It is well established that bird lungs move air in a one-way loop through their gas-exchanging tubes during inhalation and exhalation. In contrast, mammals and presumably all other vertebrates breathe tidally, bringing air into the dead-end gas-exchange structures and back out through the same pathway. One-way airflow is a highly efficient method of breathing, allowing more oxygen to be extracted per breath compared with tidal breathing. Unidirectional airflow is thought to be unique to birds, which have evolved to meet the high oxygen demands of flight. As it turns out, alligators also have a one-way path of breathing, similar to birds.

Conventional wisdom suggests that one-way air flow can only occur with the help of air sacs, bellow-like structures that are found only in birds. C. G. Farmer and Kent Sanders, from the University of Utah, USA, noticed that key features of the alligator lung looked remarkably similar to the bird lung, though alligators lack air sacs. To test the hypothesis that airflow in alligators is unidirectional, the authors performed three separate experiments. First, they artificially ventilated excised lungs from four American alligators and monitored airflow using flowmeters, termed thermistors. Next, they surgically implanted thermistors on six anaesthetized alligators and monitored airflow during normal breathing. Finally, they filled an excised alligator lung with saline fluid containing fluorescent beads and visualized the water flow during artificial ventilation.

All three experiments showed the same remarkable result, that flow occurs in one direction through the lungs of alligators. Notably, air flowed unidirectionally even in excised lungs, suggesting that one-way airflow in alligators is caused by the structural arrangement of the gas exchanging tubes themselves. These results demonstrate that unidirectional airflow can occur in the absence of air sacs, and alternative forms of breathing, such as the hepatic piston method used by alligators, do not preclude unidirectional airflow.

This extraordinary finding has important implications for how scientists think about the evolution of breathing. Farmer and Sanders suggest that unidirectional airflow evolved far earlier than previously thought. They propose that unidirectional airflow was present in basal archosaurs, the common ancestor of crocodilians, birds and dinosaurs. Mammal-like reptiles, termed synapsids, were the dominant land animals during the Permian. However, following the Permian–Triassic extinction, archosaurs emerged as the champions and dominated earth until the Cretaceous–Tertiary extinction, after which synapsids regained supremacy. Farmer and Sanders suggest that unidirectional airflow gave archosaurs a competitive advantage during the low oxygen conditions of the early Triassic by increasing their capacity for exercise. Thus, the alligator's one-way path for breathing could help explain where dinosaurs got the breath to climb to dominance of the earth.

C. G.
Unidirectional airflow in the lungs of alligators