The Problem of Size

Part of the Dinosaurs: Ancient Fossils, New Discoveries exhibition.

Big animals alive today have a lot in common. They are heavy. They have few predators. And they tend to move slowly. But have big animals always behaved this way? Scientists continue to disagree about the speed--fast or slow?--of one very big extinct animal, Tyrannosaurus rex.

Tyrannosaurus rex
Craig Chesek/AMNH

The most famous of the upright, largely meat-eating dinosaurs called theropods, T. rex would have weighed between 5,000 and 7,000 kilograms (11,000 to 15,500 pounds) with skin and flesh on its huge bones. That's about as much as the largest African elephant.

Some claim T. rex could run at speeds up to 72 kilometers (45 miles) per hour. But biomechanics researchers argue that the animal moved much more slowly, perhaps only about 16 kilometers (10 miles) per hour--about as fast as an average human runner. These experts say size slowed T. rex down. But why is it so hard to be big and fast? Bones, muscles and posture all play a part.

Fast Facts: T. rex

  • Pronunciation: "Tie-RAN-uh-sore-us rex"
  • LENGTH: 12 meters (40 feet)
  • HEIGHT: 3.6 meters (12 feet)
  • WEIGHT: 5,000-7,000 kg (about 11,000-15,500 pounds)
  • Food: Other animals
  • Lifespan: About 28 years
  • When it lived: 67-65.5 million years ago
  • Fun fact: At the hip, an adult T. rex is three times the height of an average seven-year old--but more than 260 times the weight of that child!

T. rex: Built for Speed?

Scientist John Hutchinson with ostriches.
From AMNH video © 2005

Apart from size, T. rex shares some features with fast animals alive today. Like the speedy ostrich, T. rex had long legs for its body size, especially the portion of the leg below the knee. Because it looks fast, T. rex is often shown in the somewhat crouching stance of a human sprinter at the starting line.

But judging speed from appearance is like assuming a car is fast without looking under the hood. Experts in biomechanics study the muscles and body design of living large animals--and the stress on the body caused by running. Applying this research to extinct animals, they conclude that T. rex was a slow runner, if it could run at all. More usually, it kept to a dignified walk.

Looks Can Deceive

Bones can't tell us everything about how an animal walks or runs. For example, the giraffe looks as if it might be a fast runner. It has very long legs, and skeletal details suggest the presence of large and powerful muscles. But with a top speed of perhaps 55 kilometers (35 miles) per hour, the giraffe turns out to be not much faster than the stubby-legged rhino.

In contrast, a cheetah looks like a skinny relative of the leopard, at least from its bones. You'd never know from just the body structure that this super athlete can sprint at speeds up to 110 kilometers (70 miles) per hour.

So bones can mislead--but a skeleton is all we have for most dinosaurs. That makes it tough to learn all about movement.


A Misleading Family Resemblance

Ostrich and T. rex skeletons look quite similar. This isn't surprising, though, because both T. rex and all birds arose from a common ancestor.

But are they alike in other ways? Ostriches run at up to 75 kilometers (45 miles) per hour, but the biggest ostrich weighs only about 160 kilograms (350 pounds). Compare that with T. rex's 6,000 kilograms or more (over 13,000 pounds). Today, the fastest animals tend to weigh 100 kilograms (220 pounds) or less. An extinct animal 50 times that size was probably not as fast as an ostrich.

Changing Gears

About a century ago, T. rex's discoverers wrote of its "destructive power and speed." Within decades, though, scientists had decided all large dinosaurs, including T. rex, were sluggish giants.

Later, opinions changed again. As theories about warm-blooded dinosaurs caught the public imagination, T. rex regained its reputation as a fast, fierce carnivore.

These days, T. rex may be slowing down once more. Recent biomechanical analysis suggests that while T. rex was a powerful--even "destructive"--animal, it wasn't very fast.