Urinary corticosterone metabolites and chytridiomycosis disease prevalence in a free-living population of male Stony Creek frogs (Litoria wilcoxii)

Estrogen contamination increases vulnerability of amphibians to the deadly chytrid fungus

Aquatic contaminants and infectious diseases are among the major drivers of global amphibian declines. However, the interaction of these factors is poorly explored and could better explain the amphibian crisis. We exposed males and females of the Brazilian Cururu Toad, Rhinella icterica, to an environmentally relevant concentration of the estrogen 17-alpha-ethinylestradiol (an emerging contaminant) and to the chytrid infection (Batrachochytrium dendrobatidis), in their combined and isolated forms, and the ecotoxicity was determined by multiple biomarkers: cutaneous, hematological, cardiac, hepatic, and gonadal analysis. Our results showed that Cururu toads had many physiological alterations in response to the chytrid infection, including the appearance of cutaneous Langerhans's cells, increased blood leukocytes, increased heart contraction force and tachycardia, increased hepatic melanomacrophage cells, which in turn led to gonadal atrophy. The estrogen, in turn, increased the susceptibility of the toads to the chytrid infection (higher Bd loads) and maximized the deleterious effects of the pathogen: reducing leukocytes, decreasing the contraction force, and causing greater tachycardia, increasing hepatic melanomacrophage cells, and leading to greater gonadal atrophy, which were more extreme in females. The exposure to estrogen also revealed important toxicodynamic pathways of this toxicant, as shown by the immunosuppression of exposed animals, and the induction of the first stages of feminization in males, which corroborates that the synthetic estrogen acts as an endocrine disruptor. Such an intricate relationship is unprecedented and reinforces the importance of studying the serious consequences that multiple environmental stressors can cause to aquatic populations.

The adaptive microbiome hypothesis and immune interactions in amphibian mucus

The microbiome is known to provide benefits to hosts, including extension of immune function. Amphibians are a powerful immunological model for examining mucosal defenses because of an accessible epithelial mucosome throughout their developmental trajectory, their responsiveness to experimental treatments, and direct interactions with emerging infectious pathogens. We review amphibian skin mucus components and describe the adaptive microbiome as a novel process of disease resilience where competitive microbial interactions couple with host immune responses to select for functions beneficial to the host. We demonstrate microbiome diversity, specificity of function, and mechanisms for memory characteristic of an adaptive immune response. At a time when industrialization has been linked to losses in microbiota important for host health, applications of microbial therapies such as probiotics may contribute to immunotherapeutics and to conservation efforts for species currently threatened by emerging diseases.

Effects of exogenous elevation of corticosterone on immunity and the skin microbiome of eastern newts (Notophthalmus viridescens)

The amphibian chytrid fungus, Batrachochytrium salamandrivorans (Bsal) threatens salamander biodiversity. The factors underlying Bsal susceptibility may include glucocorticoid hormones (GCs). The effects of GCs on immunity and disease susceptibility are well studied in mammals, but less is known in other groups, including salamanders. We used Notophthalmus viridescens (eastern newts) to test the hypothesis that GCs modulate salamander immunity. We first determined the dose required to elevate corticosterone (CORT; primary GC in amphibians) to physiologically relevant levels. We then measured immunity (neutrophil lymphocyte ratios, plasma bacterial killing ability (BKA), skin microbiome, splenocytes, melanomacrophage centres (MMCs)) and overall health in newts following treatment with CORT or an oil vehicle control. Treatments were repeated for a short (two treatments over 5 days) or long (18 treatments over 26 days) time period. Contrary to our predictions, most immune and health parameters were similar for CORT and oil-treated newts. Surprisingly, differences in BKA, skin microbiome and MMCs were observed between newts subjected to short- and long-term treatments, regardless of treatment type (CORT, oil vehicle). Taken together, CORT does not appear to be a major factor contributing to immunity in eastern newts, although more studies examining additional immune factors are necessary. This article is part of the theme issue 'Amphibian immunity: stress, disease and ecoimmunology'.

Introduction to the special issue Amphibian immunity: stress, disease and ecoimmunology

Amphibian populations have been declining worldwide, with global climate changes and infectious diseases being among the primary causes of this scenario. Infectious diseases are among the primary drivers of amphibian declines, including ranavirosis and chytridiomycosis, which have gained more attention lately. While some amphibian populations are led to extinction, others are disease-resistant. Although the host's immune system plays a major role in disease resistance, little is known about the immune mechanisms underlying amphibian disease resistance and host-pathogen interactions. As ectotherms, amphibians are directly subjected to changes in temperature and rainfall, which modulate stress-related physiology, including immunity and pathogen physiology associated with diseases. In this sense, the contexts of stress, disease and ecoimmunology are essential for a better understanding of amphibian immunity. This issue brings details about the ontogeny of the amphibian immune system, including crucial aspects of innate and adaptive immunity and how ontogeny can influence amphibian disease resistance. In addition, the papers in the issue demonstrate an integrated view of the amphibian immune system associated with the influence of stress on immune-endocrine interactions. The collective body of research presented herein can provide valuable insights into the mechanisms underlying disease outcomes in natural populations, particularly in the context of changing environmental conditions. These findings may ultimately enhance our ability to forecast effective conservation strategies for amphibian populations. This article is part of the theme issue 'Amphibian immunity: stress, disease and ecoimmunology'.

A Review of Non-Invasive Sampling in Wildlife Disease and Health Research: What's New?

In the last decades, wildlife diseases and the health status of animal populations have gained increasing attention from the scientific community as part of a One Health framework. Furthermore, the need for non-invasive sampling methods with a minimal impact on wildlife has become paramount in complying with modern ethical standards and regulations, and to collect high-quality and unbiased data. We analysed the publication trends on non-invasive sampling in wildlife health and disease research and offer a comprehensive review on the different samples that can be collected non-invasively. We retrieved 272 articles spanning from 1998 to 2021, with a rapid increase in number from 2010. Thirty-nine percent of the papers were focussed on diseases, 58% on other health-related topics, and 3% on both. Stress and other physiological parameters were the most addressed research topics, followed by viruses, helminths, and bacterial infections. Terrestrial mammals accounted for 75% of all publications, and faeces were the most widely used sample. Our review of the sampling materials and collection methods highlights that, although the use of some types of samples for specific applications is now consolidated, others are perhaps still underutilised and new technologies may offer future opportunities for an even wider use of non-invasively collected samples.

