Sight of a Predator Induces a Corticosterone Stress Response and Generates Fear in an Amphibian

Amphibians, like other animals, generate corticosterone or cortisol glucocorticoid responses to stimuli perceived to be threatening. It is generally assumed that the corticosterone response of animals to capture and handling reflects the corticosterone response to stimuli such as the sight of a predator that are thought to be natural stressors. Fijian ground frogs ( Platymantisvitiana ) are preyed upon by the introduced cane toads ( Rhinellamarina ), and we used ground frogs to test the hypothesis that the sight of a predator will induce a corticosterone stress response in an amphibian. Urinary corticosterone metabolite concentrations increased in male ground frogs exposed to the sight of a toad for 1, 3 or 6 h, whereas corticosterone did not change in frogs exposed to another male ground frog, a ball, or when no stimulus was present in the test compartment. The frogs exposed to a toad initially moved towards the stimulus then moved away, whereas frogs exposed to another frog moved towards the test frog and remained closer to the frog than at the start of the test. Tonic immobility (TI) was measured as an index of fearfulness immediately after the test exposure of the frogs to a stimulus. The duration of TI was longer in frogs exposed to a toad than to another frog or to a ball. The results provide novel evidence that the sight of a predator can induce a corticosterone response and lead to increased fearfulness in amphibians. In addition, they show that endemic frogs can recognise an introduced predator as a threat.     

Repeated thermal stressor causes chronic elevation of baseline corticosterone and suppresses the physiological endocrine sensitivity to acute stressor in the cane toad (Rhinella marina)

Extreme environmental temperature could impact the physiology and ecology of animals. The stress endocrine axis provides necessary physiological stress response to acute (day–day) stressors. Presently, there are no empirical evidences showing that exposure to extreme thermal stressor could cause chronic stress in amphibians. This could also modulate the physiological endocrine sensitivity to acute stressors and have serious implications for stress coping in amphibians, particularly those living in fragmented and disease prone environments. We addressed this important question using the cane toad (Rhinella marina) model from its introduced range in Queensland, Australia. We quantified their physiological endocrine sensitivity to a standard acute (capture and handling) stressor after exposing the cane toads to thermal shock at 35 °C for 30 min daily for 34 days. Corticosterone (CORT) responses to the capture and handling protocol were measured on three sampling intervals (days 14, 24, and 34) to determine whether the physiological endocrine sensitivity was maintained or modulated over-time. Two control groups (C1 for baseline CORT measurement only and C2 acute handled only) and two temperature treatment groups (T1 received daily thermal shock up to day 14 only and a recovery phase of 20 days and T2 received thermal shock daily for 34 days). Results showed that baseline CORT levels remained high on day 14 (combined effect of capture, captivity and thermal stress) for both T1 and T2. Furthermore, baseline CORT levels decreased for T1 once the thermal shock was removed after day 14 and returned to baseline by day 29. On the contrary, baseline CORT levels kept on increasing for T2 over the 34 days of daily thermal shocks. Furthermore, the magnitudes of the acute CORT responses or physiological endocrine sensitivity were consistently high for both C1 and T1. However, acute CORT responses for T2 toads were dramatically reduced between days 24 and 34. These novel findings suggest that repeated exposure to extreme thermal stressor could cause chronic stress and consequently suppress the physiological endocrine sensitivity to acute stressors (e.g. pathogenic diseases) in amphibians.     

Stress hormone dynamics: an adaptation to migration?

The hormone corticosterone (CORT) is an important component of a bird’s response to environmental stress, but it can also have negative effects. Therefore, birds on migration are hypothesized to have repressed stress responses (migration-modulation hypothesis). In contrast to earlier studies on long-distance migrants, we evaluate this hypothesis in a population containing both migratory and resident individuals. We use a population of partially migratory blue tits (Cyanistes caeruleus) in southern Sweden as a model species. Migrants had higher CORT levels at the time of capture than residents, indicating migratory preparations, adaptation to stressors, higher allostatic load or possibly low social status. Migrants and residents had the same stress response, thus contradicting the migration-modulation hypothesis. We suggest that migrants travelling short distances are more benefited than harmed by retaining the ability to respond to stress.     

