On land, cheetahs outrun all other creatures; in the air, hawks are faster than fellow fliers; and at sea, marlins outswim everything. To a point, top speed increases with the size of an animal, but it has not been clear why the largest animals on Earth aren’t also the fastest.
Myriam Hirt and her colleagues at the German Centre for Integrative Biodiversity Research in Leipzig modelled how maximum speed increases with body mass. They took into account the fact that animals have only a short time to speed up, as muscles rely on a limited energy supply during acceleration. Because larger animals are slower to accelerate, they exhaust this surge of power at a lower speed.
The team found that top speeds initially increase with size, but drop off as animals grow beyond a certain critical point. The model is backed up by data on the body size and speed limits of 474 species, ranging from flies to whales.