No Olympian: Analysis hints T. rex ran slowly, if at all

Tyrannosaurus rex, a bipedal meat eater considered by many to be the most fearsome dinosaur of its day, may not have been the swift Jeep-chaser portrayed in Jurassic Park. Scientists figure that for a 6,000-kilogram adult T. rex to dash along in high gear, as much as 86 percent of its body mass would need to be leg muscles–an unlikely pair of drumsticks, indeed.

BET ON THE CHICKEN. Unlike a 3-kilogram chicken, a 6,000-kg Tyrannosaurus rex probably didn’t have enough leg muscle to support its weight while running. Luis Rey

The leg muscles of a running, bipedal animal typically must support at least 2.5 times the animal’s body weight at the highest-stress point of its stride, says John R. Hutchinson, an evolutionary biologist now at Stanford University. That applies across the range of modern animals from chickens to ostriches to people.

Using that rule, Hutchinson and Mariano Garcia, a mechanical engineer now at Borg-Warner Automotive in Ithaca, N.Y., performed an engineering analysis of the forces that would be imposed on a T. rex‘s lower leg bones and joints during fast running.

Fossil footprints recently discovered in England suggest that cousins of T. rex could run at speeds of 29 kilometers per hour (SN: 2/23/02, p. 125: Available to subscribers at Dinosaur tracks show walking and running.). Some paleontologists have estimated that the much larger T. rex could move at speeds up to 20 meters per second, or about 72 km/hr. Hutchison and Garcia found that to sprint at that speed, the creature would have needed muscles in each leg equal to an improbable 43 percent of its entire body weight. The scientists, formerly at the University of California, Berkeley, report their results in the Feb. 28 Nature.

Simple laws of biophysics dictate that big animals need proportionately larger leg muscles to run than small animals need, says Hutchinson. The maximum force that a muscle produces when it contracts is related to its cross-sectional area, a two-dimensional parameter. However, the animal’s weight relates to its volume, a three-dimensional quantity. So, as an animal’s size goes up, the amount of muscle needed to support sprinting generally outpaces muscle performance.

Although the researchers’ analysis technique is simple, it provides results that qualitatively agree with the traits of living animals. An alligator would need rear leg muscles adding up to just over 15 percent of its body weight to run bipedally, but it actually has only half that much. And alligators don’t run on their rear legs, Hutchinson notes. Also, the analysis estimates that a quick-running chicken needs leg muscles that add up to at least 9 percent of its body mass; they actually have almost twice that much.

Even if T. rex didn’t have the stuff of an Olympic sprinter, it still had what it took to satisfy its carnivorous obsessions, says Andrew A. Biewener, a paleontologist at Harvard University. A long-striding T. rex could have walked briskly at 20 km/hr. “An animal that size moving at that pace still would have been impressive,” Biewener adds.

Furthermore, Hutchinson notes, T. rex probably preyed upon other large dinosaurs burdened with the same problem. With their immense bulk, they couldn’t run fast, either.

Says Thomas R. Holtz Jr., a paleontologist at the University of Maryland in College Park, “A T. rex didn’t have to be in a full run to outrun its prey.”

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