The selective glucocorticoid receptor antagonist CORT 108297 decreases neuroendocrine stress responses and immobility in the forced swim test

Pre-clinical and clinical studies have employed treatment with glucocorticoid receptor (GR) antagonists in an attempt to limit the deleterious behavioral and physiological effects of excess glucocorticoids. Here, we examined the effects of GR antagonists on neuroendocrine and behavioral stress responses, using two compounds: mifepristone, a GR antagonist that is also a progesterone receptor antagonist, and CORT 108297, a specific GR antagonist lacking anti-progestin activity. Given its well-documented impact on neuroendocrine and behavioral stress responses, imipramine (tricyclic antidepressant) served as a positive control. Male rats were treated for five days with mifepristone (10 mg/kg), CORT 108297 (30 mg/kg and 60 mg/kg), imipramine (10 mg/kg) or vehicle and exposed to forced swim test (FST) or restraint stress. Relative to vehicle, imipramine potently suppressed adrenocorticotropin hormone (ACTH) responses to FST and restraint exposure. Imipramine also decreased immobility in the FST, consistent with antidepressant actions. Both doses of CORT 108297 potently suppressed peak corticosterone responses to FST and restraint stress. However, only the higher dose of CORT 108297 (60 mg/kg) significantly decreased immobility in the FST. In contrast, mifepristone induced protracted secretion of corticosterone in response to both stressors, and modestly decreased immobility in the FST. Taken together, the data indicate distinct effects of each compound on neuroendocrine stress responses and also highlight dissociation between corticosterone responses and immobility in the FST. Within the context of the present study, our data suggest that CORT 108297 may be an attractive alternative for mitigating neuroendocrine and behavioral states associated with excess glucocorticoid secretion.     

Ancestry trumps experience: Transgenerational but not early life stress affects the adult physiological stress response

Exposure to stressors can affect an organism's physiology and behavior as well as that of its descendants (e.g. through maternal effects, epigenetics, and/or selection). We examined the relative influence of early life vs. transgenerational stress exposure on adult stress physiology in a species that has populations with and without ancestral exposure to an invasive predator. We raised offspring of eastern fence lizards (Sceloporus undulatus) from sites historically invaded (high stress) or uninvaded (low stress) by predatory fire ants (Solenopsis invicta) and determined how this different transgenerational exposure to stress interacted with the effects of early life stress exposure to influence the physiological stress response in adulthood. Offspring from these high- and low-stress populations were exposed weekly to either sub-lethal attack by fire ants (an ecologically relevant stressor), topical treatment with a physiologically-appropriate dose of the stress-relevant hormone, corticosterone (CORT), or a control treatment from 2 to 43 weeks of age. Several months after treatments ended, we quantified plasma CORT concentrations at baseline and following restraint, exposure to fire ants, and adrenocorticotropic hormone (ACTH) injection. Exposure to fire ants or CORT during early life did not affect lizard stress physiology in adulthood. However, offspring of lizards from populations that had experienced multiple generations of fire ant-invasion exhibited more robust adult CORT responses to restraint and ACTH-injection compared to offspring from uninvaded populations. Together, these results indicate that transgenerational stress history may be at least as important, if not more important, than early life stress in affecting adult physiological stress responses.     

