Gartner Gabriel E A, Hicks James W, Manzani Paulo R, Andrade Denis V, Abe Augusto S, Wang Tobias, Secor Stephen M, Garland Theodore
Department of Biology, University of California, Riverside, California 92521, USA.
Physiol Biochem Zool. 2010 Jan-Feb;83(1):43-54. doi: 10.1086/648509.
The cardiovascular system of all animals is affected by gravitational pressure gradients, the intensity of which varies according to organismic features, behavior, and habitat occupied. A previous nonphylogenetic analysis of heart position in snakes-which often assume vertical postures-found the heart located 15%-25% of total body length from the head in terrestrial and arboreal species but 25%-45% in aquatic species. It was hypothesized that a more anterior heart in arboreal species served to reduce the hydrostatic blood pressure when these animals adopt vertical postures during climbing, whereas an anterior heart position would not be needed in aquatic habitats, where the effects of gravity are less pronounced. We analyzed a new data set of 155 species from five major families of Alethinophidia (one of the two major branches of snakes, the other being blind snakes, Scolecophidia) using both conventional and phylogenetically based statistical methods. General linear models regressing log(10) snout-heart position on log(10) snout-vent length (SVL), as well as dummy variables coding for habitat and/or clade, were compared using likelihood ratio tests and the Akaike Information Criterion. Heart distance to the tip of the snout scaled isometrically with SVL. In all instances, phylogenetic models that incorporated transformation of the branch lengths under an Ornstein-Uhlenbeck model of evolution (to mimic stabilizing selection) better fit the data as compared with their nonphylogenetic counterparts. The best-fit model predicting snake heart position included aspects of both habitat and clade and indicated that arboreal snakes in our study tend to have hearts placed more posteriorly, opposite the trend identified in previous studies. Phylogenetic signal in relative heart position was apparent both within and among clades. Our results suggest that overcoming gravitational pressure gradients in snakes most likely involves the combined action of several cardiovascular and behavioral adaptations in addition to alterations in relative heart location.
所有动物的心血管系统都会受到重力压力梯度的影响，其强度会根据生物体特征、行为和所处栖息地的不同而变化。先前一项对蛇类心脏位置的非系统发育分析发现，蛇类经常采取垂直姿势，陆生和树栖物种的心脏位于从头算起占总身体长度的15%-25%处，但水生物种的心脏位于25%-45%处。据推测，树栖物种中位置更靠前的心脏有助于在这些动物攀爬时采取垂直姿势时降低静水压，而在重力影响不太明显的水生栖息地则不需要心脏靠前的位置。我们使用传统统计方法和基于系统发育的统计方法，分析了来自真蛇亚目五个主要科的155个物种的新数据集（真蛇亚目是蛇类的两个主要分支之一，另一个是盲蛇亚目）。使用似然比检验和赤池信息准则，比较了将对数（10）吻部到心脏的位置对对数（10）吻部到泄殖腔长度（SVL）进行回归的一般线性模型，以及对栖息地和/或类群进行编码的虚拟变量。吻端到心脏的距离与SVL呈等比例缩放。在所有情况下, 与非系统发育模型相比，在奥恩斯坦-乌伦贝克进化模型下纳入分支长度转换（以模拟稳定选择）的系统发育模型更能拟合数据。预测蛇类心脏位置的最佳拟合模型包括栖息地和类群两个方面，表明我们研究中的树栖蛇类心脏位置往往更靠后，这与先前研究中发现的趋势相反。相对心脏位置的系统发育信号在类群内部和类群之间都很明显。我们的研究结果表明，除了相对心脏位置的改变之外，蛇类克服重力压力梯度最有可能涉及多种心血管和行为适应的共同作用。
