All 2700 species of snakes lack legs and have long elongate bodies. Some species may reach a length of many meters and this morphology renders their cardiovascular system of snakes very prone to the influence of gravity. Such influence may be aggravated by the low blood pressure that are characteristic of ectothermic vertebrates and most snakes (cf. Wang et al., 2003). A long held hypothesis proposes that the heart of arboreal snakes is positioned closer to the head than terrestrial species, which was speculated to be an adaptation that secures the cerebral perfusion when arboreal snakes lift their head (Lillywhite, 1987). A recent phylogenetic analysis of more than 150 species of snakes belonging to various families, which takes the evolutionary history of the various species into account, however, shows that the heart of arboreal snakes is placed more posteriorly than terrestrial species (Gartner et al, in review). We have investigated the effects of head position on blood pressures, cardiac output and blood flow in the carotid artery of anaesthetised rattlesnakes and show that blood flow to the brain is severely compromised when the head is lifted above heart level. We also have indications that blood pools in the posterior part of the body when the rear end of the snake is lowered below heart level, which caused a decrease in cardiac filling and stroke volume.

Gartner, G.E.A., J.W. Hicks, P.R. Manzani, D.V. Andrade, A.S. Abe, T. Wang, S.M. Secor and T.Garland Jr. Phylogeny, Ecology, and Heart Position in Snakes. Physiological and Biochemical Zoology (in review).

Lillywhite, H.B. (1987). Circulatory adaptations of snakes to gravity. American Zoologist 27:81–95.

Wang, T., J. Altimiras, W. Klein and M. Axelsson (2003). Ventricular haemodynamics in Python molurus: separation of pulmonary and systemic pressures. Journal of Experimental Biology 206: 4241–4245.