Stress and translocation: alterations in the stress physiology of translocated birds

Translocation and reintroduction have become major conservation actions in attempts to create self-sustaining wild populations of threatened species. However, avian translocations have a high failure rate and causes for failure are poorly understood. While ‘stress’ is often cited as an important factor in translocation failure, empirical evidence of physiological stress is lacking. Here we show that experimental translocation leads to changes in the physiological stress response in chukar partridge, Alectoris chukar. We found that capture alone significantly decreased the acute glucocorticoid (corticosterone, CORT) response, but adding exposure to captivity and transport further altered the stress response axis (the hypothalamic–pituitary–adrenal axis) as evident from a decreased sensitivity of the negative feedback system. Animals that were exposed to the entire translocation procedure, in addition to the reduced acute stress response and disrupted negative feedback, had significantly lower baseline CORT concentrations and significantly reduced body weight. These data indicate that translocation alters stress physiology and that chronic stress is potentially a major factor in translocation failure. Under current practices, the restoration of threatened species through translocation may unwittingly depend on the success of chronically stressed individuals. This conclusion emphasizes the need for understanding and alleviating translocation-induced chronic stress in order to use most effectively this important conservation tool.     

Stress during adolescence enhances locomotor sensitization to nicotine in adulthood in female, but not male, rats

A wide body of research has indicated that perinatal exposure to stressors alters the organism, notably by programming behavioral and neuroendocrine responses and sensitivity to drugs of abuse in adulthood. Recent evidence suggests that adolescence also may represent a sensitive period of brain development, and yet there has been little research on the long-lasting effects of stressors during this period. We investigated the effects of pubertal social stress (PS; daily 1-h isolation followed by pairing with a new cage mate on postnatal days 33-48) on locomotor sensitization to injections of nicotine and corticosterone response to restraint stress when the rats were adults (approximately 3 weeks after PS). There were no differences among the groups in locomotor activity to injections of saline. However, PS females had enhanced locomotor sensitization to repeated doses of nicotine compared to control (non-stressed; NS) females, whereas PS males and NS males did not differ. PS enhanced the corticosterone response to restraint in male rats previously sensitized to nicotine and decreased the corticosterone response in nonsensitized male rats. In contrast, PS females and NS females did not differ in plasma corticosterone levels in response to restraint stress, but NS females showed enhanced corticosterone release to restraint after sensitization to nicotine. Thus, during adolescence, social stressors can have long-lasting effects, and the effects appear to differ for males and females.     

Effect of restraint stress on prolactin and corticosterone levels in streptozotocin-induced diabetic rats

Changes in neuroendocrine function have been shown to occur in diabetic animals. The aim of the present study was to examine both the prolactin (PRL) and corticosterone (CORT) responses to a short period of restraint stress after the animals had been made diabetic for six weeks. The streptozotocin - induced diabetic rats had resting CORT levels which were significantly higher than the control animals. Acute restraint significantly increased CORT levels in both the control and diabetic rats. The CORT levels after stress were higher in the diabetic rats. However, the magnitude of the response (percent increase) was less in these animals. The resting PRL levels were not significantly different in the diabetic and control animals. The PRL levels significantly increased in both the control and diabetic rats when they were exposed to the restraint stress. The PRL levels after stress were significantly less in the diabetic rats, indicating a blunted PRL stress response. These results indicate that the diabetic state can affect an animals PRL and CORT response to a new acute stress.     

Disruption of mineralocorticoid receptor function increases corticosterone responding to a mild, but not moderate, psychological stressor

Glucocorticoid negative feedback regulation of the hypothalamic-pituitary-adrenal (HPA) axis is mediated by corticosteroid receptors. It is widely thought that during stress, glucocorticoid receptors (GR) are essential for this negative feedback. In contrast, mineralocorticoid receptors (MR) are associated with HPA axis regulation in basal, nonstress conditions. Notions about the relative roles of MR and GR for HPA axis regulation during stressor challenge may not be complete. Recent work in our laboratory suggests that previous estimates of MR occupancy at resting plasma levels of corticosterone (CORT) may be overestimated. It is possible that a significant number of MR may be available to mediate negative feedback during stressor challenge. We hypothesized that this may be especially the case during mild stressor challenge when the plasma CORT response is weak. In the present studies, adult male Sprague-Dawley rats were first treated systemically or centrally with the selective MR antagonist RU28318 (50 mg/kg sc or 500 ng.10 microl(-1).2 h(-1) icv) or vehicle (300 microl propylene glycol sc or 10 microl/2 h sterile saline icv) and then challenged with 60-min novel environment or restraint. In vehicle controls, restraint resulted in a greater plasma CORT response than novel environment. Both systemic and central treatment with RU28318 significantly increased CORT responding to novel environment relative to vehicle controls. However, RU28318 treatment did not increase the CORT response to restraint. These data suggest that MR may be necessary for glucocorticoid regulation of HPA axis activity during mild stressors, but not during stressors that result in a more robust CORT response.     

