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In a sprint, put your money on the fryer, not on its distant relative, T. rex. Courtesy Luis Rey

T. rex: Walk like a tank, or sprint like a chicken?
With the last race of the winter Olympics now history, we bring good news to track and field runners training for the summer games: Tyrannosaurus rex ain't likely to qualify for the 100 meter chase-'n-chomp.

The king of the Cretaceous beasts was simply too sluggish.

Such a notion would have raised no eyebrows 20 years ago, when the fearsome carnivore was considered simply a lumbering beast. More recently, however, some scientists have argued that T. rex, the carnivore that roils the stomach like fear itself, could have burned up the track.

Now John Hutchinson and Mariano Garcia have calculated that T. rex wasn't cut out for sprinting. In fact, the ol' beast might have been outpaced by a chicken.

Hutchinson, who's now a postdoctoral researcher in biomechanics at Stanford University, brings up the subject of fried finger food because chickens are descendens of smaller T. rex relatives, and they can scoot pretty fine on two legs (which is handy, considering the rarity of the three-drumstick chicken).

(Garcia, Hutchinson's collaborator, once investigated cockroach locomotion. He now works in private industry. Extermination, for all we know...)

Because T. rexes are scarce -- and expensive -- Hutchinson and Garcia built a mathematical model of rex biomechanics.

The model correctly predicted the sprinting ability of the broiler and the alligator (another distant T. rex cousin), indicating that it would also apply to the rex.

What does biomechanics involve?

Tipping the scales
Biomechanics is the application of mechanical and mathematical principles to the movement of living stuff. Biomechanics can answer longstanding and important questions like: How did the chicken cross the road?

In this diagram of a lever, you will notice side "A" is equal to side "B". Or the input force equals the output force. But notice that side "B" is longer than side "A". Therefore the output force will be larger. This is a ratio of output force to input force called mechanical advantage. Courtesy of Journey in Time.

Answer: by using muscles to activate the system of levers and pivots in its legs and feet. You can see how this works in the drawings here.

It's the usual lever relationship -- the closer the pivot (fulcrum) is to the applied force, the stronger that force must be to activate the other end of the lever -- and the further the other end moves.

T. rexes, as any kid can tell you, were huge -- weighing more than a fully loaded SUV (up to 6,000 kilograms). That raises the crucial problem of scale. As body mass increases, muscle mass must increase even faster.

Why? Because a muscle's force is determined by its area in cross-section, and the animal's mass is related to its volume. So when animals get bigger, muscle cross-section increases as the square of the size, but mass increases as the cube of size. At a certain point -- say the size of a cheetah or so -- you pass the point of diminishing returns, and bigger animals must be slower animals.

Down at the sprinting track
So could T. rex hit the 45 miles per hour that some paleontologists have suggested? No, says Hutchinson, although he admits that the model cannot pin down the top speed, largely because the dino's running posture remains unknown.

Hutchinson did the math, and found that T. rex could run like, well, a 6,000 kilo chicken... Slowly, very slowly... Courtesy John Hutchinson.

If rexes ran in a crouch, as many paleontologists have suggested, 45 miles per hour (MPH) can "conclusively be ruled out," Hutchinson says, since that would require that 86 percent of the animal's body mass be devoted to the extensor muscles that power the legs. "That's ridiculous, it's more than any known vertebrate," he says.

Even the slower, but still dramatic speed of 25 MPH would not be realistic, he says, since in a crouch, 69 percent of body mass would be devoted to those extensors. In modern vertebrates, Hutchinson says, extensors weigh less than 20 percent of total body mass.

To a mechanical engineer, this is how the rex's propulsion mechanism looks. Courtesy John Hutchinson.

Sprint not, want not
However, if the legs were straighter, the required mass of the extensors for 25 MPH would drop to about 16 percent of total body mass. That calculation, however, depends on "some very generous assumptions," he says. "It's implausible, but you can't absolutely rule it out."

Hutchinson says his best guesstimate is that rex could reach about 10 miles per hour, about 5 meters per second. And while that's a considerably slower than many have thought, he notes that the large herbivore dinos that T. rex ate are also subject to the same limitations. They were slow-moving beasts as well.

Rex may have been too sluggish for an Olympic sprint, in other words, but it could still make a living ripping flesh apart.

Rex has long been a staple of horror films in the Hollywood imagination, but there's a good chance you could have outrun the ol' beast.

But look at it this way. Hollywood's loss is the runner's gain. Running about 23 MPH, today's world-champion sprinters could stay ahead of the ancient world's champion chomper.

-- David Tenenbaum

BIBLIOGRAPHY
Tyrannosaurus Was Not a Fast Runner, John Hutchinson and Mariano Garcia, Nature, Feb. 28, 2002.

See also Walking with Tyrannosaurs, Andrew Biewener, p. 971-2.

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