It sounds like the start of a riddle, but it's not. Between a tuna (skipjack) and a whale (large gray), which propels itself with more efficiency?

The surprising answer could have broad implications for both our understanding of animal locomotion and our development of mechanical vehicles.

To spare you any further suspense, the answer is that both the tuna and the whale move with equal efficiency. That was the finding of a team of researchers from Northwestern that set out to learn more about how energy flow changes with differences in size or mass of animals.

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While the whale is obviously heavier than the tuna (just try lifting a gray whale) and will consume more energy than the smaller creature, it turns out the whale moves just as efficiently.

How the researchers reached that conclusion ... there's the rub.

The results come courtesy of a new metric the Northwestern team created called the energy consumption coefficient. Applying it, scientists studying animals, or engineers building machines, can now make apples-to-apples comparisons of animals or vehicles, regardless of disparities in size between them.

"Our metric can be used to determine the point where an animal or a vehicle would function most efficiently. We want to know the sweet spot," lead researcher Neelesh Patankar said in a press release.

To continue with the fish theme, just imagine an underwater vehicle that could be designed to move with the precise efficiency and agility of an actual fish. That's just one of the applications that could benefit from the new metric.

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The study data was gathered by observing thousands of species of swimming and flying animals, big and small -- from larval zebrafish to whales; from insects to large flying birds.

Though the study focused on animals, the team is now looking at defining efficiency for automobiles. "As a driver," Patankar noted, "I also would like to know how efficient my car is, information currently not available to me."

The team's research will be published this week in the Proceedings of the National Academy of Sciences.