For decades, birds enjoyed the reputation of being the only known animal group to have unidirectional airflow in their lungs. This unique lung design had long been assumed to be an adaptation to the high oxygen demands of intense activity, and is thought to have enabled the evolution of flight. A unidirectional flow of air through the lungs means there is a constant stream of fresh oxygen being delivered to the bloodstream, whether the bird is breathing in or out – quite unlike that of the mammalian lung. A new study, published in the Proceedings of the National Academy of Sciences, has thrown doubt on the long-held belief that unidirectional airflow in lungs evolved for intense activity and stamina, by finding bird-like lungs in green iguanas – a species not renowned for its flight capabilities or aerobic fitness!
Colleen Farmer and colleagues, based at the University of Utah, USA, designed a series of novel methods for investigating airflow through the lungs of the iguanas. First, the team watched, via an endoscope, as the lizards inhaled harmless particle-laden theatrical smoke, allowing visualisation of the airflow through the lungs. To the team's amazement, the smoke particles only moved in one direction, regardless of whether the iguanas were breathing in or out. Following this surprise, the team then pumped water full of pollen grains through surgically removed lungs, to further clarify how the air flowed through these structures. Coupled with computer simulation models, Farmer and colleagues were able to confirm that the iguana's lungs exhibited unidirectional airflow and the lung design matched that of birds! Given the dramatic difference in natural history between the two groups, it begs the question, why would iguanas have evolved a lung morphology that is typically associated with intense aerobic activity? Moreover, Farmer adds, ‘It even suggests that unidirectional flow is not necessarily important for expanded aerobic capacity’.
These findings imply unidirectional airflow evolved not just for flight but also for some other vital life-history aspect. The authors propose that unidirectional airflow is linked to the ability of reptiles to hold their breath for long durations. Green iguanas are known to swim and dive to avoid predators, and a unidirectional lung design could potentially extend the period the lizards can stay submerged underwater. The iguana's lung design implies that unidirectional airflow evolved both before flight and the first birds and it was more than likely present in a common ancestor over 250,000 million years ago, before the Diapsida group split into the Lepidosauromorpha (tuatara, lizards, snakes) and Archosauromorpha (crocodilians, birds) that we see today. Asked whether this lung design could possibly be an example of convergent evolution, Farmer explains, ‘It would be a striking case of convergence if it turns out to be the case’.
So it appears that birds may have lost their exclusivity with respect to a unidirectional lung design and will now have to share this accolade with certain reptile species. The direction of airflow through the lungs of reptiles has largely been assumed, rather than definitively measured. Consequently, it is likely that further investigations will result in bird-like lung designs being identified in more reptile species, questioning further the exact function of unidirectional airflow.