JOHNS HOPKINS (US) — Unusual features of the bones and vascular network in their necks and skulls allow owls to spin their heads 270 degrees without dying.
Without those adaptations, turning their heads up to three-quarters of the way around would damage the birds’ delicate blood vessels and cut off the blood supply to their brains, researchers say. Humans, without features like those in the owls, are more vulnerable to neck injury.
Experts used angiography, CT scans, and medical illustrations to examine the anatomy of a dozen of the big-eyed birds. The team, led by medical illustrator Fabian de Kok-Mercado, a recent graduate student in art as applied to medicine, found four major biological adaptations that prevent injury from rotational head movements.
“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.
“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 of radiology at Johns Hopkins University and the study’s senior investigator.
Sudden gyrations of the head and neck in humans have been known to stretch and tear blood vessel linings, producing clots that can break off and cause a deadly embolism or stroke. Researchers say these injuries are commonplace, often resulting from whiplashing car accidents, but also from jarring roller coaster rides and chiropractic manipulations gone awry.
The team studied the bone structure and complex vasculature in the heads and necks of snowy, barred, and great horned owls after their deaths from natural causes.
The most striking team finding came after the researchers injected dye into the owls’ arteries, mimicking blood flow, and manually turned the animals’ heads. Blood vessels at the base of the head, just under the jaw bone, kept getting larger and larger as more of the dye entered.
Researchers say these contractile blood reservoirs allow 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, helps minimize any interruption 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, says Gailloud, the study shows “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,” he says.
The study is acknowledged in the February 1 issue of Science with a first-place in the posters and graphics category of the National Science Foundation’s 2012 International Science and Engineering Visualization Challenge.
The team found that one of the major arteries feeding the owl brain passes through bony holes in the vertebrae that are about 10 times larger than the artery. The researchers say the extra space in these transverse foraminae creates cushioning air pockets that allow the artery to move around when twisted. Twelve of the 14 cervical vertebrae in the owl’s neck 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,” de Kok-Mercado says.
The team also found that the owl’s vertebral artery enters the neck higher up than in other birds—going in at the owl’s 12th cervical vertebrae instead of the 14th—allowing for more vessel room and slack.
Among other findings were small vessel connections between the carotid and vertebral arteries—not usually seen in adult humans—that allow blood to be exchanged between the two blood vessels.
The researchers say these so-called anastomoses, including a vessel connection called a patent trigeminal artery, allow for uninterrupted blood flow to the brain, even if one route is blocked during extreme neck rotation.
Researchers next plan to examine hawk anatomy to see if other bird species possess the same adaptive features for head rotation.
The Johns Hopkins Hospital and the Vesalius Trust for Visual Communication in the Health Sciences provided funding for the study.
Source: Johns Hopkins University