CP scan images of the top and side views of an American alligator with computer renderings of unidirectional airflow in the alligator's lungs. (C.G. Farmer and Kent Sanders)

Biologists in the U.S. have found that air loops through the lungs of alligators in one direction, just as it does in the lungs of birds.

Researcher C.G. Farmer of the University of Utah and her colleagues say this unidirectional type of lung would have given the ancient ancestors of alligators an advantage in the oxygen-poor atmosphere that existed at the time.

In the lungs of humans and other mammals, the air moves like the tides: in and out, moving one direction and then the other. The air comes into the lungs, splitting off into ever small airways until they reach dead ends, called alveoli, where oxygen and carbon dioxide are exchanged with the blood.

In birds and alligators, the air comes into the lungs, and splits off into narrower airways, but there are no dead ends. Oxygen and carbon dioxide are exchanged in tubes called parabronchi, and the air flows in only one direction.

Biologists say this type of lung is highly efficient at extracting oxygen from the air and explains in part how birds can fly at high altitudes where air is much thinner.

Farmer describes alligators as the "sister" group of birds. They share a common ancestor, called archosaurs, that lived during the early Triassic period, more than 246 million years ago.

Although not much is known about the archosaurs, Farmer said it was probably "a small, relatively agile, insect-eating animal." They became the dominant land animal following the Permian extinction event 251 million years ago. It was the most extreme extinction event in Earth's history.

The fact that both birds and alligators have unidirectional airflow in their lungs suggests the trait evolved in the archosaurs, before the split in the family tree that led to alligators on one side and pterosaurs, dinosaurs and birds on the other.

If this is true, then dinosaurs had unidirectional airflow in their lungs, too, since they share this common ancestor.

Helped with competition

Following the Permian extinction, oxygen levels in the air were as low as 12 per cent, compared to 21 per cent today.

Unidirectional lungs helped the archosaurs draw oxygen from the air and out-compete the synapsids, the largest land animals prior to the extinction, and the ancestors of mammals.

"The real importance of this air-flow discovery in gators is it may explain the turnover in fauna between the Permian and the Triassic, with the synapsids losing their dominance and being supplanted by these archosaurs," said Farmer, in a statement.

Farmer and her colleagues showed that alligators have lungs similar to birds by performing three experiments, which were published Friday in the journal Science.

In the first, the researchers surgically inserted flow meters in the lungs of six live alligators and used them to measure the speed and direction of airflow.

The second experiment involved pumping air in and out of lungs removed from dead alligators recovered from a wildlife refuge in Louisiana.

In the third experiment, Farmer pumped water containing fluorescent beads through the lungs of a dead alligator, and videotaped the flow of the beads.