If you were to pit a human against a Tyrannosaurus rex in a mile-long clash of speeds, who would win? Most likely a human, according to a new formula that can predict the speed of an animal with almost 90 percent accuracy.
"Our findings help to solve one of the most challenging questions in movement ecology over recent decades: Why are the largest animals not the fastest?" write the authors.
Previous research has found that the fastest animals tend to lie smack-dab in the middle of mighty and minuscule animals, but predicting how fast an animal can fly, run, or swim has been difficult to do.
On average, the larger the animal, the faster it is, but that only works up to a point. A squirrel may be faster than a beetle, a gazelle faster than a squirrel, but an elephant is slower than a cheetah.
To uncover why this is, the team analyzed 474 species ranging in size from that of a mite to the behemoth body of a blue whale. The study, published in Nature Ecology & Evolution, found critical factors that determine why the size theory works only up to a point.
As it turns out, it’s not all about size and muscles. Big beasts may be powerful, but they often run out of anaerobic energy. This is because large animals have “fast-twitch” muscle fibers that help them accelerate to high speeds, but their tissues soon deplete of oxygen and fatigue.
Thus, how long it takes an animal to accelerate is key. Larger animals may have more muscle, but they also need more initial energy to move in the first place.
Elephants are a good example of this – by the time they reach peak speeds, their energy reserves are already rapidly depleting. Their maximum acceleration is limited by the energy storage capacity of their fast-twitch fibers.
If an animal is too small, they don’t have enough muscles, but if the animal is too big, they must overcome their mass to accelerate quickly. This sweet spot at the peak of the curve – where cheetahs and peregrine falcons lie – is therefore based on acceleration speeds and metabolic constraints.
"The theory is simple, yet extremely powerful and allows us to make predictions across the animal realm of how optimized or not species are for speed," said Professor Walter Jetz, from Imperial College London and Yale University, in a statement.
Of course, an animal’s evolution and specialized structures still need to be considered. For example, while a human weighs around the same as a cheetah, the spotted creature runs almost three times faster than us.
The formula is k=cMd-1 (k is the acceleration constant and M is the body mass). Once the researchers have the animal’s mass and the medium it travels through (ie. air, land, water), they can then predict its speed.
Such research could be used to estimate the speed of extinct species, such as dinosaurs. For example, the researchers predict that the Tyrannosaurus rex was slower than the Velociraptor – 27 and 54 kilometers per hour (17 and 33 miles per hour), respectively. That puts Tyrannosaurus rex at 1mph slower than Usain Bolt.
While some researchers are asking for more evidence of the formula's efficacy, it is at the very least a good starting point.
Lead author Myriam Hirt added: "This means that in future, our model will enable us to estimate, in a very simple way, how fast other extinct animals were able to run."
