Alligators have a one-way path for breathing that is similar to birds', new research shows. The findings, published in the Jan. 15 Science, could explain how dinosaurs' ancestors rose to prominence.
"It's absolutely transformational," comments Adam Summers of the University of Washington's Friday Harbor Laboratories. "It really makes us think hard about our interpretations of anatomy."
Unlike a mammal's breath, which exits the lungs from the same dead-end chambers it enters, a bird's breath takes a loopy one-way street through its lungs.
In mammals, air enters the lungs and flows through a network of branching tubes called bronchi, which culminate in small cul-de-sac chambers where blood vessels exchange carbon dioxide for oxygen. Air then exits the lungs via the same pathway.
But in birds' lungs, air moves constantly through a simpler network of tubes, making a single circuit before being exhaled. This unidirectional flow makes gas transfer much more efficient -- air can zip right past the blood vessels that need oxygen and then be on its way.
Conventional wisdom has held that only birds can do this because in addition to lungs, birds have air sacs that may steer the air unidirectionally through the lung. "People incorrectly believe that you must have avian-style air sacs in order to have unidirectional flow," says C.G. Farmer of the University of Utah, a coauthor of the new study. "Alligators don't have air sacs, so no one ever looked."
But a structural similarity in the way birds' and alligators' bronchi branch through the lungs caught Farmer's attention.
"If you look at the alligator lung, it's not hard to see how small modifications in this design could potentially lead to an avian lung," she says. She wondered if the similarities were more than cosmetic. Farmer and her coauthor Kent Sanders of the University of Utah inserted flow meters called thermistors into the lungs of six living alligators to see how fast and in what direction the air moved. The primary bronchi each split into two branches shortly after the point where air enters each lung. Surprisingly, air moved through the first branch in each lung in the same direction whether the gator was inhaling or exhaling.
"That's the opposite of what you expect," Farmer says.
The researchers also pumped air in and out of the lungs of four dead alligators and pumped water containing tiny fluorescent beads through the lungs of another dead gator, to measure the flow.
All three lines of evidence showed the same unidirectional path through the gators' lungs. Farmer thinks that instead of entering the first bronchial branch, which veers off at a hairpin turn, air skips this tube and enters the second branch. Its passage past the first opening creates an aerodynamic valve that sucks air out of that branch. From the second branch, air passes through small tubes called parabronchi, where carbon dioxide is traded for oxygen in the blood. Finally, air flows from the parabronchi into the first branch, and then back out through the trachea.
Having three lines of evidence and careful measurements bolsters the researchers' claim, comments Elizabeth Brainerd of Brown University.
The finding could mean that this mode of breathing is far older than scientists suspected and that it may have helped archosaurs, the common forebearers of birds, alligators and dinosaurs, rise to a dominant ecological niche millions of years ago.
Archosaurs were the largest land animals on Earth from after the Permian-Triassic extinction 251 million years ago until the group split 246 million years ago into alligators and what would become dinosaurs and birds. To rise to prominence, archosaurs had to unseat large mammal-like reptiles called synapsids. But synapsids flourished after the dinosaurs went extinct, eventually leading to today's large land mammals. How archosaurs gained their brief time on top remains a mystery.
"There's a flip-flopping from synapsids, to archosaurs, back to synapsids in this one niche. Why did this happen?" Farmer says. "Our data suggest (archosaurs) had an advantage."
The trick of one-way breathing could have given archosaurs a boost. Other research has shown that the oxygen levels at the time were about half of what they are today, even lower than the air at the top of Mount Everest. A more efficient breathing system could have allowed archosaurs to flourish in that dizzying environment.
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