Flightless Feathered Friends
New tales of penguin evolution, past and present
If a bird’s success is measured by how well it flies, then penguins rank low. Their aerial excursions are limited to hopping from one rock to another or, at best, punctuating their high-speed swims with short, arcing leaps from the ocean’s surface to take a quick breath of air. By other gauges, however, penguins are successful indeed. With their streamlined shape, waterproof plumage, and thick layers of insulating fat, penguins are tailor-made for the marine environment. As a group, they occupy prime positions in coastal ecosystems from Antarctica to the equator.
Penguins, for example, account for about 80 percent of the avian biomass in the Antarctic region. The tallest species there is the emperor penguin, which grows to 1.2 meters under some of the coldest conditions on Earth. In contrast, knee-high Galápagos penguins live astride the equator and sometimes, as dogs do, have to pant to lose heat. Macaroni penguins breed in Antarctic and South American colonies that contain millions of individuals, but members of several other species nest alone in burrows, caves, or clumps of grasses in warmer locales. All penguins dine on seafood, but while some choose fish and squid, others consume krill—the tiny, teeming crustaceans that nourish many large whales.
Scientists classify the 17 species of modern penguins into six genera. These groups seem so different from each other that scientists haven’t been sure which ones are most closely related, let alone their connections to extinct species. Bernard A. Stonehouse, an ornithologist at the University of Cambridge in England, notes that, in the penguin world, “it’s tough to tell who matches with whom.”
However, using recent fossil finds and analyses of modern species, researchers are now beginning to sketch in the branches on the penguin family tree. What’s more, DNA studies of one of the most common penguin species illustrate how forces of nature, such as climate changes during the last ice age, have shaped the penguin genome in relatively recent times.
Of a feather?
Early last century, most ornithologists reasoned that because penguins are flightless, they had evolved from other birds before those lineages took to the air. If that were the case, then the earliest penguins would have predated the 150-million-year-old Archaeopteryx, which most scientists consider to be the first bird.
Today, ornithologists agree that penguins evolved from flying ancestors much later, perhaps 80 million years ago. Their closest living relatives appear to be albatrosses, the graceful, soaring birds celebrated for their ocean-spanning trips in search of food for their young, says Marcel van Tuinen of Stanford University.
Several lines of evidence support that scenario. Fossils of early penguins, some discovered as long ago as 1859, resemble the skeletons of albatrosses. Scientists in the late 1950s noted that, for a short interval after hatching, the species known as the little penguin (Eudyptula minor), have nostril tubes similar to those characteristic of modern albatrosses and their close kin, says David Penny, an evolutionary biologist at Massey University in Palmerston North, New Zealand. Later, in the 1970s, studies of antibodies in birds’ blood supported the view that penguins’ closest modern relatives are albatrosses. More-sophisticated DNA analyses conducted in the decades since have bolstered that notion, van Tuinen notes.
Scientists are working to create a detailed family tree that maps out the penguin’s evolutionary adventure. Norberto P. Giannini and Sara Bertelli of the American Museum of Natural History in New York recently compared 70 characteristics of each penguin species. The tally included bill shape, structure, and color; patterns of down and plumage for hatchlings, juveniles, and adults; and traits such as nesting behavior and the number of eggs laid in each breeding season.
The results of that analysis bolster a family tree with six major branches. They also confirm a previously suspected close kinship between the genus containing the little penguin (Eudyptula) which breeds along the coasts of New Zealand, Tasmania, and southern Australia, and the Spheniscus genus, which includes the Magellanic, Galápagos, and jackass penguins.
The data also suggest a previously unrecognized pairing between the Eudyptes genus, which includes the macaroni and rockhopper penguins, and the Megadyptes genus, whose sole species is the rare yellow-eyed penguin of New Zealand. The researchers reported their findings last April in The Auk, an ornithology journal.
Since preparing that article, the researchers have added more information to their penguin database, nearly doubling the number of characteristics related to the species’ anatomy and including DNA data when available. The team’s latest analysis shuffles the penguin family tree somewhat. It moves the emperor and king penguins closer to the base because it appears that they evolved away from the rest of the penguins earlier than the scientists had expected. The new findings were reported in Quebec City in August at a meeting of the American Ornithologists’ Union.
Old birds
Fossils have turned up 40 or so species of extinct penguins. These flightless birds are better represented in the fossil record than are most other types of modern birds, for two reasons. First, because ancient penguins were seabirds, many of them died either on a beach or in near-shore waters, where marine sediments quickly covered their carcasses, says Nina E. Triche, a paleontologist at the University of Texas at Austin.
Second, the robust bone structure of penguins increases the likelihood that their remains will become fossilized. Early in penguin evolution, the bones, especially in the wings and hind limbs, became thick and dense. This change would have improved the ease with which the birds could dive to chase underwater prey. In contrast, the bones of flight-capable birds are highly buoyant because evolution has fine-tuned them to be thin, light, and, in some cases, filled with air. These extremely fragile bones tend to break down before they fossilize.
Besides acquiring dense bones, penguin ancestors evolved narrow wings with inflexible elbows that worked as streamlined hydrofoils, says R. Ewan Fordyce, a paleontologist at the University of Otago in Dunedin, New Zealand. Another bone—the tarsometatarsus, which actually is a set of fused anklebones—is often found in other terrestrial animals but has a distinctive shape in penguins.
Today in the wild, penguins—except for a small number that live and breed just a few kilometers north of the equator on Isabela Island in the Galápagos—live in the Southern Hemisphere. That’s also where all fossilized penguin bones have so far been found—a strong clue that the group originated somewhere in that region, Triche said in a review of penguin biogeography at the Society of Vertebrate Paleontology meeting in Denver early this month.
