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How Penguins Got Their Water Wings
30 September 2010 2:00 pm
Penguins can't fly, but don't feel sorry for them. Once they hit the water, they outperform all other aquatic birds: King penguins, for example, can dive more than 300 meters and stay submerged for as long as 7 minutes. Just how penguins evolved this underwater prowess is unclear. But the discovery of a 36-million-year-old giant penguin skeleton in Peru, with many of its feathers preserved, is providing new clues to this bird's evolutionary flight path from air to sea.
Most people, especially those who have seen the popular 2005 film March of the Penguins, probably think of these unusual birds as residents of Antarctica. But the 17 to 19 living species (depending on who's counting) actually live in a variety of habitats. And the fossil record, which includes about 50 now-extinct species, also reveals a wide geographical distribution, with specimens from Australia, New Zealand, South America, Africa, and Antarctica.
In 2007, an international team led by Julia Clarke, a paleontologist at the University of Texas, Austin, reported two new extinct species near the Pacific coast of Peru. Shortly afterwards, a team led by paleontologist Derek Briggs of Yale University showed for the first time that cellular structures called melanosomes, which contain the melanin pigments that give color to skin and hair in humans and plumage in birds, can be preserved in fossil feathers. The shape and size of melanosomes can give clues to the colors of extinct birds (and even dinosaurs). And in living penguins, the melanin pigment appears to give not only color to their wings but also the strength and rigidity required for the rigors of underwater swimming. So when Clarke returned to Peru in 2008 to look for more penguin fossils, she kept a keen lookout for penguin feathers.
It didn't take long before her team unearthed a nearly complete skeleton—with feathers—of a 36-million-year-old penguin whose swimming length measures about 1.5 meters. The new species, which the team dubbed Inkayacu paracasensis or "the water king," is one of the largest extinct penguins known (the largest living penguin, the Emperor, is about 1.2 meters long).
Clarke then called in Yale paleontologist Jakob Vinther of Briggs' group, who found melanosomes galore, as the team reports online today in Science. But the microscopic bodies looked nothing like those from living penguins, whose tightly clustered melanosomes are different from those of all other birds, according to co-author Matthew Shawkey of the University of Akron in Ohio. Shawkey, who has been helping researchers figure out the colors of dinosaurs, found that the melanosomes resembled those of most other, nonpenguin birds. And from their size and shape, the team concluded that unlike today's penguins, whose outer wings are black or brown, the wings of Inkayacu paracasensis were probably gray and reddish-brown.
That suggests that even though early penguins had the recognizable body shape of today's penguins, by 36 million years ago they had not yet evolved the kind of tightly clustered melanosomes needed to perform complex aquatic feats.
Clarke says that the first penguins might not have been divers, but perhaps swam only on the surface of the water. "We're talking about an evolutionary transition that in some ways is as dramatic" as the evolution of flight itself, Clarke says. "Penguins basically fly underwater, in a medium that is 800 times denser than air."
Gerald Mayr, an ornithologist at the Senckenberg Research Institute in Frankurt, Germany, says that the finding that this early penguin lacked the melanosome arrangement of its modern relatives is "certainly a very interesting discovery." But he questions whether the specialized melanosomes of living penguins evolved to make them better divers and swimmers. While the outer feathers of many modern penguin wings are black, their inner feathers and their bodies are white. If they were specialized for aquatic life, Mayr says, "one would expect penguins to be all black" to maximize the strength and rigidity that the melanin gives them.
Katsufumi Sato, an animal behaviorist at the University of Tokyo who studies bird flight, says he agrees with the team's "conclusion that Inkayacu paracasensis was not adapted for aquatic flight." As for why they weren't all black, Sato says that the black and white colors of today's penguins help camouflage them by reducing shadows cast on their bodies.