Mediating free glucocorticoid levels in the blood of vertebrates: are corticosteroid-binding proteins always necessary?

Down-regulating the stress axis: Living in the present while preparing for the future

The measurement of glucocorticoid (GC) hormones provides us with a window into the stress physiology of vertebrates and the adaptative responses they use to cope with predictable and unpredictable changes in the environment. Baseline GCs inform us about the metabolic demands they are subject to at that point in their yearly life-history stage, whereas GC changes (often increases) in response to acute challenges inform us on their capacity to cope with more immediate environmental challenges. However, baseline GC levels and the kinetics of GC responses to acute stressors can vary substantially among and within species, depending on individual characteristics (age, sex, condition, life-history stage). In addition, a thorough understanding of the stress status of an animal requires moving beyond the measurement of GCs alone by focusing on downstream measures of metabolic activation, such as oxidative stress. Here, we evaluated the changes in blood cortisol and oxidative stress markers in wild adult Columbian ground squirrels (Urocitellus columbianus), following a 30-min capture-handling stress performed in mid-late June. Measurements were taken when males were post-reproductive and preparing for hibernation and adult females were weaning litters. We found three key results. First, the time-course of GC increase was markedly slower (by an order of magnitude) than what is currently reported in the literature for most species of mammals, birds and reptiles. Second, there were marked differences in the male and female response, linked to differences in life-history stage: females close to weaning had abolished GC responses, whereas post-reproductive males did not. Third, there were mild to moderate increases in oxidative damage and decreases in oxidative defenses in response to our short-term challenge, consistent with the idea that short-term acute metabolic activation may carry physiological costs. However, these changes were not correlated to the changes in GCs, a novel result suggesting a disconnect between the hormonal stress response and oxidative damage.

Coping with extreme free cortisol levels: Seasonal stress axis changes in sympatric North American flying squirrels

Most environments exhibit predictable yearly changes, permitting animals to anticipate them. The hypothalamic-pituitary-adrenal (HPA) axis is a key physiological pathway that enables animals to cope with such changes. Monitoring glucocorticoid (the end products of the HPA axis) levels in wild animals throughout the year can improve our understanding of how this pathway responds to different conditions. For this study, we collected 18 months of data on two species of North American flying squirrels (Glaucomys sabrinus and G. volans) living in a southern Ontario forest where temperature and food availability fluctuate dramatically throughout the year. These squirrels are active year-round, nest communally, and rely on scatter hoarded foods in the winter months. Flying squirrels have extremely high levels of free plasma cortisol relative to other mammals, but it is unknown how these levels are affected by environmental and reproductive factors. For both species, our goals were to (1) validate an enzyme immunoassay (EIA) to measure their fecal glucocorticoid metabolite (FGM) concentrations and (2) assess yearly differences, seasonal changes, and the influence of sex, reproduction, and ambient temperature on FGM concentrations in each species. In the lab, we successfully validated the use of antibody 5α-pregnane-3β, 11β, 21-triol-20-one EIA for FGM analysis in both species. In the field, neither sex nor reproductive status (breeding condition or not) were linked to FGM concentrations in either species. FGM concentrations were higher in autumn compared to the spring and summer. There were no other seasonal differences. We discuss possible explanations for the autumn peak in FGM concentrations (increased energy expenditure and social nesting changes), as well as outline possible avenues for future research. Understanding how individuals and populations respond to environmental change is a critical goal in evolutionary ecology, particularly in the context of a rapidly changing Anthropocene.

Effects of the social environment on vertebrate fitness and health in nature: Moving beyond the stress axis

Social interactions are a ubiquitous feature of the lives of vertebrate species. These may be cooperative or competitive, and shape the dynamics of social systems, with profound effects on individual behavior, physiology, fitness, and health. On one hand, a wealth of studies on humans, laboratory animal models, and captive species have focused on understanding the relationships between social interactions and individual health within the context of disease and pathology. On the other, ecological studies are attempting an understanding of how social interactions shape individual phenotypes in the wild, and the consequences this entails in terms of adaptation. Whereas numerous studies in wild vertebrates have focused on the relationships between social environments and the stress axis, much remains to be done in understanding how socially-related activation of the stress axis coordinates other key physiological functions related to health. Here, we review the state of our current knowledge on the effects that social interactions may have on other markers of vertebrate fitness and health. Building upon complementary findings from the biomedical and ecological fields, we identify 6 key physiological functions (cellular metabolism, oxidative stress, cellular senescence, immunity, brain function, and the regulation of biological rhythms) which are intimately related to the stress axis, and likely directly affected by social interactions. Our goal is a holistic understanding of how social environments affect vertebrate fitness and health in the wild. Whereas both social interactions and social environments are recognized as important sources of phenotypic variation, their consequences on vertebrate fitness, and the adaptive nature of social-stress-induced phenotypes, remain unclear. Social flexibility, or the ability of an animal to change its social behavior with resulting changes in social systems in response to fluctuating environments, has emerged as a critical underlying factor that may buffer the beneficial and detrimental effects of social environments on vertebrate fitness and health.

