Why Don’t Penguins Fly?
by James Platt
Most of the world’s bird species have the capability of flight. In fact, of 11,000 known species of birds, there are only about 60 species including the Common Ostrich (Struthio camelus), Great Spotted Kiwi (Apteryx haastii) and 18 species of penguin that cannot fly at all, which is about 0.5% of all bird species. Why these different species evolved without flight could be due to several reasons; each one will have evolved to fit into a niche within its own environment.
Penguins were originally thought to have evolved separately from flightless birds, until quite recently when fossil records from New Zealand were discovered. These fossils revealed that they had likely descended from the order of Procellariiformes and its closest relatives including the Wandering Albatross (Diomedea exulans) and Antarctic Petrel (Thalassoica antarctica). This could be considered unexpected since the albatross is a bird that travels huge distances in the air. So why would a bird that has massive wings and uses them to glide across continents suddenly evolve into a flightless, chubby penguin?
Photo credit: Linda Martin
Why did penguins evolve to swim instead of fly?
To understand why they may have evolved in this way, first we must understand flight. Flight in birds is a tricky thing; it is a perfect balance between forces (lift, thrust, gravity and air resistance) that allow the bird to move through the air quickly and efficiently. Birds evolved hollow bones to allow them to be lighter and, therefore, lift off the ground much easier. They also have air sacs built into their body to keep a streamlined aerodynamic body shape that allows them to reduce air resistance. Birds need to stay light because the heavier they are, the more difficult it is to take off from the ground (Tobalske, 2007). In the case of some flightless birds like the ostrich, which weighs over 100kg, its wings would have to be huge to get it off the ground. Instead, they are incredibly fast runners. Similarly, with Emperor Penguins (Aptenodytes forsteri) that can weigh about 25kg, it would take large wings to fly and would not be very energy efficient. So they evolved to “fly” in the water instead. Now some penguin species have branched off and become much smaller and lighter than the Emperor Penguin, such as the Little Penguin (Eudyptula minor) at just 1.5kg, but by this time they were a fully distinct order of birds and had adapted to dominate the water.
It is believed that the Emperor Penguin is the oldest species of penguin and therefore was the first bird to try to dominate the ocean and land on the continent. But why in such a cold, harsh environment, and why would it become flightless? Well, it’s complicated and there could be many other reasons. There is a total lack of land predators which means they don’t have any immediate threats to fly away from. There is also an abundance of sea life to eat and a lack food resources on land, so they adapted to thrive off the oceans and then live on land, away from their biggest predator, the leopard seal (Hydrurga leptonyx). There are also many benefits to being flightless. Penguins have the opposite to most birds, they have incredibly dense bones that allow them to dive and swim better. Where most birds would have air sacs to stay aerodynamic, penguins can fill some of that extra space with a larger stomach and carry much more food for itself and its offspring — up to a 1/3 of its bodyweight. They can also dive much deeper than flying seabirds they may be in competition with and this allows them to make the most of what their environment has to offer (Alexander, 1999).
Part of the reason penguins swim is because flying is an energy-intensive activity
One more reason they may have lost the ability to fly is that flying is an extremely high energy activity and they need all the energy they can retain to stay warm (Elliott et al, 2013). Most birds use their energy for flying and the bird that is best at conserving its energy is the penguin’s closest relative, the Wandering Albatross. It uses wind to extend its glide times and allow it use as little energy as possible during its migration. It seems that this wasn’t energy efficient enough for some individuals and they evolved into penguins over the millennia as swimming is much more efficient because there aren’t as many forces to contend with (Culik and Wilson, 1991).
The evolution of the penguin and its loss of flight is far from a complete story and I suspect we will find out more in the coming decades as more fossils are uncovered. Leave your thoughts in the comments!
Isn’t it nice to learn why not all birds fly? Some of us might assume that just because something has wings they won’t always stay grounded.
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Culik, B., Wilson, R. and Bannasch, R., 1994. Underwater swimming at low energetic cost by pygoscelid penguins. Journal of Experimental Biology, 197(1), pp.65-78.
Alexander, R. (1999). One price to run, swim or fly?. Nature, 397(6721), pp.651-652.
Culik, B. and Wilson, R. (1991). Energetics of under-water swimming in Adelie penguins (Pygoscelis adeliae). Journal of Comparative Physiology B, 161(3), pp.285-291.
Elliott, K., Ricklefs, R., Gaston, A., Hatch, S., Speakman, J. and Davoren, G. (2013). High flight costs, but low dive costs, in auks support the biomechanical hypothesis for flightlessness in penguins. Proceedings of the National Academy of Sciences, 110(23), pp.9380-9384.
Tobalske, B. (2019). Biomechanics of bird flight. Journal of experimental biology, [online] 210(18), pp.3135-3146. Available at: https://jeb.biologists.org/content/210/18/3135.short [Accessed 8 Jul. 2019].