This evolutionary adaptation helps the birds keep their fixed-socket, binocular eyes trained on the scurrying mice and other small prey they hunt.
Fabian de Kok-Mercado, a Johns Hopkins-trained medical illustrator and an owl enthusiast, was curious. So he and a Johns Hopkins Medicine team used advanced imaging techniques to plumb the mystery.
What they found surprised them. It turns out that blood pools in an owl's neck arteries at the base of the head, possibly to provide a ready supply of oxygen-rich blood to the brain during a rapid head-turning.
"We definitely did not expect that," de Kok-Mercado said.
The team also found that owl vertebrae contain large spaces that give the vertebral artery some "wiggle room" when a bird twists its head. "We didn't expect that either," he said.
The findings were published recently in poster form in the journal Science, but the research was conducted in 2007 and 2008.
At the time de Kok-Mercado was looking for a thesis project while pursuing a master's in the art as applied to medicine program at the Johns Hopkins School of Medicine. Other students have examined topics such as liver cancer and snake anatomy. The idea is to participate in research and illustrate it.
By then, a steady diet of David Attenborough's nature films had turned de Kok-Mercado into a keen admirer of owls and other raptors such as hawks and falcons.
Fellow medical illustrator Lydia Gregg — now de Kok-Mercado's wife — had begun working with Dr. Philippe Gailloud, an interventional neuroradiologist at Hopkins. He's a blood vessel expert who treats strokes and other head and neck injuries. She introduced the two men.
"On one side, Fabian wanted to study owls and in particular their neck biomechanics," Gailloud recalled. "I do blood vessels. Then we thought, let's study the blood vessels of owls."
Even though an owl's head-rotating ability is such a notable feature, Gailloud said it's not too surprising that more wasn't understood already. He pointed out that much remains unknown about the vascular makeup of the human spinal cord despite centuries of focus on the human anatomy. "Here it's even worse: Who looks at the blood vessels of owls?"
Another reason earlier studies hadn't provided answers is that this type of research was far more difficult before the emergence of advanced CT scanners and other imaging tools.
"This imaging technology is so new it really hasn't been used on a lot of avian specimens," said Gregg, who was also involved in the research. "We'd like to start looking at different families of birds and see if this goes for all raptors or just owls."
The study examined 12 snowy, barred and great horned owls, all of which had died naturally. The team used Gailloud's CT scanners after hours. They opened up the birds' chests, then forced dye into the blood vessels to mimic blood flow.
"We were trying to see how the blood actually traveled along the neck up to the head," said de Kok-Mercado, an illustrator and animator in the department of science education at the Howard Hughes Medical Institute in Chevy Chase. "After we did that, we were able to look at the owls through dissection."
Gailloud said human neck arteries are quite fragile. Even holding a phone for 20 minutes in the crook of one's shoulder can cause problems. Whiplash from a car accident or roller coaster can easily do real damage, and arterial injuries can lead to strokes. He was intrigued to investigate how owls can turn their heads to a degree that no person ever could.
"As a joke, we always say, 'Why isn't the floor of the forest littered with stroked owls?'" he said.
Owls have an advantage in that their neck arteries travel up a central axis. In humans, the major neck arteries are positioned near the outside, making them more vulnerable to twisting or jerking.
One finding that did not surprise the research team was the existence of many connections between blood vessels in an owl's neck — alternate routes to keep blood flowing even if there's a blockage along one pathway.
But the blood-pooling was a surprise, since in people arteries tend to get smaller until they end in tiny capillaries. Also unexpected were the big gaps in neck bones that are 10 times wider than the arteries themselves. In humans, the same arteries touch bone.
"We thought maybe there was one answer; what we found is there are many answers," Gailloud said, adding that owls have developed redundant features over time. "So even if one trick does not work at some point, you have two or three other tricks. It's a set of answers."
De Kok-Mercado's poster illustrating the findings in painstaking detail won first place in the posters and graphics category of the National Science Foundation's 2012 International Science & Engineering Visualization Challenge.
For Gregg and de Kok-Mercado, the research dovetails with their love of raptors. They volunteer at an aviary for permanently damaged raptors at the Soldiers Delight Natural Environment Area in Owings Mills. They talk to elementary school students about raptors and how human behavior affects them.
Many of the injured birds were hit by cars while trying to catch mice that had been attracted to the side of the road by littered apple cores or other discarded food, Gregg said.
De Kok-Mercado said "it doesn't really matter" that the findings have no clinical relevance to human medical treatment. To him the value is that it "goes to show how amazing these creatures are and makes people more aware of this planet in general."