Metabolic Flexibility in Response to Within-Season Temperature Variability in House Sparrows

Oxidative stress across multiple tissues in house sparrows (Passer domesticus) acclimated to warm, stable cold, and unpredictable cold thermal treatments

With climate change increasing not just mean temperatures but the frequency of cold snaps and heat waves, animals occupying thermally variable areas may be faced with thermal conditions for which they are not prepared. Studies of physiological adaptations of temperate resident birds to such thermal variability are largely lacking in the literature. To address this gap, we acclimated winter-phenotype house sparrows (Passer domesticus) to stable warm, stable cold, and fluctuating cold temperatures. We then measured several metrics of the oxidative stress (OS) system, including enzymatic and non-enzymatic antioxidants and lipid oxidative damage, in brain (post-mitotic), kidney (mitotic), liver (mitotic) and pectoralis muscle (post-mitotic). We predicted that high metabolic flexibility could be linked to increases in reactive oxygen damage. Alternatively, if variation in ROS production is not associated with metabolic flexibility, then we predict no antioxidant compensation with thermal variation. Our data suggest that ROS production is not associated with metabolic flexibility, as we found no differences across thermal treatment groups. However, we did find differences across tissues. Brain catalase activity demonstrated the lowest values compared with kidney, liver and muscle. In contrast, brain glutathione peroxidase (GPx) activities were higher than those in kidney and liver. Muscle GPx activities were intermediate to brain and kidney/liver. Lipid peroxidation damage was lowest in the kidney and highest in muscle tissue.

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

Species residing across elevational gradients display adaptations in response to environmental changes such as oxygen availability, ultraviolet radiation, and temperature. Here, we study genomic variation, gene expression, and long-term adaptation in Tibetan Partridge (Perdix hodgsoniae) populations residing across the elevational gradient of the Tibetan Plateau. We generated a high-quality draft genome and used it to carry out downstream population genomic and transcriptomic analysis. The P. hodgsoniae populations residing across various elevations were genetically distinct, and their phylogenetic clustering was consistent with their geographic distribution. We identified possible evidence of gene flow between populations residing in <3,000 and >4,200 m elevation that is consistent with known habitat expansion of high-altitude populations of P. hodgsoniae to a lower elevation. We identified a 60 kb haplotype encompassing the Estrogen Receptor 1 (ESR1) gene, showing strong genetic divergence between populations of P. hodgsoniae. We identified six single nucleotide polymorphisms within the ESR1 gene fixed for derived alleles in high-altitude populations that are strongly conserved across vertebrates. We also compared blood transcriptome profiles and identified differentially expressed genes (such as GAPDH, LDHA, and ALDOC) that correlated with differences in altitude among populations of P. hodgsoniae. These candidate genes from population genomics and transcriptomics analysis were enriched for neutrophil degranulation and glycolysis pathways, which are known to respond to hypoxia and hence may contribute to long-term adaptation to high altitudes in P. hodgsoniae. Our results highlight Tibetan Partridges as a useful model to study molecular mechanisms underlying long-term adaptation to high altitudes.

Evidence for a maintenance cost for birds maintaining highly flexible basal, but not summit, metabolic rates

Reversible phenotypic flexibility allows organisms to better match phenotypes to prevailing environmental conditions and may produce fitness benefits. Costs and constraints of phenotypic flexibility may limit the capacity for flexible responses but are not well understood nor documented. Costs could include expenses associated with maintaining the flexible system or with generating the flexible response. One potential cost of maintaining a flexible system is an energetic cost reflected in the basal metabolic rate (BMR), with elevated BMR in individuals with more flexible metabolic responses. We accessed data from thermal acclimation studies of birds where BMR and/or M_sum (maximum cold-induced metabolic rate) were measured before and after acclimation, as a measure of metabolic flexibility, to test the hypothesis that flexibility in BMR (ΔBMR), M_sum (ΔM_sum), or metabolic scope (M_sum - BMR; ΔScope) is positively correlated with BMR. When temperature treatments lasted at least three weeks, three of six species showed significant positive correlations between ΔBMR and BMR, one species showed a significant negative correlation, and two species showed no significant correlation. ΔM_sum and BMR were not significantly correlated for any species and ΔScope and BMR were significantly positively correlated for only one species. These data suggest that support costs exist for maintaining high BMR flexibility for some bird species, but high flexibility in M_sum or metabolic scope does not generally incur elevated maintenance costs.

