team ran its simulations on hundreds of computers at once. Altogether, the programs ran for more than 5,000 hours. (That’s equivalent to one computer running nonstop for nearly seven months.) Over that time, the simulations “taught” the virtual dinosaur how to walk and run.

The scientists define walking as a gait in which at least one foot is always on the ground. In a run, both feet are sometimes in the air at once.

Earlier studies had suggested T. rex’s long legs helped give this creature great speed. The longer an animal’s legs, the greater the distance they can cover with each stride. But Sellers’ team also turned up a possible disadvantage to T. rex’s long legs. Leg bones experience certain stresses with the push-off and landing of each stride. And these stresses increase as bones lengthen.

If the new simulations ever suggested those bones were being stressed to the point of damage, it would flag that running speed as too high, Sellers explains. And at every speed, the stresses in a running T. rex’s leg bones were so high that they likely would have been damaged or even broken, the team found. This was especially true for the lower leg bones.

For a walking T. rex, the model found  that a safe speed maxed out at just below 28 kilometers per hour (17 miles per hour). That’s a little faster than the top speed that elite marathoners reach.

“I’m glad the team introduced bone strength as a constraint” on T. rex’s top speed, says Eric Snively. He’s a vertebrate paleontologist at the University of Wisconsin-La Crosse. But T. rex’s top speed actually could have been a little bit higher than the new estimates, he adds. That’s because the dino might have used a bouncing style of walking, called “grounded running.”

Many modern-day birds, such as sandpipers and quail, use this style of walking. (Researchers don’t know why, though, because grounded running hasn’t been studied in detail). That gait is difficult to simulate in a computer. There are signs, however, that it may be slightly faster and more efficient than a normal walk.

“This is an interesting and extremely ambitious study,” says John Hutchinson. He’s an evolutionary biologist at the Royal Veterinary College in Hatfield, England. “But it’s a bit soon to accept this conclusion,” he adds. That’s because Sellers’ team has not done similar simulations for living creatures to see if the results match reality, he says.

Such tests, plus studies of walking or jogging creatures (to verify those computer models), might reveal whether the bouncy style of walking actually reduces stresses in leg bones, says Sellers. “It would be neat to think that our modeling of dinosaurs could lead to new insights about living creatures,” he says.