December 29th, 2009
It is generally (but not universally) accepted that tyrannosaurus rex was capable of running. His anatomy certainly suggested that it ran: it had a big heavy tail for balance, large legs that appear to support massive musculature, and of course those big teeth were obviously meant for killing. So we naturally assume that tyrannosaurus rex was a runner.
But further thought suggests otherwise. Look how long that animal is: 43 feet from the nose to the tip of the tail. All that musculature must be controlled from the brain inside the skull, which means that nerve impulses must travel about 40 feet from the brain to the tail muscles. At their fastest, nerve impulses travel about 300 feet per second; it would take at least a tenth of a second for the nerve impulses to travel from the brain to the tail. In order to run without falling down, the brain must gather together all the information about the position of every part of the body. Some of that information will have to come from as far away as the tail or the toes; the travel time for those nerve impulses to reach the brain will be at least 100 milliseconds. Next, the brain must integrate together all those thousands of nerve signals to determine precisely how much each muscle in the entire body must contract or relax. That’s right -- every major muscle group must be precisely controlled to maintain balance while running. If the neck muscles relax too much and the head droops down, the center of gravity of the animal will change and the calculations for the leg muscles will be thrown off. So the brain has a huge integration task to figure out the nerve impulses for the next step. We’ll be generous and assume that all this processing takes place instantaneously. The decisions having been made, the signals must be sent to all the muscle groups. Again, at their fastest, these nerve impulses will take another 100 milliseconds to reach their destinations in the legs. Thus, at any given instant, the nervous system lags reality by about 200 milliseconds.
Now let’s consider the mechanics of bipedal running for a tyrannosaurus rex. It had a stride of about four meters; to run at a speed of 25 feet/sec (about 15 mph) the animal would have to complete two strides every second. That means that each leg was moving through a complete cycle once per second. In the time it takes for the control processes to do their job, the leg would have moved through about 20% of its complete cycle. To put it another way, the leg would move about five feet during the delay time required for processing.
I find it difficult to believe that an animal can maintain balance while running when its control responses lag body positions to such a great degree. Let’s apply the same reasoning to human running. The most remote muscles from the brain are the muscles controlling the toes, which are at most 6 feet away from the brain. At the same speed of 300 feet per second, it takes only 20 milliseconds for the nerve impulses to travel from brain to toe, or 40 milliseconds for the round trip from toe sensory nerves to brain back to toe muscles. Fast runners complete about 1.5 strides per second with a stride of about 6 feet. That means that each foot travels a distance of about 9 feet per second. During the 40 millisecond lag time required for motor control, the foot travels only about 4 inches. Thus, the spatial lag between sensation and motor control is only 4 inches. That strikes me as a reasonable figure, and running safely should certainly be possible with this kind of time lag.
We all know that massive animals move relatively more slowly than smaller animals. A hummingbird or bat can zip along faster than our eyes can track it, but the movements of a whale or an elephant are almost languid. We normally think of this as due to the enormous mass of the creature, but it is also due to the time delays arising from the slow speed of neural impulses. A large animal simply cannot control its muscles as quickly as a small animal can.
The debate over the locomotion of the tyrannosaurus rex has been going on for almost a century. Paleontologists have carried out detailed analysis of its skeletal structure, run computer simulations of the stresses on the bones at various gaits and speeds, and debated the mechanical issues in vast detail. However, I have not seen any analysis of probable nervous system constraints upon this problem. Perhaps it is because of the obvious lack of direct evidence in the form of fossils. But this line of reasoning is so powerful that I should think it would deserve serious consideration. It should settle the debate over tyrannosaurus rex locomotion once and for all.
If any readers are aware of studies of this line of reasoning, I would greatly appreciate instruction in this matter.