Context dependent variation in corticosterone and phenotypic divergence of Rana arvalis populations along an acidification gradient

Background

Physiological processes, as immediate responses to the environment, are important mechanisms of phenotypic plasticity and can influence evolution at ecological time scales. In stressful environments, physiological stress responses of individuals are initiated and integrated via the release of hormones, such as corticosterone (CORT). In vertebrates, CORT influences energy metabolism and resource allocation to multiple fitness traits (e.g. growth and morphology) and can be an important mediator of rapid adaptation to environmental stress, such as acidification. The moor frog, Rana arvalis, shows adaptive divergence in larval life-histories and predator defense traits along an acidification gradient in Sweden. Here we take a first step to understanding the role of CORT in this adaptive divergence. We conducted a fully factorial laboratory experiment and reared tadpoles from three populations (one acidic, one neutral and one intermediate pH origin) in two pH treatments (Acid versus Neutral pH) from hatching to metamorphosis. We tested how the populations differ in tadpole CORT profiles and how CORT is associated with tadpole life-history and morphological traits.

Results

We found clear differences among the populations in CORT profiles across different developmental stages, but only weak effects of pH treatment on CORT. Tadpoles from the acid origin population had, on average, lower CORT levels than tadpoles from the neutral origin population, and the intermediate pH origin population had intermediate CORT levels. Overall, tadpoles with higher CORT levels developed faster and had shorter and shallower tails, as well as shallower tail muscles.

Conclusions

Our common garden results indicate among population divergence in CORT levels, likely reflecting acidification mediated divergent selection on tadpole physiology, concomitant to selection on larval life-histories and morphology. However, CORT levels were highly environmental context dependent. Jointly these results indicate a potential role for CORT as a mediator of multi-trait divergence along environmental stress gradients in natural populations. At the same time, the population level differences and high context dependency in CORT levels suggest that snapshot assessment of CORT in nature may not be reliable bioindicators of stress.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-022-01967-1.

URINARY CORTICOSTERONE CONCENTRATIONS IN FREE-RANGING AND MANAGED CANE TOADS (RHINELLA MARINA)

Approximately 40% of amphibian species are threatened with extinction. The welfare of amphibians maintained under managed care as assurance populations is of vital importance to guard against extinction and provide a viable source population for future reintroduction. To manage amphibian species properly ex situ, it is important to understand how stress levels change over time when animals are removed from the wild and placed into managed environments. Corticosterone was analyzed in urine samples from free-ranging cane toads (Rhinella marina, n = 55) in Miami, FL, and under managed care for 22 (n = 48), 50 (n = 11), 81 (n = 25), or 119 (n = 10) days. Concentrations of corticosterone in free-ranging toad urine averaged 1.74 ± 0.195 ng/ml urine specific gravity (sp. gr.), which was greater (P < 0.05) than other time points (day 22: 0.77 ± 0.114 L; day 81: 0.85 ± 0.191 ng/ml sp. gr.; day 119: 0.58 ± 0.093 ng/ml sp. gr.), except day 50 (0.91 ± 0.274 ng/ml sp. gr.), which was not different from free-ranging or managed care values. Thus, corticosterone was lower in cane toads under managed care compared with those sampled in the wild, suggesting that managed care is not a stressor for this species.

Using dermal glucocorticoids to determine the effects of disease and environment on the critically endangered Wyoming toad

Amphibian populations are declining worldwide, and increased exposure to environmental stressors, including global climate change and pathogens like Batrachochytrium dendrobatidis (Bd), may be contributing to this decline. Our goal was to use a novel dermal swabbing method to measure glucocorticoid (GC) hormones and investigate the relationship among disease and environmental conditions in the critically endangered Wyoming toad (Anaxyrus baxteri). Our objectives were to (i) validate the use of dermal swabs to measure GCs using an adrenocorticotropic hormone (ACTH) challenge on eight captive toads (4 ACTH: 2 M, 2F and 4 saline as a control: 2 M, 2F), (ii) investigate stress physiology and disease status of toads across six reintroduction sites and (iii) compare dermal cortisol between reintroduced and captive toads. Dermal cortisol peaked immediately after the ACTH and saline injections. Faecal GC metabolites (FGMs) were significantly higher one week after the ACTH injection compared with the week before. Saline-injected toads had no change in FGM over time. Toads were only found in three reintroduction sites and dermal cortisol was similar across sites; however, reintroduced toads had higher dermal cortisol in August compared with June and compared with captive individuals. Bd status did not influence dermal cortisol concentrations. Dermal and faecal hormonal metabolite analyses can be used to study amphibian stress physiology and learn how environmental conditions are impacting population success.

Water-borne corticosterone assay is a valid method in some but not all life-history stages in Northern Leopard Frogs

There is a particular need to develop conservation tools for use in amphibian populations, which are declining rapidly. Glucocorticoid hormones like corticosterone (CORT) are often used as biomarkers of amphibian stress. A relatively new method of assessing CORT in amphibians is to measure CORT concentrations in water that has held amphibians (water-borne (WB) CORT). Here, we tested whether WB CORT is a valid measure of CORT in larval and metamorphic Northern Leopard Frogs (Lithobates pipiens). We assessed whether levels of WB CORT are different among groups of animals that should have different levels of CORT due to a handling challenge, a pharmacological challenge (ACTH), or developmental stage. We also assessed whether WB CORT was correlated with plasma CORT within individuals. Results indicated that measurement of WB CORT is valid in prometamorphic tadpoles because injection with ACTH increased WB CORT, and WB CORT and plasma CORT levels were correlated within an animal in most cases. However, were unable to fully validate the use of WB CORT in metamorphic frogs (metamorphs) because although injection with ACTH elevated levels of WB CORT, WB CORT was not correlated with plasma CORT within individual metamorphs. Also, there was no correlation between WB CORT and plasma CORT in early stage (premetamorphic) tadpoles or tadpoles undergoing metamorphic climax, indicating that WB CORT is not sensitive enough to detect natural variation of organismal CORT in these groups. Together, results indicated that WB CORT is a valid method of assessing plasma CORT in Northern Leopard Frogs, but only for some life-history stages. Our results illustrate the importance of carefully validating the use of WB CORT for appropriate interpretation of results.

