Scaling of Cardiovascular Physiology in Snakes

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Abstract
The elongate body form of snakes and the wide diversity of habitats into which they have radiated have affected the form and function of the cardiovascular system. Heart position is strongly correlated with habitat. The heart is located 15–25% of the body length from the head in terrestrial and arboreal species, but 25–45% in totally aquatic species. Semi-aquatic and fossorial species are intermediate. The viperids are exceptional, with generally more posterior hearts but arboreal species have hearts closer to the head. An anterior heartis favored when snakes climb because it reduces the hydrostatic pressure of the blood column above the heart and tends to stabilize cephalic blood pressure. In water, where hydrostatic bloodpressure is not a problem, a more centrally located heart is favored because the heart does less work perfusing the body. In terrestrial species, head-heart distance increases linearly with body length and the increased hydrostatic pressure is matched by higher resting arterial blood pressure in longer animals. Unlike mammals and birds, snakes have blood pressures that increasewith body mass. The added stress on the ventricle wall in larger snakes is correlated with ventricles that are larger than predicted by other reptiles. Heart mass scales with body mass to the 0.95 power in snakes but only 0.77–0.91 in other reptiles that are not as subject to the hydrostatic effects of gravity. The spongy hearts of reptiles do not conform well to the Principle of Laplace.

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