Blood variation implicates respiratory limits on elevational ranges of Andean birds

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.