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Why midsize animals are the fastest

New simulation suggests that speed has body mass limits

By
9:00am, August 11, 2017
cheetah and wildebeests

FULL SPEED AHEAD  The speed demons of the animal kingdom are hardly giants. A new study suggests larger animals run out of fuel for their muscles before reaching top speeds.

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Speed has its limits — on the open road and the Serengeti. Midsize animals tend to be the speedsters, even though, in theory, the biggest animals should be the fastest. A new analysis that relates speed and body size in 474 species shows that the pattern holds for animals whether they run, fly or swim (see graphs below) and suggests how size becomes a liability.

This relationship between speed and size has long stumped scientists. Big animals have longer legs or flippers to get from point A to point B. And bigger bodies have higher metabolic rates and more fast-twitch muscle cells, needed to convert chemical energy into mechanical energy and rapidly accelerate. So, why aren’t wildebeests faster than cheetahs?

The make-or-break factor is the time it takes an animal to accelerate to its top theoretical speed, an upper limit based on mass and metabolic rate, researchers report July 17 in Nature Ecology & Evolution. Fast-twitch muscle cells provide the power for acceleration but tire quickly. When an animal gets too big, it takes too long to accelerate, and these cells use up their energy before hitting top speeds. More modestly built critters need less time to accelerate to those speeds.

The researchers gathered speed and size data from past lab and field studies. The animals (some shown as icons in the slideshow below) ranged in mass from 30-microgram Spanish mites to a blue whale weighing 108 metric tons.

Citations

M. Hirt et al. A general scaling law reveals why the largest animals are not the fastest. Nature Ecology & Evolution. Published online July 17, 2017. doi: 10.1038/s41559-017-0241-4.

Further Reading

C. Martin. Brazilian free-tailed bats are the fastest fliers. Science News Online, November 21, 2016.

N. Akpan. Flying animals can teach drones a thing or two. Science News. Vol. 187, February 7, 2015, p. 15.

A. Yaeger. California mite becomes fastest land animal. Science News. Vol. 185, June 28, 2014, p. 4.

S. Milius. In the real world, cheetahs rarely go all out. Science News. Science News, July 13, 2013, p. 9.

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