Rutland C. Equine Stress: Neuroendocrine Physiology and Pathophysiology

This review presents new aspects to understanding the neuroendocrine regulation of equine stress responses, and their influences on the physiological, pathophysiological, and behavioral processes. Horse management, in essence, is more frequently confirmed by external and internal stress factors, than in other domestic animals. Regardless of the nature of the stimulus, the equine stress response is an effective and highly conservative set of interconnected relationships designed to maintain physiological integrity even in the most challenging circumstances (e.g., orthopedic injuries, abdominal pain, transport, competitions, weaning, surgery, and inflammation). The equine stress response is commonly a complementary homeostatic mechanism that provides protection (not an adaptation) when the body is disturbed or threatened. It activates numerous neural and hormonal networks to optimize metabolic, cardiovascular, musculoskeletal, and immunological functions. This review looks into the various mechanisms involved in stress responses, stress-related diseases, and assessment, prevention or control, and management of these diseases and stress. Stress-related diseases can not only be identified and assessed better, given the latest research and techniques but also prevented or controlled.

Are synthetic glucocorticoids in the aquatic environment a risk to fish?

The glucocorticosteroid, or glucocorticoid (GC), system is largely conserved across vertebrates and plays a central role in numerous vital physiological processes including bone development, immunomodulation, and modification of glucose metabolism and the induction of stress-related behaviours. As a result of their wide-ranging actions, synthetic GCs are widely prescribed for numerous human and veterinary therapeutic purposes and consequently have been detected extensively within the aquatic environment. Synthetic GCs designed for humans are pharmacologically active in non-mammalian vertebrates, including fish, however they are generally detected in surface waters at low (ng/L) concentrations. In this review, we assess the potential environmental risk of synthetic GCs to fish by comparing available experimental data and effect levels in fish with those in mammals. We found the majority of compounds were predicted to have insignificant risk to fish, however some compounds were predicted to be of moderate and high risk to fish, although the dataset of compounds used for this analysis was small. Given the common mode of action and high level of inter-species target conservation exhibited amongst the GCs, we also give due consideration to the potential for mixture effects, which may be particularly significant when considering the potential for environmental impact from this class of pharmaceuticals. Finally, we also provide recommendations for further research to more fully understand the potential environmental impact of this relatively understudied group of commonly prescribed human and veterinary drugs.

Effects of capture on stress-axis measures in endangered little brown bats (Myotis lucifugus)

Little brown bats (Myotis lucifugus) are a widely distributed species in North America that have been decimated by the fungal disease white-nose syndrome. As such, little brown bats are the focus of monitoring and research initiatives that often include capturing and handling free-ranging individuals. We examined the stress response of 198 adult female little brown bats after being captured from three bat houses, during the summer. Our objective was to inform best practices to researchers capturing and handling bats in the wild. We compared the stress response among bats held for <3 min (baseline), 15–30 min, or >30 min, and then among bats held alone or in a group with conspecifics. We measured the levels of plasma total and free cortisol, maximum corticosteroid binding capacity (MCBC), and blood glucose. Relative to baseline, total and free cortisol levels were significantly higher in bats held for 15–30 min and higher still in those held for > 30 min. Blood glucose levels were elevated after >30 min of holding. MCBC levels showed no differences among holding times. We detected a weak effect of social holding condition, with solitary-held bats having lower total cortisol levels than group-held bats, but MCBC, free cortisol, and blood glucose levels showed no effect of social holding condition. Our findings demonstrate that capture time should be minimized and suggest that little brown bats should be handled and released within 30 min of capture as means of reducing stress. Further, solitary holding did not appear to increase stress measures, which supports holding bats individually after capture, instead of in groups, to reduce risk of pathogen and parasite transmission.

