Scientists now think it’s because the muscle cells in big animals run out of fuel before they can reach their theoretical maximum speed, Science magazine said. Yet midsized animals are the fastest on Earth, a trend that researchers have long struggled to explain. Previous studies of animal speed have focused only on certain groups of animals, such as mammals. But that premise often looks at creatures within a limited size range, says Myriam Hirt, a zoologist at the German Centre for Integrative Biodiversity Research in Leipzig. That approach may also hide underlying factors by focusing on animals that are closely related, she notes.
Now, an analysis of nearly 500 species ranging from fruit flies to whales has an answer: The muscle cells in big animals run out of fuel before the creatures can reach their theoretical maximum speed. The work may also help scientists come up with estimates for the running speeds of certain dinosaurs.
To get around those previous limitations, Hirt and her colleagues looked at previously collected data for a wide variety of creatures, including ectotherms (so-called cold-blooded animals) as well as warm-blooded endotherms. The 474 species they considered included runners, swimmers, and flyers that ranged in mass from 30 micrograms to 100 metric tons.
When the scientists mapped a creature’s top speed (either measured in the wild or in a lab setting) versus its mass, they got an inverted-U–shaped graph, with moderately sized animals on top, they report today in Nature Ecology and Evolution. On the largest scale, the trend doesn’t seem to be related to biomechanics, or how an animal’s body parts are arranged and how its joints function, among other factors, Hirt says.
Just as cheetahs are fastest on land, medium-sized marlins are fastest in the sea and medium-size falcons are fastest in the air, researchers found.
The fastest animals on Earth—whether they run, swim, or fly—are midsized creatures, not the miniscule or the mighty. That trend is driven by metabolic constraints in muscle tissue, a new study suggests. Very large animals have more “fast twitch” muscle fibers needed during a sprint and can in theory accelerate for longer periods, but those tissues soon run out of oxygen and thus reach max performance long before supermassive creatures ever reach their theoretical maximum speed.
Instead, it appears to be related to a much more fundamental metabolic constraint: the length of time required for the animal to reach its theoretical maximum speed, based on the number of “fast twitch” muscle fiber cells in the creature’s muscles, as compared to the length of time it takes for those cells to run out of readily available energy. (“Fast twitch” muscle fibers contract more quickly than “slow-twitch” fibers and generate more force more quickly, but they also fatigue more quickly.) According to the researchers’ notion, the “fast twitch” muscle fibers in immense creatures such as elephants and whales run out of cellular fuel long before they can reach max speed based on the overall number of such fibers.
Hirt and her colleagues suggest their technique should apply to long-extinct dinosaurs as well. For example, the Tyrannosaurus Rex likely had a top speed of 17 mph. That’s 1 mph faster than the top speed of an average human today, and 11 mph slower than Usain Bolt, the world’s fastest man. So we may have had a fighting chance against the infamous terrible lizards.
“In the future, our model will enable us to estimate, in a very simple way, how fast other extinct animals were able to run,” Hirt said.