Like birds, green iguanas have unidirectional airflow in their lungs: When they breathe in or out, air flows in a one-way loop. “For years, people thought that the design evolved to meet the energetic demands of flight,” Colleen Farmer from the University of Utah says. “That’s all wrong. Iguanas don’t fly.”
In humans and other mammals, lungs have tree-like airways: The main trunk in each lung splits into branches and "twigs." Air flows in and out like ocean tides, and oxygen and carbon dioxide pass to and from the blood in tiny air sacs at the tips of the airway branches. The respiratory system of birds is strikingly distinct from all other animals (with a few rare exceptions). In their lungs, air loops efficiently in one direction through a series of tubes lined with blood vessels for gas exchange, and aerodynamic forces act like valves to sustain the one-way flow as they inhale and exhale. This one-way flow and its corresponding suite of anatomical features were thought to be vital to their ability to fly, an energetically demanding activity.
Recent work revealed that alligators and monitor lizards also have a bird-like pattern of airflow. “We thought we understood how these lungs work, but in fact most of us were completely wrong,” Farmer says in a news release. “People have made a lot of assumptions about how lungs work in animals such as reptiles and crocodiles but they never actually measured flow.”
Now, Farmer and colleagues show that green iguanas -- who are not known for their high-capacity aerobic fitness, and who lack all those avian adaptions above -- also have unidirectional airflow in their simple lungs. By adding yet another group of reptiles to the bird-like breathing list, a new study in Proceedings of the National Academy of Sciences suggests that we need to rethink the evolution of the vertebrate respiratory system.
The team conducted a series of experiments to visualize air moving through iguana lungs, which appear deceptively simple: a two-chambered bag with a single air tube, Science News explains. The researchers used a surgical scope to look inside the lungs of live iguanas as they inhaled harmless smoke from a theatrical fog machine. And they also used probes to measure air speed and volume in dissected lungs.
Finally, working from 3-D x-ray imaging of the contours of iguana lungs, the researchers made a computer model simulating airflow -- its predictions closely matched the flow patterns observed in real lungs. Turns out, the shapes and angles of iguana lung airways point jets of air along just one direction. Blood vessels along the walls of the lungs get rid of waste gases and pick up fresh oxygen.
These findings suggest that this style of breathing evolved in a common ancestor of lizards, crocodiles, and dinosaurs (including birds) nearly 300 million years ago. Something important -- before flight and the first birds came along -- drove the evolution of unidirectional airflow in the lungs.
Images: Bob Cieri (top), University of Utah via Phys.org (middle)