December 12, 2013
Unidirectional Breathing Technique Developed 100 Million Years Before Birds
redOrbit Staff & Wire Reports - Your Universe Online
The one-way airflow breathing technique used by monitor lizards may have originated about 20 million years earlier than previously believed, according to a new paper appearing in Wednesday’s edition of the journal Nature.
National Geographic’s Carrie Arnold called the report “surprising,” since it has been widely accepted that the unidirectional airflow breath technique of birds evolved as a result of the oxygen-heavy demands of flight. Rather than being partially filled with “stale, depleted air” like those of humans, bird lungs contain air having “a much higher average oxygen content and are much more efficient at getting oxygen to the blood.”
The monitor lizard’s breathing method “appears to be much more common and ancient than anyone thought,” senior author and Utah biology professor C.G. Farmer, said in a statement. “It has been thought to be important for enabling birds to support strenuous activity, such as flight. We now know it's not unique to birds. It shows our previous notions about the function of these one-way patterns of airflow are inadequate. They are found in animals besides those with fast metabolisms.”
However, Farmer noted that since lizard lungs are structurally different than bird and alligator lungs, the one-way airflow trait could have evolved independently approximately 30 million years ago in the predecessors of modern monitor lizards, then again 250 million years ago in archosaurs, the dinosaurs that evolved into alligators, dinosaurs and birds. In order to know for sure, she said, lizards such as iguanas and geckos need to be studied.
Research published by Farmer three years ago involved a similar analysis of unidirectional airflow in alligators, according to Discovery News. That study suggested that the breathing technique most likely evolved 250 million years ago from the archosaurs. However, the new study shows that the method could have evolved about 270 million years ago among cold-blooded diapsids, which gave rise to modern snakes and lizards.
Farmer and her colleagues performed CT scans and created 3D images of lizard lungs in order to visualize the anatomy of the organ, then surgically implanted flow meters in the bronchi of five monitor lizards in order to measure airflow direction. They measured airflow defined as they pumped air into and out of lungs removed from 10 deceased lizards, and also pumped water laced with sunflower pollen particles or plastic microspheres through lizard lungs. The movement of the pollen and spheres further demonstrated the one-directional airflow.
“Savannah monitor lizards were used in the research because they are relatively large and thus easier to study,” the university explained. “Monitor lizards also have some of the highest rates of oxygen consumption, partly because they breathe using not only their trunk muscles and ribs, but also using ‘gular pumping,’ which happens when the lizards flare out their throat and pump air into their lungs. Monitor lizards' lungs have more than a dozen chambers or bronchi in each lung. The primary airway runs the length of the lung, with lateral bronchi branching off of it.”
The research illustrated that air first enters the trachea or windpipe of the lizard, and then travels into the two primary airways, each of which enter the lung. However, rather than flowing back the same way it entered, the air instead circles back in a tail-to-head direction moving from one lateral airway to another through perforations, they said.
“The walls containing perforations that allow air to flow from one chamber to the next ‘are like lace curtains,’ Farmer says. There appear to be no mechanical valves or sphincters, so the one-way airflow appears ‘to arise simply from jetting,’ or aerodynamic valves created when air flows around bends within the lung airways,” the university added. “That is supported by the fact that one-way airflow was observed even in lungs removed from dead lizards.”