May 20, 2013 at 6:06 PM ET
Penguins have evolved into champion divers and graceful swimmers, but somewhere along the way, they lost the ability to fly. It now looks like birds are built to do one or the other — fly or swim. Penguin ancestors chose one path a long time ago and just stuck to it.
"To be an efficient swimmer you want a wing that is more like an oar — that makes it impossible to fly," Robert Ricklefs, professor of biology at the University of Missouri, told NBC News. A penguin wing designed to shove aside water, shortened for diving and swimming, comes with a price-tag: terminal inefficiency of flight.
By studying two bird species which can both dive and swim, Ricklefs and an international group of researchers are adding evidence to the theory that penguins sacrificed the ability to fly so they could move better under water.
The researchers studied murres, which roost in cliffs overlooking Arctic waters, and cormorants. Because both species are small, their wings can carry them through the air and in the sea, but bigger birds can't pull that off, the authors explain in Monday's edition of the Proceedings of the National Academy of Sciences.
Penguins have been studied aplenty. But diving fliers like the murres or cormorants haven't made the leap into flightlessness, are "getting closer to the lifestyle of a penguin," and could provide clues to how penguins got where they are now, Ricklefs says.
The group studied 41 murres roosting on the cliff-faces of Nanavuk, Canada and 22 cormorants, dwellers of Middleton Island, Alaska, for a few weeks each. They recorded energy efficiencies as the birds swam and dived and flew. The diving swimmers were just-okay fliers — and they took wing, the murres were the most energetically inefficient fliers of any known flying vertebrate.
The authors argue that penguins took that tradeoff one step further: they lost all ability to fly, but padded their bodies with muscles that would increase the power in their wingstrokes when they swam. "There aren't many flying things that are the weight of large penguins," Ricklefs said. The larger, now extinct cousins of the murres and cormorants were also flightless.
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In addition to Robert Ricklefs, the authors of High flight costs, but low dive costs, in auks support the biomechanical hypothesis for flightlessness in penguins include Kyle Elliott, Anthony Gaston, Scott Hatch, John Speakmane and Gail K. Davoren.