The Functional Significance of the Reptilian Heart: New Insights into an Old Question

Abstract

The anatomy of the nonavian reptile heart allows for mixing of oxygen rich and oxygen poor blood (cardiac shunts). The degree and the direction of the cardiac shunts are under autonomic control and changes characteristically during the intermittent ventilation of the lungs. While cardiac shunts are detrimental to oxygen transport in the endothermic birds and mammals, the potential to control the amount of mixing of blood within the nonavian ventricle has often been considered a derived trait, conveying important physiological functions favored by natural selection. Although this notion was not supported by experimental evidence, the perception that cardiac shunts represent an adaptive (Darwinian) phenotypic trait permeates much of the comparative physiological literature. Over the past two decades, a growing body of evidence from computational models and a variety of experimental approaches examining this basic tenet has altered our view of the adaptive significance of circulatory “design” of the nonavian reptile heart. Here we review the results from the various approaches to understand the functional significance of cardiac shunting. The overall conclusion is that cardiac shunting is not an adaptive feature. In contrast, the evidence supports the alternative hypothesis that cardiac anatomy and resulting cardiac shunts in nonavian reptiles are plesiomorphic traits that do not negatively impact the primary function of the cardiovascular system. Future studies, focused on investigating the genetic regulatory networks that determine the diversity of cardiac morphologies, are proposed to hold the key to ultimately understand the evolution and functional significance of the vertebrate heart.