Piotr Minias (May 2020)
A comparative study identifies major ecological and biogeographical correlates of avian blood oxygen-carrying capacity
“Ecology and evolution of blood oxygen-carrying capacity in birds”
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Physiological characters are commonly thought to be highly plastic within individuals and show great variation between individuals within populations. However, it is often still possible to identify major evolutionary forces shaping inter-specific variation in physiology. In this paper, Piotr Minias from University of Łódź in Poland, used an extensive dataset on haematocrit and blood haemoglobin concentration measurements to track the evolution of blood oxygen-carrying capacity in birds. The analysis of published data for 300 avian species revealed several macroevolutionary and macroecological patterns in the traits that play a key role in blood oxygen transport. The scientist showed that biogeographical distribution is one of the most important predictors of blood oxygen-carrying capacity in birds, as high-latitude and polar species, as well as those living at high altitudes show more efficient blood oxygen transport. These results were consistent with hypotheses that oxidative metabolism is higher in colder climate due to increased thermoregulatory costs, and that enhanced oxygen-carrying capacity of blood should compensate for reduced partial oxygen pressure at high latitudes. Also, higher metabolic performance of smaller birds favored the evolution of higher haematocrits and haemoglobin concentrations in blood. Consistently, the lowest blood oxygen-carrying capacity was recorded in large non-passerines, while the evolution of both haematocrit and haemoglobin concentration proceeded via fluctuating selection towards higher values in younger avian lineages, including many passerine families. The paper not only reconstructs evolutionary history of important physiological traits in birds, but it also gives a novel insight into their functional variation from the macroevolutionary point of view.
Abstract
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 behavior, 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.