Assay temperature affects corticosteroid-binding globulin and free corticosterone estimates across species

Glucocorticoid hormones are often measured to assess how organisms physiologically respond to challenges in their environment. In plasma, glucocorticoids circulate in two forms: bound to corticosteroid-binding globulins (CBG) or unbound (free). Measuring CBG allows us to estimate the amount of free glucocorticoids present in a plasma sample. However, free glucocorticoid estimates are affected by the assay temperature used when measuring CBG, with colder temperatures maximizing specific binding but likely underestimating glucocorticoid's affinity for CBG. Here, we test how a biologically relevant incubation temperature (41 °C) changes the disassociation constant (K_d; used to estimate free glucocorticoid levels) when compared to the traditional 4 °C incubation temperature, across four commonly studied avian species. We then apply the new K_d's calculated at 41 °C to existing data sets to examine how the change in K_d affects free corticosterone estimates and data interpretation. K_d's were generally higher (lower affinity for CORT) at warmer incubation temperatures which resulted in higher levels of estimated free CORT in all four species but differed among subspecies. This increase in free CORT levels did not qualitatively change previously reported statistical relationships, but did affect variance and alpha (P) values. We suggest that future assays be run at biologically relevant temperatures for more accurate estimates of free CORT levels in vivo and to increase the chances of detecting biological patterns of free-CORT that may not be revealed with the classic methodology that tends to underestimate free CORT levels.

Multifaceted Control of GR Signaling and Its Impact on Hepatic Transcriptional Networks and Metabolism

Glucocorticoids (GCs) and the glucocorticoid receptor (GR) are important regulators of development, inflammation, stress response and metabolism, demonstrated in various diseases including Addison's disease, Cushing's syndrome and by the many side effects of prolonged clinical administration of GCs. These conditions include severe metabolic challenges in key metabolic organs like the liver. In the liver, GR is known to regulate the transcription of key enzymes in glucose and lipid metabolism and contribute to the regulation of circadian-expressed genes. Insights to the modes of GR regulation and the underlying functional mechanisms are key for understanding diseases and for the development of improved clinical uses of GCs. The activity and function of GR is regulated at numerous levels including ligand availability, interaction with heat shock protein (HSP) complexes, expression of GR isoforms and posttranslational modifications. Moreover, recent genomics studies show functional interaction with multiple transcription factors (TF) and coregulators in complex transcriptional networks controlling cell type-specific gene expression by GCs. In this review we describe the different regulatory steps important for GR activity and discuss how different TF interaction partners of GR selectively control hepatic gene transcription and metabolism.

The stress of Arctic warming on polar bears

Arctic ecosystems are changing rapidly in response to climate warming. While Arctic mammals are highly evolved to these extreme environments, particularly with respect to their stress axis, some species may have limited capacity to adapt to this change. We examined changes in key components of the stress axis (cortisol and its carrier protein-corticosteroid binding globulin [CBG]) in polar bears (Ursus maritimus) from western Hudson Bay (N = 300) over a 33 year period (1983-2015) during which time the ice-free period was increasing. Changing sea ice phenology limits spring hunting opportunities and extends the period of onshore fasting. We assessed the response of polar bears to a standardized stressor (helicopter pursuit, darting, and immobilization) during their onshore fasting period (late summer-autumn) and quantified the serum levels of the maximum corticosteroid binding capacity (MCBC) of CBG, the serum protein that binds cortisol strongly, and free cortisol (FC). We quantified bear condition (age, sex, female with cubs or not, fat condition), sea ice (breakup in spring-summer, 1 year lagged freeze-up in autumn), and duration of fasting until sample collection as well as cumulative impacts of the latter environmental traits from the previous year. Data were separated into "good" years (1983-1990) when conditions were thought to be optimal and "poor" years (1991-2015) when sea ice conditions deteriorated and fasting on land was extended. MCBC explained 39.4% of the variation in the good years, but only 28.1% in the poor ones, using both biological and environmental variables. MCBC levels decreased with age. Changes in FC were complex, but more poorly explained. Counterintuitively, MCBC levels increased with increased time onshore, 1 year lag effects, and in poor ice years. We conclude that MCBC is a biomarker of stress in polar bears and that the changes we document are a consequence of climate warming.

