Why penguins don't fly has been answered, and the reason is swimmingly intriguing.

Penguins may not be able to fly, but they are adept swimmers and divers. It now appears that the energy required to be so proficient at diving also robs these birds of their ability to fly.

The mystery of why penguins can not fly has puzzled biologists for years. The feeding grounds for these flightless birds is often far-distant from their breeding areas. The journey between the two areas is often difficult and dangerous, and filled with predators. If penguins could fly, they might be able to make the journey in just a few minutes, flying in relative safety. So there had to be an important reason that evolution selected for those birds who could not take to the air.

Kyle Elliot, a biologist at the University of Manitoba in Canada, looked at the body structure of birds that possess the ability to dive as well as fly, including murres and cormorants.

Researchers injected the birds with isotope-tagged water, and equipped them with devices to record the depth and duration of their dives. By releasing them into the wild and recapturing the animals, the scientists were able to calculate how much energy was expended on dives and while flying by measuring the level of isotope remaining.

The results of the study showed that these birds need to use more energy to fly than another other known species. Their diving skills are also affected to the point where murres required 30% more energy to dive than a penguin of the same size.

"Basically, they have to reduce their wings or grow larger to improve their diving, and both would make flying impossible," said Robert Ricklefs, an ornithologist at the University of Missouri-St. Louis, who helped to write the findings.

Emperor penguins can hold their breath for over 20 minutes as they dive to 1,500 feet in search of food. Yet, those body features that make penguins such efficient swimmers also cost the animal too much energy to fly.

Kyle Elliot, who co-wrote the paper with Ricklefs, said, "[The] bottom line is that good flippers don't fly very well."

The results of the study were announced in the Proceedings of the National Academy of Sciences.