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Owls Spin Heads

Story at-a-glance +

  • Owls have tubular rather than round-shaped eyes, so they aren’t able to move them around much. That’s why they rotate their heads – so they can see.
  • Researchers decided to find out how owls are able to swivel their heads to such an extent without severing arteries or cutting off the blood supply to their brains.
  • As it turns out, owls have uniquely constructed necks and an arterial system that feeds blood to the brain even while they are twisting their necks to rotate their heads.
 

How Owls Spin Their Heads Around

June 19, 2013 | 6,421 views
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By Dr. Becker

Have you ever wondered why owls’ heads seem to be on a swivel? It’s because their eyes are tubular (like telescopes) rather than spherical, which gives them tremendous vision but very limited ability to move their eyeballs. They rotate their heads to compensate for the fact that their eyes have very limited mobility.

But... How Do They Do That?

A medical illustrator and a doctor at Johns Hopkins University School of Medicine decided to find out how owls are able to rotate their heads all the way around without severing arteries or cutting off the blood supply to the brain.

According to study author Dr. Philippe Gailloud of Johns Hopkins:

"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."

If a human attempted to turn his head as rapidly or as far as owls do, the linings of his arteries would rupture. This would cause the formation of clots and lead to a stroke. Not to mention he’d snap his neck.

The arteries running through the necks of most animals, including owls, are quite delicate and vulnerable to even small tears in the lining.

What the Researchers Discovered

In order to take a look at an owl’s blood vessels as the neck is turning, the researchers first injected dye into the blood vessels of 12 owls that died from natural causes and used a CT scan to watch the dye flowing through the arteries. Then they turned the owls’ heads to see what would happen.

Next the team injected a plastic-like substance into the veins of the dead owls and dissected them so they could draw the pathways and positions of the vessels. They made a number of unique discoveries, including:

  • The neck bones (vertebrae) of owls have holes much larger than those in other birds or humans. In people, the holes are about the same size as the artery; the holes in the owls’ vertebrae are 10 times the size of the artery. The researchers believe these holes hold air sacs that provide cushion when an owl twists his head around.
  • There are no holes in the bottom two vertebra of the owl’s neck, which would provide slack for the vessels during twisting motions. The extra-large holes and slack at the bottom of the neck explain why the vessels don’t break. What they don’t explain is why the blood supply isn’t cut off when the head turns, as it seems the vessels would be at least partially blocked when owls engage in exaggerated head twisting.
  • However, the vertebral artery gets slightly larger as it nears the brain, which is not seen in other animals. In fact, arteries in general get smaller the farther they are from the heart. The researchers theorize this vessel enlargement near the brain may serve to collect a certain amount of blood so the brain has a plentiful supply even when the neck is twisting.
  • Also, the blood vessels closest to the owl’s brain are highly interconnected. An artery runs between the front and back of the brain, which further supports the blood supply to the brain.

Isn’t nature incredible? Owls, like all animals, have been designed to adapt to their environment. Since their eyes are fixed, their necks are constructed so they can swivel their heads around to see.

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