On the surface, a pigeon and an alligator could hardly seem more different. While the pigeon is a flying, feather-covered creature that pecks its food with a toothless beak, an alligator is an amphibious, armored predator that crushes its prey in jaws studded with conical teeth. Despite the disparate forms, however, they are joined together by a common ancestry. Both the pigeon and the alligator are archosaurs, the group of "ruling reptiles" that also includes pterosaurs, non-avian dinosaurs, and a host of related forms that died out tens of millions of years ago. The archosaurs that exist today are only a fraction of the different forms that once existed, but a new paper in the journal Science underscores that they share some deep similarities beneath their skin.
As I breathe in and out while writing this post air enters my body via my nose, travels down into the "dead end" of my lungs, and then is exhaled after the oxygen has been absorbed. Not so in birds. Birds have a more efficient respiratory system adapted to unidirectional air flow, or, in other words, air moves in one direction through the bird's system in a kind of circuit. And, as it turns out, alligators can breathe the same way.
Scientists have known for some time that birds breathe in a different way than mammals do. What has been more difficult to determine is how alligators breathe. There had been some suggestion that alligators might breathe via unidirectional air flow, too, but no one had conclusively illustrated that this was the case. To test the hypothesis, C. G. Farmer and Kent Sanders placed sensors in two parts of the respiratory passageways of four dead alligators, artificially ventilated the lungs, and watched to see how the air moved.
The results suggested that the airflow inside the alligator's body was capable of moving in a unidirectional manner along a circuit of pathways, but would they actually breathe in such a way while alive? To find out the scientists placed airflow measurement devices in six living alligators. They found that the airflow in the alligator's respiratory system continued through the transition between inspiration (new air coming in) and expiration (old air going out). Air kept on moving through the system during both phases, again suggesting that the alligators were breathing through unidirectional airflow.
Just how the alligators are able to do this is not yet understood, but the discovery that they can breathe like birds might provide clues to some major events in the history of life on earth. If living alligators and the living theropod dinosaurs we call birds both share this physiological mechanism, then it is possible that the last common ancestor of dinosaurs and alligators was a unidirectional breather, too. If this is correct unidirectional breathing evolved in the first archosaurs over 230 million years ago in the wake of the worst mass extinction in the history of this planet.
The close of the Permian period 251 million years ago was marked by the extinction of up to 96 percent of the animals known to have lived in the seas and over 70 percent of those that lived on land. Among the survivors were the earliest archosaurs (or their close ancestors), and if they had unidirectional breathing it might have given them an advantage. Unidirectional breathing is a more efficient way of obtaining oxygen from the air than the way we breathe, and if the Permian mass extinction was triggered by major changes in the atmosphere as scientists suspect, such as the depletion of oxygen, the archosaurs may have been better able to survive than the early relatives of mammals they lived alongside. This hypothesis requires further evidence, but if correct the beginnings of the archosaur rise to dominance could all have come down to a difference in breathing.
Farmer, C., & Sanders, K. (2010). Unidirectional Airflow in the Lungs of Alligators Science, 327 (5963), 338-340 DOI: 10.1126/science.1180219