Long, long ago, O Best Beloved, the ancestor of the penguins could soar through the air. So why did the penguin give up flight? Rudyard Kipling never wrote a Just So story with an answer, but now scientists have one: The penguin doesn't fly because it would rather swim.
A new study of murres, penguinlike seabirds that retain the ability to take wing, shows just how costly and inefficient it is to be both a diver and a flyer. The new findings back the long-held hypothesis that penguins gave up the heavens more than 70 million years ago to become kings of the waves.
"This study contributes a lot by putting hard numbers on the energy costs of moving through both the aerial and aquatic realms," writes Daniel Ksepka of North Carolina State University in Raleigh, who studies penguin evolution and was not involved with the research, in an e-mail.
For insights into why ancestral penguins might have abandoned their command of the air, the researchers turned to the thick-billed murre, Uria lomvia, which nests on cliffs in Alaska, Canada, and other northerly sites. It propels itself through the water with its wings to scoop up krill and plankton, but it also flies—laboriously.
Murres "are awful flyers," says graduate student Kyle Elliott of the University of Manitoba in Winnipeg, Canada, an author of the new paper. "They beat their wings really, really fast, and they're horrible at landing."
To study murres nesting in northern Canada, the researchers carried shotguns loaded with rubber bullets to ward off the local polar bears and lived in a cabin surrounded by an electric "bear fence." Between surprise visits from bears, the scientists lassoed murres and injected them with tracer molecules to track their energy usage. They also outfitted the murres with sensors to learn how deep they dove and how much time they spent in air, underwater, and on land.
The results show that being a murre is hard work. The animals expend more energy per minute of flight than any other bird, surpassing even the previous champion, the bar-headed goose, famed for flying over the Himalayas. On the wing, murres burn energy at 31 times their rate at rest, the highest known ratio in a bird, the team reports today in the Proceedings of the National Academy of Sciences. When other vertebrates are working hardest, they burn energy at only 25 times their resting rate.
Murres fare better in the water, where they're more efficient than many other birds, but they could still use a few tips on their stroke. The researchers found that compared with penguins of the same size, murres expend far more energy while diving, indicating that giving up flight raised their efficiency.
The results show that murres "are really at the edge of what a bird can do," says University of Missouri, St. Louis, seabird ecologist Robert Ricklefs, an author of the paper. If the murre's all-purpose wing became more like a penguin's stubby flipper, swimming would be easier, because a short wing creates less drag in the water. But flying would be nearly impossible, because a short wing makes it harder to stay aloft.
The results run contrary to assumptions that "all birds had the same flight cost, more or less," Elliott says. For murres, "we were able to show that flight costs were much greater than expected … [and] demonstrate the cost of not being flightless." Even so, flight allows murres to flee predators and zip between nest and foraging grounds. For ancestral penguins, on the other hand, flightlessness was apparently a better deal, enabling them to grow larger, helping them dive deeper, swim faster, and stay underwater longer, Ricklefs says. And that meant they nabbed more and bigger prey.
The study provides valuable confirmation of the idea that ancient penguins swapped flight for underwater prowess, known as the tradeoff hypothesis, says Chris Thaxter, a seabird ecologist at the British Trust for Ornithology in Thetford, U.K. "This is a major step forward … in understanding how the tradeoff hypothesis works."