Vasopressin, protein metabolism, and water conservation

PURPOSE OF REVIEW

Highlight the mechanisms through which vasopressin and hypertonic stress regulate protein metabolism.

RECENT FINDINGS

Mammals have an 'aestivation-like' response in which hypertonic stress increases muscle catabolism and urea productionVasopressin can directly regulate ureagenesis in the liver and the kidneyIn humans chronic hypertonic stress is associated with premature aging, diabetes, cardiovascular disease, and premature mortality.

SUMMARY

There is an evolutionarily conserved 'aestivation-like' response in humans in which hypertonic stress results in activation of the vasopressin system, muscle catabolism, and ureagenesis in order to promote water conservation.

The multi-tissue gene expression and physiological responses of water deprived Peromyscus eremicus

The harsh and dry conditions of desert environments have resulted in genomic adaptations, allowing for desert organisms to withstand prolonged drought, extreme temperatures, and limited food resources. Here, we present a comprehensive exploration of gene expression across five tissues (kidney, liver, lung, gastrointestinal tract, and hypothalamus) and 19 phenotypic measurements to explore the whole-organism physiological and genomic response to water deprivation in the desert-adapted cactus mouse (Peromyscus eremicus). The findings encompass the identification of differentially expressed genes and correlative analysis between phenotypes and gene expression patterns across multiple tissues. Specifically, we found robust activation of the vasopressin renin-angiotensin-aldosterone system (RAAS) pathways, whose primary function is to manage water and solute balance. Animals reduced food intake during water deprivation, and upregulation of PCK1 highlights the adaptive response to reduced oral intake via its actions aimed at maintained serum glucose levels. Even with such responses to maintain water balance, hemoconcentration still occurred, prompting a protective downregulation of genes responsible for the production of clotting factors while simultaneously enhancing angiogenesis which is thought to maintain tissue perfusion. In this study, we elucidate the complex mechanisms involved in water balance in the desert-adapted cactus mouse, P. eremicus. By prioritizing a comprehensive analysis of whole-organism physiology and multi-tissue gene expression in a simulated desert environment, we describe the complex response of regulatory processes.

Water restriction increases oxidation of endogenous amino acids in house sparrows (Passer domesticus)

Animals can cope with dehydration in a myriad of ways, both behaviorally and physiologically. The oxidation of protein produces more metabolic water per kilojoule than that of fat or carbohydrate, and it is well established that birds increase protein catabolism in response to high rates of water loss. However, the fate of amino acids mobilized in response to water restriction has not been explicitly determined. While protein catabolism releases bound water, we hypothesized that water-restricted birds would also oxidize the resulting amino acids, producing additional water as a product of oxidative phosphorylation. To test this, we fed captive house sparrows (Passer domesticus) 13C-labeled leucine for 9 weeks to label endogenous proteins. We conducted weekly trials during which we measured the physiological response to water restriction as changes in lean mass, fat mass, metabolism and the enrichment of 13C in exhaled CO2 (δ13Cbreath). If water-restricted birds catabolized proteins and oxidized the resulting amino acids, we expected to simultaneously observe greater lean mass loss and elevated δ13Cbreath relative to control birds. We found that water-restricted birds catabolized more lean tissue and also had enriched δ13Cbreath in response to water restriction, supporting our hypothesis. δ13Cbreath, however, varied with metabolic rate and the length of the water restriction period, suggesting that birds may spare protein when water balance can be achieved using other physiological strategies.