Infection with Batrachochytrium dendrobatidis reduces salamander capacity to mount a cell-mediated immune response

The vertebrate immune system is a costly defense system that is responsible for preventing and eliminating parasites and pathogens. Theory predicts that hosts experience tradeoffs associated with immune deployment and other physiological functions. Although empirical evidence for immune-physiology tradeoffs are well documented in the literature, fewer studies have examined tradeoffs within the immune system in wild vertebrates. We explored the topic of concomitant immune challenges in amphibians by exposing salamanders (Plethodon cinereus) to a fungal pathogen Batrachochytrium dendrobatidis (hereafter "Bd") and then to phytohemagglutinin (hereafter "PHA"). We measured Bd infection using quantitative PCR and used measurements of the tail thickness at the PHA injection site as an estimate of skin swelling. We tested whether Bd reduced the salamander's capacity to mount an immune response towards PHA or whether Bd would stimulate immune activity and thereby increase the response towards PHA. Salamanders that were infected with Bd had a reduced skin-swelling when injected with PHA compared to noninfected salamanders, a result that is consistent with the hypothesis that Bd-infected salamanders have lower immunocompetence than noninfected salamanders. We also found that PHA-induced swelling response was negatively associated with Bd infection abundance (i.e., the infection burden of all exposed salamanders, including those that were exposed but not infected), indicating that salamanders with a higher infection abundance had the lowest swelling response to PHA. Our results suggest that individuals of P. cinereus might experience an energetic tradeoff between successfully fighting off Bd and mounting an immune response towards PHA.

Excreted testosterone and male sexual proceptivity: A hormone validation and proof-of-concept experiment in túngara frogs

Conventional methods for sampling hormones often preclude strong inference experimental designs, including repeated measures of both hormones and behavior and balanced or simultaneous designs for hormone-behavior sampling. In amphibians there is an opportunity to non-invasively and repeatedly sample excreted steroids in the water. We examined testosterone (T) in túngara frogs (Physalaemus (=Engystomops) pustulosus) using minimally invasive water-borne methods. First, we validated procedures for the collection, extraction and measurement of T in adult males and females using pharmacological challenges coupled with estimates of parallelism and recovery determination. Next, we extended the timeline of sampling over 9 days in order to evaluate the kinetics of excretion (baseline phase, challenge phase, recovery phase), including the estimation of individual differences during baseline sampling. We also estimated concentrations of creatinine (Cr) in each water sample and evaluated whether correcting for this proxy of urine concentration significantly decreased error variance in T estimates. Lastly, we incorporated a standardized and repeated measures assay of male sexual proceptivity (phonotaxis) during the predicted peak T and recovery T timepoints. We found strong evidence supporting the utility of these methods for precise, biologically informative estimates of T in both sexes. Males had higher T than females and responded to pharmacological challenges by elevating T substantially within 48 h of challenge (hCG, GnRH). Males exhibited repeatability in baseline T and phonotaxis frequencies were positively associated with higher T. Adjusting T levels for the simultaneous measure of Cr significantly improved model fit, which in conjunction with marked variation in urine concentration, suggests that urine likely serves as the major source of excreted T. In summary, this proof-of-concept and methods study demonstrates the utility and accuracy of measuring water-borne T and behavior in amphibians.

Relationships between glucocorticoids and infection with Batrachochytrium dendrobatidis in three amphibian species

It is often hypothesized that organisms exposed to environmental change may experience physiological stress, which could reduce individual quality and make them more susceptible to disease. Amphibians are amongst the most threatened taxa, particularly in the context of disease, but relatively few studies explore links between stress and disease in amphibian species. Here, we use the fungal pathogen Batrachochytrium dendrobatidis (Bd) and amphibians as an example to explore relationships between disease and glucocorticoids (GCs), metabolic hormones that comprise one important component of the stress response. While previous work is limited, it has largely identified positive relationships between GCs and Bd-infection. However, the causality remains unclear and few studies have integrated both baseline (GC release that is related to standard, physiological functioning) and stress-induced (GC release in response to an acute stressor) measures of GCs. Here, we examine salivary corticosterone before and after exposure to a stressor, in both field and captive settings. We present results for Bd-infected and uninfected individuals of three amphibian species with differential susceptibilities to this pathogen (Rana catesbeiana, R. clamitans, and R. sylvatica). We hypothesized that prior to stress, baseline GCs would be higher in Bd-infected animals, particularly in more Bd-susceptible species. We also expected that after exposure to a stressor, stress-induced GCs would be lower in Bd-infected animals. These species exhibited significant interspecific differences in baseline and stress induced corticosterone, though other variables like sex, body size, and day of year were usually not predictive of corticosterone. In contrast to most previous work, we found no relationships between Bd and corticosterone for two species (R. catesbeiana and R. clamitans), and in the least Bd-tolerant species (R. sylvatica) animals exhibited context-dependent differences in relationships between Bd infection and corticosterone: Bd-positive R. sylvatica had significantly lower baseline and stress-induced corticosterone, with this pattern being stronger in the field than in captivity. These results were surprising, as past work in other species has more often found elevated GCs in Bd-positive animals, a pattern that aligns with well-documented relationships between chronically high GCs, reduced individual quality, and immunosuppression. This work highlights the potential relevance of GCs to disease susceptibility in the context of amphibian declines, while underscoring the importance of characterizing these relationships in diverse contexts.

Non-invasive methods to measure inter-renal function in aquatic salamanders-correlating fecal corticosterone to the environmental and physiologic conditions of captive Necturus

This study sought to develop non-invasive techniques to monitor glucocorticoids in captive Necturus as a means to correlate inter-renal gland function in relation to environmental and physiological changes. Six individually housed breeding pairs of captive Necturus beyeri were subjected to seasonal changes in water temperature (30°F temperature differential) to stimulate natural breeding, specifically spermatophore deposition and oviposition. An enzyme immunoassay was validated for the measurement of N. beyeri faecal corticosterone metabolites (fCMs) by exhibiting parallelism and accuracy to the standard curve. Longitudinal (December 2016-October 2017) assessment of fCM concentrations and pattern of excretion from samples collected from the six breeding pairs revealed a seasonal inter-renal effect with higher concentrations (P < 0.05) excreted during months (December-March) of the year associated with breeding activity and when water temperatures were lowest. Males from each pair produced spermatophores starting on 08 December 8 2016 and ending on 05 April 2017. Females from four of the six pairs went on to successfully oviposit eggs in mid-late April 2017. One clutch was fertile, and three were non-fertile. No differences (P > 0.05) were detected in fCM concentrations between pairs in which oviposition did or did not occur. In addition, a novel waterborne corticosterone metabolite (wCM) assay was validated to overcome challenges associated with faecal collection in a group-housed amphibian. An adrenocorticotropic hormone (ACTH) challenge performed in an adult male Necturus maculosus resulted in a 50-fold increase in wCM at 4 h post-injection and marked the first demonstration of a waterborne inter-renal response to ACTH in Necturus. This study not only provides insight into inter-renal function in an aquatic salamander that exhibits marked reproductive seasonality but also confirms utility of fCM and wCM measurements as non-invasive means of assessment.

