Hummingbird blood traits track oxygen availability across space and time

• Published in: Ecology letters Vol. 26; no. 7; pp. 1223 - 1236
• Main Authors: Williamson, Jessie L., Linck, Ethan B., Bautista, Emil, Smiley, Ashley, McGuire, Jimmy A., Dudley, Robert, Witt, Christopher C.
• Format: Journal Article
• Language: English
• Published: England Blackwell Publishing Ltd 01.07.2023
• Subjects: Adaptation; Andes; Availability; Blood; blood–oxygen; Cell number; Cell size; Concentration gradient; Divergence; ecophysiology; elevation; Erythrocytes; Evolution & development; Gas exchange; haematology; haemoglobin; Hemoglobin; High altitude; hummingbird; Oxygen; Phenotypic plasticity; Phylogeny; Species; trait–environment association; Trochilidae; Variation; elevation; Andes; haemoglobin; hummingbird; haematology; phylogeny; trait-environment association; blood; ecophysiology; blood-oxygen
• ISSN: 1461-023X; 1461-0248
• DOI: 10.1111/ele.14235

Predictable trait variation across environments suggests shared adaptive responses via repeated genetic evolution, phenotypic plasticity or both. Matching of trait–environment associations at phylogenetic and individual scales implies consistency between these processes. Alternatively, mismatch implies that evolutionary divergence has changed the rules of trait–environment covariation. Here we tested whether species adaptation alters elevational variation in blood traits. We measured blood for 1217 Andean hummingbirds of 77 species across a 4600‐m elevational gradient. Unexpectedly, elevational variation in haemoglobin concentration ([Hb]) was scale independent, suggesting that physics of gas exchange, rather than species differences, determines responses to changing oxygen pressure. However, mechanisms of [Hb] adjustment did show signals of species adaptation: Species at either low or high elevations adjusted cell size, whereas species at mid‐elevations adjusted cell number. This elevational variation in red blood cell number versus size suggests that genetic adaptation to high altitude has changed how these traits respond to shifts in oxygen availability. Trait–environment associations can be caused, altered, or broken by adaptive evolution. Comparing such associations at individual versus phylogenetic scales can reveal these processes. We modelled blood traits by environment within and among 77 species of Andean hummingbirds across a 4,600‐meter elevational gradient. Unexpectedly, elevational variation in haemoglobin concentration was scale independent, reflecting physical laws of gas exchange in hummingbird respiratory systems. By contrast, species varied by mechanism of haemoglobin adjustment, revealing a signature of genetic adaptation to extreme elevations.