Insulin at the intersection of thermoregulation and glucose homeostasis

Mammals are protected from changes in environmental temperature by altering energetic processes that modify heat production. Insulin is the dominant stimulus of glucose uptake and metabolism, which are fundamental for thermogenic processes. The purpose of this work was to determine the interaction of ambient temperature induced changes in energy expenditure (EE) on the insulin sensitivity of glucose fluxes. Short-term and adaptive responses to thermoneutral temperature (TN, ∼28 °C) and room (laboratory) temperature (RT, ∼22 °C) were studied in mice. This range of temperature does not cause detectable changes in circulating catecholamines or shivering and postabsorptive glucose homeostasis is maintained. We tested the hypothesis that a decrease in EE that occurs with TN causes insulin resistance and that this reduction in insulin action and EE is reversed upon short term (<12h) transition to RT. Insulin-stimulated glucose disposal (Rd) and tissue-specific glucose metabolic index were assessed combining isotopic tracers with hyperinsulinemic-euglycemic clamps. EE and insulin-stimulated Rd are both decreased (∼50%) in TN-adapted vs RT-adapted mice. When RT-adapted mice are switched to TN, EE rapidly decreases and Rd is reduced by ∼50%. TN-adapted mice switched to RT exhibit a rapid increase in EE, but whole-body insulin-stimulated Rd remains at the low rates of TN-adapted mice. In contrast, whole body glycolytic flux rose with EE. This higher EE occurs without increasing glucose uptake from the blood, but rather by diverting glucose from glucose storage to glycolysis. In addition to adaptations in insulin action, 'insulin-independent' glucose uptake in brown fat is exquisitely sensitive to thermoregulation. These results show that insulin action adjusts to non-stressful changes in ambient temperature to contribute to the support of body temperature homeostasis without compromising glucose homeostasis.

When the tap runs dry: The multi-tissue gene expression and physiological responses of water deprived Peromyscus eremicus

The harsh and dry conditions of desert environments have resulted in genomic adaptations, allowing for desert organisms to withstand prolonged drought, extreme temperatures, and limited food resources. Here, we present a comprehensive exploration of gene expression across five tissues (kidney, liver, lung, gastrointestinal tract, and hypothalamus) and 19 phenotypic measurements to explore the whole-organism physiological and genomic response to water deprivation in the desert-adapted cactus mouse (Peromyscus eremicus). The findings encompass the identification of differentially expressed genes and correlative analysis between phenotypes and gene expression patterns across multiple tissues. Specifically, we found robust activation of the vasopressin renin-angiotensin-aldosterone system (RAAS) pathways, whose primary function is to manage water and solute balance. Animals reduce food intake during water deprivation, and upregulation of PCK1 highlights the adaptive response to reduced oral intake via its actions aimed at maintained serum glucose levels. Even with such responses to maintain water balance, hemoconcentration still occurred, prompting a protective downregulation of genes responsible for the production of clotting factors while simultaneously enhancing angiogenesis which is thought to maintains tissue perfusion. In this study, we elucidate the complex mechanisms involved in water balance in the desert-adapted cactus mouse, P. eremicus. By prioritizing a comprehensive analysis of whole-organism physiology and multi-tissue gene expression in a simulated desert environment, we describe the complex and successful response of regulatory processes.

CO2 scrubbing, zero gases, Keeling plots, and a mathematical approach to ameliorate the deleterious effects of ambient CO2 during 13 C breath testing in humans and animals

13 C breath testing is increasingly used in physiology and ecology research because of what it reveals about the different fuels that animals oxidize to meet their energetic demands. Here I review the practice of 13 C breath testing in humans and other animals and describe the impact that contamination by ambient/background CO2 in the air can have on the accuracy of 13 C breath measurements. I briefly discuss physical methods to avoid sample contamination as well as the Keeling plot approach that researchers have been using for the past two decades to estimate δ13 C from breath samples mixed with ambient CO2 . Unfortunately, Keeling plots are not suited for 13 C breath testing in common situations where (1) a subject's VCO2 is dynamic, (2) ambient [CO2 ] may change, (3) a subject is sensitive to hypercapnia, or (4) in any flow-through indirect calorimetry system. As such, I present a mathematical solution that addresses these issues by using information about the instantaneous [CO2 ] and the δ13 CO2 of ambient air as well as the diluted breath sample to back-calculate the δ13 CO2 in the CO2 exhaled by the animal. I validate this approach by titrating a sample of 13 C-enriched gas into an air stream and demonstrate its ability to provide accurate values across a wide range of breath and air mixtures. This approach allows researchers to instantaneously calculate the δ13 C of exhaled gas of humans or other animals in real time without having to scrub ambient CO2 or rely on estimated values.

