More than 5,000 male emperor penguins huddle on the sea ice, backs to the wind, heads down, sharing body heat. ‘It was a calm day,’ says Stefan, ‘but when I took off my gloves to focus the lens, the cold felt like needles piercing my fingertips.’ Antarctic winters are fierce, with temperatures below minus 40 degrees Celsius.

While the females spend two months at sea feeding, their mates care for the eggs. The male balances his precious cargo on his feet, tucked beneath a fold of skin called the brood pouch. Penguins on the windward edge of the huddle regularly peel off and join the more sheltered side, creating a constant rotation through the warm centre. Survival depends on cooperation.