Ecology and evolution of blood oxygen-carrying capacity in birds

Blood oxygen-carrying capacity is one of important determinants of oxygen amounts supplied to the tissues per unit time and plays a key role in oxidative metabolism. In wild vertebrates, blood oxygen-carrying capacity is most commonly measured with the total blood haemoglobin concentration (Hb) and haematocrit (Hct), which is the volume percentage of red blood cells in blood. Here, I used published estimates of avian Hb and Hct (nearly one thousand estimates from 300 species) to examine macroevolutionary patterns in blood oxygen-carrying capacity of blood in birds. Phylogenetically-informed comparative analysis indicated that blood oxygen-carrying capacity was primarily determined by species distribution (latitude and elevation) and morphological constraints (body mass). I found little support for the effect of life history components on blood oxygen-carrying capacity, except for a positive association of Hct with clutch size. Hb was also positively associated with diving
behaviour, but I found no effect of migratoriness on either Hb or Hct. Fluctuating selection was identified as the major force shaping the evolution of blood oxygen-carrying capacity. The results offer novel insights into the evolution of Hb and Hct in birds, as well as they provide a general, phylogenetically-robust support for some long-standing hypotheses in avian ecophysiology.

血液携氧能力（Blood oxygen-carrying capacity）是决定单位时间内向组织供氧总量的重要影响因素之一，在氧化代谢过程中发挥关键作用。在野生脊椎动物中，血液携氧能力通常通过全血血红蛋白浓度（total blood haemoglobin concentration, Hb）和血细胞比容（haematocrit, Hct）进行测定，其中血细胞比容指血液中红细胞所占的体积百分比。本研究利用已发表的鸟类Hb与Hct估算数据（涵盖300个物种的近千条记录），探究了鸟类血液携氧能力的宏观进化模式。纳入系统发育信息的比较分析（phylogenetically-informed comparative analysis）结果显示，血液携氧能力主要受物种分布（纬度与海拔）及形态约束因素（体重）调控。本研究未发现生活史特征对血液携氧能力存在显著影响，仅观察到血细胞比容与窝卵数呈正相关关系。此外，Hb与潜水行为呈显著正相关，但未发现迁徙习性对Hb或Hct存在显著作用。波动选择被确定为驱动血液携氧能力进化的主要选择压力。本研究结果为鸟类Hb与Hct的进化研究提供了全新视角，同时也为鸟类生态生理学领域的若干长期存在的假说提供了具备系统发育稳健性的普适性支持。