February 1, 2013
Study Uncovers Secret Of The Owl’s Amazing Rotating Head
April Flowers for redOrbit.com - Your Universe Online
Where do art and medical science meet? This year, apparently, they meet at the neck of an owl.
Unlike most animals which are far more vulnerable to neck injuries, the nocturnal birds are able to turn their heads nearly 270 degrees in either direction without cutting off the blood supply to their brains.
Using a groundbreaking combination of medical technologies that included angiography, CT scans and medical illustrations, the team examined the anatomy of dozens of the big-eyed birds. Led by medical illustrator Fabian de Kok-Mercado, a recent graduate student in the Department of Art as Applied to Medicine, the team found four major biological adaptations that are designed to prevent injury from the rotational head movements. These variations all concern the animals' bone structure as well as the vascular network needed to support the animal´s top-heavy body.
The findings of this study were published in a recent issue of Science as the first-place prizewinners in the posters and graphics category of the National Science Foundation's (NSF) 2012 International Science & Engineering Visualization Challenge. The competition is a collaboration between the NSF and Science to “celebrate the grand tradition of science visualization [and] to encourage its continued growth." The judges select winners in five categories that include photography, illustration, posters and graphics, games and apps, and video.
"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," says interventional neuroradiologist Philippe Gailloud of Johns Hopkins School of Medicine.
"The carotid and vertebral arteries in the neck of most animals — including owls and humans — are very fragile and highly susceptible to even minor tears of the vessel lining," adds Gailloud, an associate professor at Johns Hopkins´ Russell H. Morgan Department of Radiology.
In humans, sudden gyrations of the head and neck have been known to tear or stretch blood vessel linings. This produces clots that can break off and cause a fatal embolism or stroke. These types of injuries are commonplace, according to researchers, and often result from whiplash sustained in car accidents or roller coaster rides. They can even occur from chiropractic manipulations gone awry.
UNIQUE ANATOMICAL ADAPTATIONS
To understand how owls are able to accomplish this maneuver, the Johns Hopkins team studied the bone structure and complex vasculature in the heads and necks of snowy, barred and great horned owls after they died of natural causes. To enhance the X-ray imaging of the birds' blood vessels, the team used an injectible contrast dye. The vessels were then dissected, drawn and scanned to allow a detailed analysis.
The team made their most striking finding after injecting dye into the owls' arteries to mimic blood flow. They manually turned the animals' heads and found that blood vessels at the base of the head, just under the jawbone, continue to enlarge as more of the dye entered. The fluid then pooled in reservoirs. This varies greatly from human anatomical ability. In humans, arteries generally get smaller and smaller, and do not balloon as they branch out.
These reservoirs contract when not in use, and researchers say they act as a trade-off, allowing owls to pool blood to meet the energy needs of their large brains and eyes, while they rotate their heads. The supporting vascular network, with its many interconnections and adaptations, minimizes interruptions in blood flow.
"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," says de Kok-Mercado, now a scientific illustrator and animator at the Howard Hughes Medical Institute.
Moreover, explains Gailloud, "Our new study results show precisely what morphological adaptations are needed to handle such head gyrations and why humans are so vulnerable to osteopathic injury from chiropractic therapy. Extreme manipulations of the human head are really dangerous because we lack so many of the vessel-protecting features seen in owls."
The owl neck provides the first example of this type of anatomical variation ever discovered. One of the major arteries that feeds the brain passes through bony holes in the vertebrae, holes which are approximately 10 times larger than the artery itself. The extra space in these holes (known to researchers as transverse foraminae) creates cushioning air pockets that allow the artery to move around when twisted, explained the research team. The owls' neck has 14 cervical vertebrae, 12 of which were found to have this adaptation.
"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," says de Kok-Mercado.
Another unique anatomical variation relates to the placement of the vertebral artery. In most birds, it enters the neck around the 14th vertebrae, but in owls it enters at the 12th cervical vertebrae, providing the blood vessels with additional room and slack.
Among the other findings were small vessel connections between the carotid and vertebral arteries that allow blood to be exchanged between the two blood vessels. These so-called anastomoses — blood vessels that have branched out and then reconnected —are not usually seen in adult humans and allow for the uninterrupted blood flow to the brain even if one route is blocked during extreme neck rotation.
The cumulative effect of these unique adaptations is that these nocturnal predators can almost fully rotate their head on its axis, providing them an unusually large range of vision without the need to move their bodies. And by keeping their bodies perfectly still, they are able scope out large areas without frightening away potential prey.
The Johns Hopkins team says it plans to continue research in this field by examining hawk anatomy to see whether other bird species possess the same or similar adaptive features for head rotation.