Neck length and mean arterial pressure in the sauropod dinosaurs

Abstract

How blood was able to reach the heads of the long-necked sauropod dinosaurs has long been a matter of debate and several hypotheses have been presented. For example, it has been proposed that sauropods had exceptionally large hearts, multiple 'normal' sized hearts spaced at regular intervals up the neck or held their necks horizontal, or that the siphon effect was in operation. By means of an experimental model, we demonstrate that the siphon principle is able to explain how blood was able to adequately perfuse the sauropod brain. The return venous circulation may have been protected from complete collapse by a structure akin to the vertebral venous plexus. We derive an equation relating neck height and mean arterial pressure, which indicates that with a mean arterial pressure similar to that of the giraffe, the maximum safe vertical distance between heart and head would have been about 12 m. A hypothesis is presented that the maximum neck length in the fossil record is due to the siphon height limit. The equation indicates that to migrate over high ground, sauropods would have had to either significantly increase their mean arterial pressure or keep their necks below a certain height dependent on altitude.

Restoring Maximum Vertical Browsing Reach in Sauropod Dinosaurs

“…However, because the occasional risk of momentary brain impairment is much more acceptable in swimmers than in land giants for whom the briefest cognitive failure could result in lethal injury, and because not being able to constantly hold the head high would seriously interfere with high browsing, it is more likely that sauropods had to generate whatever pressure was necessary to fully oxygenate the brain at a given height. A more modest pressure siphon arrangement has been proposed (Hughes et al, ), but this hypothesis is vigorously opposed by Seymour and Lillywhite () who, although their statement that biosiphons are entirely impossible may be too absolute, among other items note that even a tiny open wound connecting the atmosphere to the circulatory system in the negative pressure portion of the siphon would result in a fatal entry of air into the circulation. Being fast breeding, fast growing, small brained organisms sauropods like other giant dinosaurs (Paul, ) may have accepted such high safety risks, but the Seymour and Lillywhite () objection is a serious one.…”

Cardiovascular Physiology of Dinosaurs

“…In a siphon, the potential energy used by raising the blood in one arm of the loop is reclaimed by the descending blood in the other arm, so the pressure at the heart does not have to be high. This has been proposed specifically for the sauropods, including estimates that the MAP within the head could have been Ϫ500 mmHg sub-atmospheric (4,41,48,117). However, there are several fatal problems with this reoccurring idea (101,102).…”

“…Another suggestion to permit upright necks is to surround the vasculature with a column of fluid that has a hydrostatic gradient that matches that of the blood column (47,48). This idea is similar to the water-filled "G-suit" that protects aviators from syncope by facilitating venous return.…”

A Preliminary Study on the Siphon Mechanism in Giraffe (Giraffa camelopardalis)

“…Ref. [ 23 ] commented that veins in the cranium of sauropod dinosaurs could be inhibited from characteristic collapse via a similar structure to the vertebral venous plexus. Moreover, in humans, Ref.…”

“…Ref. [ 23 ] propose that sauropods may have had mechanisms, such as intracranial sinuses and dura mater, analogous to human ones in order to successfully manage cerebral venous collapse due to negative blood pressure in the brain. Is this a possible mechanism in long-necked individuals, such as giraffes and sauropod dinosaurs, as part of the mechanisms in place to handle negative cranial pressures?…”