Hummingbird blood traits track oxygen availability across space and time

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 1,217
Andean hummingbirds of 77 species across a 4,600 m elevational gradient.
Unexpectedly, elevational variation in hemoglobin concentration ([Hb]) was
scale independent, suggesting that physics of gas exchange, rather than
species differences, determine 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.