Is Aquatic Life Correlated with an Increased Hematocrit in Snakes?

BackgroundPhysiological adaptations that allow air-breathing vertebrates to remain underwater for long periods mainly involve modifications of the respiratory system, essentially through increased oxygen reserves. Physiological constraints on dive duration tend to be less critical for ectotherms than for endotherms because the former have lower mass-specific metabolic rates. Moreover, comparative studies between marine and terrestrial ectotherms have yet to show overall distinct physiological differences specifically associated with oxygen reserves.Methodology/Principal FindingsWe used phylogenetically informed statistical models to test if habitat affects hematocrit (an indicator of blood oxygen stores) in snakes, a lineage that varies widely in habitat use. Our results indicate that both phylogenetic position (clade) and especially habitat are significant predictors of hematocrit. Our analysis also confirms the peculiar respiratory physiology of the marine Acrochordus granulatus.Conclusion/SignificanceContrary to previous findings, marine snakes have significantly–albeit slightly–elevated hematocrit, which should facilitate increased aerobic dive times. Longer dives could have consequences for foraging, mate searching, and predation risks. Alternatively, but not exclusively, increased Hct in marine species might also help to fuel other oxygen-demanding physiological adaptations, such as those involved in osmoregulation.研究背景 能够使空气呼吸脊椎动物长时间停留于水下的生理适应机制，主要涉及呼吸系统的改造，本质上通过提升氧气储备量实现。相较于恒温动物，变温动物受到的潜水时长生理约束通常更弱，这是因为变温动物的单位质量代谢率更低。此外，目前针对海洋与陆生变温动物的比较研究，尚未发现与氧气储备直接相关的系统性显著生理差异。 研究方法与主要结果 我们采用基于系统发育信息的统计模型，针对栖息地使用差异广泛的蛇类类群，检验栖息地是否会影响红细胞比容（hematocrit，后文简称Hct，血液氧气储备的指标）水平。研究结果显示，系统发育位置（演化支）尤其是栖息地类型，均为红细胞比容的显著预测因子。本分析同时证实了海洋棘鳞海蛇（Acrochordus granulatus）独特的呼吸生理特性。 研究结论与意义 与以往研究结果相悖的是，海洋蛇类的红细胞比容虽仅小幅升高，但仍具有统计学显著性，这将有助于延长其有氧潜水时长。更长的潜水时长可能会对觅食、配偶搜寻以及被捕食风险产生影响。除此之外，海洋物种红细胞比容升高也可能为其他需氧生理适应过程提供能量支持，例如与渗透压调节相关的生理过程。