Iguanas’ one-way airflow undermines usual view of lung evolution

The green iguana, which does not fly or do anything more athletic than an occasional sprint, has the simplest lungs yet found to produce a version of birds’ high-performance one-way airflow.

The lungs of a green iguana look deceptively simple, says Colleen G. Farmer of the University of Utah in Salt Lake City. Each Iguana iguana lung is just a two-chambered bag with a single air tube to bring air in and out. Using an endoscope on iguanas breathing theater fog, Farmer and her colleagues made the first measurements of air moving through the animals’ lungs. The shapes of inner landscape keep air flowing along the lung’s wall in just one direction. That wall is where blood vessels hug the lung to rid themselves of waste gases and pick up fresh oxygen.

Air flows through bird lungs in an efficient one-way path, long thought to be one of the evolutionary innovations that evolved as birds developed the sustained athletics of flight. Farmer and her colleagues have challenged that evolutionary scenario, finding one-way flows in alligators (SN: 2/13/10, p. 11) and monitor lizards (SN Online: 12/11/13). Now the simpler lung of a not-very-athletic iguana suggests that something important before flight drove the evolution of one-way lungs. They might have evolved as aids in breath-holding while hiding from predators, the researchers speculate November 17 in the Proceedings of the National Academy of Sciences.