Seasonal variation in thermoregulatory capacity of three closely related Afrotropical Estrildid finches introduced to Europe

A species' potential geographical range is largely determined by how the species responds physiologically to its changing environment. It is therefore crucial to study the physiological mechanisms that species use to maintain their homeothermy in order to address biodiversity conservation challenges, such as the success of invasions of introduced species. The common waxbill Estrilda astrild, the orange-cheeked waxbill E. melpoda, and the black-rumped waxbill E. troglodytes are small Afrotropical passerines that have established invasive populations in regions where the climate is colder than in their native ranges. As a result, they are highly suitable species for studying potential mechanisms for coping with a colder and more variable climate. Here, we investigated the magnitude and direction of seasonal variation in their thermoregulatory traits, such as basal (BMR), summit (M_sum) metabolic rates and thermal conductance. We found that, from summer to autumn, their ability to resist colder temperatures increased. This was not related to larger body masses or higher BMR and M_sum, but instead, species downregulated BMR and M_sum toward the colder season, suggesting energy conservation mechanisms to increase winter survival. BMR and M_sum were most strongly correlated with temperature variation in the week preceding the measurements. Common waxbill and black-rumped waxbill, whose native ranges encompass the highest degree of seasonality, showed the most flexibility in metabolic rates (i.e., stronger downregulation toward colder seasons). This ability to adjust thermoregulatory traits, combined with increased cold tolerance, may facilitate their establishment in areas characterized by colder winters and less predictable climates.

Mediterranean songbirds show pronounced seasonal variation in thermoregulatory traits

Addressing the patterns of variation in thermal traits is crucial to better predict the potential effects of climate change on organisms. Here, we assessed seasonal (winter vs summer) adjustments in key thermoregulatory traits in eight Mediterranean-resident songbirds. Overall, songbirds increased whole-animal (by 8%) and mass-adjusted (by 9%) basal metabolic rate and decreased (by 56%) thermal conductance below the thermoneutral zone during winter. The magnitude of these changes was within the lower values found in songbirds from northern temperate areas. Moreover, songbirds increased (by 11%) evaporative water loss within the thermoneutral zone during summer, while its rate of increase above the inflection point of evaporative water loss (i.e., the slope of evaporative water loss versus temperature) decreased by 35% during summer - a value well above that reported for other temperate and tropical songbirds. Finally, body mass increased by 5% during winter, a pattern similar to that found in many northern temperate species. Our findings support the idea that physiological adjustments might enhance the resilience of Mediterranean songbirds to environmental changes, with short-term benefits by saving energy and water under thermally stressful conditions. Nevertheless, not all species showed the same patterns, suggesting different strategies in their thermoregulatory adaptations to seasonal environments.

A multi-isotope approach reveals seasonal variation in the reliance on marine resources, production of metabolic water, and ingestion of seawater by two species of coastal passerine to maintain water balance

Tracing how free-ranging organisms interact with their environment to maintain water balance is a difficult topic to study for logistical and methodological reasons. We use a novel combination of triple-oxygen stable isotope analyses of water extracted from plasma (δ16O, δ17O, δ18O) and bulk tissue carbon (δ13C) and nitrogen (δ15N) isotopes of feathers and blood to estimate the proportional contribution of marine resources, seawater, and metabolic water used by two species of unique songbirds (genus Cinclodes) to maintain their water balance in a seasonal coastal environment. We also assessed the physiological adjustments that these birds use to maintain their water balance. In agreement with previous work on these species, δ13C and δ15N data show that the coastal resident and invertivore C. nigrofumosus consumes a diet rich in marine resources, while the diet of migratory C. oustaleti shifts seasonally between marine (winter) to freshwater aquatic resources (summer). Triple-oxygen isotope analysis (Δ17O) of blood plasma, basal metabolic rate (BMR), and total evaporative water loss (TEWL) revealed that ~25% of the body water pool of both species originated from metabolic water, while the rest originated from a mix of seawater and fresh water. Δ17O measurements suggest that the contribution of metabolic water tends to increase in summer in C. nigrofumosus, which is coupled with a significant increase in BMR and TEWL. The two species had similar BMR and TEWL during the austral winter when they occur sympatrically in coastal environments. We also found a positive and significant association between the use of marine resources as measured by δ13C and δ15N values and the estimated δ18O values of ingested (pre-formed) water in both species, which indicates that Cinclodes do not directly drink seawater but rather passively ingest when consuming marine invertebrates. Finally, results obtained from physiological parameters and the isotope-based estimates of marine (food and water) resource use are consistent, supporting the use of the triple-oxygen isotopes to quantify the contribution of water sources to the total water balance of free-ranging birds.

