Lizards Have Bird Breath: Scientists Discover Iguanas Evolved Lungs Like Birds

What do lizards and birds have in common? They have more similarities than you might think. Scientists have found that like birds, iguanas have breathing that flows in a one-directional loop through their lungs.

"We thought we understood how these lungs work, but in fact most of us were completely wrong," said Colleen Farmer, lead author of the new study, 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."

In humans and other mammals, lungs have airways with a tree-like branching structure, called alveoli. Air flows in and out of the lungs in a tidal fashion, and oxygen and carbon dioxide pass to and from blood in the alveoli at the tips of the smallest airway branches.

Yet this isn't the case in bird lungs. For birds, air loops in one direction through a series of tubes lined with blood vessels for gas exchange. In the past, scientists believed that this adaptation was due to the rigorous demands of flight.

Yet it seems as if lizards also have a bird-like pattern of airflow. Alligators have this same airflow, as do iguanas.

The researchers made this finding by using a surgical scope to look inside the lungs of live iguanas as the lizards inhaled harmless smoke from a theatrical fog machine. Then, working from 3D X-ray imaging of the contours of iguana lungs, the scientists created a model simulating airflow. This matched the patterns observed in real lungs.

The findings revealed that scientists have quite a bit of work to do in order to understand exactly what happens within the lungs of animals. Understanding the shapes and angles of lungs could inspire new ways to design devices that circulate or filter blood or other fluids without using mechanical valves.

The findings are published in the journal Proceedings of the National Academy of Sciences.