DOI: 10.5061/dryad.1g1jwsv07
title: Blood variation implicates respiratory limits on elevational ranges of Andean birds

description: 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 the sensitivity of
 hematological traits to elevation is predictive of elevational range
 breadth. Second, we asked whether variance in hematological traits changed
 as a function of distance to the nearest elevational range limit. We found
 that the correlation between hematological sensitivity and elevational
 range breadth was slightly positive, consistent with a facilitative role
 for sensitivity 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 cardiorespiratory traits. Our findings suggest that constraints on
 hematological sensitivity and local genetic adaptation to oxygen
 availability promote the evolution of the narrow elevational ranges that
 underpin tropical montane biodiversity.
description: From 2006 to 2020, we measured
 total blood hemoglobin concentration ([Hb]) and the percentage of
 hematocrit (Hct) for birds captured during collaborative fieldwork in Peru
 by the Museum of Southwestern Biology (MSB) in New Mexico, USA, and the
 Centro de Ornitología y Biodiversidad (CORBIDI) in Lima,
 Peru. As quickly as possible
 after capture, we punctured the brachial vein on the underside of each
 bird’s wing and collected whole blood using heparinized microcapillary
 tubes (for Hct) or spectrophotometer cuvettes (for [Hb]). We centrifuged
 microcapillary tubes for 5 minutes at 13,000 rpm to separate red blood
 cells from plasma and used digital calipers to quantify the relative
 volume of each, averaging two samples per bird to arrive at our final
 estimate of Hct as a percentage. We measured [Hb] (g/dL blood) for a ~5 μL
 blood sample with the Hemocue HB201 analyzer and associated hemoglobin
 photometer. As this proprietary method generates values approximately 1.0
 g/dL higher than measures from a direct cyanomethaemoglobin
 spectrophotometer (Simmons
 & Lill, 2006), we corrected the resulting estimates by
 subtracting this quantity (DuBay
 & Witt, 2014). Specimens and tissues are housed at the MSB and
 CORBIDI and specimen data are detailed in a separate data publication
 (Witt et al., in prep).
description: Raw data are stored as comma-separated values (.csv) files. All
 data processing, analyses, and figures can be produced by sequentially
 running three R Markdown (.Rmd) notebooks found in the associated Zenodo
 repository (this procedure is documented in the README).