Metabolic Profiling and Integration of Metabolomic and Transcriptomic Data From Pectoralis Muscle Reveal Winter-Adaptive Metabolic Responses of Black-Capped Chickadee and American Goldfinch

Seasonal changes, such as alterations in food availability or type and cold conditions, present challenges to free-living birds living in highly seasonal climates. Small birds respond to such challenges through seasonal metabolic flexibility, which better matches seasonal metabolic phenotypes to environmental conditions and can improve fitness. To better understand the mechanistic basis of this metabolic flexibility, we conducted a large-scale metabolic profiling of pectoralis muscle in black-capped chickadees (Poecile atricapillus) and American goldfinches (Spinus tristis), which are small, year-round bird species of temperate-zones. We analyzed muscle samples using non-biased, global metabolomics profiling technology based on UHLC/MS/MS2 platforms. A total of 582 metabolites was characterized for summer and winter season samples. Chickadees showed greater seasonal separation of global metabolite profiles than goldfinches, which is consistent with previous transcriptomic studies of pectoralis muscle in these two species. Reduced levels of amino acids during winter occurred in both species and might reflect decreasing dietary protein intake, amino acid shuttling to other pathways for thermogenesis and/or elevated rates of protein turnover in the pectoralis muscle. Concomitant decreased abundances in tricarboxylic acid cycle (TCA) metabolites suggest faster cycling of the oxidative phosphorylation pathway in winter to meet the metabolic demands of thermogenesis. Accordingly, chickadees displayed shifts toward lipid oxidation in winter, whereas goldfinches showed winter declines in ketone bodies, which suggests increased energy demand or subtle changes in substrate availability. Beyond the winter-specific changes in metabolite abundances, integration of the metabolomic and the transcriptomic data revealed a landscape of gene–metabolite associations related to the winter-adaptive metabolic response. This landscape of gene–metabolite pairs was overrepresented by pathways associated with transport of small molecules, metabolism of amino acids and derivatives, activation and biosynthesis of fatty acid derivatives, and biosynthesis and metabolism of nicotinate and nicotinamide derivatives. Collectively, our results suggest that increased levels of NADH and its derivatives in the pectoralis muscle are a potential novel mechanism for increasing winter metabolic output, fueled by lipids, for thermogenesis during winter.

Effects of acute temperature increases on House sparrow (Passer domesticus) pectoralis muscle myonuclear domain

With rapid climate change, heat wave episodes have become more intense and more frequent. This poses a significant threat to animals, and forces them to manage these physiologically challenging conditions by adapting and/or moving. As an invasive species with a large niche breadth, House sparrows (Passer domesticus) exhibit high phenotypic flexibility that caters to seasonal changes in function and metabolism. For example, their pectoral muscle complex exhibits size and mass plasticity with winter and summer acclimation. Here, we investigated the effects of acute whole-organism heat stress to 43°C on cellular-level changes in House sparrow pectoralis muscle myonuclear domain (MND), the volumetric portion each nucleus is responsible for, that have gone overlooked in the current literature. House sparrows were separated into a control group, a heat-shocked group subjected to thermal stress at 43°C for 24 h, and a recovery group that was returned to room temperature for 24 h after experiencing the same temperature treatment. Here, we found that heat-shocked and recovery groups demonstrated a decrease in number of nuclei per millimeter of fiber and increase in MND, when compared with the control. We also found a significant positive correlation between fiber diameter and MND in the recovery group, suggesting the possibility that nuclei number constrains the extent of muscle fiber size. Together, these results show that acute heat shock alters House sparrow pectoralis muscle cellular physiology in a rigid way that could prove detrimental to long-term muscle integrity and performance.