If you or I tried to swivel our head round by 270 degrees, we'd cut off the blood supply to our brains and pass out - or worse. But owls manage it: and now scientists have worked out how.
A team from Johns Hopkins has found four major biological adaptations designed to prevent the birds from injuring themselves when they rotate their heads.
"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 and 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."
And it is something that sometimes happens in humans, in situations from car accidents to roller coaster rides and chiropractic manipulations. When it does, blood vessel linings can stretch and tear, producing clots that can break off and cause a fatal embolism or stroke.
The Johns Hopkins team studied the bone structure and blood vessels of snowy, barred and great horned owls which had died from natural causes. The most striking finding came when researchers injected dye into the owls' arteries, mimicking blood flow, and manually turned their heads.
Blood vessels at the base of the head, just under the jaw bone, grew larger and larger, as more of the dye entered, and before the fluid pooled in reservoirs. This is very different to the situation in people, where arteries generally tend to get smaller and smaller, and don't balloon as they branch out. They also found differences in the owls' necks, creating a set of cushioning air pockets that allow the artery to move around when twisted.
"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 medical illustrator Fabian de Kok-Mercado.
And the owl's vertebral artery enters the neck higher up than in other birds, with small vessel connections between the carotid and vertebral arteries allowing blood to flow uninterrupted to the brain, even if one route is blocked.
The researchers say their findings are a good reason to avoid chiropractice.
"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,"says Gailloud. "Extreme manipulations of the human head are really dangerous because we lack so many of the vessel-protecting features seen in owls."