Scaling of Cardiovascular Physiology in Snakes

The elongate body form of snakes and the wide diversity of habitatsinto which they have radiated have affected the form and functionof the cardiovascular system. Heart position is strongly correlatedwith habitat. The heart is located 15–25% of the bodylength from the head in terrestrial and arboreal species, but25–45% in totally aquatic species. Semi-aquatic and fossorialspecies are intermediate. The viperids are exceptional, withgenerally more posterior hearts but arboreal species have heartscloser to the head. An anterior heartis favored when snakesclimb because it reduces the hydrostatic pressure of the bloodcolumn above the heart and tends to stabilize cephalic bloodpressure. In water, where hydrostatic bloodpressure is not aproblem, a more centrally located heart is favored because theheart does less work perfusing the body. In terrestrial species,head-heart distance increases linearly with body length andthe increased hydrostatic pressure is matched by higher restingarterial blood pressure in longer animals. Unlike mammals andbirds, snakes have blood pressures that increasewith body mass.The added stress on the ventricle wall in larger snakes is correlatedwith ventricles that are larger than predicted by other reptiles.Heart mass scales with body mass to the 0.95 power in snakesbut only 0.77–0.91 in other reptiles that are not as subjectto the hydrostatic effects of gravity. The spongy hearts ofreptiles do not conform well to the Principle of Laplace.