Inferring Whole-Organism Metabolic Rate From Red Blood Cells in Birds

Abstract

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.

Great tits differ in glucocorticoid plasticity in response to spring temperature

“…In our study, temperatures at capture ranged between ∼4-27°C (Figure S1). Assuming that the thermoneutral zone of our great tit population is between 15°C and 30°C (Broggi et al, 2005; Malkoc, Casagrande, et al, 2021), we collected two thirds of the baseline and stress-induced corticosterone samples at temperatures below the great tits’ thermoneutrality. The thermoneutral zone is typically determined in a laboratory situation and only insufficiently reflects thermoregulatory challenges and processes that animals face under natural conditions (e.g., Mitchell et al, 2018).…”

“…Individuals can differ in how strongly they respond to variation in ambient temperature, for instance in metabolic rate (Briga & Verhulst, 2017; Careau et al, 2014; Kar et al, 2021), foraging activity (Hall & Chalfoun, 2019), or incubation behavior (Cones et al, 2021). Since glucocorticoids support the regulation of such traits (Romero & Wingfield, 2016), quantifying the extent of glucocorticoid plasticity within and among individuals of wild vertebrate populations is crucial for evaluating the potential of these mechanisms to evolve (e.g., Angelier & Wingfield, 2013; Guindre-Parker, 2020; Hau et al, 2016; Malkoc, Mentesana, et al, 2021; Taff & Vitousek, 2016; Wada & Sewall, 2014). Individual differences are the raw material of selection, and for evolution to occur these individual differences have to harbor a heritable component.…”

“…Recent reports suggested that individual differences in glucocorticoid reaction norm elevations and slopes exist in wild vertebrate populations (summaries: Guindre-Parker, 2020; Malkoc, Mentesana, et al, 2021; Taff & Vitousek, 2016). However, some of these studies determined glucocorticoid metabolites from excrements (feces: Guindre-Parker et al, 2019; urine: Sonnweber et al, 2018), which can be additionally influenced by individual differences in dietary, metabolic, digestive and other processes (reviews in: Dantzer et al, 2014; Gormally & Romero, 2020; Goymann, 2012).…”

“…Within-individual changes in corticosterone concentrations with temperature: an individual bird has higher corticosterone levels when being sampled at lower than at higher ambient temperatures. In great tits, the thermoneutral zone (the range of environmental temperatures at which individuals do not expend energy on thermoregulation) ranges between 15-30°C (Broggi et al, 2005; Malkoc, Casagrande, et al, 2021). During the breeding season, birds from our population commonly experience temperatures below the thermoneutral zone (Fig.…”

Integrative Physiological Responses to Acute Dehydration in the Rufous-Collared Sparrow: Metabolic, Enzymatic, and Oxidative Traits

“…Thus, the observed relationship between BMR and COX in RBCs could reflect a synergistic response to maintain vascular and respiratory homeostasis, either because the blood cells contribute to blood flow regulating vascular tone, and/or that they are influenced by the same regulatory pathways that reflect the requirements of other energy demanding tissues (e.g., liver). Malkoc et al (2021) reported that oxygen consumption at the cellular level in RBCs of great tits (Parus major) is directly related to oxygen consumption at the organismal level. This pattern was observed only when birds did not display signs of stress as inferred from plasma corticosterone concentrations, indicating that work examining the effects of stress on metabolic rate at both the mitochondrial and organismal level warrants further exploration.…”