The Emperor penguins species is the only one that breeds during the Antarctic winter. They trek 50–120 km (31–75 mi) over the ice to breeding colonies that can contain up to several thousand individuals. The female lays a single egg, which is incubated for just over two months by the male while the female returns to the sea to feed; parents subsequently take turns foraging at sea and caring for their chick in the colony.

Both the male and female penguins lose substantial mass while raising hatchlings and incubating their egg. A male emperor penguin must withstand the extreme Antarctic winter cold for more than two months while protecting his egg. He eats nothing during this time. Most male emperors will lose around 12 kg (26 lb) while they wait for their eggs to hatch. The mean weight of males at the start of the breeding season is 38 kg (84 lb) and that of females is 29.5 kg (65 lb). After the breeding season, this drops to 23 kg (51 lb) for both sexes.

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