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 and packed at up to a hundred per square centimetre, with a layer of down beneath, 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:

• •Feathers packed more densely than in any other bird, 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.

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.