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The
upper image is a colorized CT scan showing different airways in the lung of a
monitor lizard. The bottom image shows how air flows in a mostly one-way loop
through the lizard’s lung, as measured by sensors implanted as part of a
University of Utah study. Note how the air flows through adjacent lateral
airways (blue and purple) by moving through perforations in the airways’ walls.
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Dec.
11, 2013 — Air flows mostly in a one-way loop through the lungs of monitor
lizards -- a breathing method shared by birds, alligators and presumably
dinosaurs, according to a new University of Utah study. The findings were published
online Dec. 11 in the journal Nature -- raise the possibility this breathing
pattern originated 270 million years ago, about 20 million years earlier than
previously believed and 100 million years before the first birds. Why remains a
mystery.
"It
appears to be much more common and ancient than anyone thought," says C.G.
Farmer, the study's senior author and an associate professor of biology at the
University of Utah. "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."
But
Farmer cautions that because lizard lungs have a different structure than bird
and alligator lungs, it is also possible that one-way airflow evolved
independently about 30 million years ago in the ancestors of monitor lizards
and about 250 million years ago in the archosaurs, the group that gave rise to
alligators, dinosaurs and birds. More lizard species, such as geckos and
iguanas, must be studied to learn the answer, she says.
Farmer
conducted the study with two University of Utah biologists -- first author and
postdoctoral fellow Emma Schachner and doctoral student Robert Cieri -- and
with James Butler, a Harvard University physiologist.
The
research was funded by the American Association of Anatomists, the American
Philosophical Society, the National Science Foundation and private donor Sharon
Meyer.
Tidal Versus One-Way Airflow in the Lungs
Humans
and most other animals have a "tidal" breathing pattern: Air flows
into the lungs' branching, progressively smaller airways or bronchi until
dead-ending at small chambers called alveoli, where oxygen enters the blood and
carbon dioxide leaves the blood and enters the lungs. Then the air flows back
out the same way.
Birds,
on the other hand, have some tidal airflow into and out of air sacs, but their
breathing is dominated by one-way airflow in the lung itself. The air flows
through the lung in one direction, making a loop before exiting the lung.
In
2010, Farmer published a study showing that a mostly one-way or
"unidirectional" airflow controlled by aerodynamic valves exists in
alligators. That means the breathing pattern likely evolved before 250 million
years ago, when crocodilians -- the ancestors of alligators and crocodiles --
split from the archosaur family tree that led to the evolution of flying
pterosaurs, dinosaurs and eventually birds.
The
new study found a mostly one-way, looping air flow in African savannah monitor
lizards, Varanus exanthematicus -- one of roughly 73 species of monitor lizards
-- although there was some tidal airflow in regions of the lungs. That means
one-way airflow may have arisen not among the early archosaurs about 250
million years ago, but as early as 270 million years ago among cold-blooded
diapsids, which were the common, cold-blooded ancestors of the archosaurs and
Lepidosauromorpha, a group of reptiles that today includes lizards, snakes and
lizard-like creatures known as tuataras.
One-way
airflow may help birds to fly without passing out at high altitudes, where
oxygen levels are low. Before the new study, Farmer and others had speculated
that the one-way airflow may have helped dinosaurs' ancestors dominate the
Earth when atmospheric oxygen levels were low after the Permian-Triassic mass
extinction -- the worst in Earth's history -- 251 million years ago.
"But
if it evolved in a common ancestor 20 million years earlier, this
unidirectional flow would have evolved under very high oxygen levels," she
says. "And so were are left with a deeper mystery on the evolutionary
origin of one-way airflow."
How the Study was performed
As
in her earlier research on alligators, Farmer and colleagues demonstrated
predominantly one-way airflow in the lungs of monitor lizards in several ways.
They performed CT scans and made 3-D images of lizard lungs to visualize the
anatomy of the lungs. They surgically implanted flow meters in the bronchi of
five monitor lizards to measure airflow direction.
Using
lungs removed from 10 deceased lizards, the researchers measured air flow as
they pumped air into and out of the lungs. They also pumped water laden with
sunflower pollen particles or plastic microspheres through lizard lungs, and
the movement of the pollen and spheres also showed the unidirectional airflow.
Savannah
monitor lizards were used in the research because they are relatively large and
thus easier to study, weighing about a pound and measuring roughly 15 inches
from head to tail tip. 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 is 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
study showed that air enters the lizard's trachea or windpipe, then flows into
the two primary airways, which enter the lung. But then, instead of flowing
tidally back out the same way, the air instead loops back in a tail-to-head
direction moving from one lateral airway to the next through small perforations
between them.
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. That is supported by the fact that
one-way airflow was observed even in lungs removed from dead lizards.
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