How Do Emperor Penguins Keep Their Eggs Warm? Dads on Duty

Why the egg must never touch the ice

The emperor penguin breeds on sea ice in winter, when air temperatures can drop below −40 °C (−40 °F). There is no nest, no grass and no stones to build one. An egg lying on the bare ice would freeze within a minute or two, killing the embryo inside.

The solution is to carry the egg. Both emperor penguins and their close relatives, king penguins, have a brood pouch: a loose flap of skin on the lower belly that can be lowered over an egg resting on the feet. The skin under the pouch is a bare brood patch, rich in blood vessels, which passes body heat directly to the egg.

The egg handover

The female lays a single egg in May or early June. It is pear-shaped, pale greenish-white, about 12 cm (4.7 in) long, and weighs about 450–470 g (16–17 oz). For a bird of the emperor’s size, that is a small egg, only around 2–3% of the mother’s weight. Its thick shell helps protect it from breaking.

Producing the egg uses up most of the female’s remaining reserves after weeks of courtship on the ice. Within a few hours of laying, the pair carry out a delicate handover. The female rolls the egg from her feet onto the ice and the male quickly scoops it onto his own feet with his bill and covers it with his pouch.

This moment is risky, especially for young, inexperienced pairs. If the egg rolls away or cracks, it is lost, and the pair cannot lay again that season. Once the transfer is done, the female walks back across the sea ice to open water, which can be 50 km (31 mi) or more away, to feed for about two months.

Two months on dad’s feet

The male now incubates the egg for about 65–75 days. He shuffles slowly around rather than walking normally, keeping his toes raised so the egg stays balanced and covered. If he must move, he takes small steps without lifting the egg off his feet.

This happens in the darkest, stormiest part of the year. To survive, the males crowd together into huddles of hundreds or thousands of birds, taking turns between the windy edge and the warm center. Each father keeps his egg on his feet the whole time. Read more about these huddles in how emperor penguins survive the cold.

The emperor is the only penguin in which the male alone incubates. By the time the egg hatches, he may have gone about 115–120 days without food since arriving at the colony, losing up to around 40% of his body weight.

How emperors compare with other penguins

In most penguins both parents take turns sitting on the eggs, usually in a nest of stones, pebbles, grass or a burrow. Most species also lay two eggs, while the emperor and king penguins lay just one.

The king penguin also balances its egg on its feet in a brood pouch, but the parents swap every one to three weeks so each can feed at sea. Only the emperor relies on a single, months-long fast by the father.

Hatching and the first meal

Hatching can take two to three days because the shell is thick. A newly hatched chick weighs only about 315 g (11 oz), has thin gray down and cannot keep itself warm. It stays on the father’s feet inside the pouch.

If the mother has not yet returned, the father can feed the chick a protein- and fat-rich secretion produced by a gland in his esophagus. This “crop milk” can sustain the chick for a few days. Only a handful of birds produce such a food, including pigeons and flamingos.

The mothers come back

Females return between hatching and about ten days afterward, usually in July or early August, fat and with a stomach full of food. A female finds her mate among thousands of identical-looking males by his call. She takes over the chick, transferring it from his feet to hers, and starts feeding it with regurgitated fish, squid and krill.

The thin, hungry father then leaves for the sea, where he feeds for about three to four weeks. After that, the parents take turns: one guards and broods the chick while the other forages.

From pouch to crèche

The chick spends roughly its first six to seven weeks on a parent’s feet. When it is too big for the pouch, it joins other chicks in a crèche, a group that huddles together for warmth while both parents fetch food. Parents locate their own chick by voice when they return.

Chicks molt into juvenile plumage from November and head to the sea in December or January, at about five months old. Pairs usually change partners the next year; see do penguins mate for life? for how pair bonds differ between species.