Geotimes
Web Extra
Friday March 1, 2002
Tyrannosaurus
rex: speed walker not runner
The famous portrayal of Tyrannosaurus rex chasing a speeding jeep in
Jurassic Park is scientifically disputed in this week's Nature.
In their paper entitled "Tyrannosaurus was not a fast runner,"
John R. Hutchinson and Mariano Garcia of the University of California at Berkeley
present a model that indicates T. rex's running speed was between 5 to
11 meters per second (11 to 25 miles per hour). This speed is considerably slower
than the previously estimated speeds for T. rex, ranging from 11 meters per
second (25 miles per hour) to 20 meters per second (45 miles per hour).
The model Hutchinson and Garcia created estimates the minimum mass of extensor
(supportive) muscle required to enable an animal to run quickly on two legs.
They expressed the required extensor muscle mass for fast running, where both
feet in mid-stride are off the ground, as a percentage of body mass. Animals
without sufficient leg muscle could not run bipedally. The researchers looked
at several organisms, including an alligator, a chicken, a Coelophysis (a
small dinosaur known to be capable of running), a small T. rex and an
adult T. rex.
The alligator, chicken and Coelophysis validated the model. The alligator
had only 3.7 percent body mass per leg; to run it needed 7.7 percent body mass
per leg. The chicken needed only 4.7 percent body mass per leg to run and was
found to have 8.8 percent per leg as extensor muscle, nearly double the amount
required. For T. rex, the model showed that in order to be a fast runner,
it would need approximately 86 percent of its body mass to be extensor muscle
(43 percent per leg). The current knowledge about the size and shape of T.
rex shows this is not the case.
Using isometric
scaling, the two further illustrated the inability of T. rex and other
large dinosaurs to be fast runners. Their chicken model was isometrically scaled
up to 6,000 kilograms. At that size and with normal limb orientation a chicken
would require 99 percent body mass per leg as extensor muscle in order for it
to be a fast runner, a physical impossibility. From these results, Hutchinson
and Garcia concluded that T. rex and other large dinosaurs were poor
runners.
Illustration of Tyrannosaurus
rex in a race with a 6,000 kilogram chicken. By Luis Rey
To overcome the number of unknowns in the model, such as the penetration angle
of muscle fiber, Hutchinson and Garcia changed unknown parameters to determine
what effect they have on the results. Andrew Biewener of Harvard's Concord Field
Station complimented the authors for this use of sensitivity analysis, in an
accompanying article in Nature. Using this technique, Hutchinson and Garcia
discovered that most of the unknowns did not have a significant effect on the
outcome, except they did find that the limb orientation made a significant difference
in the model.
"A Tyrannosaurus needs a lot more muscle mass to run quickly if
it used a crouched pose than if it used a more columnar pose. But in either
case, we felt we could rule out 45 miles per hour running. It required too much
muscle mass," Hutchinson says.
Fossil tracks exist that show smaller dinosaurs ran fast, but so far paleontologists
have not found running tracks of carnivorous dinosaurs the size of T. rex.
Julie Day of the University of Cambridge reported in Nature recently
about tracks of a running dinosaur one-third to one-fifth the size of T.
rex. She estimated its speed was 8 meters per second (18 miles per hour).
This speed falls in the middle of the range suggested in Hutchinson's paper,
but he points out that speed estimates from tracks are often imprecise and the
margin of error can be 50 percent. He also noted that Day's tracks were 163
million years old, while T. rex lived 65 million years ago.
It took some extra movie magic for T. rex to jog after the speeding
jeep in Jurassic Park. Animators at Industrial Light and Magic tried animating
Tyrannosaurus at 40 or more miles per hour, and it "just didn't
look right" to them, Hutchinson says. On his Web site he described the
details:
"If you watch closely, the vehicle is shifted into second, third then
fourth gears, implying a speed of 40 or more miles per hour. The actors are
screaming, the music is dramatic, the Tyrannosaurus is roaring, and the
editing cuts quickly between cameras. Take a closer look at the Tyrannosaurus
motion, though, and you spot some incongruities. It always has one foot on the
ground, and the cadence of its footfalls is rather slow, less than two steps
(one stride) per second. To be moving over 40 mph with such slow strides, the
Tyrannosaurus would have to have been taking ridiculously long steps.
A casual survey of the movie shows this is not the case. The conclusion (first
pointed out by scientist Stephen Gatesy of Brown University, and acknowledged
to us by the moviemakers), is that the scene (Tyrannosaurus and all)
was only moving at around 10 to 15 mph! Our eyes are easily fooled by movie
magic into thinking that the Tyrannosaurus was faster. Why didn't the
filmmakers just have the Tyrannosaurus moving 40+ mph? Those basic biomechanical
principles of size and locomotion just wouldn't allow it to 'look right' to
our eyes. The scene would strain credulity, and our research shows why."
Livia Levis
Geotimes contributing writer
Links:
Hutchinson's
Web page
University of California Berkeley:
Polypedal Lab
