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Why midsized animals are the fastest on Earth

By Sid Perkins Jul. 17, 2017 , 11:30 AM

An elephant should run faster than a horse—at least in theory. That’s because big creatures have more of the type of muscle cells used for acceleration. Yet midsized animals are the fastest on Earth, a trend that researchers have long struggled to explain. 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.

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.

To get around those 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.

Heft versus hustle

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.

5. Cheetah 65 kg 120 km/hr

1. Atlantic trout 7.85 kg 17.532 km/hr

3. Willow warbler 0.0087 kg 43.2 km/hr

4. Human 70 kg 43.85 km/hr

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.

The study is also a good starting point for teasing out other factors that influence a creature’s maximum speed, says Christofer Clemente, an ecophysiologist at the University of the Sunshine Coast in Maroochydore, Australia, who wasn’t involved in the research. One such unexplained trend is that warm-blooded land animals are usually faster than cold-blooded creatures of comparable size, whereas at sea the reverse is usually true.

“There’s been a big challenge in finding one overall notion of what constrains acceleration,” says John Hutchinson, an evolutionary biologist at the Royal Veterinary College in Hatfield, Hertfordshire in the United Kingdom. And although he terms the new study “bold,” the factors that limit maximum speed in small animals are likely very different than those limiting large ones, he suggests.

Hirt and her colleagues suggest their technique should apply to long-extinct dinosaurs as well. For example, a 6-metric-ton Tyrannosaurus rex’s top speed may have been about 27 kilometers per hour (slightly more than an average human’s running speed but nowhere near Jamaican sprinter and world record holder Usain Bolt’s), they estimate. Yet that figure may be a very rough estimate, because the range of max running speeds for creatures of approximately the same weight can be broad, says Thomas R. Holtz Jr., a vertebrate paleontologist at the University of Maryland in College Park.

Despite the overall conclusions of the study, within a narrow range of sizes, the way an animal is put together can indeed have a huge effect on a creature’s top running speed, Holtz notes. Humans and cheetahs are a case in point: Although the two humans included in the team’s study weighed in at 70 kilograms and had an average top speed of about 41 kilometers per hour, the heftiest cheetah weighed about 5 kilograms less but ran nearly three times as fast.

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