Cheetahs’ unrivalled speed explained by their ‘sweet spot’ size

13 March 2024

A study involving University of Queensland researchers has found why medium-sized land animals, like cheetahs, are usually faster than large animals.

While many key traits such as strength, limb length, lifespan and brain size tend to increase with animals’ size, maximum running speeds tend to be greatest in medium-sized animals.

An international team of researchers, led by the Imperial College London, developed a physical model of how muscles, the universal animal motor, set limits on land animals’ top running speeds.

The findings suggest there is not one limit to maximum running speed, but two: how fast vs 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.

Co-author Associate Professor Christofer Clemente, from the University of the Sunshine Coast and UQ, said the key to the model was understanding that maximum running speed was constrained both by how fast muscles contract, as well as by how much they can shorten during a contraction.

“Animals about the size of a cheetah exist in a physical sweet spot at around 50kg, where these two limits coincide,” Dr Clemente said.

“These animals are consequently the fastest, reaching speeds of up to 104.6km per hour.”

To test the accuracy of their model, the team 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.

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.

UQ researcher and study co-author Dr Taylor Dick said one possible explanation might be that limb muscle was 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,” Dr Dick said.

The model, combined with data from modern species, also predicted that land animals weighing heavier than 40 tonnes would be unable to move at all.

While the study looked only at land animals, the researchers will next apply their methods to animals who fly and swim.

The results are published in Nature Communications.

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