Measurement of free glucocorticoids: quantifying corticosteroid binding capacity and its variation within and among mammal and bird species

Plasma glucocorticoid (CORT) levels are one measure of stress in wildlife and give us insight into natural processes relevant to conservation issues. Many studies use total CORT concentrations to draw conclusions about animals' stress state and response to their environment. However, the blood of tetrapods contains corticosteroid-binding globulin (CBG), which strongly binds most circulating CORT. Only free CORT (CORT not bound by CBG) leaves the circulation and exerts biological effects on CORT-sensitive tissues. Measuring free CORT concentrations provides insight to an animal's stress response that cannot be revealed by simply measuring total CORT. To calculate free CORT concentrations in plasma or serum samples, one needs three measurements: the binding affinity of CBG for CORT (which varies by species), the total CORT concentration in the sample and the maximum corticosteroid binding capacity (MCBC) of CBG in the sample. Here, we detail the measurement of CBG binding capacity. We compare and contrast the three main methods to measure MCBC: charcoal, cell harvester and dialysis. Each is defined by the means by which free and bound CORT are separated. We weigh the relative merits and challenges of each. We conclude that sample volume, species and taxon binding specificity, and availability of equipment are the primary considerations in selecting the appropriate separation method. For most mammals, the charcoal method is recommended. For birds, the harvester method has critical advantages over the charcoal method. The dialysis method is widely regarded as the gold standard and has lower equipment costs but is more time-intensive and costly in terms of radioactive isotope needed and is less suited to processing large numbers of samples. The binding capacity of CBG varies tremendously within and among the bird and marine mammal species studied, and we discuss the implication of this variation for understanding the role of stress in wildlife.

Analyzing Stress as a Multivariate Phenotype

The stress phenotype is multivariate. Recent advances have broadened our understanding beyond characterizing the stress response in a single dimension. Simultaneously, the toolbox available to ecophysiologists has expanded greatly in recent years, allowing the measurement of multiple biomarkers from an individual at a single point in time. Yet these advances-in our conceptual understanding and available methodologies-have not yet been combined in a unifying multivariate statistical framework. Here, we offer a brief review of the multivariate stress phenotype and describe a general statistical approach for analysis using nonparametric multivariate analysis of variance with residual randomization in permutation procedures (RRPP) implemented using the "RRPP" package in R. We also provide an example illustrating the novel insights that can be gained from a holistic multivariate approach to stress and provide a tutorial for how we analyzed these data, including annotated R code and a guide to interpretation of outputs (Online Appendix 1). We hope that this statistical methodology will provide a quantitative framework facilitating the unification of our theoretical understanding and empirical observations into a quantitative, multivariate theory of stress.

The role of herbivory in the macroevolution of vertebrate hormone dynamics

Vertebrates have high species-level variation in circulating hormone concentrations, and the functional significance of this variation is largely unknown. We tested the hypothesis that interspecific differences in hormone concentrations are partially driven by plant consumption, based on the prediction that herbivores should have higher basal hormone levels to 'outcompete' plant endocrine disruptors. We compared levels of glucocorticoids (GCs), the hormones with the most available data, across 166 species. Using phylogenetically informed comparisons, we found that herbivores had higher GC levels than carnivores. Furthermore, we found that the previously described negative relationship between GC levels and body mass only held in herbivores, not carnivores, and that the effect of diet was greatest at extreme body sizes. These findings demonstrate the far-reaching effects of diet on animal physiology, and provide evidence that herbivory influences circulating hormone concentrations. We urge future direct testing of the relationship between phytochemical load and GC levels.

Corticosteroid-binding globulin levels in North American sciurids: implications for the flying squirrel stress axis

Corticosteroid-binding globulin (CBG) helps to regulate tissue bioavailability of circulating glucocorticoids (GCs), and in most vertebrates, ≥80%–90% of GCs bind to this protein. New World flying squirrels have higher plasma total cortisol levels (the primary corticosteroid in sciurids) than most vertebrates. Recent research suggests that flying squirrels have either low amounts of CBG or CBG molecules that have a low binding affinity for cortisol, as this taxon appears to exhibit very low proportions of cortisol bound to CBG. To test whether CBG levels have been adjusted over evolutionary time, we assessed the levels of this protein in the plasma of northern (Glaucomys sabrinus (Shaw, 1801)) and southern (Glaucomys volans (Linnaeus, 1758)) flying squirrels using immunoblotting, and compared the relative levels among three phylogenetically related species of sciurids. We also compared the pattern of CBG levels with cortisol levels for the same individuals. Flying squirrels had higher cortisol levels than the other species, but similar levels of CBG to their closest relatives (tree squirrels). We conclude that CBG levels in flying squirrels have not been adjusted over evolutionary time, and thus, the uncoupling of CBG levels from cortisol concentrations may represent an evolutionary modification in the lineage leading to New World flying squirrels.

