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. 2009 Jun 23;5(3):317-9.
doi: 10.1098/rsbl.2009.0096. Epub 2009 Apr 1.

Raising the sauropod neck: it costs more to get less

Roger S Seymour  1
Affiliations

Raising the sauropod neck: it costs more to get less

Roger S Seymour. Biol Lett. .

Abstract

The long necks of gigantic sauropod dinosaurs are commonly assumed to have been used for high browsing to obtain enough food. However, this analysis questions whether such a posture was reasonable from the standpoint of energetics. The energy cost of circulating the blood can be estimated accurately from two physiological axioms that relate metabolic rate, blood flow rate and arterial blood pressure: (i) metabolic rate is proportional to blood flow rate and (ii) cardiac work rate is proportional to the product of blood flow rate and blood pressure. The analysis shows that it would have required the animal to expend approximately half of its energy intake just to circulate the blood, primarily because a vertical neck would have required a high systemic arterial blood pressure. It is therefore energetically more feasible to have used a more or less horizontal neck to enable wide browsing while keeping blood pressure low.

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Figures

Figure 1
Figure 1
A model of the energy cost of the circulatory systems of sauropod dinosaurs in relation to the metabolic cost of maintaining the body alone. The assumptions of the model are that total basal metabolic rate in an animal is equivalent to watts=3.6 mass0.71, an allometric relationship derived from mammals (White et al. in press). The heart consumes 10%, and the body 90%, of the total metabolic rate, if the blood pressure is 100 mm Hg. A blood pressure of 750 mm Hg causes the circulatory cost to equal approximately that of the rest of the body. Because circulatory work is a constant proportion of metabolic rate at a given blood pressure, assuming a lower metabolic rate of an ectotherm or a higher rate of an active animal in the field has no effect other than to change the scale of the y-axis.
Figure 2
Figure 2
Model of the effects of raising the neck on the arterial blood pressure at the heart and the potential volume of food available to a stationary sauropod. Blood pressure is assumed to be regulated at 100 mm Hg when the neck is horizontal or lowered. Raising the head above horizontal increases central blood pressure according to the density of blood and the vertical distance above the chest region. The feeding volume increments are the volumes of the ‘spherical segments’, i.e. stacked slices of the sphere enclosed by horizontal planes arbitrarily set 1 m apart. The radius of the sphere is assumed to be 9 m (the distance from the heart to the head), and the heart is placed 3 m above the ground. The graph is illustrative of the diminishing returns from lifting the head; it is not assumed that the entire volume of each segment is available for feeding, but only an arbitrary fraction of it.
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