Ask anyone ‘what is the fastest animal on earth?’. A guaranteed popular response will be ‘a cheetah!’. But, if you ask ‘why are cheetahs the fastest animals?’, you might receive some quizzical looks and uncertain shoulder shrugs. Until recently, most scientists also felt this way. The largest animals should be able to move the fastest because they have more muscle to propel them to top speed, in theory. Why then are the largest animals – think elephants – outpaced by mid-sized animals like cheetahs? This conundrum doesn't just hold for land-running animals but also for those that swim and fly: being mid-sized truly is the sweet spot for moving fast.

Myriam Hirt from the German Centre for Integrative Biodiversity Research and her colleagues from Friedrich Schiller University, Yale University, and Imperial College London, sought to solve the perplexing mystery behind animal speed. Hirt's team looked at whether the muscles that power movement may actually slow down the largest animals. The researchers focused on ‘fast-twitch’ muscle fibres, which animals use when pulling off their fastest performance. Though this type of muscle helps speedsters move quickly, fast-twitch fibres consume energy stores more quickly than they can be replaced, leading eventually to fatigue. The heavier an animal is, the longer it will take to reach its fastest speed, and the more energy it will require to reach that speed. Hirt and colleagues predicted that the biggest animals never reach their true top speed because they burn up their fuel supplies before attaining their fastest pace.

Using the prediction that larger animals would tire out before hitting their potential top speed, the researchers modelled the maximum speeds that animals could attain based on their body mass and mode of locomotion (whether an animal travels on land, in water or in air). Hirt's new muscle-based model was incredibly precise and predicted animal top speeds with 90% accuracy in the 474 different species that the team tested. Impressively, the model was accurate for animals from 30 μg (mite) to 108 400 kg (blue whale) in body size and for animals from vastly different habitats. On land, under water and in the air, Hirt and colleagues’ model showed that mid-sized animals outpaced their larger counterparts. The researchers suggest that mid-sized animals have enough heft and power to move fast, but not so much that they are held back.

One exciting application for an all-encompassing muscle model such as Hirt's is that researchers can use it to estimate speeds for long-extinct species. The researchers investigated whether their model could stand the test of time and predict top speeds for six different dinosaurs. The model showed that a nimble, mid-sized Velociraptor would have moved almost twice as fast as a massive Tyrannosaurus at their fastest paces, confirming the expectations of previous researchers, who thought Tyrannosaurus to be relatively slow.

The team's research also has exciting applications for currently living animals. Hirt's work could be a valuable starting point for investigating why some animals differ from their model's prediction by being either faster or slower than expected.

M. R.
B. C.
A general scaling law reveals why the largest animals are not the fastest
Nat. Ecol. Evol.
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