Image: Oligocene penguins
Geology Museum, University of Otago (N.Z.)
Palaeeudyptes, one of the "giant" penguins lived during the Oligocene, about 28 million years ago. Bones in this bird and its relatives show clear evidence of a heat-conserving structure known as a humeral arterial plexus.
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updated 12/21/2010 10:10:15 PM ET 2010-12-22T03:10:15

Those tuxedo-wearing birds that inhabit Earth's coldest continent may have evolved a means of retaining heat when they were still living in warm climates, scientists now suggest.

A key adaptation that helped modern penguins to invade the cold waters of Antarctica within the last 16 million years is the so-called humeral arterial plexus, a network of blood vessels that limits heat loss through the wings.

The plexus routes blood coming into the body from the wings past the blood traveling from the body to the wings. As such, the cooler blood from the wings, which get cold in the water, is heated up by warmer blood from the body, thus conserving heat.

To find out more about how this anatomical structure evolved, scientists investigated seven live penguin species and 19 fossil ones. In live specimens, they found the plexus leaves behind grooves in the upper arm bone called the humerus. As such, they could see when this structure began appearing in extinct penguin species from the fossil record.

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Surprisingly, they found the plexus arose at least 49 million years ago, when the planet was going through a warm "greenhouse Earth" phase due to vast amounts of global warming gases that got pumped into the atmosphere, perhaps by volcanism.

"I began this work thinking we would relate heat retention in penguins to the global cooling that took place at the Eocene-Oligocene boundary [about 34 million years ago], whereas in fact, penguins were cold-water-tolerant millions of years earlier," researcher Daniel Thomas, a paleontologist at the University of Cape Town in South Africa, told LiveScience.

The earliest-known penguins to feature the plexus lived on the lost continent of Gondwana, on what is now Seymour Island in Antarctica. Back then, the waters there were 59 degrees Fahrenheit (15 degrees Celsius), compared with the water's current average temperature of 34 degrees F (1 degree C). (Scientists can deduce ancient temperatures by looking at the chemistry of fossils — for instance, magnesium levels in the shells of certain organisms rise as temperatures go up.)

The researchers suspect the plexus first evolved to help penguins save energy during long foraging trips in the cold water, as the structure evolved in concert with dramatic skeletal changes that promoted buoyancy and reduced drag, thus improving deep-diving and long-distance swimming. As global climate cooled, the plexus then found a new use, proving key to the penguins' invasion of Antarctic ice sheets.

"Penguins have occupied much of the Southern Hemisphere in the last 40 million years because of their tolerance for cold water," Thomas said.

Thomas and his colleagues Dan Ksepka and Ewan Fordyce detailed their findings online Wednesday in the journal Biology Letters.

© 2012 LiveScience.com. All rights reserved.

Explainer: 10 secrets of the deep ocean

  • Mark Spear / Woods Hole Oceanogr

    The oceans cover more than 70 percent of the earth's surface, yet their depths remain largely unknown. It's a frontier that scientists are racing to explore using tools such as the deep-ocean submersible Alvin, shown here. Click the "Next" arrow above to learn about 10 deep-ocean secrets that have come to light.

  • Deep-ocean octopuses have Antarctic origins

    Image: Megaleledon setebos
    Census of Marine Life

    Many deep ocean octopuses trace their origins back to relatives that swam in the waters around Antarctica. The migration began about 30 million years ago when the continent cooled and large ice sheets grew, forcing octopuses there into ever deeper waters. The climate shift also created a northbound flow of deep, cold water that carried the cephalopods to new habitats. As they adapted to new niches, new species evolved. Many lost their defensive ink sacs because the pitch-black ocean depths required no camouflage screen. The species known as Megaleledon setebos, shown here, is the closest living relative of the deep-sea octopuses' common ancestor.

  • 'Brittlestar City' found on undersea mountain

    Census Of Marine Life  /  AP

    The orange and red starfish relatives called brittlestars have managed to defy the odds and colonize the flanks of a giant, underwater peak on the Macquarie Ridge, an 870-mile-long underwater mountain range that stretches south from New Zealand to just short of the Antarctic Circle. The peak, known as a seamount, juts up into a swirling circumpolar current that flows by at 2.5 miles per hour, delivering ample food for the brittlestars to grab while sweeping away fish and other would-be predators. Another brittlestar species has settled on the seamount's flat summit, a habitat normally settled by corals and sponges.

  • Deep Antarctic waters, cradle of marine life

    Wiebke Brokeland / GCMB

    This pale crustacean from the genus Cylindrarcturus is one of more than 700 species new to science found scurrying, scampering and swimming in the frigid waters between 2,000 and 21,000 feet below the surface of the Weddell Sea off Antarctica. The discoveries were part of a research project to determine how species at different depths are related to each other there, and to other creatures around the world. "The Antarctic deep sea is potentially the cradle of life of the global marine species," team leader Angelika Brandt, an expert from the Zoological Institute and Zoological Museum at the University of Hamburg, said in a statement announcing the discoveries.