Out in the cold and sick: Low temperatures and fungal infections impair a frog's skin defenses

Amphibians worldwide continue to battle an emerging infectious disease, chytridiomycosis, caused by Batrachochytrium dendrobatidis (Bd). Southern leopard frogs, Rana sphenocephala, are known to become infected with this pathogen, yet they are considered ‘of least concern’ for declines due to chytridiomycosis. Previous studies have shown that R. sphenocephala secretes four antimicrobial peptides (AMPs) onto their skin which may play an important role in limiting susceptibility to chytridiomycosis. Here we examined the (1) effects of temperature and AMP depletion on infections with Bd and (2) effects of temperature and Bd infection on the capacity to secrete AMPs in juvenile leopard frogs. Pathogen burden and mortality were greater in frogs exposed to Bd at low temperature but did not increase following monthly AMP depletion. Both low temperature and Bd exposure reduced the capacity of juvenile frogs to restore peptides after monthly depletions. Frogs held at 14°C were poorly able to restore peptides in comparison with those at 26 °C. Frogs held at 26 °C were better able to restore their peptides, but when exposed to Bd, this capacity was significantly reduced. These results strongly support the hypothesis that both colder temperatures and Bd infections impair the capacity of juvenile frogs to produce and secrete AMPs, an important component of their innate defense against chytrid fungi and other pathogens. Thus, in the face of unpredictable climate changes and enzootic pathogens, assessments of disease risk should consider the potential for effects of environmental variation and pathogen exposure on the quality of host defenses.

Review of the Amphibian Immune Response to Chytridiomycosis, and Future Directions

The fungal skin disease, chytridiomycosis (caused by Batrachochytrium dendrobatidis and B. salamandrivorans), has caused amphibian declines and extinctions globally since its emergence. Characterizing the host immune response to chytridiomycosis has been a focus of study with the aim of disease mitigation. However, many aspects of the innate and adaptive arms of this response are still poorly understood, likely due to the wide range of species' responses to infection. In this paper we provide an overview of expected immunological responses (with inference based on amphibian and mammalian immunology), together with a synthesis of current knowledge about these responses for the amphibian-chytridiomycosis system. We structure our review around four key immune stages: (1) the naïve immunocompetent state, (2) immune defenses that are always present (constitutive defenses), (3) mechanisms for recognition of a pathogen threat and innate immune defenses, and (4) adaptive immune responses. We also evaluate the current hot topics of immunosuppression and immunopathology in chytridiomycosis, and discuss their respective roles in pathogenesis. Our synthesis reveals that susceptibility to chytridiomycosis is likely to be multifactorial. Susceptible amphibians appear to have ineffective constitutive and innate defenses, and a late-stage response characterized by immunopathology and Bd-induced suppression of lymphocyte responses. Overall, we identify substantial gaps in current knowledge, particularly concerning the entire innate immune response (mechanisms of initial pathogen detection and possible immunoevasion by Bd, degree of activation and efficacy of the innate immune response, the unexpected absence of innate leukocyte infiltration, and the cause and role of late-stage immunopathology in pathogenesis). There are also gaps concerning most of the adaptive immune system (the relative importance of B and T cell responses for pathogen clearance, the capacity and extent of immunological memory, and specific mechanisms of pathogen-induced immunosuppression). Improving our capacity for amphibian immunological research will require selection of an appropriate Bd-susceptible model species, the development of taxon-specific affinity reagents and cell lines for functional assays, and the application of a suite of conventional and emerging immunological methods. Despite current knowledge gaps, immunological research remains a promising avenue for amphibian conservation management.

Chytridiomycosis causes catastrophic organism-wide metabolic dysregulation including profound failure of cellular energy pathways

Chytridiomycosis is among several recently emerged fungal diseases of wildlife that have caused decline or extinction of naïve populations. Despite recent advances in understanding pathogenesis, host response to infection remains poorly understood. Here we modelled a total of 162 metabolites across skin and liver tissues of 61 frogs from four populations (three long-exposed and one naïve to the fungus) of the Australian alpine tree frog (Litoria verreauxii alpina) throughout a longitudinal exposure experiment involving both infected and negative control individuals. We found that chytridiomycosis dramatically altered the organism-wide metabolism of clinically diseased frogs. Chytridiomycosis caused catastrophic failure of normal homeostatic mechanisms (interruption of biosynthetic and degradation metabolic pathways), and pronounced dysregulation of cellular energy metabolism. Key intermediates of the tricarboxylic acid cycle were markedly depleted, including in particular α-ketoglutarate and glutamate that together constitute a key nutrient pathway for immune processes. This study was the first to apply a non-targeted metabolomics approach to a fungal wildlife disease and specifically to dissect the host-pathogen interface of Bd-infected frogs. The patterns of metabolite accumulation we have identified reveal whole-body metabolic dysfunction induced by a fungal skin infection, and these findings have broad relevance for other fungal diseases.

