Photo: Wikimedia Commons

The humongous grey whale and the skipjack tuna, though of contradicting sizes, both employ similar propelling mechanisms through water. Pound per pound, however, which of the two animals is most energy efficient? Engineers at Northwestern University have developed a new metric for analyzing such problems and found that the two marine animals are almost just as energy efficient despite the great difference in mass. This newly developed metric, or standard, can be applied to almost any animal, be it marine, terrestrial or flying, as well as anthropocentric machinery, like transportation vehicles.  This way, the researchers note, they can understand where a car starts becoming less efficient once it crosses a certain weight barrier and thus design better vehicles.

Whale or tuna: whose muscles are more efficient?

“Our study is about how energy flow changes with size or mass,” said Neelesh A. Patankar, who led the research. “This is good insight to have in the transportation field, whether you are working with cars, ships or planes. What are the limits of how good you can become? 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.”

A truck needs more fuel compared to a small car to cover the same distance – everyone knows this this. Likewise, the husky whale consumes more energy to travel the same distance underwater as a tuna. Does it mean that the muscular “engine” propelling the whale is less efficient compared to the tuna or is the higher fuel consumption of the whale an unavoidable consequence of the laws of physics? The whale’s higher fuel consumption is unavoidable, the researchers report, and the engine efficiencies of the whale and tuna are similar.

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To reach this conclusion, Patankar and team developed an energy consumption coefficient similarly to the drag coefficient employed in aerospace, which takes into account metabolic rate, muscle mass and physics. The metric was then applied to data of energy consumption by thousands of species of swimming and flying animals, ranging from tiny larval zebrafish to massive mammalian swimmers such as dolphins and whales.

The new metric successfully collapsed energy consumption data on to a single trend with respect to mass — mass that varied almost a trillion times from the smallest to the largest animal. The key idea was not to plot the energy consumption itself versus mass but instead to plot energy consumption normalized by an appropriate scale that accounts for the size of the animal.

“The study helps quench my curiosity about how nature works, but, as an engineer, I also want to see utility,” Patankar said. “The energy consumption coefficient can be an important tool in designing self-propelled underwater vehicles as well as aerial vehicles. And, as a driver, I also would like to know how efficient my car is, information currently not available to me.”

The new Northwestern metric for efficiency that enabled this comparison could be extremely useful in designing underwater vehicles — such as those used to study fragile coral reefs, repair damaged deep-sea oil rigs or investigate sunken ships — to be as efficient and agile as a real fish. Of course, motor vehicles design could also benefit from the findings reported in the journal Proceedings of the National Academy of Sciences (PNAS).