In their correspondence ‘Why vascular siphons with sub-atmospheric pressures are physiologically impossible in sauropod dinosaurs’, Seymour and Lillywhite discuss a number of physiological problems with a siphon operating in sauropods. They cite three main objections: (1) collapse of blood vessels in the head and neck outside the protection of the cerebral spinal fluid; (2) outgassing of oxygen and nitrogen bubbles due to low hydrostatic pressure; and (3) several problems associated with capillary function and vascular damage if interstitial fluid created a protective head-to-claw hydrostatic gradient.
If these issues are insurmountable, then it would be reasonable to conclude that the natural pose of the sauropod neck was horizontal. However, the purpose of our paper (Hughes et al., 2016) is to stimulate discussion about whether vertical necks really are impossible or whether there is a solution. Our paper postulates that a parallel pathway in the neck, i.e. veins and vertebral venous plexus, would provide a continuous fluid stream back to the heart so that a siphon could operate. If this was the case, the left ventricle of the sauropod heart would not need to generate pressures of the order 760 mmHg to overcome gravity. Another important consideration is energy. If a siphon were in operation, the sauropod heart would only need to supply sufficient energy to overcome vascular friction and not gravity.
An important recent development of relevance to the head high/low discussion is a paper by Smerup et al. (2016), referenced in our paper, who hypothesise that giraffe hearts have a small intraventricular cavity and a relatively thick ventricular wall to generate high arterial pressures, albeit with normal left ventricular wall tension.
This at least raises the possibility that sauropod hearts were actually able to generate higher pressures than previously thought without a huge heart out of proportion to extant animals. The paper by Smerup et al. (2016) raises the question of whether the giraffe heart could be extrapolated to generate the pressure and flow required for a high head. If not, are giraffe necks at the maximum height allowed by physiology?
Many extant organisms are adapted to extreme environments, for example, extreme cold (Duman, 2015) or extremely low barometric pressure (Scott, 2011). High-altitude birds are particularly relevant to the current discussion. Rüppell's vulture purportedly holds the height record at 11,278 m. The bar-headed goose flies over the Himalayas as it migrates between Tibet and India, and has been seen flying over Everest at 9000 m.
At 10,000 m, the barometric pressure is 25% of the pressure at sea level, equivalent to the hydrostatic pressure inside a siphon 7.5 m above the upper reservoir. High-altitude birds have to cope with two main challenges – flapping their wings more vigorously to stay aloft in the rarefied air and extracting oxygen from air with only one-quarter the amount per volume at sea level.
Suppose there were no extant high-altitude birds, the bar-headed goose was extinct and only fossilised remains of the bar-headed goose had been found on the top of Everest. It might be reasonable to conclude that because no extant birds fly over Everest, the bar-headed geese had been blown on top of the mountain by a storm. However, the bar-headed goose exists and studies have revealed various high-altitude flying ‘survival features’, such as haemoglobin with an increased affinity for oxygen. Did the sauropods have ‘survival’ features allowing a high-altitude head?
In the case of high-altitude birds, because the barometric pressure can be as low as 0.25 bar, the absolute partial pressure of oxygen in the tissues will also be 75% lower. The relative partial pressure of oxygen between the red blood cells and the tissues will be similar to that at low altitude, and so oxygen detaches from the haemoglobin and diffuses across to the tissues. Could sauropods have had a similar type of haemoglobin to high-altitude birds?
The following is a hypothesis that could resolve all three of Seymour and Lillywhite's objections to a sauropod siphon. The hypothesis is that the interstitial fluid above the heart of the sauropods had the same or similar pressure reference as the hydrostatic indifferent point (HIP). If this were the case, the interstitial pressure would reduce at the same rate as in the circulation. Because the relative partial pressure of oxygen between the blood and tissues would be similar at different levels, oxygen would not leave the haemoglobin ‘early’ as blood ascended to the head. The same goes for dissolved nitrogen. This resolves issue 1 in the list above.
Another advantage of this schema is that all of the small blood vessels out to the surface of the skin would be protected from collapse by the interstitial fluid. This resolves issue 2. Also, because of the negative hydrostatic pressure of the interstitial fluid above the HIP, the fluid would not bear down on the vasculature below the HIP. Effectively, the neck of the sauropod would be like a barometer. In a barometer, the column of water is supported by the difference in pressure between the atmosphere and low pressure (vapour pressure of water) at the top. The hydrostatic pressure in the column is negative above the level of the water in contact with the atmosphere, which means the pressure directly beneath the barometer column is the same as at any other point at the same depth.
If this were the case in the sauropod neck, it would mean that weight of the interstitial fluid would be supported by the ambient atmospheric pressure (via the lungs) and so would not increase the hydrostatic pressure in the limbs. In the case of the sauropods there would also be some support from hydrogen bond attraction, which would ultimately be supported by the skeleton. As a result, pressures below the HIP for a sauropod with a raised head would be the same as for a horizontal neck. This resolves issue 3.
Of course, it could be countered that there is no evidence for the above hypothesis. However, in the presumed absence of Jurassic Park-style cloning, at some point in the future it might be possible to generate a physiologically realistic computer simulation to test the viability of this hypothesis.