The reason why penguins cannot fly has finally been laid to rest, researchers claim.
Penguins cannot fly because they are such good swimmers - and no bird can excel at both, said scientists.
They started to fly through the ocean instead of the air as they developed wing-propelled diving skills that allowed them to forage for food at increasing depths.
Penguins lost the ability to fly as they developed more efficient wing-propelled diving skills that allowed them to better forage for food at increasing depths of the ocean
The evolution of flightlessness in penguins has mystified scientists for decades.
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But now a study of guillemots - which closely resemble penguins in their diving and swimming, but can still fly - shows there is no such thing as a wing that's good for both.
Scientists discovered that guillemots on Coats Island, Northern Canada, used substantially less energy than most other birds when they were diving.
But the energy they needed for flight was the highest ever reported for a flying bird - 31 times greater than expended when at rest.
Professor Speakman said: 'Guillemots resemble penguins in their diving and swimming behaviour, but differ in that they are still able to fly.
Scientists have been mystified by the penguin's inability to fly until now
'In many respects they are like a modern equivalent to the ancient ancestors of penguins before they lost the ability to fly.'
Paleontologists have proved penguins could once fly by discovering the density of bone has been increasing over the last 36 million years.
What this means is a much heavier penguin today compared to its ancestor. Penguin bone used to be hollow. Birds today are able to fly in part due to the fact that they have hollow bones.
Hollow bones are lighter then dense bones making it easier to fly. Penguins changed into dense boned, grounded birds, after they followed their food out of the air into the sea.
Kyle Elliott, a zoologist at Manitoba University in Winnipeg, Canada, said: 'Clearly, form constrains function in wild animals, and movement in one medium creates tradeoffs with movement in a second medium.
'Bottom line is that good flippers don't fly very well.'
The study was published in Proceedings of the National Academy of Sciences.
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