Monitor Lizard One-Way Airflow Evolved Before Dinosaurs?

Monitor lizard one way airflow evolved before dinosaurs

New findings from researchers at the University of Utah suggests that monitor lizard breathing patterns may have evolved before dinosaurs. As with birds, alligators and seemingly dinosaurs, the air flows in a “one-way loop” through the tiny lungs of the monitor lizard.

The group’s findings were published in the Dec. 11 issue of the journal Nature, positing that this pattern of breathing had originated approximately 20 million years earlier than previously estimated, some 270 million years ago.

Monitor lizards are generally large lizards, possessing long necks, sturdy limbs and very powerful tails. Most of the species are terrestrial, but some are known to travel through trees and semiaquatic environments.

The lizards are thought to possess a high degree of intelligence and cooperate when scavenging for food sources. They are found in Africa, Australia, New Guinea and many parts of Asia.

Associate Professor of Biology Colleen Farmer, based at the University of Utah, and lead author of the latest study, explained that this form of breathing was far more commonplace than scientists had initially suspected. According to Farmer, the consensus of opinion amongst the scientific community, was that one-way airflow was an important feature in birds to support strenuous activity. She elaborates on this contention:

“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.”

Farmer offers a word of caution in interpreting these findings, however, highlighting the difference in lung structure between lizards and alligators; on this basis, she considers the possibility that one-way airflow evolved independently in the predecessors of today’s monitor lizards, around 30 million years ago, whereas this breathing pattern may have evolved 250 million years ago in archosaurs, whose modern day representatives comprise of crocodiles and birds.

Tidal Flow and Unidirectional Airflow

There are some significant differences in the way humans, birds and lizards breath. Humans – as is the case with most other animals – have what is known as a “tidal” breathing pattern. During inhalation, air flows into the trachea and through the ever-branching airways of the lungs, until finally reaching the small alveoli sacs; these numerous chambers are designed to increase the surface area to improve the rate of diffusion of gases to and from the bloodstream.

At the alveoli, oxygen enters the bloodstream, whilst carbon dioxide leaves the blood and enters the lungs. During exhalation, air flows back out through the airway passages, in much the same way as air enters.

Meanwhile, birds are known to have some tidal airflow into and out of air sacs, but their breathing pattern is predominately one-way airflow into the lung itself. The airflow is described as unidirectional, traveling in a single circuit before exiting the lung.

The latest study concluded that African savannah monitor lizards (Varanus exanthematicus) possess a looping airflow system, with some tidal airflow present within particular regions of the lung. Using this knowledge, the group theorize the one-way airflow breathing pattern to have plausibly arisen as early as 270 million years ago, predating early archosaurs. If this were true, the pattern may have manifested among cold-blooded diapsids – creatures that developed temporal fenestrae on either side of their skulls. These diapsids were the common ancestors of the Lepidosauromorpha, which includes lizards, snakes and tuataras, as well as archosaurs.

Monitoring the Monitor Lizard’s Airflow

Farmer’s team demonstrated unidirectional airflow using a number of methods. Firstly, computerized tomography (CT) scanners were employed to generate 3-dimensional images of the lung anatomy of monitor lizards. Airflow patterns in the reptilian creatures were measured by using flow meters, which were surgically implanted into five separate specimens.

In addition to this, the lungs were extracted from 10 dead lizards and studied in even greater detail. The researchers observed the airflow within the lungs upon pumping air into and out of them.  During another experiment, they pumped pollen or plastic microspheres suspended in water through the lungs and observed their movement.

CT scans of the monitor lizard lungs
Top image: colorized CT scan of the monitor lizard’s lung anatomy. Bottom image: diagram of unidirectional airflow, with some air returning through adjacent lateral airways (blue and purple).

Ultimately, the weight of evidence pointed towards monitor lizards displaying unidirectional airflow. The air travels down the monitor lizard’s trachea, through the two primary airways and then into the lung. The air doesn’t flow back in the same direction, however. Instead, it travels from one lateral airway to the next, via tiny perforations between each passage.

The group point out the evolutionary advantages that this one-way airflow system conferred for different animals. In birds, the researchers argue it may have prevented the creatures from passing out when flying at high altitudes. In a previous study, Farmer and her colleagues had contemplated the notion that the unidirectional airflow pattern could have evolved in the dinosaurs’ ancestors, during a time when atmospheric oxygen levels were much lower.

In a recent press release, Farmer recently discussed a possible discrepancy between the point at which her team believes this evolutionary change to have transpired and the level of atmospheric oxygen that was available at that time:

“But if it evolved in a common ancestor 20 million years earlier, this unidirectional flow would have evolved under very high oxygen levels… And so were are [sic] left with a deeper mystery on the evolutionary origin of one-way airflow.”

So, it appears that scientists are now fully aware of the airflow pattern in the lungs of monitor lizards. However, much mystery remains over precisely when this evolutionary adaptation took place.

By James Fenner

University of Utah Press Release

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