Evaluating the Stress Response as a Bioindicator of Sub-Lethal Effects of Crude Oil Exposure in Wild House Sparrows (Passer domesticus)

Petroleum can disrupt endocrine function in humans and wildlife, and interacts in particularly complex ways with the hypothalamus-pituitary-adrenal (HPA) axis, responsible for the release of the stress hormones corticosterone and cortisol (hereafter CORT). Ingested petroleum can act in an additive fashion with other stressors to cause increased mortality, but it is not clear exactly why—does petroleum disrupt feedback mechanisms, stress hormone production, or both? This laboratory study aimed to quantify the effects of ingested Gulf of Mexico crude oil on the physiological stress response of house sparrows (Passer domesticus). We examined baseline and stress-induced CORT, negative feedback, and adrenal sensitivity in house sparrows given a 1% oil or control diet (n = 12 in each group). We found that four weeks on a 1% oil diet did not alter baseline CORT titers or efficacy of negative feedback, but significantly reduced sparrows'' ability to secrete CORT in response to a standardized stressor and adrenocorticotropin hormone injection, suggesting that oil damages the steroid-synthesizing cells of the adrenal. In another group of animals on the same 1% oil (n = 9) or control diets (n = 8), we examined concentrations of eight different blood chemistry parameters, and CORT in feathers grown before and during the feeding experiments as other potential biomarkers of oil exposure. None of the blood chemistry parameters differed between birds on the oil and control diets after two or four weeks of feeding, nor did feather CORT differ between the two groups. Overall, this study suggests that the response of CORT to stressors, but not baseline HPA function, may be a particularly sensitive bioindicator of sub-lethal chronic effects of crude oil exposure.     

The breeding season duration hypothesis: acute handling stress and total plasma concentrations of corticosterone and androgens in male and female striped plateau lizards (Sceloporus virgatus)

Acute glucocorticoid elevations can be adaptations to short-term stressors. The breeding season hypothesis predicts reduced glucocorticoid responsiveness to acute stressors in populations or species with short breeding seasons. The striped plateau lizard (Sceloporus virgatus) has a short breeding season in Arizona. We measured plasma corticosterone and total androgen levels (dihydrotestosterone and testosterone) following one of the four stress-handling treatments (0, 10, 60, or 180 min). In both sexes, longer handling stress yielded higher corticosterone; females had higher corticosterone than males at all time points. Androgens did not vary with handling duration, in either sex. Combining treatments, plasma androgens correlated positively with corticosterone (CORT) in females but not in males; plasma CORT and body mass residuals were negatively correlated in both sexes, suggesting lizards in poor body condition and/or not investing heavily in reproduction (follicle mass) have higher acute corticosterone. Total plasma androgens and body mass residuals were positively associated in males, but showed no association in females. The maximal CORT elevation after handling stress in this single-clutching species was of comparable magnitude to responses in related multi-clutching lizard species with longer breeding seasons. Using data from studies of multiple populations of three Sceloporus species, we found no relationship between the relative magnitude of the CORT increase and either latitude or elevation, two variables in the literature correlated with duration of the breeding season, and only weak relationships with geographic elevation and actual (not relative) stress-elevated CORT values in this multi-population comparison.     

