Blood variation implicates respiratory limits on elevational ranges of Andean birds

Abstract

The extent to which species ranges reflect intrinsic physiological tolerances is a major, unsolved question in evolutionary ecology. To date, consensus has been hindered by the limited tractability of experimental approaches across most of the tree of life. Here, we apply a macrophysiological approach to understand how hematological traits related to oxygen transport shape elevational ranges in a tropical biodiversity hotspot. Along Andean elevational gradients, we measured traits that affect blood oxygen-carrying capacity--total and cellular hemoglobin concentration and hematocrit--for 2,355 individuals of 136 bird species. We used these data to evaluate the influence of hematological traits on elevational ranges. First, we asked whether hematological plasticity is predictive of elevational range breadth. Second, we asked whether variance in hematological traits changed as a function of distance from the midpoint of the elevational range. We found that the correlation between hematological plasticity and elevational range breadth was slightly positive, consistent with a facilitative role for plasticity in elevational range expansion. We further found reduced local variation in hematological traits near elevational range limits and at high elevations, patterns consistent with intensified natural selection, reduced effective population size, or compensatory changes in other cardiohematological traits with increasing distance from species-specific optima for oxygen availability. Our findings suggest that constraints on hematological plasticity and local genetic adaptation to oxygen availability promote the evolution of the narrow elevational ranges that underpin tropical montane biodiversity.

Genomic Variation, Population History, and Long-Term Genetic Adaptation to High Altitudes in Tibetan Partridge (Perdix hodgsoniae)

“…Furthermore, prolonged exposure to low PO 2 is associated with long-term genetic changes involving, e.g. elevated hemoglobin concentration ( Beall 2000 ; Linck et al 2021 ), increased hemoglobin-oxygen affinity ( Storz et al 2007 ; Projecto-Garcia et al 2013 ; Natarajan et al 2015 ), changes in hematology, relative heart mass ( DuBay and Witt 2014 ), increased capillarization of muscle fibers ( Monge and Leon-Velarde 1991 ; Scott et al 2009 ; Butler 2010 ), changes in breathing patterns ( Scott 2011 ; Lague et al 2017 ), or additional changes that optimize metabolic functions ( Storz et al 2010 ; Dawson et al 2020 ).…”

Hummingbird blood traits track oxygen availability across space and time

“…The macro-physiological ‘rules’ governing blood trait variation over phylogenetic and population timescales have remained elusive because adequate comparative data are scarce. Blood traits are known to vary among avian clades (e.g., [28,29]) and exhibit elevational variation both within species (e.g., [30–32]) and among species (e.g., [30,31]). At least two additional recent studies have conducted broad-scale interspecific comparative analyses of avian [Hb] and Hct data, but both datasets were compiled from heterogeneous published data and lacked elevational sampling sufficient to estimate elevational effects [33,34].…”