Mystery revealed of how owls can spin their heads

Undated composite image showing the 'extreme neck rotation' of a barred owl on Amherst Island, Ontario. (CTV/ Hester Riches)

Of all the amazing things that owls can do, one of the most fascinating – and creepiest – is their ability to turn their heads almost completely around.

Owls can rotate their necks 270 degrees – more than twice as much as a human can turn. This ability is needed to help them hunt for prey because their large, tubular-shaped eyes cannot move.

But what’s always baffled scientists is how owls can spin their heads to such extremes without tearing the delicate blood vessels in their necks, or pinching off the blood supply to their brains.

In a human, such large and sudden neck twists would almost certainly rip a blood vessel, which would be either instantly fatal or would cause a blood clot and eventually, a stroke.

Now, a team of researchers at Johns Hopkins University thinks they know how owls twist their necks without injury.

The team studied the bone structure and blood vessel systems in the heads and necks of snowy owls, barred owls and great horned owls, using birds that had died of natural causes and been collected by nature centres.

They injected contrast dye into the birds’ vessels and used X-ray imaging and CT scans while they manually turned their heads, to understand what makes owl necks so special.

What they found were four major adaptations.

The most striking was that the blood vessels at the base of an owl’s head, just under the jaw bone, grow larger as they branch out and can pool into reservoirs. We humans, on the other hand, have neck arteries that generally tend to get smaller as they go up into our brain.

The reservoirs in owl necks allow for blood flow to continue into the brain, even if the arteries are pinched lower down by a neck twist.

As well, they found that when owls’ arteries thread through the vertebrae in their necks, the holes they go through are much wider than in humans. In fact, the bone cavities are about 10 times larger in diameter than the artery going through it. This extra space creates a kind of cushioning air pocket that allows the artery to move around when twisted without being damaged.

This is an important feature, says one of the team’s researchers Fabian de Kok-Mercado, who’s a medical illustrator and who helped illustrate the team’s findings.

"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," he said in a release from Johns Hopkins.

The team also found that owls’ main neck artery leaves the vertebrae and enters the neck higher up than in other birds -- going in at the owl's 12th cervical vertebra instead of the 14th -- allowing for more vessel slack.

Finally, they found that there were unique vessel connections between two major neck arteries -- the carotid and vertebral arteries – connections that humans don’t have. These connections appear to allow blood to be exchanged between the two blood vessels, allowing for even more uninterrupted blood flow to the brain 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 study senior investigator, Dr. Philippe Gailloud, an interventional neuroradiologist, says this study answers a lot of questions.

"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," he said in a statement.

The team won first prize in the posters and graphics category of the National Science Foundation's 2012 International Science & Engineering Visualization Challenge. The win is noted in the Feb.1 issue of the journal Science.

The research team plans to publish their full study findings soon.