The axial level of the heart in snakes

Abstract

Snakes are remarkable for their extremely elongated body plan, loss of limbs and increase in numbers of vertebrae. These adaptations have a number of functional advantages including the ability to enter the underground burrows of prey, to move around the tree canopies with relative ease, to strike at prey and to swim in water. Compared to four-legged squamate ancestors, snakes show a remarkable displacement and elongation of their viscera along their extended body axes. The heart in snakes varies widely in position along the body axis, and this variation in axial level may be correlated with phylogeny and/or with habitat. Here, we review the existing literature and examine the variation in the axial level of the heart in a sample of 15 mature snake specimens from arboreal, terrestrial, subterrestrial, semi-aquatic and aquatic habitats. Alcohol-preserved animals were dissected and examined radiographically. The results show that there is a trend towards a more rostral positioning of the heart along the body axis in arboreal snakes. However, this axial level falls within the range of that observed in terrestrial species. Phylogeny does not fully account for the observed axial level of the heart in the species examined. We conclude that a combination of habitat (both arboreal and aquatic) and phylogeny may explain the axial level of the heart in snakes.