The ability to fly is an adaptive trait in birds, but at least five lineages of seabirds have lost this skill over time, penguins among them.
It was previously thought that as many flightless birds live in areas with few predators and low food supplies that they needed to direct their energies towards collecting food from the sea - so evolving to dive rather than fly.
But birds like penguins also have to travel huge distances between their feeding and breeding grounds (a task much more easily achieved by flight); and some flightless diving seabirds live in areas with good food supplies. So Kyle Elliott and the team from the University of Manitoba in Canada thought there must be another reason why birds would evolve to lose this useful skill.
They looked at the energy usage of two types of birds, thick-billed murres who use their wings to propel themselves through the water and Pelagic Cormorants who use their feet.
The paper, published in PNAS this week, revealed that the thick-billed murres had an exceptionally high energy expenditure when flying (around 31 times their metabolic rate at rest), much higher than the Cormorants who are less specialised to dive.
They concluded that, by evolving to be good at diving owing to a decreased wingspan, larger wing bones, an increased body mass and muscles specialised to beat at low frequencies, the energy expenditure required for flying increases. They therefore hypothesised that there is a fitness valley between seabirds specialised to fly and dive and at a certain point if a bird specialises for diving too much they lose the ability to fly, just like penguins.