Researchers from the University of Manitoba have shown that birds can either be very good at flying or swimming, but not both. And they’ve been studying a very awkward seabird to prove it.
Animals that can fly really have it good. Flight allows for quick getaways, aerial view hunting, expanded territorial ranges, and the ability to travel vast distances when making seasonal migrations.
So, for a species to give up flight, there better be a damned good reason.
And indeed, the penguin did exactly such a thing about 70 million years ago, foregoing flight in favor of swimming. Scientists aren’t really sure why penguins made the switch, but they suspect it had something to do with a lack of land-based predators.
But a new theory, the biomechanical hypothesis, suggests that nature cannot provide a seabird with a wing that is proficient at both flying and swimming. For penguins, selectional pressures eventually tipped over in the direction of swimming, resulting in a vestigial wing that could propel the bird not through air, but through water. As a consequence, flight had to be completely abandoned lest the species remain mediocre at both — a condition that nature, with its preference for fitness peaks, will not endure.
To reach this conclusion, a research team led by Kyle Elliott studied two birds that are closely related to the penguin, the thick-billed murres (Uria lomvia), which are wing-propelled divers, and pelagic cormorants (Phalacrocorax pelagicus) which are foot-propelled divers. The murres proved to be particularly interesting — it’s a bird that’s reasonably good at swimming, but absolutely dreadful at flying.
Evolutionarily speaking, the murres is not quite ready to give up on flight just quite yet — but it’s not too far off, either. Without a doubt, the murres is one of the most awkward flying seabirds to ever be documented by scientists. In order to fly, it must beat its wings like crazy, expending energy at 31 times their resting state. Most animals burn energy at about 25 times their resting rate when going full-tilt.
The murres is also really terrible at landing.
At the same time, they’re so-so swimmers. Compared to penguins of the same size, the murres has to expend about 30% more energy. The scientists theorize that the dual-purpose feathered wings are causing too much drag for the bird underwater. The penguins' smooth and stubby flippers, which propels them through the water, is highly specialized for the task.
The murres, on the other hand, needs a wing that’s good at both, making specialization impossible. This bird is obviously well suited to its environment right now, but like the penguin, selection pressures may eventually tip it towards one of two possible directions.
Read the entire study at PNAS: “High flight costs, but low dive costs, in auks support the biomechanical hypothesis for flightlessness in penguins.”
Images: Kyle H. Elliot; inset: Christian Musat.