Insulin at the Intersection of Thermoregulation and Glucose Homeostasis

Mammals are protected from changes in environmental temperature by altering energetic processes that modify heat production. Insulin is the dominant stimulus of glucose uptake and metabolism, which are fundamental for thermogenic processes. The purpose of this work was to determine the interaction of ambient temperature induced changes in energy expenditure (EE) on the insulin sensitivity of glucose fluxes. Short-term and adaptive responses to thermoneutral temperature (TN, ~28°C) and room (laboratory) temperature (RT, ~22°C) were studied in mice. This range of temperature does not cause detectable changes in circulating catecholamines or shivering and postabsorptive glucose homeostasis is maintained. We tested the hypothesis that a decrease in EE that occurs with TN causes insulin resistance and that this reduction in insulin action and EE is reversed upon short term (<12h) transition to RT. Insulin-stimulated glucose disposal (Rd) and tissue specific glucose uptake were assessed combining isotopic tracers with hyperinsulinemic-euglycemic clamps. EE and insulin-stimulated Rd are both decreased (~50%) in TN-adapted vs RT-adapted mice. When RT-adapted mice are switched to TN, EE rapidly decreases and Rd is reduced by ~50%. TN-adapted mice switched to RT exhibit a rapid increase in EE, but whole body insulin-stimulated Rd remains at the low rates of TN-adapted mice. In contrast, whole body glycolytic flux rose with EE. This higher EE occurs without increasing glucose uptake from the blood, but rather by diverting glucose from glucose storage to glycolysis. In addition to adaptations in insulin action, 'insulin-independent' glucose uptake in brown fat is exquisitely sensitive to thermoregulation. These results show that insulin action adjusts to non-stressful changes in ambient temperature to contribute to the support of body temperature homeostasis without compromising glucose homeostasis.

Prey consumption does not restore hydration state but mitigates the energetic costs of water deprivation in an insectivorous lizard

To cope with limited availability of drinking water in their environment, terrestrial animals have developed numerous behavioral and physiological strategies including maintaining an optimal hydration state through dietary water intake. Recent studies performed in snakes, which are generalist carnivorous reptiles, suggest that the benefits of dietary water intake are negated by hydric costs of digestion. Most lizards are generalist insectivores that can shift their prey types, but firm experimental demonstration of dietary water intake is currently missing in these organisms. Here, we performed an experimental study in the common lizard Zootoca vivipara, a keystone mesopredator from temperate climates exhibiting a great diversity of prey in its mesic habitats, in order to investigate the effects of food consumption and prey type on physiological responses to water deprivation. Our results indicate that common lizards cannot improve their hydration state through prey consumption, irrespective of prey type, suggesting that they are primarily dependent upon drinking water. Yet, high-quality prey consumption reduced the energetic costs of water deprivation, potentially helping lizards to conserve a better body condition during periods of limited water availability. These findings have important implications for understanding the physiological responses of ectotherms to water stress, and highlight the complex interactions between hydration status, energy metabolism and feeding behavior in insectivorous lizards.

Whole-genome sequencing reveals adaptations of hairy-footed jerboas (Dipus, Dipodidae) to diverse desert environments

BACKGROUND

Environmental conditions vary among deserts across the world, spanning from hyper-arid to high-elevation deserts. However, prior genomic studies on desert adaptation have focused on desert and non-desert comparisons overlooking the complexity of conditions within deserts. Focusing on the adaptation mechanisms to diverse desert environments will advance our understanding of how species adapt to extreme desert environments. The hairy-footed jerboas are well adapted to diverse desert environments, inhabiting high-altitude arid regions, hyper-arid deserts, and semi-deserts, but the genetic basis of their adaptation to different deserts remains unknown.

