Not So Fast, T. Rex ! Theory Slows Toothy Tyrant to Trot

He was big, and he was mean. But was he fast?

For years, scientists--and movie makers--have speculated about the top speed of Tyrannosaurus rex, the fearsome predator of the Cretaceous. Some paleontologists have theorized the creature could gallop after its prey at 25, 30, even 45 mph.

Slow down, say a pair of researchers.

Using computerized biomechanical models, the two scientists, specialists in biomechanics, have reduced T. rex to no more than a trotter. To actually run, according to their studies, which are published in the journal Nature, an animal as big and heavy as the Tyrannosaurus would need leg muscles so massive they might make up as much as 86% of its body mass--leaving little room for lungs, a skeleton or much of anything else. Typically no more than 50% of an animal’s body mass is composed of muscle and usually only 5% to 40% is allocated to the legs.

Researchers John R. Hutchinson and Mariano Garcia used a mathematical model and information from dinosaurs’ living relatives--birds--to determine how much muscle would be necessary for T. rex to achieve a fast run. Hutchinson, now at Stanford and Garcia, now at Borg-Warner Automotive, performed the research while both were at UC Berkeley.

Early assumptions about dinosaur speed were often made based on physical similarities to living animals--if ornithomimus looked like an ostrich, it must run as fast as one.

Other methods to determine top speeds were hampered by holes in the fossil record.

Some scientists, for example, have calculated speeds based on fossilized footprints. A paper published in Nature last month clocked megalosaurus, a large European dinosaur, at 8 meters per second (roughly 18 mph), said Neill Alexander, a researcher at the University of Leeds in Britain who specializes in animal and human movement. He first developed the formula for calculating speed from footprints in 1976.

But footprints are few and far between. Only one known T. rex footprint exists, Hutchinson said, making it impossible to estimate the speed of the giant dinosaurs that way.

Calculating the realm of what is physically possible in terms of scale may be a better way of attacking the problem, Hutchinson said.

“If an animal is going to move a certain speed its muscles need to generate enough force to maintain that speed,” Hutchinson said.

Muscle strength is not proportional to body size; in fact, just the contrary. The strength of a muscle depends on its length and width, but the mass of an animal depends on three dimensions: length, width and height. If one animal is twice the size of another, its muscles would be four times stronger, but its mass would be eight times greater. In proportion to body size, the smaller animal would be much stronger.

“Ants can lift things two times their body weight for essentially the same reason that T. rex couldn’t run,” said Tom Roberts, a biomechanist at Oregon State University.

“The dramatic effect of scaling”--the way strength and weight change as the scale of an animal’s body increases--”simply makes it hard to build a vertebrate animal this size that could run,” said Andrew Biewener, a scientist at Harvard University who published a response to Hutchinson’s paper in the same issue of Nature.

Chicken Legs and Alligators Yield Clues

To calculate the amount of force needed to produce a fast run, Hutchinson and Garcia developed stick-figure models of the animals. The models allowed them to visualize all the rotational forces that acted on the dinosaur’s joints. Once they calculated the force needed for the dinosaur to balance itself, they calculated how much more force would be needed for it to run. They based their calculations on the knowledge that animals, from ants to horses, generally need to produce about 2 1/2 times more force to run than they do to support themselves while standing still.

Once they calculated the required force, their models allowed the two researchers to estimate the minimum amount of muscle mass the dinosaur would need.

Hutchinson and Garcia first confirmed the validity of their model by using its estimates to calculate the minimum muscle mass needed for chickens and alligators to run.

Chickens, like humans and other good runners, have more leg muscle than the model indicates they absolutely need in order to run--about 17% of body mass. Alligators, which are not strong runners, have half the estimated minimum leg muscle needed to run well--about 7%.

The researchers also rebut another assumption about T. rex: its posture.

In science fiction depictions and elsewhere, the dinosaur is typically pictured running in a crouched position, Hutchinson said. But larger animals are able to support their weight more easily in an upright position than in a crouch--a principle that anyone who has stood in place for a long time can understand.

“You straighten your legs because it requires the least exertion,” he said. “The muscles need to be tensed to maintain a crouched posture.”

Using their models, Hutchinson and Garcia tested a number of different postures and a variety of speeds.

It Might Have Feasted on Predeceased Prey

For T. Rex, the crouched running positions all required too much leg muscle to be feasible. And even standing straight, a speed faster than 25 mph would require an untenable amount of muscle mass in the leg, they found.

“Regardless of what posture we put in, there was no way a Tyrannosaur could run 45 miles an hour,” Hutchinson said.

Even a 25-mph top speed seems unlikely, he said. To hold the muscle mass to no more than 16% to 20% of the animal’s total would require a posture so upright that it would be difficult for the dinosaur to walk, Hutchinson said.

A further implication of the work: If T. Rex could not chase down its prey, it is possible, some paleontologists say, that the creature actually got much of its food not by predation, but by eating carrion. On the other hand, Hutchinson said, many dinosaurs that were possible T. Rex victims, such as triceratops, would also be hindered by their own bulk.

To further emphasize their point, Hutchinson and Garcia studied the mechanics of a chicken scaled up to the size of a T. rex.

The gigantic chicken would need to have more than 99% of its body mass composed of leg muscle to produce enough force for it to run--a physical impossibility, although perhaps a dream for lovers of dark meat.

Whether the new information will change the movies remains to be seen. By now, T. rex is “also a movie actor, and you see human actors doing things normal humans can’t do, if a director wants it that way,” said Ken Bryan, a modeling supervisor for Industrial Light & Magic, which handled the animations for the “Jurassic Park” movies, among others.

But at least the movie makers will be well informed. On the strength of his research, Hutchinson served as an informal consultant for “Jurassic Park 3.”

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