July 6, 2010
Tyrannosaurus Didn’t Have the Nerve to Run Fast
It was one of the most memorable scenes in Jurassic Park—a hungry Tyrannosaurus rex chasing after Ian Malcolm, Ellie Sattler and Robert Muldoon as they make their escape in a Jeep. It was also among the moments that probably made paleontologists in the audience facepalm. Tyrannosaurus was fearsome, but it was way too big to be that fast, and a study just published in the Proceedings of the Royal Society B helps to explain why.
As the interdisciplinary team of scientists state in the introduction of the paper, the ability of animals to coordinate their movements greatly relies upon two factors—the speed with which an animal can respond to stimuli and its ability to react appropriately to those stimuli. These factors are influenced by the length and density of nerves in the animal’s body. Among larger animals there is a trade-off between the speed with which an animal can respond and their sensitivity to stimuli, and to gain a better understanding of this the scientists looked at the nerves of shrews and elephants.
Taken together, shrews and elephants represent the extremes of the animal world—elephants are the largest terrestrial mammals, and shrews are among the smallest. By experimentally stimulating the sciatic nerve in the hindlimb and figuring out the speed with which the signal caused a reaction in the medial gastrocnemius muscle, the scientists could determine how size affects an animal’s ability to respond quickly and appropriately to its environment. When they compared the results, the scientists found that the speed with which the signals were transmitted through the nerves of both animals were essentially constant. Since the elephant is so much larger than a shrew, it took much longer for the nerve impulse to elicit a reaction from the muscle (100 milliseconds in the elephant versus 1 millisecond in the shrew). In other words, one of the reasons that an elephant can’t run too fast is that its nervous system needs more time for information from the body to get to the brain, and if it ran ahead before these signals were processed it could trip itself up and become seriously hurt.
If the elephant had much larger neurons, it could transmit information faster. But the researchers also found that long neural projections called axons were only about twice as wide in elephants as in shrews. That’s presumably because if neurons get too wide, fewer of them can project to a given part of the body, cutting down on how sensitive the nervous system can be to external stimuli.
Now, even though the scientists did not study Tyrannosaurus or any other dinosaur, these findings have implications for how the largest of dinosaurs would have moved. Just as it is with elephants, large body size among dinosaurs would have caused a trade-off between sensitivity to their environment and speed of nerve impulses which would have required them to be slower than their smaller relatives. As disappointing as it may be to some movie-goers, Tyrannosaurus may have been able to overtake a person on foot, but not keep up with a car at high speed. Given that Tyrannosaurus was after large prey, however, it would not have had to have been a speed demon in an absolute sense—it just had to be faster than its slowest prey.
More, H., Weber, D., Hutchinson, J., Aung, S., Collins, D., & Donelan, M. (2009). Scaling of sensorimotor control in terrestrial mammals Comparative Biochemistry and Physiology – Part A: Molecular & Integrative Physiology, 153 (2) DOI: 10.1016/j.cbpa.2009.04.510
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Dear sir, just three points:
1) The post has not made clear which the journal that has published the original work is, CBP A or PRS b.
2) When you say “the extremes of the animal world”, I suppose that, actually, you mean “the extremes of the mammal world”, because it is obvious that there are smaller animals than a shrew, even among vertebrates.
3) The stimulus conduction speed was probably the same or similar but it should not be, because if fiber diameter is twice in the elephant, stimulus speed should be higher.
If this study proves accurate, then wouldn’t that mean dinosaurs like Sauroposeidon, Amphicoelias, and their kin — the largest animals to have ever lived and many times larger than even T. rex — would have been essentially immobile?
Giraffes, anyone? They can run upwards of 30 mph, and while I realize they use quite a different gait than T. rex or elephants, the nerves are still a bit longer.
Also, did T. rex and other theropods have a large pelvic ganglia?
I’m not sure what should be so objectionable about the scene in JP. The T-Rex was chasing a jeep that was driving in the mud while in a heavy rain storm. I doubt the Jeep would have been going very fast. I certainly didn’t have the impression that it was when I saw the movie.
Girraffes are generally light weight which would explain why they can run faster than an elephant.
But… an elephant is not as slow as you think, it can charge pretty fast and can turn on a dime. Rhino, same story, they are heavier than a girrafe, but can run even faster, they can also turn on a dime.
One other factor to consider is the oxygen levels of that time were (apparantly) much higher than today, which was one of the reasons they grew to such extreme heights. More oxygen means more energy, better blood circulation and may even have caused certain nerve systems to respond quicker.
So (in my opinion) the problem here is not the speed of the dino, it’s the size. The oxygen level had a direct effect on the animals growth rate. If dinosaurs lived today their mass would be halved, perhaps even more because the atmosphere simply can’t give them high levels they need to grow as fast.
That’s why they had such big plants, insects and millions of other animals that grew to such extreme heights.
Just something I heard a long time ago, and sorry for all the typo’s.
[...] Tyrannosaurus didn’t have the nerve to run fast. Those scared to death by the t-rex in Jurassic Park can breathe a sigh of relief — recent research suggests that the “tyrant lizard” couldn’t move nearly as fast as depicted. Brian Switek of Dinosaur Tracking explains the reassuring details. [...]
A shrew can run 16 mph. An elephant can run 25 mph. If we follow this series we would imagine a T rex could run even faster than an elephant, nerves or no nerves. Remember, walking can be handled semi-autonomously since it is a rhythmic action modulated by the elasticity of the muscles. Local neurons could control the muscles based on sensed tension, position and pressure.
Animals don’t have to use their brains for each and every step. Some animals can even walk after you disconnect their brains from their bodies by cutting their heads off. I agree that it is harder to make path corrections or respond to a varying surface if you have a long way from your feet to your brain, but a shrew probably has to do a lot of adjusting while an elephant can often just knock stuff aside. If nothing else, an elephant can see, and so plan, farther. A T rex could probably plan a path and pour on the speed.
I wouldn’t be surprised at all if a T rex could go faster than 25 mph, though I imagine one might take advantage of its slow brain time to escape. At 25 mph I doubt a T rex could turn on a dime.