Why aren’t the biggest animals the fastest?

It’s common knowledge that bigger animals like dogs and horses are faster than little creatures like ants and mice, so why aren’t elephants the fastest runners?

As animals get bigger, their muscles get larger and more powerful, and contain more of the fast-twitch fibres needed for acceleration and high speed. In theory, this means that huge animals like elephants should be capable of incredible speeds, but in reality they don’t come close to the midsized cheetah.

 

In a new study, researchers plotted 474 animals’ top speeds against their masses to find the explanation for this phenomenon. The species ranged from 30 micrograms to 100 tonnes (98.4 tons) and included those that fly, swim and run. The graph they produced has an upside-down U-shape; above a certain weight, something starts to cancel out the effects of being more muscular.

a, Comparison of flying, running and swimming. b–d, Differences between each species illustrated separately for flying (b), running (c) and swimming (d) animals. Credit: Myriam Hirt

 

The problem for big beasts comes in the form of inertia – the tendency for something to remain in its current state unless something changes it. This principal means that animals need power to get moving, and bigger animals need more power. An elephant takes much longer than a smaller animal to get anywhere near a sprint, and by the time it does it’s used up a lot of energy and can’t ever reach the speed its muscles might be capable of.

 

Short, fast sprints are powered by anaerobic metabolism – using a limited amount of stored fuel in the muscles tissues and no oxygen – and by the time a large animal has got its bulky body going, much of that fuel is already gone. It’s the same for flying and swimming animals, which is why the peregrine falcon could fly rings around the 20 kilogram (44 pound) kori bustard and why the black marlin (a medium-sized fish) is much faster than a whale. Medium-sized animals have the best balance of muscle power and weight, making them the fastest whether they’re on land, in the water or in the air.

 

Using their formula, the scientists can predict the top speed of a species from its mass with 90 per cent accuracy – some animals are exceptions, like the horse fly capable of flying at 155kph/90mph – so this discovery could allow them to work out how fast extinct species could travel.

 

You can read the original report in Nature Ecology and Evolution

Peregrine falcon photo: Ron Knight/flickr