DOI: 10.5061/dryad.ncjsxksvh
title: Inferring whole-organism metabolic rate from red blood cells in birds

description: Metabolic rate is a key ecological variable that quantifies the energy
 expenditure needed to fuel almost all biological processes in an organism.
 Metabolic rates are typically measured at the whole-organism level (woMR)
 with protocols that can elicit stress responses due to handling and
 confinement, potentially biasing resulting data. Improved, non-stressful
 methodology would be especially valuable for measures of field metabolic
 rate, which quantifies the energy expenditure of free-living individuals.
 Recently, techniques to measure cellular metabolic rate (cMR) in
 mitochondria  of blood cells have become available, suggesting
 that blood-based cMR can be a proxy of organismal aerobic performance.
 Aerobic metabolism actually takes place in the mitochondria. Quantifying
 cMR from blood samples offers several advantages such as direct estimates
 of metabolism and minimized disturbance of individuals. To our knowledge,
 the hypothesis that blood-based cMR correlates with woMR has not yet been
 directly tested. We measured cMR in red blood cells of captive great tits
 (Parus major), first during their morning activity period and second after
 subjecting them to a 2.5 h day-time respirometry protocol to quantify
 woMR. We predicted cMR to decrease as individuals transitioned from an
 active to a resting state. In the two blood samples we also assessed
 circulating corticosterone concentrations to determine the perceived
 disturbance of individuals. From respirometry traces we extracted initial
 and final woMR measures to test for a predicted positive correlation with
 cMR measures, while accounting for corticosterone concentrations. Indeed,
 cMR declined from the first to the second measurement. Furthermore, woMR
 and cMR were positively related in individuals that had relatively low
 corticosterone concentrations and displayed little locomotor activity
 throughout respirometry. By contrast, woMR and cMR covaried negatively in
 birds that increased corticosterone concentrations and activity levels
 substantially. Our results show that red blood cell cMR represents a proxy
 for woMR when birds do not display signs of stress, i.e. either before
 increases in hormonal or behavioral parameters have occurred or after they
 have abated. This method represents a valuable tool for obtaining
 metabolic data repeatedly and in free-living individuals. Our findings
 also highlight the importance of accounting for individual stress
 responses when measuring metabolic rate at any level. Metabolic rate is a
 key ecological variable that quantifies the energy expenditure needed to
 fuel almost all biological processes in an organism. Metabolic rates are
 typically measured at the whole-organism level (woMR) with protocols that
 can elicit stress responses due to handling and confinement, potentially
 biasing resulting data. Improved, non-stressful methodology would be
 especially valuable for measures of field metabolic rate, which quantifies
 the energy expenditure of free-living individuals. Recently, techniques to
 measure cellular metabolic rate (cMR) in mitochondria  of blood
 cells have become available, suggesting that blood-based cMR can be a
 proxy of organismal aerobic performance. Aerobic metabolism actually takes
 place in the mitochondria. Quantifying cMR from blood samples offers
 several advantages such as direct estimates of metabolism and minimized
 disturbance of individuals. To our knowledge, the hypothesis that
 blood-based cMR correlates with woMR has not yet been directly tested. We
 measured cMR in red blood cells of captive great tits (Parus major), first
 during their morning activity period and second after subjecting them to a
 2.5 h day-time respirometry protocol to quantify woMR. We predicted cMR to
 decrease as individuals transitioned from an active to a resting state. In
 the two blood samples we also assessed circulating corticosterone
 concentrations to determine the perceived disturbance of individuals. From
 respirometry traces we extracted initial and final woMR measures to test
 for a predicted positive correlation with cMR measures, while accounting
 for corticosterone concentrations. Indeed, cMR declined from the first to
 the second measurement. Furthermore, woMR and cMR were positively related
 in individuals that had relatively low corticosterone concentrations and
 displayed little locomotor activity throughout respirometry. By contrast,
 woMR and cMR covaried negatively in birds that increased corticosterone
 concentrations and activity levels substantially. Our results show that
 red blood cell cMR represents a proxy for woMR when birds do not display
 signs of stress, i.e. either before increases in hormonal or behavioral
 parameters have occurred or after they have abated. This method represents
 a valuable tool for obtaining metabolic data repeatedly and in free-living
 individuals. Our findings also highlight the importance of accounting for
 individual stress responses when measuring metabolic rate at any level.