RESULTS

Here, we sequenced the whole genome of 83 hairy-footed jerboas from distinct desert zones in China to assess how they responded under contrasting conditions. Population genomics analyses reveal the existence of three species in hairy-footed jerboas distributed in China: Dipus deasyi, Dipus sagitta, and Dipus sowerbyi. Analyses of selection between high-altitude desert (elevation ≥ 3000m) and low-altitude desert (< 500m) populations identified two strongly selected genes, ATR and HIF1AN, associated with intense UV radiation and hypoxia in high-altitude environments. A number of candidate genes involved in energy and water homeostasis were detected in the comparative genomic analyses of hyper-arid desert (average annual precipitation < 70mm) and arid desert (< 200mm) populations versus semi-desert (> 360mm) populations. Hyper-arid desert animals also exhibited stronger adaptive selection in energy homeostasis, suggesting water and resource scarcity may be the main drivers of desert adaptation in hairy-footed jerboas.

CONCLUSIONS

Our study challenges the view of deserts as homogeneous environments and shows that distinct genomic adaptations can be found among desert animals depending on their habitats.

Behavioural hydroregulation protects against acute effects of drought in a dry-skinned ectotherm

During extreme climate events, behavioural thermoregulation may buffer ectotherms from thermal stress and overheating. However, heatwaves are also combined with dry spells and limited water availability, and how much individuals can behaviourally mitigate dehydration risks through microclimate selection remains largely unknown. Herein, we investigated the behavioural and physiological responses to changes in air and microhabitat humidity in a terrestrial ectotherm, the asp viper (Vipera aspis). We exposed individuals to a simulated heatwave together with water deprivation for 3 weeks, and manipulated air water vapour density (wet air vs. dry air) and microclimate (wet shelter vs. dry shelter) in a two-by-two factorial design. Dry air conditions led to substantial physiological dehydration and muscle wasting. Vipers exposed to dry air used more often a shelter that offered a moist microclimate, which reduced dehydration and muscle wasting at the individual level. These results provide the first experimental evidence that active behavioural hydroregulation can mitigate specific physiological stress responses caused by a dry spell in an ectotherm. Future studies investigating organismal responses to climate change should consider moisture gradient in the habitat and integrate both hydroregulation and thermoregulation behaviours.

Evaluating the effects of water and food limitation on the life history of an insect using a multiple-stressor framework

Many environmental stressors naturally covary, and the frequency and duration of stressors such as heat waves and droughts are increasing globally with climate change. Multiple stressors may have additive or non-additive effects on fitness-related traits, such as locomotion, reproduction, and somatic growth. Despite its importance to terrestrial animals, water availability is rarely incorporated into multiple-stressor frameworks. Water limitation often occurs concurrently with food limitation (e.g., droughts can trigger famines), and the acquisition of water and food can be linked because water is necessary for digestion and metabolism. Thus, we investigated the independent and interactive effects of water and food limitation on life-history traits using female crickets (Gryllus firmus), which exhibit a wing dimorphism mediating a life-history trade-off between flight and fecundity. Our results indicate that traits vary in their sensitivities to environmental factors and factor-factor interactions. For example, neither environmental factor affected flight musculature, only water limitation affected survival, and food and water availability non-additively (i.e., interactively) influenced body and ovary mass. Water availability had a larger effect on traits than food availability, affected more traits than food availability, and mediated the effects of food availability. Further, life-history strategy influenced the costs of multiple stressors because females investing in flight capacity exhibited greater reductions in body and ovary mass during stress relative to females lacking flight capacity. Therefore, water is important in the multiple-stressor framework, and understanding the dynamics of covarying environmental factors and life history may be critical in the context of climate change characterized by concurrent environmental stressors.

Disentangling environmental drivers of circadian metabolism in desert-adapted mice

Metabolism is a complex phenotype shaped by natural environmental rhythms, as well as behavioral, morphological and physiological adaptations. Metabolism has been historically studied under constant environmental conditions, but new methods of continuous metabolic phenotyping now offer a window into organismal responses to dynamic environments, and enable identification of abiotic controls and the timing of physiological responses relative to environmental change. We used indirect calorimetry to characterize metabolic phenotypes of the desert-adapted cactus mouse (Peromyscus eremicus) in response to variable environmental conditions that mimic their native environment versus those recorded under constant warm and constant cool conditions, with a constant photoperiod and full access to resources. We found significant sexual dimorphism, with males being more prone to dehydration than females. Under circadian environmental variation, most metabolic shifts occurred prior to physical environmental change and the timing was disrupted under both constant treatments. The ratio of CO2 produced to O2 consumed (the respiratory quotient) reached greater than 1.0 only during the light phase under diurnally variable conditions, a pattern that strongly suggests that lipogenesis contributes to the production of energy and endogenous water. Our results are consistent with historical descriptions of circadian torpor in this species (torpid by day, active by night), but reject the hypothesis that torpor is initiated by food restriction or negative water balance.

