Why Can't Penguins Fly?

On a beach, a gentoo penguin looks like a design error. It rocks from foot to foot on stubby legs, flippers held out for balance, and every few steps it seems about to tip over. Follow the same bird into the sea and the impression flips. It accelerates to thirty-six kilometres per hour, faster than any other bird ever measured beneath the surface and quicker than most fish of its size.

That contrast holds the real answer to why penguins cannot fly. Their ancestors had flight. The lineage spent it, one bone and one feather at a time, on becoming the most capable diving birds on Earth.

The short answer

Penguins cannot fly because their bodies were rebuilt for swimming. Roughly sixty million years ago their ancestors were still flying seabirds, but life in the water stiffened the wing into a rigid flipper, packed the bones dense so the bird could sink, and compressed the feathers into a tight waterproof coat. Those three changes make a superb underwater engine and a hopeless flying machine, so a penguin still beats its wings, only now it does it in the ocean.

Penguin ancestors really could fly

Penguins descend from seabirds that flew, and the skeleton records the switch without ambiguity. Inside every flipper sits the same arm and hand structure a gull uses to stay aloft, flattened and locked stiff. Somewhere around sixty million years ago, the ancestors of today's penguins began spending more of their lives chasing fish underwater. From then on, natural selection paid for diving performance instead of lift, and every generation that swam a little better left more descendants than the ones that flew a little better.

Air and water make opposite demands on a body. Air is thin, so a flier needs to be light and needs broad, flexible wings to catch enough of it. Water is thick and pushes a floating body back up, so a diver needs weight and a short, stiff blade to push against it. Penguins committed fully to the second set of demands.

Three changes that traded the sky for the sea

The sea rewrote the penguin body in three moves. Each one made the bird a better swimmer and moved flight further out of reach.

The wing became a flipper

A flying wing flexes, folds, and relies on long flight feathers to hold the bird up. The penguin version is a short, rigid paddle. Its joints have stiffened so the whole limb acts as a single blade, which is exactly what you want for levering against water and exactly wrong for staying airborne.

Hollow bones turned solid

Flying birds save weight with light, air-filled bones. Penguins went the other way and packed their bones dense. That extra mass works like a diver's weight belt. A bird whose lungs and plumage hold air would otherwise bob at the surface, and heavy bone helps it sink and stay down. For a flier, the same skeleton would be a crippling load.

Feathers shrank into a waterproof shell

Penguin feathers are short, stiff, and overlapping, and an emperor carries seventy to a hundred of them in every square centimetre, an area you could cover with a fingernail tip. The outer surface sheds water like a shell. Beneath it, a thin layer of trapped air insulates the skin. It is a coat engineered for the ocean, and it has none of the long, broad feathers a bird needs to generate lift in air.

How a penguin flies underwater

Underwater, the flipper does the job a wing does in air, and it does it on both halves of every beat. The downstroke drives the bird forward, and the upstroke drives it forward again, so there is no wasted recovery stroke. The feet stay tucked flat against the body while the bird cruises, keeping the outline smooth, and flare out as rudders when it needs to turn.

The result is a range of performance tuned to each species' way of hunting:

• Adélie penguins cruise at a steady five to eight kilometres per hour, with short bursts near twenty-five when chasing prey.
• Gentoo penguins hold the underwater speed record for any bird, at thirty-six kilometres per hour.
• Emperor penguins trade speed for depth, with a confirmed dive to 565 metres.

How deep and how long can a penguin dive?

The emperor's record dive went down 565 metres and lasted twenty-two minutes, and no other bird has been recorded going deeper. Holding a breath that long takes a complete rewrite of the oxygen budget, and the emperor manages it in three ways.

First, its blood is unusually rich in haemoglobin, the oxygen-binding protein, so the bird dives with a larger store on board. Second, its heart slows dramatically. A pulse of roughly two hundred beats a minute at the surface can fall to as low as fifteen on a deep dive, which cuts how fast that store is spent. Third, the bird rations what is left. Limb muscles are allowed to run into oxygen debt on purpose, working on their own reserves while the oxygen in the blood is held back for the two organs that cannot go without it: the heart and the brain.

Why do penguins waddle?

The waddle is the bill for all that engineering. A body shaped to slip through water carries its weight high on very short legs. Standing upright on land, the bird can only take short steps, and it rocks its whole body from side to side to swing each foot forward. What looks clumsy on the ice is the visible cost of a streamlined swimmer, paid every time the bird walks home from the sea.

Common questions

Are all penguins flightless?

Yes. All eighteen living species are flightless, and all of them are wing-propelled divers that use the flipper stroke described above.

Do penguins only live in Antarctica?

Every penguin species is native to the Southern Hemisphere, but their range runs from the deep Antarctic to the equator. The Galápagos penguin sits almost on the equator, and its daily struggle is shedding heat. The emperor breeds in winter darkness at temperatures that reach minus sixty with wind chill. Most species live between those extremes, along cool coasts in South America, South Africa, and New Zealand, and on the islands that circle Antarctica.

Key terms

Flipper

A penguin's wing, flattened and stiffened into a rigid paddle that pushes against water on both the downstroke and the upstroke.

Haemoglobin

The protein in red blood cells that binds oxygen. Emperor penguins carry more of it, which stretches each breath across a longer dive.

Oxygen debt

A state in which muscles keep working after their oxygen runs short, leaving a deficit the animal repays once it can breathe again.

Related field notes

For how the same body copes with cold, salt, and crowded colonies, browse more Penguins Decoded field notes.

This article is adapted from "The Bird That Walked Into the Sea," the opening chapter of Penguins Decoded, which follows the whole rebuild from flying ancestor to the fastest bird underwater.