Long-term effects of adolescent stress on neophobic behaviors in zebra finches are modulated by social context when in adulthood

Experiencing stress during adolescence can increase neophobic behaviors in adulthood, but most tests have been conducted in the absence of conspecifics. Conspecifics can modulate responses to stressors, for example by acting as ‘social buffers’ to attenuate the aversive appraisal of stressors. Here, we investigate the long-term effects of adolescent stress on the behavioral responses to novel stimuli (a mild stressor) across social contexts in an affiliative passerine bird, the zebra finch. During early (days 40–60) or late (days 65–85) adolescence the birds (n = 66) were dosed with either saline or the hormone corticosterone (CORT). CORT was given in order to mimic a physiological stress response and saline was given as a control. In adulthood, the birds' behavioral responses to a novel environment were recorded in both the presence and absence of conspecifics. An acute CORT response was also quantified in adolescence and adulthood. Our findings show clear evidence of social context mediating any long-term effects of adolescent stress. In the presence of familiar conspecifics no treatment effects were detected. Individually, birds dosed with CORT in early adolescence were slower to enter a novel environment, spent more time perching in the same novel environment, and, if female, engaged in more risk assessment. Birds dosed in late adolescence were unaffected. No treatment effects were detected on CORT, but adolescents had a higher CORT concentration than adults. Our results are the first to suggest that familiar conspecifics in adulthood can buffer the long-term effects of stress that occurred during early adolescence.     

Corticosterone manipulations alter morph-specific nestling provisioning behavior in male white-throated sparrows,Zonotrichia albicollis

In the polymorphic white-throated sparrow (Zonotrichia albicollis), tan-striped males provision nestlings at higher rates than do white-striped males. In a previous study, we found that tan-striped males had lower baseline corticosterone levels than white-striped males during the nestling stage. To determine if this variation in corticosterone influences morph-specific differences in nestling provisioning behavior, we used intraperitoneal osmotic pumps to increase baseline corticosterone levels in tan-striped males (TS CORT) and administer RU486, a glucocorticoid receptor antagonist, in white-striped males (WS RU486). These manipulations essentially reversed morph-specific nestling provisioning behavior in males. TS CORT males fed nestlings at lower rates than TS controls (vehicle-only implant), and at similar rates to WS controls (vehicle-only implant), while WS RU486 males fed nestlings at higher rates than WS controls, and at similar rates to TS controls. These results demonstrate that (1) increases in baseline corticosterone (i.e., below concentrations associated with the adrenocortical response to stress) can directly or indirectly inhibit nestling provisioning behavior, and (2) corticosterone influences morph-specific variation in parental behavior in male white-throated sparrows. This study contributes to the growing evidence that modulating baseline CORT mediates parental care and self-maintenance activities in birds, and thus may serve as a mechanism for balancing current reproductive success with survival.     

Effects of captivity and body condition on plasma corticosterone, locomotor behavior, and plasma metabolites in curve-billed thrashers

The acute stress response involves the secretion of catabolic glucocorticoids, such as corticosterone (CORT) in birds, that mobilize intrinsic energy stores primarily through a gluconeogenic pathway involving fat breakdown, thus linking body condition and stress. We measured changes in CORT and gluconeogenic metabolites (triglycerides, free glycerols, glucose) during handling stress in curve-billed thrashers Toxostoma curvirostre from two habitats (urban vs. desert) that may differ in food abundance in the wild, in captivity, and in response to both food restriction and subsequent recovery. Urban thrashers were heavier and secreted more CORT than desert birds in the field, but differences did not persist in captivity. Decreased access to food resulted in decreased body mass and a diminished ability to elevate plasma CORT in response to handling stress. However, the opposite effect was observed as these birds recovered from food restriction. Plasma levels of glucose and triglycerides did not change with stress. Food restriction also increased locomotor activity, which likely further exacerbated energy loss. These observations suggest that body condition and stress differences between urban and desert birds may be related to differences in their relative energetic states, possibly due to food availability. Body condition may affect the extent to which an individual can elevate CORT and use free glycerol as energy during acute stress.     

