Cool Birds

How can emperor penguins live like that?

Holding an unhappy penguin can drive even a careful person to take risks. In her first field season in the Antarctic, Barbara Wienecke was struggling to fasten a small radio transmitter to a penguin that was struggling to get away. To finish the job quickly, Wienecke took off her gloves for a few minutes.

TOUGH LIFE. Emperor penguins go to extraordinary lengths—walking over kilometers of ice, going months without food—to raise their families in one of the toughest places on Earth. Adapting to life in Antarctica has given these birds physiological and behavioral novelties of great interest to science. Corbis
PLACE TO BE. At some 40 spots around Antarctica, penguins gather in dense crowds to court and raise young. Unlike most birds, emperor penguins incubate their eggs during winter. Corbis
DOWN UNDER. Emperor penguins, the deepest avian divers known, take advantage of cracks in the ice to go fishing (left). They can keep swimming even when their bodies’ oxygen stores have been depleted beyond levels that would knock out a human diver. After feeding, the penguins speed to the surface (right) and can shoot out onto the ice. Kooyman/Scripps Inst. of Oceanography

Corbis

DAY CARE. Emperor penguin chicks, whose parents have gone fishing, snuggle together to keep warm. Kooyman/Scripps Inst. of Oceanography

As she worked frantically, her fingers paled and then “went from something white colored, which still looked a bit like skin, to something rather waxy that looked nothing like skin,” she says. The fingers then started to swell, a sign that ice crystals were forming, and soon went numb. “Then, thank God, the bird was ready to go,” she says. She’d exposed her hands for only 7 minutes or so, but the temperature hung near –25°C and the wind was up.

Although the technique is not ideal, Wienecke dipped her hands into a bucket of seawater scooped from a hole in the ice and waited to see if her fingers would regain feeling. “Initially, there was just nothing, and then suddenly—the only thing I can think of that would give you an idea of the pain was if anybody were to pour boiling oil over your fingers,” she says.

Wienecke, a biologist at the Australian Antarctic Division in Kingston, Australia, still has all her fingers. She also has a profound appreciation for the emperor penguin, the only vertebrate besides people to spend winters on the skirt of solid ice around Antarctica.

Male emperor penguins, which stand 1 meter tall, spend the whole winter barefoot on the ice (SN: 11/27/04, p. 346: Available to subscribers at Flightless Feathered Friends). Their only protection against cold is to huddle together. Indeed, the emperors do more than just survive. Unlike most birds and all other penguins, the emperors court and then lay and incubate their eggs in winter. And during those 4 months, the dads eat only snow.

The birds’ survival in the face of such extremes of family life made a hit of this year’s documentary film March of the Penguins. These unusual skills have also inspired scientists. Just about every phase of the penguins’ annual cycle offers something unusual to study, whether it’s the birds’ diving prowess and feeding binges of fall, their winter courtship and chick rearing, or their abrupt molts in summer. And the birds survive it all without nests or burrows. Or gloves.

Depth findings

Emperor penguins fish for a living, and right before the breeding season, they binge fish. The first scientist to see how deep their fishing dives take them was physiologist Gerald Kooyman of the Scripps Institution of Oceanography in La Jolla, Calif.

In the late 1960s, he attached a depth recorder to an emperor penguin and made the first measurements of this species’ diving capacity. That bird plunged as deep as 265 m. It was the deepest avian dive recorded, although mammal champs such as sperm whales can go deeper.

Now, Kooyman dismisses 265 m as “modest.” Later studies observed emperor penguins going as deep as 500 m. Most dives are in the modest range and last only 5 or 6 minutes, but the birds have been known to remain underwater as long as 20 minutes.

“Hydrodynamically, penguins are better designed than most marine mammals,” Kooyman says. Seals, for example, flex their bodies dramatically as they swim, and that flexing increases drag. In contrast, emperor penguins motor along with their bodies almost rigid.

Paul Ponganis, a Scripps researcher and a practicing anesthesiologist—for people—in San Diego, has joined Kooyman in studying how deep-diving penguins manage their oxygen. First, he says, they load up on it by taking advantage of “the major adaptation” in some animals for epic dives, an abundance of the compound myoglobin in their tissues.

