Animal speed limits explained

An upper limit on the time it takes an animal to reach maximum speed explains why the fastest animals on Earth are not the biggest, finds a paper published online this week in Nature Ecology & 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.

Animals of intermediate size, such as cheetahs, are the fastest on land, despite having smaller muscles and fewer muscle fibres than larger animals. The same pattern also occurs in air or water, as seen in medium-sized species such as marlins and peregrine falcons.

Myriam Hirt and colleagues built a general theoretical model to show that, across all animals, it is the time needed to accelerate that ultimately limits the maximum speed an animal can achieve. They find this is because the acceleration phase requires muscles to function anaerobically, in which only limited stores of energy are available. Larger animals take longer than smaller ones to accelerate to their maximum speed, so if acceleration time in the anaerobic phase exceeds the amount of energy that can be made available to muscles, the maximum achievable speed reaches an upper limit.

Comparing the output of their model to data from 474 animal species ranging from molluscs to whales, and including flying animals too, the authors find a good fit to their predictions that maximum speed drops off steeply as animals grow beyond intermediate sizes.

In an accompanying News & Views, Christofer Clemente and Peter Bishop write: “The exciting part of this proposal is that it applies equally well to animals on land, in the air and in water. However, variation in locomotor performance in similarly sized animals needs further reconciliation.”