Seasonal changes in acute stressor-mediated plasma glucocorticoid regulation in New World flying squirrels

Southern flying squirrels have higher circulating cortisol levels than most vertebrates. However, regulation of tissue exposure to cortisol by the hormone's carrier protein, corticosteroid-binding globulin (CBG), appears to be altered due to lower-than-expected CBG expression levels, and a reduced affinity for cortisol. To assess the capacity of flying squirrels to regulate acute stress-mediated cortisol levels, we used the dexamethasone (DEX) suppression test followed by the adrenocorticotropic hormone (ACTH) stimulation test in both the breeding and non-breeding seasons, and quantified resultant changes in plasma cortisol and relative CBG levels. Regulation of cortisol via negative feedback, and the acute stress response appeared to function as they do in other vertebrates during the breeding season, but response to DEX in the non-breeding season showed that the sensitivity of the negative feedback mechanism changed across seasons. The relatively high concentrations of DEX required to induce negative feedback suggests that southern flying squirrels have a reduced sensitivity to cortisol compared with other vertebrates, and that high circulating cortisol levels may be required to compensate for low target tissue responsiveness in this species. Cortisol, but not CBG levels, were higher during the non-breeding than breeding season, and females had higher cortisol and CBG levels than males. Our data suggest that flying squirrel cortisol levels are regulated by negative feedback at a higher set point than in related species. Seasonal changes in cortisol levels, target tissue sensitivity to DEX, and in the capacity to respond to stressors appear to be part of the underlying physiology of southern flying squirrels, and may be required to maximize fitness in the face of tradeoffs between survival and reproduction.

Comprehensive endocrine response to acute stress in the bottlenose dolphin from serum, blubber, and feces

Several hormones are potential indicators of stress in free-ranging animals and provide information on animal health in managed-care settings. In response to stress, glucocorticoids (GC, e.g. cortisol) first appear in circulation but are later incorporated into other tissues (e.g. adipose) or excreted in feces or urine. These alternative matrices can be sampled remotely, or by less invasive means, than required for blood collection and are especially valuable in highly mobile species, like marine mammals. We characterized the timing and magnitude of several hormones in response to a stressor in bottlenose dolphins (Tursiops truncatus) and the subsequent incorporation of cortisol into blubber, and its metabolites excreted in feces. We evaluated the endocrine response to an acute stressor in bottlenose dolphins under managed care. We used a standardized stress protocol where dolphins voluntarily beached onto a padded platform and remained out of water for two hours; during the stress test blood samples were collected every 15 min and blubber biopsies were collected every hour (0, 60, and 120 min). Each subject was studied over five days: voluntary blood samples were collected on each of two days prior to the stress test; 1 and 2 h after the conclusion of the out-of-water stress test; and on the following two days after the stress test. Fecal samples were collected daily, each afternoon. The acute stressor resulted in increases in circulating ACTH, cortisol, and aldosterone during the stress test, and each returned to baseline levels within 2 h of the dolphin's return to water. Both cortisol and aldosterone concentrations were correlated with ACTH, suggesting both corticosteroids are at least partly regulated by ACTH. Thyroid hormone concentrations were generally unaffected by the acute stressor. Blubber cortisol increased during the stress test, and fecal GC excretion was elevated on the day of the stress test. We found that GCs in bottlenose dolphins can recover within hours of acute stress, and that cortisol release can be detected in alternate matrices within a few hours-within 2 h in blubber, and 3.5-5 h in fecal samples.

Eye region surface temperature reflects both energy reserves and circulating glucocorticoids in a wild bird

Body temperature of endotherms shows substantial within- and between-individual variation, but the sources of this variation are not fully understood in wild animals. Variation in body temperature can indicate how individuals cope with their environment via metabolic or stress-induced effects, both of which may relate to depletion of energy reserves. Body condition can reflect heat production through changes to metabolic rate made to protect energy reserves. Additionally, changes in metabolic processes may be mediated by stress-related glucocorticoid secretion, which is associated with altered blood-flow patterns that affect regional body temperatures. Accordingly, both body condition and glucocorticoid secretion should relate to body temperature. We used thermal imaging, a novel non-invasive method of temperature measurement, to investigate relationships between body condition, glucocorticoid secretion and body surface temperature in wild blue tits (Cyanistes caeruleus). Individuals with lower body condition had lower eye-region surface temperature in both non-breeding and breeding seasons. Eye-region surface temperature was also negatively correlated with baseline circulating glucocorticoid levels in non-breeding birds. Our results demonstrate that body surface temperature can integrate multiple aspects of physiological state. Consequently, remotely-measured body surface temperature could be used to assess such aspects of physiological state non-invasively in free-living animals at multiple life history stages.

