Why Tyrannosaurus was a slow runner and why the largest are not always the fastest

There is a parabola-like relationship between the body mass of animals and the maximum speed they can reach. For the first time, researchers are able to describe how this comes about, thanks to a simple mathematical model.

No other animal on land is faster than a cheetah -- the elephant is indeed larger, but slower. For small to medium-sized animals, larger also means faster, but for really large animals, when it comes to speed, everything goes downhill again. For the first time, it is now possible to describe how this parabola-like relationship between body size and speed comes about. A research team under the direction of the German Centre for Integrative Biodiversity Research (iDiv) and the Friedrich Schiller University Jena (Germany) have managed to do so thanks to a new mathematical model, and also published their findings in the journal Nature Ecology and Evolution.

A beetle is slower than a mouse, which is slower than a rabbit, which is slower than a cheetah... which is slower than an elephant? No! No other animal on land is faster than a cheetah -- the elephant is indeed larger, but slower. For small to medium-sized animals, larger also means faster, but for really large animals, when it comes to speed, everything goes downhill again. For the first time, it is now possible to describe how this parabola-like relationship between body size and speed comes about.

The model is amazingly simple: The only information that it must be 'fed' with is the weight of a particular animal as well as the medium it moves in, so either land, air or water. On this basis alone, it calculates the maximum speed that an animal can reach with almost 90% accuracy. "The best feature of our model is that it is universally applicable," says the lead author of the study, Myriam Hirt of the iDiv research centre and the University of Jena. "It can be performed for all body sizes of animals, from mites to blue whales, with all means of locomotion, from running and swimming to flying, and can be applied in all habitats." Moreover, the model is by no means limited to animal species that currently exist, but can be applied equally well to extinct species.

Tyrannosaurus reached a speed of only 17 miles/hour

"To test whether we can use our model to calculate the maximum speed of animals that are already extinct, we have applied it to dinosaur species, whose speed has up to now been simulated using highly complex biomechanical processes," explains Hirt. The result is that the simple model delivered results for Triceratops, Tyrannosaurus, Brachiosaurus and others that matched those from complex simulations -- and were not exactly record-breaking for Tyrannosaurus, who reached a speed of only 27 km/h (17 mi/h). "This means that in future, our model will enable us to estimate, in a very simple way, how fast other extinct animals were able to run," says the scientist.

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