Is Aquatic Life Correlated with an Increased Hematocrit in Snakes?

Background
Physiological 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 Findings
We 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/Significance
Contrary 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
空气呼吸脊椎动物若要实现长时间水下停留，其生理适应机制主要涉及呼吸系统的改造，核心途径为提升氧气储备量。相较于恒温动物（endotherms），变温动物（ectotherms）所受的潜水时长生理限制通常更弱，这是因为变温动物的单位质量代谢率更低。此外，当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明确相关的整体性生理差异。
研究方法与主要发现
本研究采用系统发育关联统计模型（phylogenetically informed statistical models），针对栖息地使用类型差异显著的蛇类类群，检验栖息地是否会影响其红细胞比容（hematocrit，血液氧储备的指示指标）。研究结果显示，系统发育位置（进化支clade）尤其是栖息地，均为红细胞比容的显著预测因子。本分析同时证实了海洋物种小粒棘鳞蛇（Acrochordus granulatus）独特的呼吸生理特征。
研究结论与意义
与既往研究结果相悖，海洋蛇类的红细胞比容虽仅小幅升高，但仍具有显著统计学意义，这一特征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行为、配偶搜寻以及捕食风险产生相应影响。此外（并非唯一诱因），海洋物种红细胞比容的升高，或许还可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。