If you’ve ever wondered how an owl can contort its head into some of the most uncomfortable looking positions, you’re not alone; scientists and animal experts have wondered the same thing or years. Recently, they had an opportunity to find answers to this mysterious question through dissection and analysis of several owls that had died from natural causes.
In most animals, humans included, carotid and vertebral arteries are fragile structures that are highly susceptible to tears and stretches. This is especially true where quick, jarring motions are involved – a car accident, a roller coaster ride, even a chiropractic movement done incorrectly. In some cases, the condition is minor, but in others, the tearing or stretching can cause a clot to form. If the clot breaks free, it can result in an embolism, stroke, or even death.
While the vessels are just as delicate in owls, there seems to be some protective mechanisms that allow owls to rotate their necks up to 270 degrees in either direction, and they can make that movement rapidly. What’s even more interesting is that, even during these rapid and extreme rotations, blood supply to the brain is maintained. But until now, no one understood how.
“Until now, brain imaging specialists like me who deal with human injuries caused by trauma to the 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,” Dr. Philippe Gailloud, an interventional neuroradiologist at John Hopkins and senior researcher on the study told The Smithsonian Magazine in a statement.
The study, which won first place in the 2012 International Science and Engineering Vizualation Challenge, included the dissection and analysis of 12 snowy, barred, and great horned owls (all three of which are native to the Americas – from the southernmost tip of the South American mainland to Alaska and Canada) that had died from natural causes. Bone structures and vascular structures were examined with the help of CT scans and angiography (a test that provides an X-ray image of the inside of blood vessels).
To mimic blood flow so they could see how the blood vessels worked, researchers injected dye into the owls’ arteries. They then turned their heads manually. They found that blood vessels at the base of their heads (the area just below their jawbones), kept expanding as more dye flowed in. Once full, the fluid pooled into tiny reservoirs. This is a complete contrast when compared to human arteries, which do not have a pooling reservoir and tend to get smaller during a head rotation. Researchers felt that this function was crucial to the rotation function of the owls’ heads because it helped to minimize interruption of blood flow to the brain and eyes. But there’s more to this story…
In addition to the increased blood flow to the vessels, the owls were found to have larger transverse foraminae (hallow cavities that encase major arteries that feed the brain). In fact, these cavities were ten times bigger in owls than they are in humans. Researchers said that this extra space creates multiple air pockets that help cushion the artery so that it can move safely during extreme head rotations.
The unique structures were found in 12 of the 14 vertebrae in the owls’ necks, and their arteries entered into the neck higher up than other birds (for most birds, arteries enter the neck at the 14th vertebrae, but in owls, they entered in at the 14th). It is thought that this higher position gives the vessels in owls’ necks more room to move. Additionally, small vessel connections were found between the carotid and vertebral arteries to enable uninterrupted blood flow to the brain – this was true, even when researchers delivered extreme pronation to the owls’ necks.
“In humans, the vertebral artery really hugs the hallow 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,” Fabian de Kok-Mercado, lead researcher on the study and a medical illustrator at Howard Hughes Medical Institute in Maryland told The Smithsonian.
“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.
The team plans to study hawk anatomy – another hunting bird that has the ability to look far left and far right – to see if there are similar structures that prevent injury to the head and neck.