How did sauropods get blood up their necks?

Source: Wikipedia

Sauropods are the largest animals to have ever lived, surpassing whales even though they lived on land. Their unique anatomy, including their extremely long neck, has long been the subject of speculation and study, as their physiology must have stretched the limits of regular vertebrate capabilities. The topic of circulation is one central issue: how did they get blood all the way up several meters of neck? This is the topic of today’s post.

ResearchBlogging.org I am basing this post heavily on Seymour (2009).

The topic cannot be discussed alone, as one needs to get into the issue of blood pressure, which ties into heart structure and metabolism. A minimum blood pressure can be estimated by measuring the distance between the head and the heart – for dinosaurs, we approximate its location. Seymour (1976) provides several dinosaur numbers for this measure, and what he finds is that most dinosaurs, including the large theropods, had similar ranges to mammals and birds, with blood pressures much higher than fish and amphibians. This is one of the key evidences that points to dinosaurs having been endothermic, i.e. “warm-blooded”, using its own heart and metabolism to maintain body temperature.

Being endothermic means that dinosaurs most likely had a four-chambered heart, similar to the one found in birds and mammals. The reason is purely functional. A very high blood pressure can be sustained by most of the body’s blood vessels, but sending blood at such high pressures into the lungs would be disastrous, as lung capillaries are very thin. The result would be a pulmonary edema, the lungs filling with fluid, and death. A four-chambered heart circumvents this by allowing two separate streams: one low-pressure bloodstream sent from the left ventricle to the lungs, which comes back oxygenated to the other side of the heart to get pumped from the right ventricle to the rest of the body at a high pressure.

This high blood pressure also means that the hearts of endotherms are very large, as determined by Laplace’s Law (modified to apply to hearts). The ventricle wall’s thickness is proportional to pressure and radius, so a large ventricle with thicker walls is needed to produce a high blood pressure. This is why the left and right sides of the heart have walls of differing thicknesses: the left side produces the low blood pressure and is thin, relative to the thickened right side that sends high pressure to the rest of the body. Similarly, as body size increases, the hearts get larger and thicker.

And so we come to the sauropods and their gigantism. A sauropod needs to get a blood pressure of ~700 mm Hg to get blood to the top of its neck. For comparison, the average mammal has a blood pressure of 90 mm Hg. A fin whale produces “only” 100 mm Hg, although whales cheat by being in water. To produce a sauropod’s blood pressure, the whale would need a right ventricle that’s fifteen times heavier (up to two tons), with a wall five times thicker. This would effectively double the metabolic rate, meaning it would have to feed twice as much to be able to sustain itself.

This should give you an idea of the problem we face with sauropods. Sure, they could have had a heart that weighed several tons. But then they would have had to be sedentary, unless they had a source of very high-quality food (something I explored briefly here) that allowed them to maintain the resultant metabolic rate.

This is one of the reasons why sauropods are now reconstructed with their necks more or less horizontal. Without the need to fight against gravity for several meters up the neck, the blood pressure can get down to whale-level, with a similar heart. However, this brings up another issue: feeding and walking. A sauropod taking one step takes a huge amount of energy, as does raising its neck. So unless the saurood is sedentary and merely moves its neck from side to side to feed (we have fossilised sauropod trackways, so at least we know they walked), the problem of needing a super-large heart is not completely solved.

Source: Choy & Altman (1992)
Source: Choy & Altman (1992)

Several solutions have been proposed over the years. The most imaginative, in every sense of the word, is the hypothesis that sauropod necks had “accessory hearts”, little hearts that helped pump the blood up the neck. See Bakker (1978) and especially Choy & Altman (1992) for the reasonings on this. Creative, but there is a lack of any sort of evidence for this: multiple hearts aren’t known in any vertebrate, no vertebrate has arterial valves, the neck is really not wide enough to support multiple hearts, and this would have required quite a lot of innovations in the nervous system to allow for it to be controlled. While it would solve the problem from a physics point of view, the hypothesis just requires too many unevidenced assumptions to be taken seriously.

Dennis et al. (1992) and Badeer & Hicks (1996) provide another physics-informed hypothesis, based on the siphon principle, whereby the blood pressure drops to well below ambient levels at the upper reaches of the neck, so allowing for the blood to get “sucke up”. But there are several problems with this. The vessels would have to be extemely rigid, but this is plausible if completely unevidenced – veins are always flexible rather than rigid. Tt would be a very risky and tenuous set-up in any case. Interestingly though, any wound to the neck would mean air getting sucked into the wound, as well as no bleeding out (bleeding happens because of high-pressure blood squirting out). While physically-sound, it’s simply biologically unsound, so this hypothesis will need some extraordinary evidence to get supported.

Seymour & Lillywhite (2000) proposed the possibility that the sauropod heart pumps small amounts of blood faster, instead of pumping all the blood from a ventricle as happens normally. However, this is very inefficient, and still requires a very thick ventricle wall, and doesn’t solve the metabolic rate problems. So it’s not really a solid solution.

In fact, the only solution is to postulate that they just never raised their necks, as Stevens & Parrish (1999) and Seymour & Lillywhite (2000) point out. No need to get blood all the way up the neck means no problem getting blood to the head. Otherwise, one can go very old-skool, back to Edward Cope in the 19th century, who proposed that sauropods spent a large amount of time in the water, where gravity plays no effect due to the balancing of the internal and external hydrostatic pressures.

In other words, palaeontology still doesn’t have a satisfactory answer. The various hypotheses proposed all depend on too many biological assumptions not borne by phylogeny or actualistic comparisons (e.g. with giraffe necks), or they require knowledge of their ethology we do not yet have – we don’t know if they really floated around or never raised their necks. All we can do is research more.

References:

Badeer HS & Hicks JW. 1996. Circulation to the head of Barousaurus revisited: Theoretical considerations. Comparative Biochemistry and Physiology A 114, 197-203.

Bakker RT. 1978. Dinosaur feeding behaviour and the origin of flowering plants. Nature 274, 661-663.

Choy DSJ & Altman P. 1992. The cardiovascular system of Barosaurus: an educated guess. The Lancet 340, 534-536.

Dennis JM, Taylor MA, Hicks JW & Badeer HS. 1992. Barosaurus and its circulation. The Lancet 340, 1228-1229.

Seymour RD. 1976. Dinosaurs, endothermy and blood pressure. Nature 262, 207-208.

Seymour RD. 2009. Raising the sauropod neck: it costs more to get less. Biology Letters 5, 317-319.

Seymour RD & Lillywhite HB. 2000. Hearts, neck posture and metabolic intensity of sauropod dinosaurs. Proc. R. Soc. B 267, 1883-1887.

Stevens KA & Parrish JM. 1999. Neck Posture and Feeding Habits of Two Jurassic Sauropod Dinosaurs. Science 284, 798-800.

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