Limited Evidence for Parallel Evolution Among Desert-Adapted Peromyscus Deer Mice

Warming climate and increasing desertification urge the identification of genes involved in heat and dehydration tolerance to better inform and target biodiversity conservation efforts. Comparisons among extant desert-adapted species can highlight parallel or convergent patterns of genome evolution through the identification of shared signatures of selection. We generate a chromosome-level genome assembly for the canyon mouse (Peromyscus crinitus) and test for a signature of parallel evolution by comparing signatures of selective sweeps across population-level genomic resequencing data from another congeneric desert specialist (Peromyscus eremicus) and a widely distributed habitat generalist (Peromyscus maniculatus), that may be locally adapted to arid conditions. We identify few shared candidate loci involved in desert adaptation and do not find support for a shared pattern of parallel evolution. Instead, we hypothesize divergent molecular mechanisms of desert adaptation among deer mice, potentially tied to species-specific historical demography, which may limit or enhance adaptation. We identify a number of candidate loci experiencing selective sweeps in the P. crinitus genome that are implicated in osmoregulation (Trypsin, Prostasin) and metabolic tuning (Kallikrein, eIF2-alpha kinase GCN2, APPL1/2), which may be important for accommodating hot and dry environmental conditions.

When it's hot and dry: life-history strategy influences the effects of heat waves and water limitation

The frequency, duration and co-occurrence of several environmental stressors, such as heat waves and droughts, are increasing globally. Such multiple stressors may have compounding or interactive effects on animals, resulting in either additive or non-additive costs, but animals may mitigate these costs through various strategies of resource conservation or shifts in resource allocation. Through a factorial experiment, we investigated the independent and interactive effects of a simulated heat wave and water limitation on life-history, physiological and behavioral traits. We used the variable field cricket, Gryllus lineaticeps, which exhibits a wing dimorphism that mediates two distinct life-history strategies during early adulthood. Long-winged individuals invest in flight musculature and are typically flight capable, whereas short-winged individuals lack flight musculature and capacity. A comprehensive and integrative approach with G. lineaticeps allowed us to examine whether life-history strategy influenced the costs of multiple stressors as well as the resulting cost-limiting strategies. Concurrent heat wave and water limitation resulted in largely non-additive and single-stressor costs to important traits (e.g. survival and water balance), extensive shifts in resource allocation priorities (e.g. reduced prioritization of body mass) and a limited capacity to conserve resources (e.g. heat wave reduced energy use only when water was available). Life-history strategy influenced the emergency life-history stage because wing morphology and stressor(s) interacted to influence body mass, boldness behavior and immunocompetence. Our results demonstrate that water availability and life-history strategy should be incorporated into future studies integrating important conceptual frameworks of stress across a suite of traits - from survival and life history to behavior and physiology.

How dryland mammals will respond to climate change: the effects of body size, heat load and a lack of food and water

Mammals in drylands are facing not only increasing heat loads but also reduced water and food availability as a result of climate change. Insufficient water results in suppression of evaporative cooling and therefore increases in body core temperature on hot days, while lack of food reduces the capacity to maintain body core temperature on cold nights. Both food and water shortage will narrow the prescriptive zone, the ambient temperature range over which body core temperature is held relatively constant, which will lead to increased risk of physiological malfunction and death. Behavioural modifications, such as shifting activity between night and day or seeking thermally buffered microclimates, may allow individuals to remain within the prescriptive zone, but can incur costs, such as reduced foraging or increased competition or predation, with consequences for fitness. Body size will play a major role in predicting response patterns, but identifying all the factors that will contribute to how well dryland mammals facing water and food shortage will cope with increasing heat loads requires a better understanding of the sensitivities and responses of mammals exposed to the direct and indirect effects of climate change.

