title: Hemoglobin-oxygen affinity in high-altitude vertebrates: is there evidence for an adaptive trend?
abstract: In air-breathing vertebrates at high altitude, fine-tuned adjustments in hemoglobin (Hb)-O2affinity provide an energetically efficient means of mitigating the effects of arterial hypoxemia. However, it is not always clear whether an increased or decreased Hb-O2affinity should be expected to improve tissue O2delivery under different degrees of hypoxia, due to the inherent trade-off between arterial O2loading and peripheral O2unloading. Theoretical results indicate that the optimal Hb-O2affinity varies as a non-linear function of environmental O2availability, and the threshold elevation at which an increased Hb-O2affinity becomes advantageous depends on the magnitude of diffusion limitation (the extent to which O2equilibration at the blood-gas interface is limited by the kinetics of O2exchange). This body of theory provides a framework for interpreting the possible adaptive significance of evolved changes in Hb-O2affinity in vertebrates that have colonized high-altitude environments. To evaluate the evidence for an empirical generalization and to test theoretical predictions, I synthesized comparative data in a phylogenetic framework to assess the strength of the relationship between Hb-O2affinity and native elevation in mammals and birds. Evidence for a general trend in mammals is equivocal, but there is a remarkably strong positive relationship between Hb-O2affinity and native elevation in birds. Evolved changes in Hb function in high-altitude birds provide one of the most compelling examples of convergent biochemical adaptation in vertebrates.
authors: School of Biological Sciences, University of Nebraska, Lincoln, NE 68588, USA jstorz2@unl.edu.
authors: Jay F
authors: 0000-0001-5448-7924
authors: JF
authors: Storz
mesh_terms: D000222:Adaptation, Physiological; D000531:Altitude; D000818:Animals; D006454:Hemoglobins / Q000378:metabolism*; D000860:Hypoxia / Q000503:physiopathology; D010100:Oxygen / Q000378:metabolism*; D014714:Vertebrates / Q000502:physiology*
chemical_list: D006454:Hemoglobins; D010100:Oxygen
keywords: Biochemical adaptation; Blood oxygen transport; Hemoglobin; High-altitude adaptation; Hypoxia; Physiological adaptation