The fastest animals are neither large elephants nor tiny ants, but intermediately sized, like cheetahs (Acinonyx jubatus). Why does running speed break with the regular patterns that govern most other aspects of animal anatomy and performance? New research suggests that there is not one limit to maximum running speed, as previously thought, but two: how fast versus by how far, muscles contract; the maximum speed an animal can reach is determined by whichever limit is reached first — and that limit is dictated by an animal’s size.

The cheetah (Acinonyx jubatus).

“The key to our model is understanding that maximum running speed is constrained both by how fast muscles contract, as well as by how much they can shorten during a contraction,” said Professor Christofer Clemente, a researcher at the University of the Sunshine Coast and the University of Queensland.

“Animals about the size of a cheetah exist in a physical sweet spot at around 50 kg, where these two limits coincide. These animals are consequently the fastest, reaching speeds of up to 105 km per hour (65 mph).”

The first limit, termed the ‘kinetic energy capacity limit,’ suggests that the muscles of smaller animals are restrained by how quickly they can contract.

Because small animals generate large forces relative to their weight, running for a small animal is a bit like trying to accelerate in a low gear when cycling downhill.

The second limit, called the ‘work capacity limit,’ suggests that the muscles of larger animals are restrained by how far their muscles can contract.

Because large animals are heavier, their muscles produce less force in relation to their weight, and running is more akin to trying to accelerate when cycling up a hill in a high gear.

“For large animals like rhinos or elephants, running might feel like lifting an enormous weight, because their muscles are relatively weaker and gravity demands a larger cost,” said Dr. Peter Bishop, a researcher at Harvard University.

“As a result of both, animals eventually have to slow down as they get bigger.”

To test the accuracy of their model, the authors compared its predictions to data on land animal speed and size collected from more than 400 species, from large mammals, birds and lizards to tiny spiders and insects.

The model accurately predicted how maximum running speeds vary with body size for animals that differ by more than 10 orders of magnitude in body mass — from tiny 0.1 milligram mites to six-tonne elephants.

Their findings shed light on the physical principles behind how muscles evolved and could inform future designs for robots that match the athleticism of the best animal runners.

In addition to explaining how fast animals can run, the new model may also provide critical clues for understanding differences between groups of animals.

Large reptiles, such as lizards and crocodiles, are generally smaller and slower than large mammals.

“One possible explanation for this may be that limb muscle is a smaller percentage of reptiles’ bodies, by weight, meaning that they hit the work limit at a smaller body weight, and thus have to remain small to move quickly,” said Dr. Taylor Dick, a researcher at the University of Queensland.

Combined with data from modern species, the team’s model also predicted that land animals weighing heavier than 40 tons would be unable to move.

The heaviest land mammal alive today is the African elephant at around 6.6 tons — yet some land dinosaurs, like Patagotitan, likely weighed much more than 40 tons.

“This indicates that we should be cautious to estimate the muscular anatomy of extinct animals from data on non-extinct ones,” the researchers said.

“Instead, the data indicate that extinct giants might have evolved unique muscular anatomies, which warrant more study.”

“Our study raises lots of interesting questions about the muscle physiology of both extinct animals and those that are alive today, including human athletes,” said Dr. David Labonte, a researcher at Imperial College London.

“Physical constraints affect swimming and flying animals as much as running animals — and unlocking these limits is next on our agenda.”

A paper on the findings was published in the journal Nature Communications.

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D. Labonte et al. 2024. Dynamic similarity and the peculiar allometry of maximum running speed. Nat Commun 15, 2181; doi: 10.1038/s41467-024-46269-w



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