Penguins are flightless seabirds of the southern hemisphere, around eighteen species, which gave up flight in the air for flight underwater. They are not confined to ice: most species live in temperate or even tropical water, and only two breed on the Antarctic continent proper. One of them does so in the middle of the Antarctic winter.
A penguin's wing is a flipper: flattened, stiff, with the joints largely fused so it cannot fold. It is driven by a very large keel and flight muscles, and the stroke generates thrust on both the downbeat and the upbeat, which is the same solution hummingbirds reached in air.
Penguin bones are dense rather than pneumatised. Flying birds have air-filled bones to save weight; penguins reversed this, because a diving animal wants to be less buoyant, not more.
The feathers are short, stiff and overlapping, more like scales than like the feathers of a flying bird, and they are packed densely and coated with oil from the preen gland. Beneath them is a layer of down and then fat. Penguins also run a countercurrent heat exchange in the flippers and legs, so blood going out to the extremities is cooled by blood returning, and heat is not thrown away into the water.
They drink seawater. A supraorbital gland above the eye concentrates salt and drips it out of the bill.
The distribution is southern hemisphere, and the popular association with ice is mostly wrong. Only the emperor and the Adélie breed on the Antarctic mainland. The others occupy the sub-Antarctic islands, the coasts of South America, southern Africa, Australia and New Zealand, and the Galapagos.
There are no penguins in the Arctic, and there never were. The word penguin originally belonged to the great auk of the North Atlantic, a large flightless black-and-white diving bird that was hunted to extinction by 1844; European sailors reaching the southern oceans applied the familiar name to unrelated birds that looked and behaved similarly. The resemblance is convergence, and the original penguin is gone.
The emperor penguin does something no other bird attempts. It breeds through the Antarctic winter, on sea ice, in temperatures below minus forty with winds over a hundred kilometres an hour.
The female lays a single egg and transfers it to the male, then leaves for the sea to feed. The male balances the egg on his feet under a fold of skin and stands there for around sixty-five days in darkness without eating, having already fasted through courtship. By the time the female returns with food, he has been without food for roughly four months and has lost close to half his body weight.
They survive it by huddling. Thousands of birds pack together at densities where the interior of the huddle is warm enough that birds must rotate outward to cool down, and the whole mass moves in slow travelling waves that circulate individuals from the exposed edge to the middle. It is a cooperative solution, and no bird holds a permanent advantage in it.
Penguins are among the deepest-diving birds. Emperors have been recorded to 564 metres and for more than twenty minutes on a single breath.
The physiology is closer to a seal's than a flying bird's. They exhale before diving rather than filling the lungs, tolerate very low blood oxygen, shunt circulation to the heart and brain, and slow the heart to a few beats per minute in the deepest dives. Myoglobin concentration in the muscle is extremely high, storing oxygen where it is used.
Penguins are frequently used as a single indicator of Antarctic change, and the picture does not resolve that neatly.
The emperor's trend is the clearest and the worst. It depends on stable fast ice for the full breeding cycle, and if the ice breaks up before chicks fledge they drown, which was recorded across several Bellingshausen Sea colonies in 2022. Projections under continued warming show severe declines, and the species was listed as threatened under the US Endangered Species Act in 2022 on that basis. Satellite detection of colonies by their guano staining has meanwhile found colonies nobody knew existed, so the baseline itself is still moving.
Adélies and chinstraps diverge. Both have declined sharply on the rapidly warming Antarctic Peninsula, where chinstrap colonies have lost more than half their birds; Adélies in the colder Ross Sea have increased. Krill availability, sea ice extent and competition with recovering whale populations all feature in the explanations, and their relative weight is not settled.
The species in the worst position are not polar at all. The African penguin was uplisted to Critically Endangered in 2024 after losing more than ninety-seven per cent of its historical population, driven by commercial purse-seine fishing removing the sardine and anchovy it depends on, compounded by the historical removal of guano that it used to burrow into for shelter.
The penguin is the most recognisable animal of the southern ocean and one of the most misunderstood, mostly through a persistent association with ice that fits two species out of eighteen. The ones in real trouble are largely the temperate ones, and their problem is fishing rather than temperature.