The fossil record suggests that penguins first appeared in or around New Zealand and spread from there, says the University of Cambridge’s Stonehouse. Even in the warm seas of that region, a diving bird would have needed a significant layer of insulation to maintain its 39°C to 40°C body temperature, which is a couple of degrees warmer than a person’s normal body temperature.
Such insulation could have provided the basis for later adaptations suitable for colder seas, regardless of whether a temperature drop stemmed from a move to higher latitudes or from the climate change associated with the formation of the Antarctic ice sheet, Stonehouse notes. Moving to the cooler waters nearer Antarctica would have enabled penguins to exploit the ecosystems where upwelling provided more food and predators were few.
The oldest penguin fossils—including three nearly complete but disarticulated skeletons from New Zealand—date to between 60 million and 58 million years ago. Scientists are still working to remove these fossils from rock and describe them.
None of the three skeletons includes a complete skull, but the remains “certainly look like an archaic penguin,” Fordyce notes. Bones in the forelimbs of the species are shorter and broader than those found in modern albatrosses but aren’t as short and broad as those in today’s penguins. Also, the ancient creatures’ elbows seem to have had some flexibility, and other bones farther out the forelimbs closely resemble those in a flight-capable bird, says Fordyce.
Bones of a single penguin recently excavated from sandstone rocks in Tierra del Fuego, at the southern tip of South America, have almost doubled the history of penguins on that continent. The 37-to-40-million-year-old fossils include several leg bones and part of a pelvis, says Julia A. Clarke of North Carolina State University in Raleigh. Some features of the upper-leg bone are similar to those of that bone in albatrosses. Details about where muscles had attached to the bone hint that the ancient creature used its legs when swimming in a different way than modern penguins do. Clarke and her Argentine colleagues describe the fossil in the Dec. 9 American Museum Novitates, a publication of the American Museum of Natural History in New York.
The newly described South American species, which would have been slightly smaller than today’s emperor penguins, doesn’t appear to be part of a lineage that survived to the modern day, says Clarke. Nevertheless, she notes, the find does provide clues about the distribution of early penguins at an important time in the group’s evolution. The seabirds would have inhabited an extremely southern region at a warm period in Earth’s history, before the world’s climate cooled and a massive ice sheet covered the Antarctic continent.
Bones to pick
“The fossil record is interesting, but it raises more questions than it solves,” says Stonehouse. For example, if penguins could spread north to the equator, why didn’t they spread to the Northern Hemisphere? Even though the North and South Pacific Oceans are divided only by a line on the map, there may have been—and still may be—an ecological barrier between the hemispheres, says James L. Goedert of the Burke Museum of Natural History and Culture at the University of Washington in Seattle.
It’s not that flightless seabirds such as penguins couldn’t live in the North Pacific, says Goedert. An analogue to penguins, the extinct plotopterids, appeared in the North Pacific about 37 million years ago. This group of birds, which at first glance must have looked like long-necked penguins, had dense bones and inflexible elbows, just as modern penguins do. However, they were descendants of pelicans and not closely related to the ancestors of penguins.
Plotopterids didn’t make it in the long haul, however. The plotopterid species with the biggest body size died out about 25 million years ago, just as sea lions came on the scene. While these aquatic mammals may have preyed on the plotopterids, they may also have competed for food with the birds, says Goedert. Or, he notes, the sea lions might have preferred the same sites for breeding grounds, thereby displacing the seabirds and eventually causing their populations to plummet beyond recovery.
The smallest-bodied species of plotopterids disappeared from the fossil record about 19 million years ago, in the same era that a group of toothed whales went extinct. Whether these simultaneous die-offs are related remains unknown, says Goedert.
In the Southern Hemisphere, there are fewer large mammals that eat or compete with penguins.
Evolution marches on
The bones, feathers, and breeding habits of modern penguins aren’t the only features yielding clues to the seabirds’ evolutionary history. DNA from penguins that died hundreds or thousands of years ago is revealing population distributions.
Blood samples from hundreds of Adélie penguins in Antarctica indicate genetic differences that reflect two distinct lineages. One group, dubbed the Ross Sea lineage, lives and breeds only at sites along its namesake’s coastline. The other, found at locales all around the continent, is referred to as the Antarctic lineage.
Penguins from the two groups look alike, often live together, and even interbreed, says David Lambert of Massey University in Auckland, New Zealand. However, the two lineages’ mitochondrial DNA—which is passed down only through female penguins—suggests that, in the past, the species lived in two separate populations for an extended period.
To look into the history of these penguins, Lambert and his colleagues scrutinized DNA samples extracted from 96 sets of Adélie penguin remains buried in sediments at 17 Antarctic breeding sites. Carbon dating of the remains indicates when these penguins died, from 275 to about 6,400 years ago. The mutation rates of the mitochondrial DNA suggest that the two groups last had a common ancestor about 75,000 years ago.
That most recent common ancestor probably lived around the middle of the last ice age, when ice would have blanketed coastal Antarctica, says Lambert. Ross Island, which is near the Antarctic coast and today is home to several hundred thousand breeding pairs of the penguins, then would have been iced in and almost 900 km from icefree ocean—and therefore uninhabitable for Adélies which don’t nest on ice. Lambert says that, as the ice age began, the Adélies, probably moved northward to remote islands that were warmer and therefore icefree.
The penguins of the two lineages would have lived and bred on isolated patches of rock in separate regions until the big thaw. Then, when sea ice retreated, the penguins moved southward again and together repopulated the fringes of the continent. Although the lineages can’t be easily distinguished by looks or behavior, a genetic memory of their ice age exile remains.
So, even though penguins have been immensely successful, the forces of evolution continue to sculpt their genome.
