Why Can't Penguins Fly? Trading Flight for Swimming

Penguins traded the sky for the sea

Every bird faces a basic trade-off. Wings that are good for flying in air are large and light, while wings that work well underwater are small, firm and narrow, because water is about 800 times denser than air. A wing cannot be excellent at both.

Penguins took the underwater option completely. Their flippers are true wings, containing the same upper arm, forearm and hand bones as other birds, but the bones are flattened and broad, and the elbow and wrist joints barely bend. The result is a rigid paddle that the bird moves from the shoulder. Short, scale-like feathers cover the flipper instead of long flight feathers.

Penguins beat these flippers up and down much as a flying bird beats its wings, and they push against the water on both the upstroke and the downstroke. That is why their swimming looks so much like flight in slow motion.

Heavy bones and heavy bodies

Most flying birds have hollow, air-filled bones that keep their weight down. Penguins have dense, solid bones instead. Extra weight would be a burden in the air, but in the water it acts like a diver’s weight belt, reducing buoyancy and making it easier to dive deep.

Size matters too. An emperor penguin weighs 22–45 kg (49–99 lb) and a king penguin 9.3–18 kg (21–40 lb). Very few flying birds weigh more than about 15–20 kg (33–44 lb), and even they need large wings and long take-off runs. A big body is useful in cold water, because it loses heat more slowly and can store more oxygen and fat for long dives, but it rules out flight with small flippers.

What studies of diving seabirds show

Scientists have tested the trade-off by studying birds that still do both: the auks of the Northern Hemisphere, such as murres and puffins. Auks fly in the air and also swim underwater with their wings, but their wings are a compromise, short and stiff for diving.

A 2013 study by Kyle Elliott and colleagues, published in the Proceedings of the National Academy of Sciences, measured the energy used by thick-billed murres. Flying turned out to be extremely expensive for them, among the highest costs measured for any vertebrate, while diving was comparatively cheap. The researchers concluded that once wings become small enough for very efficient diving, flying becomes so costly that giving it up is the better option. This supports the idea that the ancestors of penguins passed the same tipping point.

The extinct great auk of the North Atlantic shows the same pattern. It was a large, flightless auk that swam with its wings, and it was the original bird called a “penguin” before the name was given to the southern birds.

Penguins came from flying ancestors

Penguins descend from flying seabirds. Genetic studies suggest their closest living relatives are the tubenoses, the group that includes albatrosses and petrels. However, even the oldest penguin fossils known, such as Waimanu from New Zealand, about 60–62 million years old, were already flightless and had wings adapted for diving. The flying “proto-penguin” has not been found.

One reason flightlessness could work for penguins is that most of them breed on islands and coasts in the Southern Hemisphere with few land predators. In the Arctic, where foxes and polar bears roam, the auks that fill a similar role have kept the ability to fly.

How fast and how deep penguins swim

Giving up flight paid off in the water. Most penguins cruise at about 6–10 km/h (4–6 mph), fast enough to cover long distances efficiently. The gentoo penguin is usually described as the fastest, with short bursts of up to about 36 km/h (22 mph). Penguins also “porpoise”, leaping out of the water as they swim so they can breathe without losing speed.

The emperor penguin is the deepest-diving bird known. You can read about its records in how deep penguins can dive.

Penguins and other flightless birds

Penguins are among roughly 60 living bird species that cannot fly. Most of the others, such as ostriches, emus, kiwis and the kakapo, lost flight on land, usually where there were few ground predators or where running was more useful. Penguins are unusual because they lost flight in exchange for a different kind of flying. Meet the others in our list of flightless birds.