  • Northernmost black smokers discovered

    Credit: Center for Geobiology/U. of Bergen

    Scientists working deep inside the Arctic Circle have discovered a cluster of five hydrothermal vents, also known as black smokers, which spew out liquid as hot as 570 degrees Fahrenheit. The vents are 120 miles further north than the closest known vents, which tend to occur where the seafloor spreads apart at a quicker pace. This image shows the arm of a remotely operated vehicle reaching out to sample fluids billowing from the top three feet of the tallest vent, which reaches four stories off the seafloor. The chimney is covered with white bacteria that feast on the freshly delivered minerals.

  • Black smoker fossils hint at life's beginnings

    Timothy Kusky / Gondwana Research

    The discovery of primitive bacteria on 1.43 billion-year-old black-smoker fossils – a crosscut is shown here – unearthed from a Chinese mine adds weight to the idea that life may have originated in deep-sea hydrothermal vents, according to geologist Timothy Kusky at Saint Louis University. The ancient microbe dined on metal sulfide that lined the fringes of the chimneys. The oldest-known life forms on Earth are 3.5 billion-year-old clumps of bacteria found in Western Australia. That find suggested that shallow seas, not the deep oceans, were the birthplace of life. Neither discovery, however, serves as the definitive answer about life's origins.

  • Microbes feast on ocean-bottom crust

    Image: Basalt rocks
    NOAA/WHOI

    Once thought barren and sparsely populated, the deep-ocean floor is home to rich and diverse communities of bacteria. In fact, scientists have found that the seafloor contains three to four times more bacteria than the waters above, raising the question of how the organisms survive. Lab analyses suggest that chemical reactions with the rocks themselves provide the fuel for life. The discovery is another tantalizing hint that life could have originated in the ocean depths. In a statement about the find, the University of Southern California's Katrina Edwards said: "I hope that people turn their heads and notice: There's life down there."

  • Where do deep-sea fish go to spawn?

    Harbor Branch / E.widder

    Life in the dark, cold and vast depths of the sea was long thought to be lonely for the few fish that dared eke out an existence there, mostly from organic detritus that sinks from shallower waters. That picture began to change in 2006, when researchers probing the Mid-Atlantic Ridge discovered that fishes may occasionally gather at features such as seamounts to spawn. The evidence for these gatherings comes from the sheer volume of fish collected at seamounts – much higher than would have been expected if the fish were purely nomadic wanderers. What's more, images made from acoustical "scatterings" are suggestive of a massive fish aggregation. The 35-pound anglerfish shown here is one of the rare species hauled up from the deep during the project.

  • Colossal squid has, well, colossal eyes

    Image: New Zealand colossal squid
    Ross Setford  /  AP

    What did you expect? Would a colossal squid have anything but eyes big enough to generate a few over-the-top superlatives? Probably not - but still, when researchers thawed out this squid in New Zealand, the wow factor was undeniable. The creature's eye measured about 11 inches across; its lens was the size of an orange. Scientists suspect the big eye allows the huge squid to capture a lot of light in the dark depths in which it hunts. The squid weighed about 1,000 pounds when caught in the Antarctic's Ross Sea and measured 26 feet long. Scientists believe the species, which can descend to 6,500 feet, may grow as long as 46 feet.

  • Deep-sea corals record history

    Image: Scuba divers collect coral samples
    Rob Dunba  /  Stanford University

    Some coral reefs are found thousands of feet below the ocean surface, where they have grown amid frigid waters for millennia. Like tree rings, they serve as a faithful archive of global environmental change, according to Robert Dunbar, a professor of geological and environmental sciences at Stanford University. His team travels the world to collect samples of these corals, such as this one from a colony near Easter Island. In 2007, the team published a 300-year archive of soil erosion in Kenya, as recorded by coral samples collected from the bottom of the Indian Ocean. They are now analyzing 4,000-year-old corals discovered off Hawaii to create an archive of climate change.

  • Trawling destruction visible from space

    Sky Truth

    Some scientists are working urgently to expose more secrets of the deep ocean before unexplored treasures are plundered. Their biggest concern is the fishing practice known as bottom trawling. This image shows the billowing plumes of sediment left in the wake of trawlers dragging giant nets across the ocean floor in the Gulf of Mexico. The practice has been shown to strip coral reefs bare and ravage underwater ecosystems such as seamounts, where thousands of species are known to gather. Though the practice is increasingly restricted, tens of thousands of trawlers continue to ply the deep oceans.

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