Penguins lost their ability to fly millions of years ago, and now a new study explains why -- the birds became lean and mean diving machines, trading flight for such skills.

The study, published in the latest Proceedings of the National Academy of Sciences, points out that good flippers don’t fly very well.

"Once penguins gave up flight, changes to wing structure and overall body size and shape probably followed rapidly because flying no longer placed constraints to body form," co-author Robert Ricklefs told Discovery News.

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"Note that penguins are much more at risk of predation in the water than they are on land, and so there has been strong selection to make their swimming and diving as efficient as possible," added Ricklefs, who is a professor of biology at the University of Missouri at St. Louis.

Ricklefs, lead author Kyle Elliott and their team at first wondered why the ubiquitous black and white birds lost their ability to fly millions of years ago, given how beneficial flying can be. Emperor penguins laboriously walk over 32 miles between their rookeries and the sea. The journey takes them several days, which could be reduced to just a few hours if they could fly. Why then don’t they?

To solve the mystery, the researchers focused on birds-- especially the murre -- that both fly and dive. The scientists equipped 41 such wild-caught birds with equipment to measure avian energy expenditure. In doing so, the researchers came up with a new world’s record. Murres and pelagic cormorants turn out to have the highest expenditure ever recorded for any flying animal.

"The costs are incurred in providing lift in air," Ricklefs explained, adding that overcoming drag in the air is also energy costly to the birds.

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While murres can both fly and dive, there appears to be a threshold where one activity overtakes the other in evolution. If a bird needs to fly more, it will lose more of its diving and swimming ability. Conversely, if a bird greatly relies upon swimming and diving for its hunting and survival, then it will tend to lose its flight skills. In the case of penguins, those skills completely disappeared, with the wings evolving into marine mammal-type flippers.

The study also sheds light on what prehistoric flying penguins looked and acted like.

"The flying ancestors of penguins were probably not much different in general appearance than murres and their relatives, and probably behaved in much the same way," Ricklefs said.

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The findings could help explain how other birds lost their ability to fly. There is a flightless cormorant in the Galapagos Islands, and steamer ducks of the southern oceans are also flightless.

The reasons for flightlessness are different for ostriches and emus, which do not dive. These big birds instead traded flight for running ability. It’s likely that the ancestors of ostriches and emus did not have to migrate. They perhaps lived in the southern continents with relative few predators. Running with their powerful legs sufficed, versus needing to rely upon flight to take them up and away.

Tony Diamond of the University of New Brunswick, James Lovvorn at Southern Illinois University, and Daniel Roby of the Department of Fisheries and Wildlife all told Discovery News that they agree with the conclusions of the new study.

Diamond said the study "draws on the unique diversity of mobility-modes in birds -- walking, running, swimming, flying -- to clarify and explain evolutionary patterns that are otherwise puzzling."

Rory Wilson of Swansea University, had a more measured response, saying the authors of the study "are probably right, but the result would be more definitive if they compared auks with diving ducks." He explained that some birds have very different types of plumage that can affect heat loss. Heat loss, in turn, can affect the bird’s energy costs when flying and diving.