How Do Penguins Survive the Antarctic Winter? Feathers, Fat and the Huddle

Penguins are birds that gave up flying to swim, and everything about them is shaped by the sea. The wings became stiff flippers, the bones became solid to reduce buoyancy, and the body became a streamlined torpedo that an emperor can drive to 500 metres down and hold for twenty minutes on one breath. Their problem is that water draws heat from a body twenty-five times faster than air, and the Southern Ocean sits near minus two degrees, so the first line of defence is against the water rather than the wind. A penguin's feathers are short, stiff and dense, overlapping like tiles over a layer of down, and they trap a film of air that the water cannot reach; the bird oils them from a gland at the tail and preens constantly to keep the seal.

Beneath the feathers is fat, a layer two or three centimetres thick in a well-fed emperor, which insulates and also fuels the fast. Then come the tricks of the blood. Penguin flippers and feet are thin and would lose enormous heat if warm blood flowed through them freely, so the arteries running into them lie alongside the veins running back, and the outgoing blood gives up its heat to the returning blood before it reaches the extremities. The feet are kept just above freezing, cold enough to lose almost nothing to the ice, warm enough not to freeze, and the same counter-current exchange runs in the nasal passages, where exhaled breath warms the incoming air and reclaims most of the moisture. The main defences:

• •Stiff overlapping feathers over a layer of down, trapping air against skin and water
• •A thick layer of fat that insulates and is burned during the fast
• •Counter-current heat exchangers in the flippers, legs and nose
• •A large body: the emperor, at up to 40 kilograms, is the biggest penguin, and a large body loses heat more slowly in proportion to its mass
• •Behaviour: turning the back to the wind, tucking the flippers, and the huddle

Alone, an emperor would burn through its fat before the winter ended. Together, the males form a huddle of thousands of birds pressed so tightly, about ten per square metre, that the temperature inside rises above twenty degrees and the birds at the centre can overheat. The huddle is not still. Every thirty to sixty seconds a bird takes a small step, and the step propagates through the mass like a wave through a traffic jam, so that the whole formation slowly creeps and rotates, moving birds from the cold windward edge to the warm interior and back out again; the sharing is not perfectly fair but it is enough that every bird spends time in the warmth. Physicists who filmed the huddles from above found that the waves follow the same rules as a crowd or a jam of cars, and that the penguins pack in a near-perfect hexagonal lattice. The huddle cuts each bird's energy use by about half.

The female lays a single egg in May and passes it to the male, who tucks it onto his feet under a fold of skin, and then she walks up to a hundred kilometres to the open sea to feed. The male stands with the egg through June and July. He has been fasting since he arrived at the colony in March, and by the time the chick hatches and the female returns he has gone about 115 days without food and lost nearly half his weight. He can still produce a secretion from his crop to feed the chick for its first days if she is late. Then he walks to the sea himself, and the parents alternate for the rest of the season, the chick growing in a creche of its own kind until, in December, the summer ice breaks up and the whole colony can go to sea.

The emperor's whole life cycle depends on sea ice that forms early, stays fast to the coast through the winter and does not break up before the chicks have fledged. In 2022 the ice in the Bellingshausen Sea broke up in November, and satellite images showed that four of the five colonies there lost every chick; the same has happened elsewhere since. Models of the shrinking ice suggest that most colonies could be effectively extinct by the end of the century if warming continues at its current rate, and the emperor was listed as threatened in the United States in 2022 on that basis. The bird that outlasts the worst winter on Earth is not equipped for a shorter one.

The emperor is the exception, not the rule. Of the eighteen or so penguin species, only a few breed in Antarctica, and most live much further north, on the coasts of South America, southern Africa, Australia, New Zealand and the sub-Antarctic islands. The Galapagos penguin lives almost on the equator, kept alive by cold currents welling up from the deep. The African penguin nests in burrows on beaches near Cape Town, and its problem is overheating: it pants in the sun and has bare pink patches of skin above the eyes that help it shed warmth.

Even the Adelie penguin, the other truly Antarctic species, handles winter differently from the emperor. It breeds on the coast in the short summer, raising its chicks in months of near-constant daylight, and then leaves. For the winter, Adelies travel hundreds of kilometres north to the edge of the pack ice, where the sea is open enough to feed and there is still some daylight to hunt by. The emperor's strategy of breeding through the dark, which gives its large, slow-growing chicks time to be ready by summer, is unique among birds.

The emperor's winter breeding made it one of the hardest birds on Earth to study. In June 1911, during Robert Falcon Scott's last expedition, Edward Wilson, Henry Bowers and Apsley Cherry-Garrard set out to walk to the colony at Cape Crozier in near-total darkness to collect eggs, hoping the embryos might shed light on how birds evolved from reptiles. In temperatures that fell close to minus sixty, they spent about five weeks on the journey and brought back three eggs. Cherry-Garrard's account of it is called The Worst Journey in the World.

Today much of the counting is done from orbit. In 2009 researchers showed that emperor colonies can be spotted in satellite images by the brown stain their droppings leave on the white sea ice, and a later census using very high-resolution images counted the birds themselves. It found several colonies nobody had known about and put the total population at close to 600,000 birds, roughly twice earlier estimates. The same method is how the breeding failures in the Bellingshausen Sea were detected: colonies that should have been dotted with chicks simply vanished from the images when the ice broke up early.

Because emperors build no nest and stand in a constantly shifting crowd, they cannot find a partner or chick by returning to a fixed spot, as most birds do. They rely on voice. Each adult has a distinctive call, and the emperor, like its close relative the king penguin, exploits an unusual feature of the bird voice box. The syrinx sits where the windpipe divides into two, and each side can produce sound separately. The two sides sound at slightly different frequencies at the same time, and the interaction between them creates a beating pattern that is individual to each bird.

Experiments in which researchers played recorded calls back to the birds showed how much that pattern matters. When one of the two voices was removed from a recording, partners and chicks largely stopped responding, while penguins that nest in fixed sites, such as the Adelie, rely more on location and call with a single voice. The double voice seems to act as a signature that survives the din of thousands of calling birds and the roar of the wind. A returning female walks into the colony calling, the males answer, and a pair separated for two months can find each other among birds that look, to us, completely identical.

Emperor penguins survive the Antarctic winter with the densest feathers of any bird trapping air against the skin, a thick layer of fat, counter-current blood vessels that keep the flippers and feet just above freezing without losing heat, a large body, and above all the huddle, a slowly rotating mass of thousands of birds that halves each one's energy use while the males fast for two months with an egg on their feet. The whole cycle depends on stable sea ice, which is now failing in the warmest years.