Myoglobin has an even stronger affinity for oxygen than does its close relative hemoglobin. This attraction makes it possible for tissues to receive the oxygen that hemoglobin carries in the bloodstream. Deep-diving mammals such as Weddell seals have unusually high concentrations of myoglobin in their cells, and so do emperor penguins.

Also, emperors have some 100 milliliters of blood per kilogram of body weight, says Ponganis. That’s not as much as extreme-diver seals, with 220 ml/kg, but more than a person’s 70 ml/kg.

The penguins also ration these oxygen stores using physiological tricks unavailable to landlubbers. A person plunging into the water and swimming downward feels a rise in heartbeat. When an emperor penguin dives, however, its heart slows, as does its use of oxygen. A penguin heart may pound at 180 to 200 beats per minute when the bird is on land, but during a dive, the heart rate drops to as low as 60 beats per minute, Ponganis says.

Plenty of mysteries remain about penguins’ diving prowess. Ponganis, his Scripps colleague Torrence Knower Stockard, and an international team found that when diving, penguins nearly deplete the oxygen in the special avian air sacs attached to their lungs. The team measured oxygen remaining in the air sacs during the last 15 seconds of an emperor penguin’s dive.

For 42 percent of about 70 dives made by four test birds, air-sac oxygen dropped below 20 millimeters of mercury (mmHg), the researchers report in the Aug. 1 Journal of Experimental Biology. Should a person’s lung-oxygen supply drop below 25 mmHg, “we’re passing out,” says Ponganis.

Coming up from an extreme dive in theory should present penguins with the same risks that beset stroke patients when tissues starved for oxygen suddenly get blood after a blockage is removed. This rush produces chemically damaging free radicals that typically do more damage than the initial oxygen starvation did.

Penguins don’t seem to suffer from their dives, and researchers are now searching for enzymes that protect the animals.

Emperor penguins don’t seem bothered by the bends, either. Human divers who plunge to penguin depths and then surface as briskly as these birds do develop dangerous nitrogen bubbles in their blood. Kooyman doesn’t have an explanation for the birds’ resistance to the bends. “I could wave my arms around,” he offers.

The birds not only avoid the bends but also end their dives with flair. Ponganis and his colleagues have seen many of the birds’ acrobatic exits from water onto ice.

To study diving, the researchers corral the penguins near two isolated holes in the ice. When the birds, often with expensive instrumentation attached, plunge through the holes to fish, researchers wait nearby anticipating that the birds will return through the same hole.

Leopard seals, connoisseurs of penguin meat, often lurk near these ice holes too. To avoid predators, penguins jetting up from a hole get out of the way as quickly as possible. “They hit the ice and keep scooting,” Ponganis says.

Extreme dieting

In fall, after a frenzy of fishing, Antarctica’s emperor penguins splash out of the water and begin their storied long march across the ice to some 40 breeding sites typically well away from the water. Some spots attract only a few hundred bird pairs, but 10,000 couples meet at others.

In 1994, when Wienecke herself migrated to a penguin colony, daytime shrank until she and the birds had only 3 hours of dim light each day between the hours of total blackness. She reports that it wasn’t all bad. “You could step outside at 10 o’clock in the morning and see raging auroras,” she recalls.

As the looming winter chills Antarctica, seawater around it freezes, nearly doubling the summertime surface area of the continent. This ice ruff around solid ground will melt as spring returns, so the penguins have to hike far enough for their breeding ground to stay solid until the chicks can cope with water. Penguins travel south for dozens of kilometers by waddling upright and sledding on their bellies.

Physiologist René Groscolas of the Center for Ecology and Metabolic Physiology in Strasbourg, France, has watched the penguins march in Antarctica. “It was very impressive to see living beings appearing at the horizon and walking in a complete silence,” he says.


Corbis

When emperor penguins start marching after weeks of foraging at sea, they’re as fat as they ever get.

Once the penguins mate at their breeding site, the female lays one egg and then heads back to sea. The male stands on the ice snuggling the egg under his belly and on top of his feet for 7 weeks or so. If a chick hatches early, its dad feeds it a liquid secreted by his esophagus.

If all goes well, the father can keep junior and himself alive until his mate shows up with a bellyful of baby food.

