Emperor Penguin

Diving Where the Blood Runs Cold

Emperor penguins can dive deeper than 500 meters and stay under for more than 23 minutes. One reason is their hemoglobin, the protein that carries oxygen in blood, which holds onto oxygen unusually tightly, like that of high-altitude birds and other penguins. That lets a diving penguin carry more oxygen when blood oxygen runs low and use up more of the oxygen it breathed in before the dive. The blood in its arteries stays almost fully saturated with oxygen through much of a dive, and only drops significantly on the final climb to the surface.

A diving penguin’s wings run cold. In a study of emperors diving from an isolated hole in the sea ice, the average temperature inside a wing vein during dives was about 7.6 degrees Celsius, while the stomach averaged 37.1 and the chest muscle 37.8. Those readings fit a warm core and a cooled outer shell, caused by narrowed blood vessels near the body’s surface (peripheral vasoconstriction), feathers that insulate less during a dive, and heat lost to the water.

Big Birds, Big Weight Swings

Emperor penguins stand 101 to 132 centimeters tall. They gain and lose weight rapidly across the breeding and feeding seasons: around hatching time, adults weigh 22 to 30 kilograms, and before their yearly molt they have to almost double their body mass. Wild emperors typically live 15 to 20 years, though some pass 40.

Silverfish, Squid, and a Spiky Tongue

Emperor penguins are carnivores, eating mostly fish, crustaceans and molluscs from the waters around Antarctica. Which prey is plentiful varies from place to place, but some research has found Antarctic silverfish among their most frequently eaten, with Nototheniidae fish a staple and Antarctic krill and glacial squid eaten regularly. A rough, spiky-textured tongue helps them catch and eat slippery fish and squid. They breed along the Antarctic coastline between 66 and 78 degrees south, where winter temperatures fall as low as minus 40 degrees Celsius with wind chills reaching minus 60. Killer whales and leopard seals have been seen preying on emperor penguins, and skuas are known to steal and eat their eggs and chicks. Most dives are shallow and fairly quick, which makes it harder for predators to reach them, and countershading, a dark back and pale belly, makes swimming penguins harder to make out.

Fasting Fathers of the Ice

Breeding starts just as the Antarctic winter does: penguins arrive at their breeding grounds on the sea ice in early to mid-April. Females lay a single egg in late April or May and pass it to their mate almost immediately, then travel about 50 to 120 kilometers back to open water to feed. Males hold the egg on top of their feet inside a fold of warm skin called a brood pouch, low on the belly, for roughly 65 to 75 days until it hatches. Across the four-month winter fast, a male starts at about 38 kilograms, and the leanest are down to an estimated 18 by the time they head back to sea to feed. To save energy through the fast, males pack into huddles that can hold hundreds of birds at densities of up to 10 per square meter. Chicks hatch in mid-July. Their mother’s return can come before hatching or as much as a month after it, and the father feeds them a nutrient-rich, milky substance he produces in his own esophagus until she is back.