Why fast birds, fish and animals are never too small or big

An animal’s maximum speed is based on how fast it can accelerate, which explains why the largest animals are not the fastest

Most of us know a cheetah can beat an elephant in a footrace. But unravelling the mystery of why certain animals move faster than others is something scientists have struggled with for years. Now it seems size might be the answer.

Animals’ speed limits affect how they migrate, interact with their environment – and whether they eat or are eaten. Speed is determined by how far an animal moves in a given amount of time. So, it may seem intuitive that animals with longer limbs or fins will travel further and reach higher speeds.

But all the evidence says it isn’t so: today’s fastest animals aren’t the elephants, condors or blue whales of the world, but instead the cheetahs, falcons and marlins. Some scientists have suggested this is because the bones and muscles of very large animals are unable to withstand the forces experienced during rapid locomotion.

But Myriam Hirt at the German Centre for Integrative Biodiversity Research in Leipzig thinks there is something else going on.

By examining data from 474 running, flying and swimming species, Hirt has created a model that seems to confirm that the fastest species in each locomotion category are those with a body mass that falls in the middle of the range. For running and swimming animals that “middle” body mass is about 100 kilograms; for flying animals it is about 1 to 10 grams.

Her team suggests being “middle-sized” is advantageous because of basic physical considerations. Animals have a limited amount of time to accelerate up to high speeds before they run out of the quickly available energy stored in muscle fibres called fast twitch fibres.

Larger animals have more muscle tissue, and therefore more energy available for sprinting. But, due to their large body mass, they can’t start moving as quickly as small animals and take a longer time to reach a given absolute speed. By the time they get up to higher speeds, they have little of this quickly available energy left.

The model shows that maximum speed decreases steeply as animals grow beyond intermediate sizes, and matches what we know about different species’ speeds.

Hirt was surprised by the model’s universal applicability. “It works for animals of different ecosystems, locomotion types and even for larger extinct species,” she says. “We can also make highly accurate predictions of maximum speeds of currently living species.”

She says the model can also allow us to predict extinct species’ speeds. The running speeds of dinosaurs has been a contentious point of debate among palaeontologists for some time. Hirt’s model predicts lower speeds for Tyrannosaurus than its smaller carnivorous counterpart, the Velociraptor.

But some scientists are sceptical of the model until more evidence is presented. Mauricio Fuentes is an independent researcher from Santiago, Chile, who focuses on the biomechanics of fast running speeds. He would like to see more evidence that the model applies to swimming and flying animals.

“A good correlation does not necessarily mean the model is correct, but it certainly is a good signal,” says Fuentes.

Read more: Cheetahs win races on acceleration but not speed

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