Quantification of a Glucocorticoid Profile in Non-pooled Samples Is Pivotal in Stress Research Across Vertebrates

Vertebrates are faced continuously with a variety of potential stressful stimuli and react by a highly conserved endocrine stress response. An immediate catecholamine mediated response increases plasma glucose levels in order to prepare the organism for the "fight or flight" reaction. In addition, in a matter of minutes after this (nor)adrenaline release, glucocorticoids, in particular cortisol or corticosterone depending on the species, are released through activation of the hypothalamic-pituitary-interrenal (HPI) axis in fish or hypothalamic-pituitary-adrenal (HPA) axis in other vertebrates. These plasma glucocorticoids are well documented and widely used as biomarker for stress across vertebrates. In order to study the role of glucocorticoids in acute and chronic stress and gain in-depth insight in the stress axis (re)activity across vertebrates, it is pivotal to pin-point the involved molecules, to understand the mechanisms of how the latter are synthesized, regulated and excreted, and to grasp their actions on a plethora of biological processes. Furthermore, in-depth knowledge on the characteristics of the tissues as well as on the analytical methodologies available for glucocorticoid quantification is needed. This manuscript is to be situated in the multi-disciplinary research topic of glucocorticoid action across vertebrates which is linked to a wide range of research domains including but not limited to biochemistry, ecology, endocrinology, ethology, histology, immunology, morphology, physiology, and toxicology, and provides a solid base for all interested in stress, in particular glucocorticoid, related research. In this framework, internationally validated confirmation methods for quantification of a glucocorticoid profile comprising: (i) the dominant hormone; (ii) its direct precursors; (iii) its endogenously present phase I metabolites; and (iv) the most abundant more polar excreted exogenous phase I metabolites in non-pooled samples are pivotal.

A state of non-specific tension in living matter? Stress in Australian animals

Evidence of stress responses in Australian animals is reviewed through a series of case studies involving desert frogs and lizards, small carnivorous marsupials, desert wallabies, a dwarf kangaroo species, the quokka wallaby and a small nectarivorous bird. An operational definition of stress as "the physiological resultant of demands that exceed an animal's homeostatic capacities" is used to identify instances of stress responses in the field, and to gauge their intensity. Clear evidence of stress responses is found in small dasyurid marsupial carnivores, and desert agamid lizards, both of which are semelparous. Other instances of seasonal stress responses include the Rottnest Island quokka, the Barrow Island euro kangaroo and a small nectarivorous bird, the Silvereye. The review also highlights the high level of physiological adaptation of some desert wallabies, such as the Spectacled hare wallaby, which is able to maintain physiological homeostasis in the field when challenged by conditions of extreme drought. The importance of thermal and hygric refugia for the long-term survival or rock wallabies, which apparently lack any hormonal control of renal function, is also highlighted.

Coping with pregnancy after 9months in the dark: Post-hibernation buffering of high maternal stress in arctic ground squirrels

Pregnancy and lactation are key times in the life of female mammals when energetic resources must be brought to bear to produce and nurture offspring. Changes in glucocorticoid (GC) levels are central to this objective, due to their roles in modulating development and physiology and in mediating energetic tradeoffs. We examined GC changes over reproduction in a species living in a harsh seasonal environment: the arctic ground squirrel (Urocitellus parryii). Females become pregnant immediately after emerging from a ∼9month hibernation, and then must begin this pregnancy during an additional month of freezing temperatures and limited food availability. We measured plasma levels of total cortisol, corticosteroid-binding globulin (CBG), and free cortisol in unstressed females at three stages: not-visibly pregnant, visibly pregnant, and lactating. Total cortisol levels were similar in all stages, but CBG levels increased 4-fold from the not-visibly pregnant stage to visibly pregnant and lactating stage. As a result, the free cortisol fraction declined from 51% of total cortisol when females were not-visibly pregnant to only 5% when they were visibly pregnant (remaining low and stable throughout pregnancy) and 10% when they were lactating. This pattern is markedly different from that seen in other mammals, where 10% or less of GCs are free and these tend to increase during gestation. We postulate that the high free cortisol just prior to visible pregnancy is a seasonal adaptation relating either to the pronounced physiological changes the female must undergo after emerging from hibernation and immediately getting pregnant, or to the mobilization of body reserves for energy to permit pregnancy, or both. Thereafter, high CBG levels may shield the developing offspring from the negative effects of cortisol overexposure.

