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They have a complex, adaptive network of protective blood vessels
that make the structures in our necks look puny–a network that researchers have now dissected, mapped and illustrated for the first time.
“Until now, brain imaging specialists like me who deal with human
injuries caused by trauma to arteries in the head and neck have always
been puzzled as to why rapid, twisting head movements did not leave
thousands of owls lying dead on the forest floor from stroke,” said Dr. Philippe Gailloud, an interventional neuroradiologist at Johns Hopkins and a senior researcher on the study, in a statement. A poster depicting these findings won first place in the 2012 International Science and Engineering Visualization Challenge, the journal Science announced yesterday.
The carotid and vertebral arteries in the neck of most animals,
including owls and humans, are delicate and fragile structures. They’re
highly susceptible to minor tears and stretches of vessel linings. In
humans, such injuries can be common: whiplash sustained in a car
accident, a back-and-forth jarring roller coaster ride or even a
chiropractic maneuver gone wrong. But they’re also dangerous. Blood
vessel tears caused by sudden twisting motions produce clots that can
break off, sometimes causing an embolism or stroke that could prove
fatal.
Owls, on the other hand, can rotate their necks up to 270 degrees
in either direction without damaging the vessels running below their
heads, and they can do it without cutting off blood supply to their
brains.
Using medical illustations, CT scans and angiography,
which produces X-ray images of the inside of blood vessels, researchers
studied the bone structure and vascular structure in the heads and
necks of a dozen snowy, barred and great horned owls after their deaths
from natural causes. All three species are native to the Americas, their
habitats stretching from Tierra del Fuego, the southernmost tip of the
South American mainland, to the Arctic tundra of Alaska and Canada.
When researchers injected dye into the owls’ arteries to mimic blood
flow and then manually turned the birds’ heads, they saw mechanisms at
play that contrasted greatly with humans’ head-turning ability. Blood
vessels at the base of the owls’ heads, just below the jawbone, kept
expanding as more of the dye flowed in. Eventually, the fluid pooled
into tiny reservoirs. Our arteries tend to get smaller during head
rotations and don’t balloon in the same way.
Researchers believe this feature is crucial to support the top-heavy
winged creatures. While they twist theirs heads back and forth, the
owls’ reservoirs allow the birds to pool blood to sustain the function
of their eyes and brain, which are both relatively large compared to the
size of their heads. This interconnected vascular network helps
minimize interruption of blood flow.
But these silent hunters’ head-on-a-swivel ability continued to be
more complex, researchers found. In owls’ necks, one of the major
arteries feeding the brain passes through bony holes in the birds’
vertebrae. These hollow cavities, known as the transverse foraminae,
were ten times bigger in diameter than the artery passing through it.
The researchers say the roomy extra space creates multiple air pockets
that cushion the artery and allow it to travel safely during twisting
motions.
“In humans, the vertebral artery really hugs the hollow cavities in
the neck. But this is not the case in owls, whose structures are
specially adapted to allow for greater arterial flexibility and
movement,” said lead researcher Fabian de Kok-Mercado in the statement. De Kok-Mercado is a medical illustrator at Howard Hughes Medical Institute in Maryland.
This adaptation appeared in 12 of the 14 vertebrae in the owls’
necks. The vertebral arteries entered their necks higher up than in
other birds, introduced at the 12th vertebrae (when counted from the
top) instead of the 14th, which gives the vessels more
slack and room to breathe. Small vessel connections between the carotid
and vertebral arteries, called anastomoses, let blood flow
uninterrupted to the brain, even when owls’ necks were contorted into
the most extreme twists and turns.
“Our in-depth study of owl anatomy resolves one of the many
interesting neurovascular medical mysteries of how owls have adapted to
handle extreme head rotations,” de Kok-Mercado said.
Up next for the team is studying hawk anatomy to find out if other
bird species possess owls’ adaptive features for looking far left and
right.
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