Validation of water-borne steroid hormones in a tropical frog (Physalaemus pustulosus)

Minimally invasive methods for estimating hormone concentrations in wild vertebrates offer the opportunity to repeatedly measure behavior and hormone concentrations within individuals while minimizing experimenter interference during sample collection. We examined three steroid hormones (corticosterone, CORT; 17-β estradiol, E₂; progesterone, PROG) in túngara frogs (Physalaemus pustulosus) using non-invasive water-borne methods. Using solid-phase extraction of water samples and liquid extraction of plasma and homogenate samples, coupled with enzyme immunoassays, we complimented the conventional validation approaches (parallelism, recovery determination) with dose-response assays that incorporated pharmacological challenges with adrenocorticotropic hormone (ACTH) and human chorionic gonadotropin (HCG). We also compared steroid concentrations in water to those observed in plasma and whole body homogenates. Lastly, we identified the constituent steroids in each sample type with a panel targeting 30 steroid species using high performance liquid chromatography-mass spectrometry (HPLC-MS). We found that a 60-min water-bath captures physiologically relevant changes in concentrations of CORT, E₂ and PROG. Peak levels of water-borne CORT were found at approximately 2 h after ACTH injection. Water-borne CORT and E₂ concentrations were positively correlated with their plasma and homogenate equivalents, while water-borne PROG was uncorrelated with homogenate PROG concentrations but negatively correlated with homogenate E₂ concentrations. Together, our findings indicate that sampling water-borne hormones presents a non-invasive and biologically informative approach that will be useful for behavioral endocrinologists and conservation physiologists.

A novel method for the measurement of glucocorticoids in dermal secretions of amphibians

Amphibians have been declining in both diversity and abundance due in large part to habitat degradation and the prevalence of emerging diseases. Although stressors can suppress the immune system, affecting an individual's health and susceptibility to pathogens, established methods for directly collecting stress hormones are not suitable for rapid field use or for use on threatened and endangered species. To overcome these challenges, we are developing an innovative method to collect and measure amphibian glucocorticoid secretions using non-invasive dermal swabs. We tested this methodology using multiple terrestrial, semi-aquatic and fully aquatic species. We swabbed the dorsal side of each animal six times and then induced a stressor of either hand-restraint, ACTH injection, or saline as a control. We then repeated swab collection immediately after the stressor and at 15, 30, 45, 60, 90 and 120 min intervals. Cortisol enzyme immunoassay detected changes in cortisol post-stressor. We also tested this methodology in the field and were successfully able to detect glucocorticoids from multiple species at varying life stages. When using in the field, capture technique should be considered since it may impact stress levels in certain species. Upon further testing, this novel method may be used to greatly increase our understanding of amphibian health especially as disease and environmental changes continue to impact fragile populations.

Evaluation of physiological stress in Australian wildlife: Embracing pioneering and current knowledge as a guide to future research directions

Australia has a rich terrestrial and marine biodiversity and high species endemism. However, the oceanic continent is facing the biodiversity extinction crisis. The primary factors are anthropogenic induced environmental changes, including wildlife habitat destruction through urbanisation and predation by feral animals (e.g. red foxes and feral cats), increased severity of diseases (e.g. chytridiomycosis and chlamydia), and increased occurrence of summer heat waves and bush fires. Stress physiology is a dynamic field of science based on the studies of endocrine system functioning in animals. The primary stress regulator is the hypothalamo-pituitary adrenal (interrenal) axis and glucocorticoids (corticosterone and/or cortisol) provide stress index across vertebrate groups. This review paper focuses on physiological stress assessments in Australian wildlife using examples of amphibians, reptiles, birds and marsupials. I provide a thorough discussion of pioneering studies that have shaped the field of stress physiology in Australian wildlife species. The main findings point towards key aspects of stress endocrinology research, such as quantification of biologically active levels of glucocorticoids, development of species-specific GC assays and applications of stress physiology approaches in field ecology and wildlife conservation programs. Furthermore, I also discuss the importance of chronic stress assessment in wildlife populations. Finally, I provide a conceptual framework presenting key research questions in areas of wildlife stress physiology research. In conclusion, wildlife management programs can immensely benefit from stress physiology assessments to gauge the impact of human interventions on wildlife such as species translocation and feral species eradication.

Effects of corticosterone on infection and disease in salamanders exposed to the amphibian fungal pathogen Batrachochytrium dendrobatidis

Although it is well established that glucocorticoid hormones (GCs) alter immune function and disease resistance in humans and laboratory animal models, fewer studies have linked elevated GCs to altered immune function and disease resistance in wild animals. The chytrid fungal pathogen Batrachochytrium dendrobatidis (Bd) infects amphibians and can cause the disease chytridiomycosis, which is responsible for worldwide amphibian declines. It is hypothesized that long-term exposure to environmental stressors reduces host resistance to Bd by suppressing host immunity via stress-induced release of GCs such as corticosterone (CORT). We tested whether elevation of CORT would reduce resistance to Bd and chytridiomycosis development in the red-legged salamander Plethodon shermani. Plasma CORT was elevated daily in animals for 9 d, after which animals were inoculated with Bd and subsequently tested for infection loads and clinical signs of disease. On average, Bd-inoculated animals treated with CORT had higher infection abundance compared to Bd-inoculated animals not treated with CORT. However, salamanders that received CORT prior to Bd did not experience any increase in clinical signs of chytridiomycosis compared to salamanders not treated with CORT. The lack of congruence between CORT effects on infection abundance versus disease may be due to threshold effects. Nonetheless, our results show that elevation of plasma CORT prior to Bd inoculation decreases resistance to infection by Bd. More studies are needed to better understand the effects of CORT on animals exposed to Bd and whether CORT variation contributes to differential responses to Bd observed across amphibian species and populations.

Amphibian immunity-stress, disease, and climate change

Like all other vertebrate groups, amphibian responses to the environment are mediated through the brain (hypothalamic)-pituitary-adrenal/interrenal (HPA/I) axis and the sympathetic nervous system. Amphibians are facing historically unprecedented environmental stress due to climate change that will involve unpredictable temperature and rainfall regimes and possible nutritional deficits due to extremes of temperature and drought. At the same time, amphibians in all parts of the world are experiencing unprecedented declines due to the emerging diseases, chytridiomycosis (caused by Batrachochytrium dendrobatidis and Batrachochytrium salamandrivorans) and ranavirus diseases due to viruses of the genus Ranavirus in the family Iridoviridae. Other pathogens and parasites also afflict amphibians, but here I will limit myself to a review of recent literature linking stress and these emerging diseases (chytridiomycosis and ranavirus disease) in order to better predict how environmental stressors and disease will affect global amphibian populations.

