Birds, crocodiles and dinosaurs have much in common – including, it turns out, their breath. The hyper-efficient breathing system of birds is shared with alligators, and probably evolved in archosaurs, the common ancestor of crocodilians, birds and dinosaurs.
The researchers who have discovered the system in alligators believe it may have given dinosaurs the competitive edge over the ancestors of mammals following the mass extinction at the end of the Permian period, 250 million years ago.
Thanks to one-way airflow, birds are far more efficient breathers than mammals. When they breathe in, air does not go directly into the lungs. Instead, it enters the air sacs, where it is stored briefly before passing into the lungs at the next inhalation. In this way, air enters and exits a bird's lungs at different points – in via the air sacs, out via the windpipe – allowing them to maintain near-constant, one-way airflow through their lungs and extract up to two-and-a-half times as much oxygen per breath as a mammal.Previous research has suggested that one-way air flow is unique to birds and evolved specifically to allow them to make oxygen-demanding flights. It was also thought that the bellows-like air sacs were critical for pumping air one way.
Those conclusions have been toppled by the new study, in which devices to measure airflow were surgically implanted into the lungs of live alligators.
Gators flayedColleen Farmer of the University of Utah in Salt Lake City also placed the flow meters into the dissected lungs of four dead alligators, which the team then filled with air using a giant syringe. What they found was that alligator lungs function very much like birds'.
Alligators don't have air sacs like birds, but the researchers think an unusual airway that sits on either side of the alligator trachea may do the same job.
Per Ahlberg of Uppsala University in Sweden agrees that the efficient breathing system probably evolved in a common archosaur ancestor of crocodilians, birds and dinosaurs. "Swift, long-legged animals that might have relied on the rich oxygen supply provided by a one-way airflow were abundant in the early lineages of these groups," he says.
Farmer goes one step further and argues that the system would have allowed archosaurs to outcompete synapsids – the ancestors of mammals – following the end-Permian mass extinction."Oxygen levels were really low during the early Triassic [after the Permian], so the evolution of unidirectional airflow, which boosts oxygen delivery to muscles, may have made archosaurs more capable of vigorous exercise than synapsids," she says
Journal reference: Science, DOI: 10.1126/science.1180219