Why cheetahs are the fastest animals on land

Medium-sized animals dominate the top spots in the running portion of the Animal Olympics, and now we know why, thanks to a new muscle performance model.

For many important criteria, animals thrive on being large. Insects can lift astonishing weights for their size, but in absolute terms the largest animals are also the strongest and usually live the longest. Many measures of animal performance can be predicted from their size using a simple power law—not perfect, but pretty good.

But when it comes to speed, cheetahs can outrun much larger animals, and they have a medium-sized pack that ranks near the top of the list, an anomaly that has puzzled zoologists enough to inspire many papers.

“Why does running speed break the regular patterns that govern most other aspects of animal anatomy and performance?” asked Dr. David Labonte of Imperial College London said in a statement. Labonte has helped answer that question by showing that there are two limiting factors on walking speed, rather than one as had been assumed.

Labonte and co-authors examined the way muscles contract and used this to build a physical model that predicts an animal’s walking speed. They confirmed its accuracy by comparing predictions with measurements others have collected on more than 400 species, from insects weighing 10,000e of a gram (0.000004 ounces) for the largest mammals.

“The key to our model is understanding that maximum running speed is limited by both the speed at which the muscles contract and the extent to which they can shorten during a contraction,” explains co-author Professor Christofer Clemente from the University of the Sunshine Coast out. “Animals the size of a cheetah physically exist in a sweet spot, around 50 kilograms [110 pounds], where these two boundaries coincide. These animals are the fastest and can reach speeds of up to 65 miles per hour [105 kilometers per hour].”

Smaller animals are held back by the first limit Clemente points to: their muscles simply can’t contract fast enough to bother their medium-sized counterparts. The forces generated by muscle contraction are large relative to the weight of the animals, which is good for overcoming opposing forces – such as when running uphill – but limits top speeds.

Large animals are reaching the “work capacity limit”; they simply cannot produce forces large enough to accelerate their large masses to high speeds.

“For large animals like rhinos or elephants, running can feel like lifting a huge weight because their muscles are relatively weaker and gravity comes at a higher cost. As a result of both, animals eventually have to slow down as they grow,” explains Dr. Peter Bishop of Harvard University.

The authors compare the situation with the Reynolds number in fluid dynamics, where viscosity dominates on small scales and inertial forces on larger scales.

Cheetahs may be just the right size for maximum speed, but there are other reasons they’ve reached the top. One factor, as anyone who has seen a documentary filmed on the African savannah can tell you, is that they have evolved into sprinters rather than distance runners, focusing entirely on fast-twitch muscles.

Cheetahs are also spared from competition from some similarly sized creatures. Large reptiles such as crocodiles cannot match the speeds of their mammals.

This fact could be yet another piece of science from the creators of the Jurassic Park/World franchise will ignore it, but the authors of this study would like to know why, although they don’t have all the answers so far.

“A possible explanation for this could be that limb muscle mass makes up a smaller percentage of reptiles’ bodies by weight, meaning they reach their working limit at a smaller body weight, and thus need to remain small to move quickly,” Dr. Taylor Dick from the University of Queensland said. However, this only moves the question back one step: why, given the immense advantages of speed, have large reptiles not evolved limb muscles similar to those of mammals?

But perhaps those filmmakers are right not to take the model as gospel. While the model does an excellent job of matching the speed capabilities of living species, the model runs into problems at the larger end of the scale, as it predicts that it should be impossible for a land animal weighing more than 40 tons to move. We know this isn’t true because the titanosaurs were somehow able to move, and the largest weighed an estimated 70 tons. The authors think there must have been something different about the muscle structure of the largest dinosaurs, compared to that of all living things, but they don’t know what it was.

The research has been published open access in Nature Communications.

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