Differences in chytridiomycosis infection costs between two amphibian species from Central Europe

Batrachochytrium dendrobatidis (Bd) causes the disease chytridiomycosis associated with amphibian declines. Response and costs of infection varies greatly between species. Bd can induce a stress response in amphibians resulting in elevated corticosterone (CORT). We exposed Bombina variegata and Hyla arborea tadpoles to Bd+ or Bd- Salamandra salamandra larvae and measured CORT release rates, Bd infection loads, and survival through metamorphosis. Tadpoles of both species exposed to Bd+ larvae had elevated CORT release rates compared to tadpoles exposed to Bd- larvae. Bombina variegata appear less resistant to infection than H. arborea, showing higher Bd loads and more infected individuals. Within species, we did not find differences in cost of infection on survival, however more B. variegata tadpoles reached metamorphosis than H. arborea. The differences in resistance may be species specific, owing to higher immunity defenses with H. arborea having higher overall CORT release rates, and differences in antimicrobial peptides, or to differences in Bd strain or other unexplored mechanisms.

Assessing host response to disease treatment: how chytrid-susceptible frogs react to increased water salinity

Context The severity and prevalence of the amphibian fungal pathogen, Batrachochytrium dendrobatidis (Bd) is correlated with several environmental variables, including salinity, temperature, and moisture content, which influence the pathogen’s growth and survival. Habitats that contain these environmental variables at levels outside of those optimal for Bd growth and survival may facilitate the survival of susceptible host species. Therefore, manipulation of environmental salinity is a potential management strategy to help conserve Bd-susceptible species. However, host behaviour also influences disease dynamics, and the success of habitat manipulation programs depends on how hosts use this altered habitat. Aims To assess if the Bd-susceptible green and golden bell frog, Litoria aurea, will select waterbodies with a salinity increased to S=3; if this selection is affected by infection; and if a frog’s time in a waterbody of this salinity affects infection load or blood physiology. Methods We conducted a filmed choice experiment and a 3-year field study where infected and uninfected frogs could choose between fresh or saline waterbodies. Key results In both the laboratory experiment and field study, Bd-infected L. aurea spent a significantly greater amount of time in or closer to a waterbody than uninfected frogs. Experimentally infected frogs tended to prefer the saline water over fresh, but their choice of water usage did not differ statistically from uninfected frogs. In the field, frogs began to avoid ponds when salinities rose above S=5. Conclusions Because both wild and captive, and infected and uninfected L. aurea readily selected waterbodies with a salinity of S=3, this salinity could potentially be used as a passive method for reducing the severity of Bd when managing this species. However, further testing is needed to understand the efficacy of this treatment, and care must be taken to prevent salinities rising above S=5, because this level seems to produce an avoidance response and therefore may not be suitable in every location. Implications Manipulation of aquatic habitats may be a worthwhile focus for Bd management in habitats where water level fluctuations are minimal.

Does physiological response to disease incur cost to reproductive ecology in a sexually dichromatic amphibian species?

It is well known that the disease chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd) has contributed to amphibian declines worldwide. The impact of Bd varies, with some species being more susceptible to infection than others. Recent evidence has shown that Bd can have sub-lethal effects, whereby increases in stress hormones have been associated with infection. Could this increased stress response, which is a physiological adaptation that provides an increased resilience against Bd infection, potentially be a trade-off with important life-history traits such as reproduction? We studied this question in adult male frogs of a non-declining species (Litoria wilcoxii). Frogs were sampled for (1) seasonal hormone (testosterone and corticosterone), color and disease profiles, (2) the relationship between disease infection status and hormone levels or dorsal color, (3) subclinical effects of Bd by investigating disease load and hormone level, and (4) reproductive and stress hormone relationships independent of disease. Testosterone levels and color score varied seasonally (throughout the spring/summer months) while corticosterone levels remained stable. Frogs with high Bd prevalence had significantly higher corticosterone levels and lower testosterone levels compared to uninfected frogs, and no differences in color were observed. There was a significant positive correlation between disease load and corticosterone levels, and a significant negative relationship between disease load and testosterone. Our field data provides novel evidence that increased physiological stress response associated with Bd infection in wild frogs, could suppress reproduction by down-regulating gonadal hormones in amphibians, however the impacts on reproductive output is yet to be established.

Exposure to Corticosterone Affects Host Resistance, but Not Tolerance, to an Emerging Fungal Pathogen

Host responses to pathogens include defenses that reduce infection burden (i.e., resistance) and traits that reduce the fitness consequences of an infection (i.e., tolerance). Resistance and tolerance are affected by an organism's physiological status. Corticosterone (“CORT”) is a hormone that is associated with the regulation of many physiological processes, including metabolism and reproduction. Because of its role in the stress response, CORT is also considered the primary vertebrate stress hormone. When secreted at high levels, CORT is generally thought to be immunosuppressive. Despite the known association between stress and disease resistance in domesticated organisms, it is unclear whether these associations are ecologically and evolutionary relevant in wildlife species. We conducted a 3x3 fully crossed experiment in which we exposed American toads (Anaxyrus [Bufo] americanus) to one of three levels of exogenous CORT (no CORT, low CORT, or high CORT) and then to either low or high doses of the pathogenic chytrid fungus Batrachochytrium dendrobatidis (“Bd”) or a sham exposure treatment. We assessed Bd infection levels and tested how CORT and Bd affected toad resistance, tolerance, and mortality. Exposure to the high CORT treatment significantly elevated CORT release in toads; however, there was no difference between toads given no CORT or low CORT. Exposure to CORT and Bd each increased toad mortality, but they did not interact to affect mortality. Toads that were exposed to CORT had higher Bd resistance than toads exposed to ethanol controls/low CORT, a pattern opposite that of most studies on domesticated animals. Exposure to CORT did not affect toad tolerance to Bd. Collectively, these results show that physiological stressors can alter a host’s response to a pathogen, but that the outcome might not be straightforward. Future studies that inhibit CORT secretion are needed to better our understanding of the relationship between stress physiology and disease resistance and tolerance in wild vertebrates.

The relationship between physiological stress and wildlife disease: consequences for health and conservation

Wildlife populations are under increasing pressure from a variety of threatening processes, ranging from climate change to habitat loss, that can incite a physiological stress response. The stress response influences immune function, with potential consequences for patterns of infection and transmission of disease among and within wildlife, domesticated animals and humans. This is concerning because stress may exacerbate the impact of disease on species vulnerable to extinction, with consequences for biodiversity conservation globally. Furthermore, stress may shape the role of wildlife in the spread of emerging infectious diseases (EID) such as Hendra virus (HeV) and Ebola virus. However, we still have a limited understanding of the influence of physiological stress on infectious disease in wildlife. We highlight key reasons why an improved understanding of the relationship between stress and wildlife disease could benefit conservation, and animal and public health, and discuss approaches for future investigation. In particular, we recommend that increased attention be given to the influence of anthropogenic stressors including climate change, habitat loss and management interventions on disease dynamics in wildlife populations.

