What determines the maximum running speed of animals?

In the vast expanse of the animal kingdom, a curious pattern exists where traits like strength and brain size generally increase with an animal’s size, yet the peak running speeds are often found in medium-sized creatures. 

To uncover the reason behind this anomaly, an international research team, including members from Imperial College London (ICL), Harvard University, The University of Queensland, and The University of the Sunshine Coast (USC), embarked on a study to investigate how muscle capabilities limit the top speeds of land animals.

Running speeds have two distinct limits

“The fastest animals are neither large elephants nor tiny ants, but intermediately sized, like cheetahs,” said lead author David Labonte, an expert in bioengineering at ICL. “Why does running speed break with the regular patterns that govern most other aspects of animal anatomy and performance?”

The investigations reveal that maximum running speed isn’t constrained by a single factor but by two distinct limits related to muscle function: the speed of muscle contraction and the extent of muscle shortening during a contraction. An animal’s maximum speed is capped by the first limit it encounters, which varies according to its size.

“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 65 miles per hour,” explained co-author Christofer Clemente, a researcher at both Queensland and USC.

Animal size and running speed

The study introduces two theoretical limits: the ‘kinetic energy capacity limit’ for smaller animals, which are limited by the speed at which their muscles can contract, and the ‘work capacity limit’ for larger animals, constrained by the degree to which their muscles can contract. 

“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 co-author Peter Bishop, a postdoctoral fellow in organismic and evolutionary biology at Harvard.

Physical principles governing muscle evolution

The team’s model, tested against data from over 400 species ranging vastly in size, accurately predicted the variation in maximum running speeds across the animal kingdom, highlighting the underlying physical principles governing muscle evolution. 

This insight could potentially guide the development of robots that emulate the athleticism of nature’s finest runners.

Limb muscle mass and body weight

Furthermore, the model sheds light on differences between animal groups, suggesting, for example, that reptiles’ smaller limb muscle mass, relative to body weight, may explain why larger reptiles tend to be 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 study co-author Taylor Dick, a senior lecturer in biomedical sciences at Queensland.

Muscle physiology of massive animals

The findings also suggest that the largest land mammals alive today, like the African elephant, are well below the theoretical weight limit beyond which land animals would be immobile. 

This casts doubt on the muscular anatomy of extinct giants such as the Patagotitan, which likely exceeded 40 tons, and prompts further investigation into their unique physiological adaptations.

“Our study raises lots of interesting questions about the muscle physiology of both extinct animals and those that are alive today, including human athletes. Physical constraints affect swimming and flying animals as much as running animals – and unlocking these limits is next on our agenda,” Labonte concluded.

The study is published in the journal Nature Communications.

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