Some of my favorite things in Antarctica are the penguins. In every way, these birds are so full of surprises and contradictions. On the one hand, they look and sometimes act like soft and pudgy stuffed animals. On the other hand, they are amazing athletes - stunningly efficient at swimming and flying. Wait, did I just say 'flying' ??
Well, penguins sometimes do fly - or at least glide. Its called porpoising.
Penguin's wings are meant for water, not air, and their dense bones would make true flying very difficult but these birds do get their air time. Just hang around a swimming penguin a little while and you probably see it leap out of the water, soaring in a graceful arc before plunging into the water again. Scientists and others have argued about the purpose of the leaps. It enables the penguin to breathe without slowing their swimming speed, so maybe they do it to get a good breath of air. But, penguins don't porpoise very often (usually only for a few minutes at a time), and they can hold their breath a long time, even while swimming hard, so easy breathing isn't the whole story.
Another possibility is that penguins porpoise to conserve energy. Because penguins have to come to the surface to breathe, they travel close to the surface. But swimming in that boundary layer - defined as 1 to 3 body diameters under the water - creates a lot of drag. Because air is so much less dense than water, gliding through the air from time to time gives the penguins a little break. One more explanation (and the one I favor) has it that porpoising penguins are on the lookout for predators like Orcas and seals. Leaping in and out of the water makes them a tough target. Whatever else it is, leaping out of the water is a social activity, usually I see a whole group of penguins, ten or twenty or more, porpoising in time with each other.
To read more about penguin swimming and diving, check out this kid-friendly website by SeaWorld:
And, for anyone who wants more, this paper by Japanese researchers describes tagged Adelie penguins.