Wouldn't the March of the Penguins have been a whole lot easier as Flight of the Penguins? Perhaps—but if penguins were able to fly, they might have to give up their incredible swimming ability, scientists say: It's a tradeoff. A team looked at the guillemot, a relative of the penguin that can both swim and fly, in order to figure out why penguins can't achieve liftoff. For guillemots, the researchers found, "the energy costs (of flying) are very, very high."
Diving, however, is fairly easy for the birds, the BBC notes. That seems to support the "biomechanical hypothesis" as to why penguins can't fly. The theory suggests that flying and diving are so different that "you cannot build a wing that is good at doing both," says a professor. For evolving penguins, "as the wings became more and more efficient for them to dive, they became less and less efficient for them to fly," he notes. "At some point it became so 'expensive' for them to fly that it was better to give up flying altogether and make the wings into small flippers."