What if penguins could fly?

Penguins are birds. They have wings and feathers, like other birds.

But penguins do not fly in the sky. What if they could? What would a flying penguin need?

Penguins are birds, with feathers, beaks and wings, yet not one of them can fly. Instead, they zoom through the ocean. The Smithsonian Ocean team writes that penguins spend up to 75 percent of their lives in the water.

So what if a penguin could fly? In this thought experiment, you can explore penguin wings, penguin bones, and a seabird that does both jobs, then decide what a flying penguin might gain and lose.

Penguins (order Sphenisciformes) are flightless seabirds that spend up to 75 percent of their lives at sea, according to the Smithsonian's Ocean portal. Their wings still exist, but as stiff flippers.

Why would a bird give up flight? Flight is costly to build and to run, and the body plan that suits air is not the one that suits water. This thought experiment uses real measurements of penguins and of murres, which both fly and dive, to ask what a flying penguin would have to give up.

On land, penguins waddle. Their legs are far back on their bodies.

In the water, they are fast! Their wings are flippers. They flap them to swim.

On land, penguins waddle, because their legs and feet sit far back on their bodies. Underwater, those same feet steer like rudders while the flippers push.

The Smithsonian says the emperor penguin can reach 14 kilometers per hour when it hurries, and most midsize penguins swim at about 8. On average, penguins dive 9 to 18 meters deep, but king penguins often go to 91 meters and emperor penguins can reach about 518 meters.

Penguins have torpedo-shaped bodies, legs set far back that act as rudders underwater, and flippers that produce thrust on both the downstroke and the upstroke, unlike flying birds, which rely on the downstroke.

The Smithsonian gives swimming speeds of up to 14 km/h for emperor penguins (they prefer about 11), about 8 km/h for midsize species and 1.5 km/h for little penguins. Typical dives reach 9 to 18 m; king penguins often reach 91 m, emperors about 518 m, and a 2018 study recorded an emperor penguin dive of over 32 minutes.

To fly, a bird needs long wings and a light body.

Penguins have short, stiff wings and heavy bones. A flying penguin would need a very different body!

Flying birds have long wings and light bones with air spaces inside. Penguins have short flippers and heavy, solid bones. To fly, a penguin might need longer wings, lighter bones and a smaller body.

But every one of those changes might make it a worse diver. Light, air-filled bones might make it float more easily. Long wings are hard to push through water, which is much thicker than air. A flying penguin might not reach the deep places where emperor penguins find their food today.

Scientists studying diving seabirds describe a tradeoff: features that help in water work against flight. Penguins evolved shorter wingspans, larger wing bones, bigger bodies and more oxygen-storing myoglobin in their muscles, according to Elliott and colleagues (2013).

A flying penguin would need the reverse: longer, lighter wings and a lower mass. The study notes that wing-propelled diving birds are flightless above about 1 kilogram and able to fly below it. The largest auk and the smallest penguin both weigh about 1 kilogram, so a flying penguin might have to be smaller than any penguin alive today, and a lighter, flying version might dive less deeply and less cheaply.

Flying uses lots of energy. Swimming with flippers uses less.

Penguins are very good at swimming. That is how they catch their food.

Murres are seabirds that fly through the air and also use their wings to swim underwater. In 2013, scientists measured how much energy murres use. Flying cost them more energy than scientists had ever recorded for an animal with a backbone.

Diving was cheap for murres, but still about 30 percent more costly than for a penguin of the same size. A wing that is good for diving makes flying very expensive. Scientists think that is one big reason penguins' ancestors gave up flying and became expert swimmers.

Elliott, Ricklefs, Gaston and colleagues measured free-living thick-billed murres in Nunavut and pelagic cormorants in Alaska. The murres' flight cost, about 141 W for a 1 kg bird, was the highest recorded for any vertebrate, and murres have the highest wing loading of any flying bird.

Murre dives cost about 30 percent more than expected for a similar-sized penguin. The authors argue this supports the biomechanical hypothesis: a wing that works well in water makes flight very costly, so large wing-propelled divers lose flight. They also note that flightlessness has evolved in at least five lineages of seabirds.

Puffins can fly. They can also swim underwater with their wings!

Puffins are small and light. Penguins are bigger and better at diving.

Some seabirds do both jobs. The Cornell Lab of Ornithology says Atlantic puffins fly underwater, using their wings to push and their feet to steer, and they can dive to about 60 meters. Puffins weigh only about 310 to 550 grams.

The 2013 study points out that the largest auk, the family that includes puffins and murres, and the smallest penguin both weigh about 1 kilogram. Lighter wing-swimmers can still fly; heavier ones cannot. So the real world already has small, flying penguin look-alikes, and large, deep-diving penguins.

Auks (puffins, murres, razorbills) are the Northern Hemisphere's wing-propelled divers. Cornell's All About Birds notes that Atlantic puffins 'fly' underwater using their wings for propulsion and feet as a rudder, reaching around 200 feet.

Elliott and colleagues place flying auks and flightless penguins on either side of a 'fitness valley': living and fossil wing-propelled divers are flightless above about 1 kg and fly below it. The great auk, about 5 kg, was a flightless auk. The study suggests that losing flight was followed by the evolution of larger bodies, which dive longer and more efficiently.