Scientists discover how owls rotate their heads without cutting off blood supply

It always appeared to fly in the face of logic.
But now, the biological secrets that allow owls to rotate their heads without cutting off their blood supply have finally been unravelled.
Scientists discovered four major adaptations in owls designed to prevent injury when the animals rotate their overly large heads by up to 270 degrees.
The study found that the birds' unique bone structures and vascular systems let them move with increased flexibility. Scientists at Johns Hopkins University School of Medicine in the US studied snowy, barred and great horned owls after their deaths from natural causes.
They found that the vertebral artery enters the neck higher than in other birds, creating more slack.
Unlike humans, owls were found to have small vessel connections between the carotid and vertebral arteries, allowing blood to be exchanged between the two blood vessels.
This creates an uninterrupted blood flow to the brain, even if one route is blocked during extreme neck rotation.
The adaptation gives the birds a huge range of vision without having to move their bodies and arouse detection by prey.
The lack of similar adaptations in humans could explain why humans are more vulnerable to neck injury, the experts concluded.
Study senior investigator Doctor Philippe Gailloud, said: '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.
'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.'
To solve the puzzle, the researchers studied the bone and blood vessel structures in the heads and necks of the birds.
An injectable contrast dye was used to highlight the birds' blood vessels, which were then dissected, drawn and scanned to allow detailed analysis. The most striking finding came after 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, and before the fluid pooled in reservoirs.
This contrasted starkly with human anatomical ability, where arteries generally tend to get smaller and smaller, and do not balloon as they branch out.
Researchers say these contractile blood reservoirs 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, helps minimise any interruption in blood flow. Dr Gailloud added: '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.'
Medical illustrator Fabian de Kok-Mercado said: '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.'
The team's findings were published in the journal Science.