Does corticosterone regulate the onset of breeding in free-living birds?: The CORT-Flexibility Hypothesis and six potential mechanisms for priming corticosteroid function

For many avian species, the decision to initiate breeding is based on information from a variety of environmental cues, including photoperiod, temperature, food availability, and social interactions. There is evidence that the hormone corticosterone may be involved in delaying the onset of breeding in cases where supplemental cues, such as low food availability and inclement weather, indicate that the environment is not suitable. However, not all studies have found the expected relationships between breeding delays and corticosterone titers. In this review, we present the hypothesis that corticosterone physiology mediates flexibility in breeding initiation (the "CORT-Flexibility Hypothesis"), and propose six possible corticosterone-driven mechanisms in pre-breeding birds that may delay breeding initiation: altering hormone titers, negative feedback regulation, plasma binding globulin concentrations, intracellular receptor concentrations, enzyme activity and interacting hormone systems. Based on the length of the breeding season and species-specific natural history, we also predict variation in corticosterone-regulated pre-breeding flexibility. Although few studies thus far have examined mechanisms beyond plasma hormone titers, the CORT-Flexibility Hypothesis is grounded on a solid foundation of research showing seasonal variation in the physiological stress response and knowledge of physiological mechanisms modulating corticosteroid effects. We propose six possible mechanisms as testable and falsifiable predictions to help clarify the extent of HPA axis regulation of the initiation of breeding.

Non-invasive monitoring of stress hormones in the bat Eptesicus isabellinus - Do fecal glucocorticoid metabolite concentrations correlate with survival?

Chronic stress may negatively impact fitness and survival in wildlife. Stress hormone analysis from feces is a non-invasive tool for identifying stressors and deducing about individual and population level fitness. Although many bat populations are endangered, fecal stress hormone analysis has not been established in bats as a method for focusing conservation efforts. The isabelline serotine bat, Eptesicus isabellinus, is exposed to human disturbance as its roosts are mostly found in anthropogenic structures. Moreover, this bat is host to various diseases and survival rates between colonies may vary significantly. To validate the analysis of fecal glucocorticoid metabolites, we applied an adrenocorticotropic hormone (ACTH) challenge and tested four different enzyme immunoassays (EIA) for measuring glucocorticoid concentrations. Cortisol and its metabolites showed the highest increase in blood and feces after the ACTH challenge, but corticosterone and its metabolites also increased significantly. Baseline fecal cortisol metabolite (FCM) concentrations did not increase until 1.5h after the animals were captured, which is a convenient time lag for sample collection from captured animals. We furthermore compared baseline FCM concentrations between five colonies of E. isabellinus in Andalusia, Spain, and tested for their correlation with survival rates. FCM concentrations did not vary between colonies, but FCM levels increased with the animals' age. FCM analysis may prove a useful tool for identifying bat colonies that experience uncommon environmental stress. However, inter-individual variation in hormone secretion, due to factors such as age, may require additional information to properly interpret differences in hormone concentrations.

Responses of New World flying squirrels to the acute stress of capture and handling

Northern ( Glaucomys sabrinus ) and southern ( G. volans ) flying squirrels have glucocorticoid (GC; stress hormone) levels higher than most vertebrates but virtually no binding capacity for these GCs via the carrier protein, corticosteroid-binding globulin. Thus, their total GCs are essentially all free and biologically active. However, the GC estimates come from blood samples taken after squirrels had been in live traps, and thus in a stress-induced state. Obtaining baseline values for physiological variables is valuable for assessing the response of vertebrates to stressors in their environment. We compared baseline plasma total cortisol levels (within 3min of capture) to stress-induced levels (after 30min of trap restraint) in both flying squirrel species. We recorded baseline cortisol levels that were some of the highest ever reported for mammals, indicating their stress axes operate at a higher set point than most other species. As part of the stress response, we also measured 4 indices in addition to cortisol. Total cortisol and free fatty acids increased in both species, as predicted. In contrast with our predictions, blood glucose and neutrophil/lymphocyte ratio showed no overall change, and hematocrit decreased significantly. New World flying squirrels therefore appear to have a stress response that differs from many other mammals. The selective forces driving the physiology of these animals remain elusive, but this lineage may provide an interesting comparative system for the study of stress axis function and its evolution among vertebrates.