Part of what enables the male to limit his energy expenditure is huddling behavior. Hundreds, sometimes thousands of emperor penguins crowd together conserving body heat. Densities can reach up to 10 birds per square meter, not counting the babies, says Groscolas. In such close quarters, the need for warmth seems to override male-bird territoriality.

Physiologists have calculated that a lone male has the energy to keep himself warm only if the temperature stays above –10°C. Groscolas says that a huddling male can drop his metabolic rate 40 percent compared with that of a lone dad. A huddler loses no more energy than he would standing around in a temperate environment.

“Actually, it appears that emperors may get too warm while in a huddle and have to regularly go outside it to refresh,” Groscolas says.

“They’re toasty,” Wienecke agrees, speaking relatively. Temperatures inside the huddle reach about 20°C. Emperor penguins now and then stretch their highly extensible necks above the hunkered mass. “They’re like periscopes,” says Wienecke. “And you can see this puff of hot air.”

When the female returns, she takes over chick care and lets her mate waddle back to the sea for his first meal in 4 months. He also picks up the next load of baby food.

Yet Groscolas and his colleagues argued in 2001 that full-bellied parents arrive at the colony starving themselves metabolically.

When the team analyzed various metabolites, such as triglycerides, in the birds’ blood, the biochemical profiles didn’t fit those of animals digesting food after plentiful meals. Emperor penguins seem to block their digestion and fast with their stomachs full.

The only other animals known to do this are king penguins brooding chicks and gastric-brooding frogs in Australia, Groscolas says. The possibly extinct frogs shut down digestion while their tadpoles use their mom’s stomach as a nursery.

Eventually, the emperor penguin chick gets big enough to regulate its own body temperature, and both parents can go off foraging. They leave their chick in an avian version of a playgroup, which ornithologists call a crèche. If a storm blows in, the youngsters crowd into a junior huddle of gray fluff.

The spring weather can actually be tough on animals as well insulated as emperor penguins are. On a sunny day with no wind, the temperature can soar to –10°C to –5 °C. “If it’s a hot day, they’ll be gobbling snow,” says Ponganis. Chicks throw themselves down on the ice with their stubby wings outstretched.

Beyond extreme

Fasting isn’t over for the penguins. They again can’t eat during their summertime molt because they can’t swim without a fully protective body suit. To prepare, penguins leave their chicks to fend for themselves and take off on long fishing trips.

To see where penguins go to fatten up, Wienecke’s group fastened satellite trackers and dive recorders onto birds at two colonies along the Mawson Coast. Instrument packages on nine penguins showed the birds swimming north on a great feeding expedition of 22 to 38 days. On the longest trip, a penguin traveled up to 1,900 kilometers and reached points 600 km from the breeding ground.

Rather than returning to the nesting colony site to molt, the tagged emperor penguins settled on some large, free-floating hunks of ice, Wienecke and her colleagues report in the January 2004 Polar Biology. Then, the birds molted.

Emperors, like other penguins, grow much denser feather coats than do flying birds. An emperor’s feathers keep an air layer for insulation on land. During diving, the water compresses the outer layer of oiled feathers so that it loses much of its insulating power. Even a modest dive subjects the birds to 20 atmospheres of pressure.

The compressed feathers provide a waterproof dive suit, explains Wienecke. It protects the skin from direct contact with the chilly water and keeps the undercoat of down from getting waterlogged.

Airborne birds lose and replace feathers gradually so that they’re never grounded. Emperors and other penguins, however, replace all their feathers during a single month. A molting penguin will actually drown if it ventures into the water, Wienecke says. Molting birds stroll around or stand in “pools of feathers,” says Wienecke.

As each new feather works its way up through the skin, it pushes the old one aside. Just before the old feather drops off, it loses its sleek penguin styling and sticks straight out. The birds never get naked, but the new feathers grow in patchily. The birds look “terribly raggedy and moth-eaten,” says Wienecke. “It must itch something horrendous.”

Yet most of the birds succeed in growing back their feathers in time to go back to sea and fatten up before beginning—again—their long march.

Kooyman welcomes the enthusiasm for emperor penguins that The March of the Penguins has engendered. He says, “I’ve been telling people they’re remarkable for years.”


Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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