Baseline and stress-induced plasma corticosterone concentrations of mice selectively bred for high voluntary wheel running

The hypothalamic-pituitary-adrenal (HPA) axis is important in regulating energy metabolism and in mediating responses to stressors, including increasing energy availability during physical exercise. In addition, glucocorticoids act directly on the central nervous system and influence behavior, including locomotor activity. To explore potential changes in the HPA axis as animals evolve higher voluntary activity levels, we characterized plasma corticosterone (CORT) concentrations and adrenal mass in four replicate lines of house mice that had been selectively bred for high voluntary wheel running (HR lines) for 34 generations and in four nonselected control (C) lines. We determined CORT concentrations under baseline conditions and immediately after exposure to a novel stressor (40 min of physical restraint) in mice that were housed without access to wheels. Resting daytime CORT concentrations were approximately twice as high in HR as in C mice for both sexes. Physical restraint increased CORT to similar concentrations in HR and C mice; consequently, the proportional response to restraint was smaller in HR than in C animals. Adrenal mass did not significantly differ between HR and C mice. Females had significantly higher baseline and postrestraint CORT concentrations and significantly larger adrenal glands than males in both HR and C lines. Replicate lines showed significant variation in body mass, length, baseline CORT concentrations, and postrestraint CORT concentrations in one or both sexes. Among lines, both body mass and length were significantly negatively correlated with baseline CORT concentrations, suggesting that CORT suppresses growth. Our results suggest that selection for increased locomotor activity has caused correlated changes in the HPA axis, resulting in higher baseline CORT concentrations and, possibly, reduced stress responsiveness and a lower growth rate.     

Sustained release of corticosterone in rats affects reactivity, but does not affect habituation to immobilization and acoustic stimuli

Depression is often preceded by stressful life events and accompanied with elevated cortisol levels and glucocorticoid resistance. It has been suggested that a major depressive disorder may result from impaired coping with and adaptation to stress. The question is whether or not hypothalamus-pituitary-adrenal (HPA)-axis dysfunction influences the process of adaptation. We examined the effect of a dysregulated HPA-axis on the adaptation to acoustic stimuli in rats with or without preceding restraint stress. HPA-axis function was altered via slow release of corticosterone (CORT, 90 mg) from subcutaneously implanted pellets for 7 or 14 days. The rate of body temperature increases during restraint (10 min) and the response to acoustic stimuli (of 80+120 dB) were used to quantify daily stress reactivity. Rats habituated to either stress regardless of CORT treatment. CORT treatment combined with restraint decreased the initial reactivity and the variability in response, but the rate of habituation was not influenced. These results show that suppressing normal HPA-axis function by chronic exposure to CORT does affect the course of habituation, but not habituation per se. This implies that altered HPA-axis function in depressed patients may not be causally related to stress coping, but instead may influence the course of the disorder.     

Adrenocortical stress responses influence an invasive vertebrate's fitness in an extreme environment

Continued range expansion into physiologically challenging environments requires invasive species to maintain adaptive phenotypic performance. The adrenocortical stress response, governed in part by glucocorticoid hormones, influences physiological and behavioural responses of vertebrates to environmental stressors. However, any adaptive role of this response in invasive populations that are expanding into extreme environments is currently unclear. We experimentally manipulated the adrenocortical stress response of invasive cane toads (Rhinella marina) to investigate its effect on phenotypic performance and fitness at the species'' range front in the Tanami Desert, Australia. Here, toads are vulnerable to overheating and dehydration during the annual hot–dry season and display elevated plasma corticosterone levels indicative of severe environmental stress. By comparing unmanipulated control toads with toads whose adrenocortical stress response was manipulated to increase acute physiological stress responsiveness, we found that control toads had significantly reduced daily evaporative water loss and higher survival relative to the experimental animals. The adrenocortical stress response hence appears essential in facilitating complex phenotypic performance and setting fitness trajectories of individuals from invasive species during range expansion.     

