For anyone who has sat through March of the Penguins, a documentary film following the arduous journey that Emperor penguins make between the ocean and their ancestral breeding ground, and wondered, ‘Why can't penguins just make life easier for themselves and fly?’, a recent paper published in PNAS should help to answer the gnawing question. An international team of researchers set out to understand why certain seabird species may have evolved in such a way that a journey through the air (taking hours) would be deemed less advantageous than a journey on land (lasting several days).

The team hypothesized that certain seabirds may have lost the ability to fly as a trade-off with another activity. For example, moving between air and water, flying and diving, requires optimized wing designs – perhaps a wing good for diving is a terrible wing for flight, and vice versa. The scientists wondered whether, as flightlessness evolved in certain lineages of seabirds, such as in penguins, their wings became more biomechanically adapted to swimming and diving and less and less able to sustain flight. Using the thick-billed murre (Uria lomvia) and the foot-propelled pelagic cormorant (Phalacrocorax pelagicus), two diving seabird species that are also able to fly and are considered counterparts to ancient ancestors of penguins, the scientists were able to test this biomechanical hypothesis and to compare flying seabirds with flightless seabirds.

The murre and pelagic cormorant are similar to penguins in their diving and swimming behavior, so the research team decided to measure the energy cost of flight as well as the energy cost of diving in these birds. During the summer of 2006 and 2012, the researchers captured 41 thick-billed murres in Canada and 22 pelagic cormorants in Alaska, respectively, and fitted them with specialized sensors that measure the birds' energy expenditure at the same time as following their diving and flight behavior using data loggers. Both species dive underwater to hunt for prey – the thick-billed murre are wing-propelled divers, while the pelagic cormorant are foot-propelled drivers – yet both are considered only adequate fliers. The team was able to measure how much energy the birds need to expend during flight and during swimming dives and found that the birds used more energy to fly compared with any other known bird and used more energy to swim compared with penguins of similar size. Furthermore, the authors found that the ‘flight cost’ for the thick-billed murre and pelagic cormorant is the highest recorded so far for vertebrates.

Just as flight has been shown in many instances to have evolved independently, so too has flightlessness. The authors suggest that flightlessness in penguins evolved as a result of biomechanical pressures on the wings. In order to become expert divers, they argue, penguins gave up the ability to fly in favor of increasing dive endurance. As seals, sharks, sea lions and killer whales all pose a risk to diving penguins, a wing designed for aquatic expertise becomes a must. On land, penguins have less predation to fear – a wing designed for flight is less beneficial. Thick-billed murres and pelagic cormorants, in contrast, have many land-based predators, including foxes and wolves, leading researchers to suggest that the pressure to maintain wings optimized for flight in these species is strong. The authors conclude that there must be a compromise in wing design – a wing cannot excel equally well in both air and water – and that trade-offs are inevitable.

K. H.
R. E.
A. J.
S. A.
J. R.
G. K.
High flight costs, but low dive costs, in auks support the biomechanical hypothesis for flightlessness in penguins
Proc. Natl. Acad. Sci. USA