Phenotypic flexibility in heat production and heat loss in response to thermal and hydric acclimation in the zebra finch, a small arid-zone passerine

To maintain constant body temperature (T_b) over a wide range of ambient temperatures (T_a) endothermic animals require large amounts of energy and water. In hot environments, the main threat to endothermic homeotherms is insufficient water to supply that necessary for thermoregulation. We investigated flexible adjustment of traits related to thermoregulation and water conservation during acclimation to hot conditions or restricted water availability, or both, in the zebra finch, Taeniopygia guttata a small arid-zone passerine. Using indirect calorimetry, we measured changes in whole animal metabolic rate (MR), evaporative heat loss (EHL) and T_b before and after acclimation to 23 or 40 °C, with different availability of water. Additionally, we quantified changes in partitioning of EHL into respiratory and cutaneous avenues in birds exposed to 25 and 40 °C. In response to heat and water restriction zebra finches decreased MR, which together with unchanged EHL resulted in increased efficiency of evaporative heat loss. This facilitated more precise T_b regulation in heat-acclimated birds. Acclimation temperature and water availability had no effect on the partitioning of EHL into cutaneous or respiratory avenues. At 25 °C, cutaneous EHL accounted for ~ 60% of total EHL, while at 40 °C, its contribution decreased to ~ 20%. Consistent among-individual differences in MR and EHL suggest that these traits, provided that they are heritable, may be a subject to natural selection. We conclude that phenotypic flexibility in metabolic heat production associated with acclimation to hot, water-scarce conditions is crucial in response to changing environmental conditions, especially in the face of current and predicted climate change.

Osmolyte Depletion and Thirst Suppression Allow Hibernators to Survive for Months without Water

Thirteen-lined ground squirrels (Ictidomys tridecemlineatus) are obligatory hibernators who can survive over 6 months of the year in underground burrows or laboratory hibernaculum without access to food or water [1]. Hibernation consists of prolonged periods of torpor, lasting up to 18 days, which are characterized by low body temperature and suppressed metabolism. This torpidity is interspersed with short periods of interbout arousal, lasting up to 48 h, during which squirrels temporarily return to an active-like state and lose small amounts of water to urination and evaporation [2]. Water is also lost during torpor due to a positive vapor pressure difference created by the slightly higher temperature of the body compared to its surroundings [2, 3]. Here, we investigate the physiological mechanism of survival during prolonged water loss and deprivation throughout hibernation. By measuring hydration status during hibernation, we show that squirrels remain hydrated during torpor by depleting osmolytes from the extracellular fluid. During brief periods of arousal, serum osmolality and antidiuretic hormone levels are restored, but thirst remains suppressed. This decoupling of thirst and diuresis enables water retention by the kidney while suppressing the drive to leave the safety of the underground burrow in search of water. An acute increase in serum osmolality reinstates water-seeking behavior, demonstrating preservation of the physiological thirst circuit during hibernation. Better mechanistic understanding of internal osmolyte regulation and thirst suppression could translate to advancements in human medicine and long-term manned spaceflight. VIDEO ABSTRACT.

The role of humidity and metabolic status on lean mass catabolism in migratory Swainson's thrushes (Catharus ustulatus)

Migratory birds use protein as a fuel source during flight, but the mechanisms and benefits of protein catabolism during migration are poorly understood. The tissue-specific turnover rate hypothesis proposes that lean mass loss depends solely on the constitutive rate of protein degradation for a given tissue, and is therefore independent of metabolic rate or environmental stimuli. However, it has been demonstrated that environmental stressors such as humidity affect the rate of lean mass catabolism during flight, a finding that seemingly contradicts the tissue-specific turnover rate hypothesis. In order to resolve this, we placed migratory Swainson's thrushes in either high (HEWL) or low (LEWL) evaporative water loss conditions at rest and while undergoing simulated migratory flight at 8 m s^-1 in a wind tunnel to test the impact of both environmental stressors and metabolic rate on the rate of protein breakdown. The total quantity and rate of lean mass loss was not different between flight and rest birds, but was affected by humidity condition, with HEWL losing significantly more lean mass. These results show that the rate of protein breakdown in migratory birds is independent of metabolic rate, but it can be augmented in response to environmental stressors.

