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The Science of Sue
Walking Tall or Born To Run?
One problem associated with giant body sizes is how such animals could have moved about. A basic rule of muscle physiology, namely that the strength of a muscle only increases by about two thirds as its mass is doubled, becomes a major obstacle as animals get larger. As dinosaurs evolved very large sizes, muscle mass would have significantly outpaced muscle strength, resulting in proportionately less force available to move their giant bodies. This is why very large animals, such as elephants, are slower than mid-size animals like horses, despite their longer legs. So how agile was SUE, one of the largest bipedal animals to have ever lived?
This question was recently tackled by John Hutchinson and Marciano Garcia, who estimated how much force the muscles in a SUE-sized T. rex leg would have to exert at mid-stride, as the other leg was off the ground. This force can be translated into required muscle mass, which in turn can be compared to total body mass to determine whether the values are realistic when compared to running animals of today.
In addition to body size, ground speed and joint angles are important parameters for obtaining estimates of locomotory capabilities. Actual speed and joint angle values are unknown, but using a range of realistic joint angles determined from how the hindlimb bones articulate, Hutchinson and Garcia predicted how much muscle was needed to keep SUE’s leg upright at different speeds. Altering the parameters provides a wide range of values, but all lie well outside of the expected range of muscle to body mass ratios seen in extant running animals, and don’t support some earlier reconstructions of galloping Tyrannosaurus. Indeed, their calculations for a “galloping” T. rex show that 86% of the total body mass would have to lie in just those leg muscles that keep the joints straight during the footfall phase of the stride cycle, without even considering the rest of the skeleton, musculature, organs and skin. Obviously, such a value is biologically impossible.
Applying values from living animals to a SUE-sized T. rex indicates very limited to non-existent running capabilities. According to the Hutchinson and Garcia model, SUE’s maximum speed would have about 11 mph (18 kph). T. rex youngsters weighing less than a ton may have been more adept runners, however, and possibly attained greater speeds such as those calculated for ornithomimids and other fleet-footed, medium-sized theropods. Another result of this study is that less muscular force would have been required for walking straight-legged with the bones carrying the animal’s weight rather than with highly flexed joints – SUE most probably walked with straight knees rather than the crouched, stalking pose SUE is mounted in.
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