This blog will discuss why the owl twists his head and how it is able to do it without cutting of his brain blood supply. It was triggered by the fascinating new research by Fabian de Kok-Mercado, Michael Habib, Tim Phelps, Lydia Gregg, and Philippe Gailloud of the Johns Hopkins University School of Medicine.
We, like the owl, have our eyes in front of our heads & thus view objects with both eyes simultaneously. This is “binocular vision” (Figure 1). The advantage of this is that we can see in 3D and thus more accurately judge distances & depths. Obviously 3D vision is a major advantage in predators (as well as sportsmen etc). Many animals & birds have their eyes on either side of their heads i.e. monocular vision. The advantage of this type of vision is that wide visual fields enable detection the approach of danger from many directions.
Animals with binocular vision must also be able to scan larger areas without having to turn their whole body. We do this by moving our eyeballs using 6 muscles & by turning our heads 90 degrees to the right or left (Figure 2)
The owl has evolved such large eyes that there is simply not enough room in the head for eye muscles. Their tubular shaped eyes are fixed in the sockets & to scan a wider area it must turn its head. This it can do up to an incredible 270 degrees! This is possible because they have 14 cervical vertebrae compared just 7 in man. Furthermore the surface of the bodies of the interlinking vertebrae is more flexible in owls: the surface of their vertebral bodies is saddle shaped (“heterocoelous). In mammals, including humans, it is flat (“acelous”) & best adapted for weight bearing.
Thus the highly flexible neck required in owls could at times limit brain blood flow. We have already with our simple milk shake analogy pointed out the blood flow depends on the balance between 3 factors a)pressure b) resistance: diameter & length of a vessel, & c) viscosity of blood. The blood vessels can kink when the neck twists & the decreased diameter would raise the resistance to blood flow. This necessitated co-evolutionary adaptations in the owl to prevent rotation from kinking their cerebral blood vessels or provision of alternative methods to maintain the flow. The researchers from John Hopkins showed this is done by several fascinating adaptations.
- The carotid arteries of humans are positioned lateral to the bodies of the cervical vertebra whereas they are central & anterior to the vertebral bodies in the owl (Figure 3).
The latter position produces less torsion on the arteries when the neck is rotated to extreme positions
- The vertebral arteries are also an important route of blood flow to the brain. They run through the foramina of the lateral spinal processes of the vertebrae. In the owl these arteries do not enter the foramina in the first three cervical vertebra. The diameter of these foramina is much larger relative to the diameter of their vertebral arteries compared to humans & this space is filled with air sacs (Figure 3). This cushioned larger space limits arterial compression when the head swivels.
- If extreme rotation does kink the arteries circulation is maintained by two mechanisms (Figure 4).
i) Owls have an abundant collateral circulation to help bypass any obstruction. ii) The jugular arteries have contractile reservoirs so if a kink occurs & pressure falls proximal to their position they will contract & temporarily keep blood flowing.
It is interesting to note that the above research did not look at blood flow in cerebral veins. Veins are thinner walled than arteries and kink & compress more easily. Thus it is possible that when the owl twists its neck that venous return is compromised. Perhaps because the duration is relatively short lived the distensible venous compartments can expand to accommodate the temporary increase in volume. I did find a very old publication on internet that did discuss the extensive venous collaterals in the neck of birds that would allow flow to bypass temporary obstructions. However is will email the above authors to ask about venous flow. I will update the blog if I receive extra information.
Further reading: Podulka et al. Handbook of bird biology 2001 Princeton University Press