Environmental temperature alters the overall digestive energetics and differentially affects dietary protein and lipid use in a lizard

Processing food (e.g. ingestion, digestion, assimilation) requires energy referred to as specific dynamic action (SDA) and is at least partially fuelled by oxidation of the nutrients (e.g. proteins and lipids) within the recently ingested meal. In ectotherms, environmental temperature can affect the magnitude and/or duration of the SDA, but is likely to also alter the mixture of nutrients that are oxidized to cover these costs. Here, we examined metabolic rate, gut passage time, assimilation efficiency and fuel use in the lizard Agama atra digesting cricket meals at three ecologically relevant temperatures (20, 25 and 32°C). Crickets were isotopically enriched with 13C-leucine or 13C-palmitic-acid tracers to distinguish between protein and lipid oxidation, respectively. Our results show that higher temperatures increased the magnitude of the SDA peak (by 318% between 32 and 20°C) and gut passage rate (63%), and decreased the duration of the SDA response (by 20% for males and 48% for females). Peak rate of dietary protein oxidation occurred sooner than peak lipid oxidation at all temperatures (70, 60 and 31 h earlier for 20, 25 and 32°C, respectively). Assimilation efficiency of proteins, but not lipids, was positively related to temperature. Interestingly, the SDA response exhibited a notable circadian rhythm. These results show that temperature has a pronounced effect on digestive energetics in A. atra, and that this effect differs between nutrient classes. Variation in environmental temperatures may thus alter the energy budget and nutrient reserves of these animals.

Muscles provide an internal water reserve for reproduction

The use of fat to support the energy needs of reproduction (i.e. capital breeding) has been studied in a diversity of taxa. However, despite reproductive output (i.e. young or eggs) being approximately 70% water, little is known about the availability of internal resources to accommodate the hydric demands of reproduction. Recent research suggests that dehydration increases the catabolism of muscle as a means of maintaining water balance. Accordingly, we investigated the interactive effects of reproductive investment and water deprivation on catabolism and reproductive output in female Children's pythons (Antaresia childreni). Both reproductive and non-reproductive females were either provided water ad libitum or were water-deprived for three weeks at the time when reproductive females were gravid. We found that water-deprived reproductive females had, in general, greater body mass loss, epaxial muscle loss, plasma osmolality and plasma uric acid concentrations relative to the other groups. Furthermore, water-deprived females had similar clutch sizes compared with females with access to water, but produced lighter eggs and lower total clutch masses. Our results provide the first evidence that selective protein catabolism can be used to support water demands during reproduction, and, as a result, these findings extend the capital breeding concept to non-energetic resources.

Learning to starve: impacts of food limitation beyond the stress period

Starvation is common among wild animal populations, and many individuals experience repeated bouts of starvation over the course of their lives. Although much information has been gained through laboratory studies of acute starvation, little is known about how starvation affects an animal once food is again available (i.e. during the refeeding and recovery phases). Many animals exhibit a curious phenomenon – some seem to ‘get better’ at starving following exposure to one or more starvation events – by this we mean that they exhibit potentially adaptive responses, including reduced rates of mass loss, reduced metabolic rates, and lower costs of digestion. During subsequent refeedings they may also exhibit improved digestive efficiency and more rapid mass gain. Importantly, these responses can last until the next starvation bout or even be inherited and expressed in the subsequent generation. Currently, however, little is known about the molecular regulation and physiological mechanisms underlying these changes. Here, we identify areas of research that can fill in the most pressing knowledge gaps. In particular, we highlight how recently refined techniques (e.g. stable isotope tracers, quantitative magnetic resonance and thermal measurement) as well as next-generation sequencing approaches (e.g. RNA-seq, proteomics and holobiome sequencing) can address specific starvation-focused questions. We also describe outstanding unknowns ripe for future research regarding the timing and severity of starvation, and concerning the persistence of these responses and their interactions with other ecological stressors.

