The enormous, carnivorous dinosaurs portrayed in Jurassic Park run fast enough to easily chase down humans. But based on a new study out this week in Nature Ecology & Evolution, T. rex may not be the Jeep-catching monster we’ve seen in the movies.
In an extensive cross-species study, a team of German scientists compared almost 500 different animals, ranging in size from tiny snails weighing 30 micrograms to massive whales of up to 100 metric tons. The researchers created a mathematical model to describe how animals move, considering their weight and the characteristics of their environment—air, land, or water—to calculate how quickly they could accelerate for short sprints.
The study revealed a fundamental constraint to animal speed: larger animals have a greater number of “fast-twitch” muscles that can generate force quickly, but these muscles also tire easily. During sprints, fast-twitch muscles can only store a limited amount of energy, so smaller animals can quickly reach their maximum speed. The trade-off between increasing size and muscle mass and increasing inertia means that cheetahs are much speedier than the infamous giant lizards.
As a result, animal speed increases, to a point. Beyond the weight of cheetahs (on land), marlins (in water), or peregrines (in air), an animal’s top speed starts to taper off. The researchers described the relationship between body mass and top speed using a relatively simple parabolic arc. With it, they claim to predict an animal’s speed regardless of whether it flies, swims, or runs.
Matt Burgess interviewed Myriam Hirt, lead author on the paper for Wired:
Hirt, from the German Centre for Integrative Biodiversity Research, explains the top speeds of an animal—including humans—comes down to acceleration. “While the largest animals, in theory, could be the fastest, the energy and time required to accelerate their larger bodies keeps them from ever attaining it.”
Theoretically, Hirt says, if scaling of size and speed were linear an elephant would be able to reach a top speed of [373 mph]. In reality they max out at about [21 mph]. “Larger animals run out of energy supplied by the muscles before being able to reach their theoretically possible maximum speed,” Hirt says.
The model holds true for extinct species as well. T. rex was previously thought to run up to 45 mph, but given its nine-ton frame, the therapod probably clocked in at about 16.5 mph. The average adult human can run at about 15 mph, and Usain Bolt, on record as the fastest human alive, can run up to an impressive 27 mph. While Bolt can easily outpace the T. rex, he may have faced a serious threat from the Velociraptor, which could run up to 34 mph, according to the research.
Other dinosaurs would be equally, perhaps disappointingly, slow. Much like larger elephants and whales, the Brachiosaurus would have stomped about at less than 11 mph, and the Apatosaurus (popularly known as Brontosaurus) would tire at 7.6 mph.
Ultimately, though, weight is not the only deciding factor. The study found that on land, warm-blooded animals can move faster than comparable cold-blooded ones, though in the ocean the opposite is true.
The biomechanics of each animal are also critical—the humans in the study weighed only 11 pounds more than the largest cheetah, but the wild cat can run almost three times as fast.