SECOR, Stephen M.; University of Alabama: Heart position in snakes, ontogenetic shifts and correlation with other organs

It has been hypothesized for snakes that heart position is adaptively link to habitat. From aquatic to terrestrial to arboreal habitats as snakes are more often vertically oriented, there is an increase tendency for their hearts to be positioned closer to the head. With a more anterior-positioned heart, adequate circulation can be maintained to the head when the body is positioned upright. Given the attention directed at the generality of this feature over a wide diversity of snake species, it has not been asked whether relative heart position varies ontogenetically or is linked to the position of other organs. For the semi-aquatic diamondback water snake (Nerodia rhombifer), the terrestrial sidewinder (Crotalus cerastes) and the semi-arboreal Burmese python (Python molurus), I measured the position of internal organs of neonate to adult individuals spanning respective 260-, 40-, and 600-fold range in body mass. For these snakes, the heart on average was positioned 19.5%, 38.0%, and 23.5% of snout-vent length from the snout, respectively. Each species exhibited a significant decrease in the relative position of the heart with body size, the shift equaling as much as 6% of SVL. Heart position was linked to the position of other organs intra- and interspecifically. Individual snakes whose hearts were positioned anterior or distal to a predicted location tended to have other organs positioned accordingly. Among three species, the more distal the heart is located, the further back are positioned the other organs, with a decrease in the distance relative to SVL between the heart and those organs.