What Is the Key to Predict Animal Speed?

The biggest animals aren’t always the fastest

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If it came to pure muscle mass, the African elephant would be the fastest animal on Earth. But that isn't the case. As Marlowe Hood reports for Agence France-Presse, it turns out that body size, not muscle mass, that can predict animal speed. And the winners of the rase are usually those with medium-sized bodies.

A recent study published in the journal Nature Ecology & Evolution digs into the math behind speedy creatures. The results suggest that mid-sized critters—such as cheetahs, springboks, falcons and marlins—lie in a body-size sweet spot: They have plenty of muscle fiber to move fast but not too much body mass to fatigue their muscles and slow acceleration.

Myriam Hirt, a biologist at the German Centre for Integrative Biodiversity Research in Leipzig was looking for a formula that could predict an animal’s speed. But her initial attempts, relying on muscle mass, were a big flop. Based on this method, elephants would win the race with a calculated speed of 373 miles per hour, "which of course is not true,” she tells Hannah Lang at National Geographic. An elephant’s true top speed is little under 25 miles per hour.

To solve the mystery, Hirt and her colleagues began analyzing the fastest animals on earth, searching for patterns or commonalities. As Sid Perkins at Science reports, they analyzed 474 species, including critters like fruit flies, whales, warblers, trout, humans and hundreds of others. By plotting the top speeds on a graph, they found an inverted U-shaped curve, with many of the moderately-sized speedy species at the top.

Based on the data, they couldn't find any mechanical reason for the speed. For instance, all the fast fish didn’t have a certain fin shape. Instead, the researchers found a correlation between the time it would take an animal to bring its mass to its theoretical top speed and how quickly its “fast twitch” muscle fibers—used for things like sprinting—take to reach exhaustion.

In creatures like elephants and whales, these muscle fibers poop out way before the creatures can exert enough energy to accelerate to anything close to 300 miles per hour. According to Hood, the researchers developed a formula that can be used to calculate an animal’s maximum speed with 90 percent accuracy based on its mass and whether it moves through air, water or on land.

Theoretically, the formula should also work on extinct animals as well, and could give researchers new insights into dinosaur speeds. For instance, according to the formula, velociraptors likely darted along at 31 miles per hour while T-Rex lumbered around at about half that speed, Hood reports. The insights into speed also gives researchers clues into each animals favored prey.

While the formula is a good start for understanding speed, Thomas R. Holtz Jr., a paleontologist at the University of Maryland, who not involved in the study, tells Perkins that it does not explain all the differences. The two humans included in the study averaged 154 pounds and topped out at 25.4 miles per hour. The chunkiest cheetah in the study, which weighed just eleven pounds less, still can sprint over 60 miles per hour, meaning structural differences likely play a role in max speed as well.

The researchers suggest that those differences may be the result of evolutionary pressure. Humans, it seems, spent much of their energy outsmarting their prey with things like traps or spears for hunting. But many other species, like cheetahs, instead evolved to become increasingly fast. But this means that their prey likely evolved to be faster as well. “Species that gain the most selective advantage—predators and prey with few places to hide, for example—will approach the predicted maximum speeds,” Hirt explains in the release.

That raises a new question: Who's been chasing Usain Bolt?

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