Image copyright Getty Images Image caption The cheetah has a recorded speed of 68-75 mph (109.4-120.7 km/h)

They're the sprinters of the animal world - cheetahs on land, falcons in the air and marlins in the sea.

But, why are they so fast when bigger, more muscular animals might be expected to outpace them?

Now, scientists have come up with a new theory to explain the gold medal-winning performance of animal athletes.

It appears it is all down to the energy required to get off the starting blocks.

"Scientists have long struggled with the fact that the largest animals are not the fastest," said Prof Walter Jetz, from the US's Yale University.

"In our work, we explain this with the simple fact that animals run out of readily mobilised energy before they are able to get their bodies to the maximum possible speed.

"So, while the largest animals in theory could be the fastest, the energy and time required to accelerate their larger bodies keep them from ever attaining it. "

The theory, outlined in the journal Nature Ecology & Evolution, explains why lean, medium-sized animals are generally built for speed.

Image copyright Getty Images Image caption The black marlin is one of the fastest fish species in the ocean

It applies to animals from fruit flies to blue whales and could even help predict the maximum running speed of animals that have long disappeared.

Results from the model were compared with data on hundreds of animal species, from flying animals to whales.

Researchers found the data generally fits with their predictions that maximum speed drops off sharply as animals grow beyond medium sizes.

"The theory is able to explain the maximum speeds of over 450 terrestrial, aerial and aquatic species varying from less than a gram to 10 tonnes in size," said Prof Jetz, who worked on the study with colleagues at the EcoNetLab in Germany.

"It is also able to predict maximum speeds of long extinct species such as birds and dinosaurs."

Speedy animals like the cheetah have evolved bodies primed to catch prey. They are the optimal size for maximum speed.

Accelerating takes a lot of energy, and muscles can only function at such intensity for a short time.

Thus, an elephant will never be able to overtake a cheetah because the fuel for acceleration (involving anaerobic respiration) will run out before it has reached its maximum (theoretical) speed.

The information will help scientists understand more about the ecology and behaviour of a species, including diet, hunting, migration and the search for a mate.

Image copyright Getty Images Image caption The peregrine is renowned for its speed, reaching more than 320 km/h (200 mph)

"The exciting part of this proposal is that it applies equally well to animals on land, in the air and in water," write Australian scientists Peter Bishop and Christofer Clemente in a commentary in the journal.

"With this refined explanation of why animals move as fast as they do (or don't), we can expect to gain further insight into how locomotion and ecology has evolved in various groups throughout the history of life."

There are a few finer differences to iron out, however, including our own performance.

Humans fall short of the capability of our animal cousins of a similar size.

"In body mass, we humans are actually not too far from cheetahs," said Prof Jetz.

"But obviously, being primates rather than part of the cats family, our body type has not, over millions of years, adapted to outrun fast prey.

"Our limbs and bipedal movement instead signal a less specialised diet and many other trade-offs that ultimately have us readily overtaken - or worse - by a lion or cheetah, whose body types are fully optimised for speed."

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