Measurement of free glucocorticoids: quantifying corticosteroid-binding globulin binding affinity and its variation within and among mammalian species

Plasma glucocorticoids (GCs) are commonly used as measures of stress in wildlife. A great deal of evidence indicates that only free GC (GC not bound by the specific binding protein, corticosteroid-binding globulin, CBG) leaves the circulation and exerts biological effects on GC-sensitive tissues. Free hormone concentrations are difficult to measure directly, so researchers estimate free GC using two measures: the binding affinity and the binding capacity in plasma. We provide an inexpensive saturation binding method for calculating the binding affinity (equilibrium dissociation constant, K d) of CBG that can be run without specialized laboratory equipment. Given that other plasma proteins, such as albumin, also bind GCs, the method compensates for this non-specific binding. Separation of bound GC from free GC was achieved with dextran-coated charcoal. The method provides repeatable estimates (12% coefficient of variation in the red squirrel, Tamiasciurus hudsonicus), and there is little evidence of inter-individual variation in K d (range 2.0-7.3 nM for 16 Richardson's ground squirrels, Urocitellus richardsonii). The K d values of 28 mammalian species we assessed were mostly clustered around a median of 4 nM, but five species had values between 13 and 61 nM. This pattern may be distinct from birds, for which published values are more tightly distributed (1.5-5.1 nM). The charcoal separation method provides a reliable and robust method for measuring the K d in a wide range of species. It uses basic laboratory equipment to provide rapid results at very low cost. Given the importance of CBG in regulating the biological activity of GCs, this method is a useful tool for physiological ecologists.

Stress physiology in marine mammals: how well do they fit the terrestrial model?

Stressors are commonly accepted as the causal factors, either internal or external, that evoke physiological responses to mediate the impact of the stressor. The majority of research on the physiological stress response, and costs incurred to an animal, has focused on terrestrial species. This review presents current knowledge on the physiology of the stress response in a lesser studied group of mammals, the marine mammals. Marine mammals are an artificial or pseudo grouping from a taxonomical perspective, as this group represents several distinct and diverse orders of mammals. However, they all are fully or semi-aquatic animals and have experienced selective pressures that have shaped their physiology in a manner that differs from terrestrial relatives. What these differences are and how they relate to the stress response is an efflorescent topic of study. The identification of the many facets of the stress response is critical to marine mammal management and conservation efforts. Anthropogenic stressors in marine ecosystems, including ocean noise, pollution, and fisheries interactions, are increasing and the dramatic responses of some marine mammals to these stressors have elevated concerns over the impact of human-related activities on a diverse group of animals that are difficult to monitor. This review covers the physiology of the stress response in marine mammals and places it in context of what is known from research on terrestrial mammals, particularly with respect to mediator activity that diverges from generalized terrestrial models. Challenges in conducting research on stress physiology in marine mammals are discussed and ways to overcome these challenges in the future are suggested.

Do smart birds stress less? An interspecific relationship between brain size and corticosterone levels

Vertebrates respond to unpredictable noxious environmental stimuli by increasing secretion of glucocorticoids (CORT). Although this hormonal stress response is adaptive, high levels of CORT may induce significant costs if stressful situations are frequent. Thus, alternative coping mechanisms that help buffer individuals against environmental stressors may be selected for when the costs of CORT levels are elevated. By allowing individuals to identify, anticipate and cope with the stressful circumstances, cognition may enable stress-specific behavioural coping. Although there is evidence that behavioural responses allow animals to cope with stressful situations, it is unclear whether or not cognition reduces investment in the neuroendocrine stress response. Here, we report that in birds, species with larger brains relative to their body size show lower baseline and peak CORT levels than species with smaller brains. This relationship is consistent across life-history stages, and cannot be accounted for by differences in life history and geographical latitude. Because a large brain is a major feature of birds that base their lifetime in learning new things, our results support the hypothesis that enhanced cognition represents a general alternative to the neuroendocrine stress response.