Penguins are best known for their swimming, rock hopping and all around adorableness, but people forget that they are birds and they have wings. Why can't they fly? Scientists reasoned that the emperor penguins of the Antarctic could have made their yearly trek much faster if they flew rather than marching for days, with many penguins dying on the way. Penguins would be much safer from their primary predator the leopard seal who will often snatch an unsuspecting penguin right off the water's edge.
It's long been thought that the penguins' environment was responsible for their wings evolving to better suit streamlined diving rather than flight, because so much of their food source lay deep in the frigid water. Some of today's modern penguin species can hold their breath underwater for up to twenty minutes and dive to depths of 1500 feet, making them excellent fishers.
A study that examined energy costs in living birds that both fly and dive for food has shed light on how penguins lost their ability to fly.
"Clearly, form constrains function in wild animals, and movement in one medium creates tradeoffs with movement in a second medium," study co-author Kyle Elliott, of the University of Manitoba, said in a statement.
Researchers looked to penguinlike seabirds in the Northern Hemisphere that still use their wings to dive and to fly. The team examined thick-billed murres at a colony in Nunavut, Canada, and pelagic cormorants at Middleton Island, Alaska. Outfitting them with location trackers and measuring their energy expenditure with injections of tracer isotopes, which are variations of an element with different numbers of neutrons. The cormorants can be considered biomechanical models for the lifestyle energy use of an ancient penguin ancestor that was the last of its line to take flight. The team also fitted them with time-budget devices that track those activities-recording movements, speeds, and other data much like pedometers do.
"Basically the birds do only three things: sit, swim, and fly. So by measuring lots of birds and combining their time budgets with the total costs of living from the isotope measures, it is possible to calculate how much each component of the budget costs," explained study co-author John Speakman, who leads the Energetics Research Group at the University of Aberdeen, Scotland.
It is believed that today's penguins evolved into birds that excelled in diving, but lost their ability to fly as result.
"This would involve a progressive reduction in wing size, which makes diving more efficient and flying less so. Penguin bones also thickened over the ages, as lighter bones that make it easier for birds to fly gave way to more dense bones, which may have helped make them less buoyant for diving." Speakman believes the wing changes were the primary adaptation.
"What we do know is that in the radiation of the mammals after the K-T event, there suddenly [in geological terms] appear a whole load of mammals that would have been serious competitors for aquatic resources [like] cetaceans and pinnipeds," Speakman said. The K-T event is thought to be an asteroid impact that raised radiation levels and eliminated about 70% of species on Earth. After this event penguins may have needed their diving skills more than their ability to fly.
"So this new competitive environment may have placed a greater benefit on being more efficient swimmers and divers for aquatic seabirds. That push toward being more efficient in the aquatic environment may have been enough to tip them over the edge into flightlessness."
The results of the study were published in the journal Proceedings of the National Academy of Sciences.
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