What do penguins eat?

An Emperor Penguin can dive to 565 metres. He can hold his breath for up to 22 minutes. He hunts in water around -1.8 C, the freezing point of seawater. The mass of fish, krill and squid he consumes in a working day is roughly two to three kilograms, which he metabolises into both his own survival and into a crop-stored slurry he later regurgitates to feed a chick that has not seen its mother in two months.

The penguin diet is not a list of foods. It is the most extreme underwater hunting routine of any bird, performed daily, in conditions that would kill almost anything else.

Diet by species

Species	Primary food	Notable behaviour
Emperor Penguin	Antarctic silverfish, krill, squid	Deepest diving bird on the planet
King Penguin	Lantern fish, squid	Builds fat reserves on squid through winter
Adelie Penguin	Antarctic krill, small fish	1 to 2 kg of krill daily
Chinstrap Penguin	Krill, small fish	Pursuit-dives in coastal waters
Gentoo Penguin	Crustaceans, fish, squid	Most opportunistic feeder, diet shifts seasonally
Macaroni Penguin	Krill, fish, cephalopods	Single species eats more biomass of marine prey than any other seabird
Magellanic Penguin	Cuttlefish, squid, krill	Has a salt-secreting supraorbital gland
Humboldt Penguin	Anchovy, sardine, hake	Vulnerable to El Niño disruptions in fish populations
Yellow-eyed Penguin	Red cod, opal fish, sprat, squid	Population collapse correlated with overfishing
Galapagos Penguin	Sardines, anchovies	Only penguin in the Northern Hemisphere

The Macaroni Penguin line is worth dwelling on. There are approximately 18 million Macaroni Penguins. Each eats around a kilogram of krill and small fish per day. The species alone consumes more marine biomass than any other seabird in the world. The penguins of the Southern Ocean are not bystanders in that ecosystem - they are a significant fraction of the predator pressure on its smaller fish and crustaceans.

The dive

The dive is the part that makes penguins different. A Mallard duck dives 4 metres. A loon dives 60. An Emperor Penguin dives 565.

To do this, the bird has solved several problems that vertebrates usually cannot.

• Oxygen storage. Penguins have unusually high concentrations of myoglobin in their muscle tissue (about 30 times that of a chicken). The muscle itself is a battery.
• Bradycardia. During a dive, heart rate drops from around 175 beats per minute to roughly 15. Blood is shunted away from non-essential organs.
• Reduced metabolic rate. Body tissues other than brain and heart receive less oxygen and do less work during the dive.
• Pressure tolerance. At 500 metres the pressure is 50 atmospheres. Penguin air sacs and lungs compress without rupturing. Nitrogen dissolves into tissue without producing the bends, partly because the air supply is finite and partly because the bird exhales before descending.
• Cold tolerance. A counter-current heat exchanger in the flippers and feet keeps the bird’s core warm while the extremities run close to ambient water temperature.

The result is a working dive cycle that looks, from above water, like a duck. From below it is more like a submarine. The flippers that make them the only birds that cannot fly but swim like this are the reason they hunt where they do.

The feeding strategies that are not just fish

Three species deserve their own paragraph.

Bearded - wait, wrong list. Let me restart that one.

Emperor Penguin males fast for 115 days. The breeding cycle is structured so that the female lays a single egg, transfers it to the male, and walks back to the sea to feed for two months. The male incubates the egg on top of his feet under a fold of belly skin in temperatures of -40 C. He does not eat. He metabolises his body fat. When the female returns and regurgitates squid into the chick’s mouth, the male has lost roughly 45 per cent of his body weight and walks back to the sea himself.

Adelie Penguins steal nest stones. Adelies build pebble nests on bare rock. Pebbles are surprisingly valuable - a good pebble has been used for years and is well-shaped. A 1998 paper by Hunter in the journal The Auk documented Adelie penguins explicitly stealing pebbles from neighbours’ nests when the neighbour was looking elsewhere. The pebble economy has its own moral structure.

Gentoos dive in coordinated groups. Pursuit-dive together in a shoal of fish, herd the fish into a ball, and take fish from the edge of the ball. Cooperative hunting in penguins is documented but rare. Gentoos appear to be the species that does it most.

What zoo penguins eat

Captive penguins eat dead fish - herring, capelin, smelt, anchovies - hand-fed once or twice a day. The fish is supplemented with vitamins, particularly thiamine, because frozen fish degrades the vitamin during storage. A medium penguin eats roughly 0.5 to 1 kg of fish per day in captivity, less than wild birds because activity levels are lower.

The smell of a penguin enclosure is distinctive. It is the smell of fish, guano and seawater, in roughly equal parts.

Why this should matter to a non-penguin person

Penguins are an underestimated bellwether for the health of the Southern Ocean. Their numbers track the abundance of krill and small fish, which track the productivity of the polar food web. The current rate of Antarctic krill decline - linked to warming sea-surface temperatures and reduced sea ice - is a problem for several penguin populations, particularly Adelie and Chinstrap, whose breeding success in the West Antarctic Peninsula has dropped sharply since 1980.

The penguin in the David Attenborough documentary is also the gauge needle on the world’s largest marine ecosystem. The needle is moving.