What structure lets a male emperor penguin keep an egg warm on his feet?

A male emperor penguin keeps an egg warm on his feet beneath a brood pouch.

After the female lays a single egg, she carefully transfers it to the male. He balances the egg on top of his feet and covers it with a fold of warm abdominal skin called the brood pouch. This prevents the egg from touching the freezing ice.

The male remains with the egg during the Antarctic winter while the female travels to sea to feed. When she returns, the parents exchange care of the chick, allowing the male to go to sea after his long fast.

The brood pouch is not a separate external organ like a bird’s crop. It is a specialized skin fold that works with the feet and body heat to protect the egg.