Corticosterone induces rapid spinogenesis via synaptic glucocorticoid receptors and kinase networks in hippocampus

Background

Modulation of dendritic spines under acute stress is attracting much attention. Exposure to acute stress induces corticosterone (CORT) secretion from the adrenal cortex, resulting in rapid increase of CORT levels in plasma and the hippocampus.

Methodology/Principal Findings

Here we demonstrated the mechanisms of rapid effect (∼1 h) of CORT on the density and morphology of spines by imaging neurons in adult male rat hippocampal slices. The application of CORT at 100–1000 nM induced a rapid increase in the density of spines of CA1 pyramidal neurons. The density of small-head spines (0.2–0.4 µm) was increased even at low CORT levels (100–200 nM). The density of middle-head spines (0.4–0.5 µm) was increased at high CORT levels between 400–1000 nM. The density of large-head spines (0.5–1.0 µm) was increased only at 1000 nM CORT. Co-administration of RU486, an antagonist of glucocorticoid receptor (GR), abolished the effect of CORT. Blocking a single kinase, such as MAPK, PKA, PKC or PI3K, suppressed CORT-induced enhancement of spinogenesis. Blocking NMDA receptors suppressed the CORT effect.

Conclusions/Significance

These results imply that stress levels of CORT (100–1000 nM) drive the spinogenesis via synaptic GR and multiple kinase pathways.     

Elevated Corticosterone Levels and Changes in Amphibian Behavior Are Associated with Batrachochytrium dendrobatidis (Bd) Infection and Bd Lineage

Few studies have examined the role hormones play in mediating clinical changes associated with infection by the parasite Batrachochytrium dendrobatidis (Bd). Glucocorticoid (GC) hormones such as corticosteroids (CORT) regulate homeostasis and likely play a key role in response to infection in amphibians. We explore the relationship between CORT release rates and Bd infection in tadpoles of the common midwife toad, Alytes obstetricians, using a non-invasive water-borne hormone collection method across seven populations. We further examined whether tadpoles of A. muletensis infected with a hypervirulent lineage of Bd, BdGPL, had greater CORT release rates than those infected with a hypovirulent lineage, BdCAPE. Finally, we examined the relationship between righting reflex and CORT release rates in infected metamorphic toads of A. obstetricans. We found an interaction between elevation and Bd infection status confirming that altitude is associated with the overall severity of infection. In tandem, increasing elevation was associated with increasing CORT release rates. Tadpoles infected with the hypervirulent BdGPL had significantly higher CORT release rates than tadpoles infected with BdCAPE showing that more aggressive infections lead to increased CORT release rates. Infected metamorphs with higher CORT levels had an impaired righting reflex, our defined experimental endpoint. These results provide evidence that CORT is associated with an amphibian’s vulnerability to Bd infection, and that CORT is also affected by the aggressiveness of infection by Bd. Together these results indicate that CORT is a viable biomarker of amphibian stress.     

Stress,song and survival in sparrows

The stress response—increases in circulating glucocorticoids following a stressor—is typically considered adaptive, but few studies address the fitness consequences of individual variation in stress response. Generally, due to negative consequences of prolonged elevation of glucocorticoids, animals should have a transient stress response just sufficient to cope with the stressor. In rodents, stress responsiveness is affected by early developmental experience, and hyper-responsiveness to stress is linked to morbidity and mortality. We assessed individual variation in stress responses in free-living song sparrows, Melospiza melodia, in relation to fitness-related measures including song and overwinter survival. Birds with greater increases in corticosterone 30 min following restraint stress were less likely to return to breed the following year. Stress responsiveness was also correlated with song complexity: males with fewer syllables in their song repertoires had greater stress reactivity. Our findings support the hypothesis that developmental stressors both impair song development and affect the adult stress response. Thus, individual variation in the stress response may relate to variation in fitness.