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There is another mechanism besides muscle that gives energy to running and hopping animals. It enables such animals as kangaroos to run faster than their muscles alone can take them. What happens is the tendons act as springs that stretch and then snap back. There is no reason to think that a large and heavy dinosaur couldn’t make use of such a tendon-spring mechanism to go a high speed, although not for long.

Brian Hanley
Greenbrae, Calif.

In a recent reply to a letter about the T. rex Sue being lame and her survival being attributed to group care, Gordon M. Burghardt responded that all of these behaviors have been observed in living “reptiles” (See letter appended to “Turn Your Head and Roar,” SN: 12/15/01, p. 376: Turn Your Head and Roar .). My understanding was that dinosaurs had been concluded to be the precursors of birds, not reptiles. This would seem to apply to the question of how fast tyrannosaurs ran, as well. If they were built like a bird, then maybe they weren’t as massive as the researchers think.

Shad Augenstein
Delaware, Ohio

Your article reports that T. rex couldn’t run, but “Dinosaur tracks show walking and running” (SN: 2/23/02, p. 125: Available to subscribers at Dinosaur tracks show walking and running .) shows that Megalosaurus , a comparable animal of comparable size, both walked and ran. It would seem that something’s amiss. What if T. rex “only” ran at half of what some paleontologists have estimated, 72 kilometers per hour, and probably only for short bursts? It would most likely have developed only enough speed to catch its prey, and I doubt it ever needed 72 km/hour for that purpose.

McClellan Blair
Indiana, Pa.

In fact, the largest tyrannosaurs were about 4 meters longer than Megalosaurus and probably weighed about 5 times as much. T. rex ‘s walking speed was a large fraction of Megalosaurus ‘ running pace primarily because the larger animal had a longer stride. –S.P. One letter, above, suggests the possibility of energy storage in elastic tendons. I would like to offer a quick reply. While it is true that elastic energy storage can play a role in the running gaits of various animals (including humans), our analysis is unchanged by its presence. The force in question is transmitted through the muscle and tendon in series and would be essentially independent of tendon stiffness. Our calculations were based on the necessary muscle force and limiting stress, not the necessary energy input. Elastic energy storage would make the animal more efficient, but it would not allow the muscles to withstand significantly higher leg forces.

Mariano Garcia
Ithaca, N.Y.