Almost exactly a year ago, I was introduced to scuba diving and the astonishing submarine sights of corals, colorful fish, sea lions, flightless cormorants and even penguins.
Despite those pleasures, being under pressure under water just doesn't seem a tenable state for this particular human. However, numerous other creatures have taken to diving as an integral part of their lifestyle, even though they, too, evolved from terrestrial or at least surface species.
Northern bottlenose whales seem to be foremost in this respect, diving repeatedly and for prolonged periods to depths of more than 1,000 meters. Sperm whales have set the depth record at more than 1,200 meters, and though they don't regularly venture so far, they certainly stay under for a long time. I timed several dives in the waters between Tierra del Fuego and the Antarctic Peninsula at more than 50 minutes each.
Walrus, which feed on the floor of the Arctic Ocean, and Weddell seals, common in Antarctica, both exhale fully before diving, and have flexible rib cages that compress during a dive.
Some birds, too, have adopted a diving lifestyle. Two groups, the loons and penguins, have taken these skills to the extreme. Some scientists consider them sister groups, both originating in the Southern Hemisphere. It is thought loons moved into the Northern Hemisphere in the early Tertiary Period (around 50 million years ago) to avoid competition with the emerging penguins. These then flourished in the Southern Hemisphere, thriving around Antarctica. However, loons constrained their diving ability and kept their capacity for flight, while penguins pursued more effective underwater locomotion.
Flight is physiologically expensive. In response, secondary flightlessness has evolved independently across a wide range of avian genera, commonly in situations of geographic isolation where ground predators were few or absent, and where the energetic costs of flight could safely be avoided.
Penguins have an ancestry dating back at least 40 million years to the late Eocene Epoch. Though flightless (in one sense), they have retained the deep keel, or breastbone, a feature commonly lost among other flightless species. This is because penguins are wing-propelled divers that use their wings to "fly" underwater. Their flipper-like forelimbs cannot be folded, and they also lack flight feathers. In these ways, they are unlike the flightless cormorants of the Galapagos Islands, the only other surviving flightless seabird, which has a very poorly developed sternum and still has ragged wing quills.
However, like their sister group the loons, penguins have webbed feet on strong legs set so far back that they stand and walk erectly, albeit with that familiar comical waddle. Once in water, though, penguins are no longer comical; they are masterful. Having snorkeled with them, I can assure you that their underwater capabilities are awesome.
Wing-propelled diving is by no means unique to penguins; at least three other evolutionary lineages have achieved it too -- the diving petrels of the Southern Hemisphere, the auks and alcids of the Northern Hemisphere, and an extinct group known as the Plotopterids. Penguins certainly do it well, though.
The king penguins I mentioned recently in this column, breeding in places such as the Falkland Islands and South Georgia, have often been recorded diving to 100 meters, and have been known to reach 240 meters. Their larger cousins, the emperor penguins that breed in winter on the Antarctic ice shelves, have been recorded down to an astonishing 534 meters. However, records are one thing and regular behavior is another, since these birds are more frequently found going down to 20-40 meters, and may dive more than 200 times in a day but only remain underwater for 4-5 minutes each time.
The diving lifestyle of the penguins is one that is supported by a number of secondary adaptations. For one thing, the lightweight, hollow bones that are so important in reducing the weight but maintaining the skeletal strength of flying birds, would be an impediment to birds wanting to dive. For them, reduced buoyancy is a benefit.
Their feather structure differs from flying birds, too. Penguin feathers are extremely specialized, forming a dense coat of short, stiff plumes that are curved and overlap each other to aid insulation and water- and wind-resistance. Even strong Antarctic winds do not ruffle their feathers, while they are further helped by a thick layer of subcutaneous fat -- something else that is untenable for flying birds.
Nevertheless, the insulating and aquadynamic "skin" provided by the feather coat must still be shed, or molted, as in other birds. There are few things as abjectly pathetic as a molting penguin hunched in a summer snowstorm, unable to flee into the ocean, and surrounded by heaps of its own shed feathers.
Watching penguins underwater is, however, both a joy and an education. Their flipper movements are similar to those of a bird in flight, though the denser medium of water gives them a degree of control unimaginable among even the finest aerialists. As a result, they can perform rapid twists and turns, loops, and jinks while in pursuit of their fast-moving fish prey, with their feet and stiffened tails providing the steering. Their adaptations to the diving lifestyle mean they can travel far from their colonies. The rockhopper, for example, may go 250 km from its nest on foraging trips, though the record goes to king penguins, which have been logged up to 900 km from their colony.
Of course, prolonged travel in search of food by one parent means the other must stay without food for several days while incubating the eggs. Then, if the hunting parent does not return, the other must abandon its eggs or young and head out to sea to feed. Hence, penguins are unable to rear their young alone.
Penguin species get larger the farther south you go into colder regions, following a principle known as Bergmann's Rule. In parallel, their feathers become longer, too, and not surprisingly the only species to live on the Equator, the Galapagos penguin, has the shortest feathers, while the Adelie penguin of Antarctica has the longest.
There was greater diversity among the 32 species of extinct penguins that we know of from fossils than among the surviving 17 or 18 species -- and some among them were giants. One stood more than 170 cm tall and had a long dagger-like beak perhaps used to spear fish. I can't help wondering whether a penguin much larger than the emperor could dive even deeper -- but we will never know.
