Why Lizards Have Bird Breath

It has long been thought by scientists that birds' one-directional loop through their lungs was a unique characteristic, but a new study shows that lizards too may share this type of breathing.

If University of Utah researchers are accurate with their findings, it would mean this unidirectional flow evolved long before the first birds, arising nearly 300 million years ago in a common ancestor of lizards, snakes, crocodiles and dinosaurs including birds.

"We thought we understood how these lungs work, but in fact most of us were completely wrong," lead author Colleen Farmer said in a statement. "People have made a lot of assumptions about how lungs work in animals such as reptiles and crocodiles but they never actually measured flow," she says.

Unlike birds and apparently green iguanas, humans and other mammals have airways with a tree-like branching structure. Air flows in and out just like the ocean tide, where oxygen enters blood vessels as carbon dioxide leaves them.

But for birds, air loops in one direction in their lungs through a series of tubes lined with blood vessels for gas exchange. Aerodynamic forces act like valves to sustain the one-way flow whether birds are breathing in or out.

"For years, people thought that the design evolved to meet the energetic demands of flight," Farmer explained. "That's all wrong. Iguanas don't fly."

Previous research had demonstrated bird-like breathing in alligators and monitor lizards, and now after finding the same in green iguanas, the new study further supports the idea that the breathing technique evolved in birds and reptiles separately, originating in a common ancestor.

During the study, the results of which were published in the journal Proceedings of the National Academy of Sciences, Farmer and her colleagues found a way to visualize breathing in iguanas. They used a surgical scope to look inside the animals' lungs as they inhaled harmless smoke from a fog machine. In addition, probes were used to measure air speed and lung volume.

Based on subsequent 3D X-ray imaging and computer models simulating airflow, the team found that their model's predictions were "dead-on with the directions of flow" they observed in their experiment, according to Farmer.

So when scientists had once thought that iguanas and other reptiles breathed as a result of air moving down a pressure gradient, now they have more reason to believe that these vertebrates, though wingless, are more like birds than they previously thought.