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.

Effects of acute restraint stress, prolonged captivity stress and transdermal corticosterone application on immunocompetence and plasma levels of corticosterone on the cururu Toad (Rhinella icterica)

Glucocorticoid steroids modulate immunocompetence in complex ways with both immunoenhancing and immunosuppressive effects in vertebrates exposed to different stressors. Such bimodal effects have been associated with variation in duration and intensity of the stress response. Given that natural populations have been exposed to a multitude of stressors, a better understanding of the functional association between duration and intensity of the stress response, the resulting changes in glucocorticoid plasma levels and their impact on different aspects of immunocompetence emerges as a cornerstone for vertebrate conservation strategies. We investigated the effects of a restraint challenge (with and without movement restriction), long-term captivity, and transdermal corticosterone application on plasma levels of corticosterone (hereinafter referred to as CORT) and different parameters of innate immunocompetence in the male cururu toads (Rhinella icterica). We show that for R. icterica restraint for 24h proved to be a stressful condition, increasing CORT by 3-fold without consistent immunological changes. However, the application of a more intense stressor (restraint with movement restriction), for the same period, potentiated this response resulting in a 9-fold increase in CORT, associated with increase Neutrophil/Lymphocyte ratio (N:L) and a lower bacterial killing ability (BKA). Transdermal application of corticosterone efficiently mimics repeated acute stress response events, without changing the immune parameters even after 13 days of treatment. Interestingly, long-term captivity did not mitigate the stress response, since the toads maintained 3-fold increased CORT even after 3 months under these conditions. Moreover, long-term captivity in the same condition increased total leukocyte count (TLC) and generated an even greater decrease in BKA, suggesting that consequences of the stress response can be aggravated by time in captivity.

Defects in host immune function in tree frogs with chronic chytridiomycosis

The amphibian chytrid fungus Batrachochytrium dendrobatidis (Bd) has caused mass mortality leading to population declines and extinctions in many frog species worldwide. The lack of host resistance may be due to fungal immunosuppressive effects that have been observed when Bd is incubated with cultured lymphocytes, but whether in vivo host immunosuppression occurs is unknown. We used a broad range of hematologic and protein electrophoresis biomarkers, along with various functional tests, to assess immune competence in common green (Litoria caerulea) and white-lipped (L. infrafrenata) tree frogs experimentally infected with Bd. Compared with uninfected frogs, Bd infection in L. caerulea caused a reduction in immunoglobulin and splenic lymphocyte responses to antigenic stimulation with sheep red blood cells, along with decreased white blood cell and serum protein concentrations, indicating possible impaired immune response capability of Bd-infected frogs. This is the first in vivo study suggesting that infection with Bd causes multiple defects in systemic host immune function, and this may contribute to disease development in susceptible host species. Although L. infrafrenata failed to maintain Bd infection after exposure, white blood cell and serum globulin concentrations were lower in recovered frogs compared with unexposed frogs, but antigen-specific serum and splenic antibody, and splenic cellular, responses were similar in both recovered and unexposed frogs. This may indicate potential systemic costs associated with infection clearance and/or redirection of host resources towards more effective mechanisms to overcome infection. No clear mechanism for resistance was identified in L. infrafrenata, suggesting that localized and/or innate immune defense mechanisms may be important factors involved in disease resistance in this species.

A review of factors influencing the stress response in Australian marsupials

Many Australian marsupials are threatened species. In order to manage in situ and ex situ populations effectively, it is important to understand how marsupials respond to threats. Stress physiology (the study of the response of animals to challenging stimuli), a key approach in conservation physiology, can be used to characterize the physiological response of wildlife to threats. We reviewed the literature on the measurement of glucocorticoids (GCs), endocrine indicators of stress, in order to understand the stress response to conservation-relevant stressors in Australian marsupials and identified 29 studies. These studies employed a range of methods to measure GCs, with faecal glucocorticoid metabolite enzyme immunoassay being the most common method. The main stressors considered in studies of marsupials were capture and handling. To date, the benefits of stress physiology have yet to be harnessed fully in marsupial conservation. Despite a theoretical base dating back to the 1960s, GCs have only been used to understand how 21 of the 142 extant species of Australian marsupial respond to stressors. These studies include merely six of the 60 marsupial species of conservation concern (IUCN Near Threatened to Critically Endangered). Furthermore, the fitness consequences of stress for Australian marsupials are rarely examined. Individual and species differences in the physiological stress response also require further investigation, because significant species-specific variations in GC levels in response to stressors can shed light on why some individuals or species are more vulnerable to stress factors while others appear more resilient. This review summarizes trends, knowledge gaps and future research directions for stress physiology research in Australian marsupial conservation.

Evaluation of amphotericin B and chloramphenicol as alternative drugs for treatment of chytridiomycosis and their impacts on innate skin defenses

Chytridiomycosis, an amphibian skin disease caused by the emerging fungal pathogen Batrachochytrium dendrobatidis, has been implicated in catastrophic global amphibian declines. The result is an alarming decrease in amphibian diversity that is a great concern for the scientific community. Clinical trials testing potential antifungal drugs are needed to identify alternative treatments for amphibians infected with this pathogen. In this study, we quantified the MICs of chloramphenicol (800 μg/ml), amphotericin B (0.8 to 1.6 μg/ml), and itraconazole (Sporanox) (20 ng/ml) against B. dendrobatidis. Both chloramphenicol and amphotericin B significantly reduced B. dendrobatidis infection in naturally infected southern leopard frogs (Rana [Lithobates] sphenocephala), although neither drug was capable of complete fungal clearance. Long-term exposure of R. sphenocephala to these drugs did not inhibit antimicrobial peptide (AMP) synthesis, indicating that neither drug is detrimental to this important innate skin defense. However, we observed that chloramphenicol, but not amphotericin B or itraconazole, inhibited the growth of multiple R. sphenocephala skin bacterial isolates in vitro at concentrations below the MIC against B. dendrobatidis. These results indicate that treatment with chloramphenicol might dramatically alter the protective natural skin microbiome when used as an antifungal agent. This study represents the first examination of the effects of alternative antifungal drug treatments on amphibian innate skin defenses, a crucial step to validating these treatments for practical applications.

