Solving the mystery of why cheetahs are so fast unlocks clues to the speeds of dinosaurs

Victoria Woollaston July 17, 2017

Since the time of the dinosaurs, the largest animals have not been the fastest, despite their increased muscle mass and potential power. In fact, many are the slowest within their respective classes and, while you’d expect lumbering beasts to be slower than nimble creatures, the mechanism behind this behaviour has divided scientists for decades.

Solving the mystery of why cheetahs are so fast unlocks clues to the speeds of dinosaurs

Now, researchers led by Myriam Hirt from the German Centre for Integrative Biodiversity Research, have discovered that the answer may lie in each animal’s acceleration speeds. Put simply, the time it takes an animal to accelerate determines their maximum overall speed. During acceleration, the body converts chemical, metabolic energy into mechanical energy used for movement. This process occurs in so-called fast-twitch muscle fibres through a method known as anaerobic metabolism.

Animals only have a finite amount of time to accelerate from a standing start before they can accelerate no longer. In particular, they can accelerate until the point at which these fibres run out of their “metabolic fuel” meaning the time available for acceleration is limited by the amount of these fibres.

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As larger animals have more fast-twitch muscle fibres, they can accelerate for longer, however, the mass of these animals means that they take longer to reach an absolute speed compared to smaller species. “At some point, the amount of time required to accelerate to faster speeds will exceed the finite amount of time available for acceleration, and so faster speeds are never reached.” Mid-sized animals, such as cheetahs, have a perfect balance of mass versus the number of fast-twitch muscles to achieve these absolute speeds.

What’s more, the research shows this discovery is true for both swimming and flying animals – a point at which previous hypotheses have fallen down.

To test their model predictions, Hirt and her colleagues gathered data on the maximum speeds of 474 running, flying and swimming animals including mammals, fish and bird species but also reptiles, molluscs and arthropods. Body masses of these species ranged from molluscs to whales.

“Our findings help to solve one of the most challenging questions in movement ecology over recent decades: why are the largest animals not the fastest?” wrote Hirt in her paper A general scaling law reveals why the largest animals are not the fastest published in the journal Nature Ecology and Evolution. “By measuring body size alone, the new model can accurately predict the speed limits of animals ranging from fruit flies to blue whales, and explains why medium-sized animals are generally the fastest.”

These findings can also be used to predict the speeds of extinct species. For example, palaeontologists have long debated the potential running speeds of large birds and dinosaurs. Hirt’s time-dependent model shows that the Tyrannosaurus Rex would have run at speeds of around 27.05 km/h. The Triceratops maxes out at 24.36 km/h.

Images: Wikimedia Commons/Hirt et al./Nature Ecology & Evolution