Penguins stopped flying

Penguins stopped flying, A team of biologists has published evidence supporting the biomechanical hypothesis that penguins lost their ability to fly because no bird can be both an expert diver and flier.
"Biomechanical models of energy costs during flying and diving suggest that a wing designed for optimal diving performance should lead to enormous energy costs when flying in air," explain the authors, in a paper published in the journal Proceedings of the National Academy of Sciences.
The widely-held theory is mainly only countered by an opposing hypothesis that flightless birds thrived in areas where they were no longer under threat by predators, and thus did not need a quick escape route. It's of course entirely possible the answer was an evolutionary combination of the two, but a team studying diving birds believes it has some of the most solid empirical evidence that the biomechanical hypothesis rules.
The team studied two species, the pelagic cormorant and the thick-billed murre. While both dive underwater, the former uses its webbed feet while the latter flaps its wings. Neither bird is the best flyer. The murre in particular has a gait that doesn't afford it the most elegant takeoff -- rather, it has to flap its small wings at great speeds to remain airborne, as is seen in this video (one minute in).
To measure just how much energy the birds need to expend to remain airborne and to dive, the biologists fitted the murres with recording devices to measure the time, depth and temperature of dives, while the cormorants were fitted with data-loggers to account for acceleration changes, depth and temperature, reports Nature. The biologists also measured how much carbon dioxide they expelled. They found that the birds used more energy to fly than any other known bird, while energy expenditure in diving was far higher than in penguins, relative to size. The birds only managed to gain some semblance of speed once they got fairly deep into the water, as their wings became more streamlined and created less drag.
"We show that flight costs for thick-billed murres, which are wing-propelled divers, and pelagic cormorants (foot-propelled divers), are the highest recorded for vertebrates," write the authors.
Because the birds live in cold climates and are constantly in the water, it's unclear if a large amount of this energy burn-off is in fact due to the birds trying to keep warm. They do not weigh as much as penguins, and have to burn energy more rapidly to stay warm. It's an area to follow-up on, to clear up any hint of a suggestion that might detract from the biomechanical hypothesis, but it looks like the team of ornithologists has come close to finding evolutionary proof for why penguins lost their ability to fly.
Neither an expert diver, nor an expert flyer, murres in particular look to be on the precipice of evolutionary change. Their wings are incredibly strong, and great for diving, but their cumbersome size -- good for flying -- means they are not yet matching the diving ability of small-winged penguins.
"Basically, they have to reduce their wings or grow larger to improve their diving, and both would make flying impossible," Robert Ricklefs of the University of Missouri-St. Louis and co-author on the paper told Nature.
One argument for why murres and cormorants haven't sacrificed their ability to fly is that they are still under attack from multiple land-based predators. Penguins, of course, have to contend with plenty of predators in the water, from seals and sharks to sea lions and killer whales, while on land their predators are few and far between. So it makes sense that they would have dropped the high-energy-expending practice of flying to flee predators faster while submerged in the water.

Fossils have shown that penguins probably evolved into flightless birds around 50 million years ago, but there are many gaps in our timeline of this transformation.

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