Increase in branched-chain amino acids due to acidemia in neonatal calves with diarrhoea

The aim of this study was to investigate the relationships between acid-base status and plasma branched-chain amino acids (BCAA) concentration in calves with diarrhoea for intravenous nutrition, especially with amino acid solution in calves with diarrhoea. Thirty-four Holstein calves aged 11.0±5.9 days old were enrolled in this study. In 10 of 34 calves exhibiting clinical signs of diarrhoea, severe dehydration and acidemia were observed (severe group: pH: 7.04±0.11, base excess (BE): −17.4±4.5) based on blood gas analysis. In 7 of 34 calves exhibiting clinical signs of diarrhoea, mild dehydration and acidemia were observed (mild group: pH: 7.29±0.06, BE: 0.0±5.2). The other 17 calves did not exhibit dehydration or acidemia (pH: 7.41±0.02, BE: 11.2±3.5) based on clinical signs and blood gas analysis. The plasma concentration of BCAA was significantly higher in the severe group than in the other groups. In addition, the blood pH and plasma concentrations of BCAA (r=−0.41, P<0.05) were significantly and negatively correlated. As calves with metabolic acidosis have increased plasma BCAA concentrations due to hypermetabolic states of proteolysis, amino acid solutions containing low concentrations of BCAA may be useful to gradually correct the negative nitrogen balance.

Specialist-generalist model of body temperature regulation can be applied at the intraspecific level

According to theoretical predictions, endothermic homeotherms can be classified as either thermal specialists or thermal generalists. In high cost environments, thermal specialists are supposed to be more prone to using facultative heterothermy than generalists. We tested this hypothesis at the intraspecific level using male laboratory mice (C57BL/cmdb) fasted under different thermal conditions (20 and 10°C) and for different time periods (12-48 h). We predicted that variability of body temperature (T_b) and time spent with T_b below normothermy would increase with the increase of environmental demands (duration of fasting and cold). To verify the above prediction, we measured T_b and energy expenditure of fasted mice. We did not record torpor bouts but we found that variations in T_b and time spent in hypothermia increased with environmental demands. In response to fasting, mice also decreased their energy expenditure. Moreover, animals that showed more precise thermoregulation when fed had more variable T_b when fasted. We postulate that the prediction of the thermoregulatory generalist-specialist trade-off can be applied at the intraspecific level, offering a valid tool for identifying mechanistic explanations of the differences in animal responses to variations in energy supply.

Oxidation of linoleic and palmitic acid in pre-hibernating and hibernating common noctule bats revealed by 13C breath testing

Mammals fuel hibernation by oxidizing saturated and unsaturated fatty acids from triacylglycerols in adipocytes, yet the relative importance of these two categories as an oxidative fuel may change during hibernation. We studied the selective use of fatty acids as an oxidative fuel in noctule bats (Nyctalus noctula). Pre-hibernating noctule bats that were fed 13C-enriched linoleic acid (LA) showed 12 times higher tracer oxidation rates compared to conspecifics fed 13C-enriched palmitic acid (PA). After this experiment, we supplemented the diet of bats with the same fatty acids on 5 subsequent days to enrich their fat depots with the respective tracer. We then compared the excess 13C enrichment (APE) in breath of bats for torpor and arousal events during early and late hibernation. We observed higher APE values in breath of bats fed 13C-enriched LA than in bats fed 13C-enriched PA for both states, torpor and arousal, and also for both periods. Thus, hibernating bats oxidized selectively endogenous LA instead of PA, most likely because of faster transportation rates of PUFA compared with SFA. We did not observe changes in APE values in the breath of torpid animals between early and late hibernation. Skin temperature of torpid animals increased by 0.7°C between early and late hibernation in bats fed PA, whereas it decreased by -0.8°C in bats fed LA, highlighting that endogenous LA may fulfil two functions when available in excess: serving as an oxidative fuel and supporting cell membrane functionality.