Over-wintering tadpoles of Mixophyes fasciolatus act as reservoir host for Batrachochytrium dendrobatidis

Batrachochytrium dendrobatidis (Bd), a cutaneous amphibian fungus that causes the lethal disease chytridiomycosis, has been implicated as a cause of many amphibian declines. Bd can tolerate low temperatures with an optimum thermal range from 17-24°C. It has been shown that Bd infection may result in species extinction, avoiding the transmission threshold presented by density dependent transmission theory. Prevalence of Bd during autumn and winter has been shown to be as low as 0% in some species. It is currently unclear how Bd persists in field conditions and what processes result in carry-over between seasons. It has been hypothesised that overwintering tadpoles may host Bd between breeding seasons. The Great Barred Frog (Mixophyes fasciolatus) is a common, stable and widespread species in Queensland, Australia, and is known to carry Bd. Investigation into Bd infection of different life stages of M. fasciolatus during seasonally low prevalence may potentially reveal persistence and carry-over methods between seasons. Metamorphs, juveniles, and adults were swabbed for Bd infection over three months (between March and May, 2011) at 5 sites of varying altitude (66 m-790 m). A total of 93 swabs were analysed using Polymerase Chain Reaction (PCR) real-time analysis. PCR analysis showed 6 positive (1 excluded), 4 equivocal and 83 negative results for infection with Bd. Equivocal results were assumed to be negative using the precautionary principle. The 5 positive results consisted of 4 emerging (Gosner stage 43-45) metamorphs and 1 adult M. fasciolatus. Fisher's exact test on prevalence showed that the prevalence was significantly different between life stages. All positive results were sampled at high altitudes (790 m); however prevalence was not significantly different between altitudes. Infection of emerging metamorphs suggests that individuals were infected as tadpoles. We hypothesise that M. fasciolatus tadpoles carry Bd through seasons. Thus, Mixophyes fasciolatus may act as disease reservoirs at multiple life stages.

Acute thermal stressor increases glucocorticoid response but minimizes testosterone and locomotor performance in the cane toad (Rhinella marina)

Climatic warming is a global problem and acute thermal stressor in particular could be considered as a major stressor for wildlife. Cane toads (Rhinella marina) have expanded their range into warmer regions of Australia and they provide a suitable model species to study the sub-lethal impacts of thermal stressor on the endocrine physiology of amphibians. Presently, there is no information to show that exposure to an acute thermal stressor could initiate a physiological stress (glucocorticoid) response and secondly, the possible effects on reproductive hormones and performance. Answering these questions is important for understanding the impacts of extreme temperature on amphibians. In this study, we experimented on cane toads from Queensland, Australia by acclimating them to mildly warm temperature (25°C) and then exposing to acute temperature treatments of 30°, 35° or 40°C (hypothetical acute thermal stressors). We measured acute changes in the stress hormone corticosterone and the reproductive hormone testosterone using standard capture and handling protocol and quantified the metabolites of both hormones non-invasively using urinary enzyme-immunoassays. Furthermore, we measured performance trait (i.e. righting response score) in the control acclimated and the three treatment groups. Corticosterone stress responses increased in all toads during exposure to an acute thermal stressor. Furthermore, exposure to a thermal stressor also decreased testosterone levels in all toads. The duration of the righting response (seconds) was longer for toads that were exposed to 40°C than to 30°, 35° or 25°C. The increased corticosterone stress response with increased intensity of the acute thermal stressor suggests that the toads perceived this treatment as a stressor. Furthermore, the results also highlight a potential trade-off with performance and reproductive hormones. Ultimately, exposure acute thermal stressors due to climatic variability could impact amphibians at multiple eco-physiological levels through impacts on endocrine physiology, performance and potentially fitness traits (e.g. reproductive output).

Changes in serum and urinary corticosterone and testosterone during short-term capture and handling in the cane toad (Rhinella marina)

Non-invasive endocrine monitoring with minimally invasive biological samples, such as urine, is being used widely for conservation biology research on amphibians. Currently, it is unknown how closely urinary measurements correspond with the traditional serum hormone measurements. We compared urinary and serum concentrations of corticosterone (CORT) and testosterone (T) in adult male cane toads (Rhinella marina) using a standard capture and handling (short-term stressor) protocol. Free-living male cane toads were captured and sampled for baseline urine (0h) with a second urine sample taken at 0.5h and hourly between 1 and 8h. A single blood sample was collected from each toad after the final urine sampling and capture handling. The mean serum CORT concentration increased between 0 and 0.5h, reaching the highest level between 6 and 8h. The mean urinary CORT concentration increased with a lag-time of 1h and continued to increase up to 8h. The mean level of serum T decreased between 0 and 7h and increased between 7 and 8h. Mean urinary T concentration decreased with a lag-time of 0.5h. Urinary T levels did not change between 4 and 8h. Mean serum T levels reached 50% of the original 0h value at 1h while mean serum CORT levels reached 200% of the original 0h value within 0.5h. Mean urinary T levels reached 50% of the original 0h value within 3h while mean urinary CORT levels reached 200% of the original 0h value within 3h. The inter-individual variation in baseline serum and urinary CORT and T levels were highly comparable, suggesting that baseline urine sample provides a reliable indicator of the physiological status of the animal. Overall, the results have demonstrated that urine sampling and standard capture handling protocol provide reliable measures of baseline corticosterone and testosterone, as well as short-term stress hormone responses in amphibians.

Non-invasive monitoring of glucocorticoid physiology within highland and lowland populations of native Australian Great Barred Frog (Mixophyes fasciolatus)

This study used non-invasive endocrinology to examine baseline corticosterone at different altitudes in a free-living Australian amphibian: the Great Barred Frog (Mixophyes fasciolatus). An enzyme immunoassay (EIA) was performed on urine samples and validated biologically using an adrenocorticotropic hormone (ACTH) challenge. Frogs were injected with ACTH on day 0 and recaptures occurred 1-10days post injection. Urine samples and body condition measurements were collected from lowland (60m) and highland (660m and 790m) sub-populations of M. fasciolatus in South East Queensland (SEQ), close to their post-breeding period during autumn 2011. We simultaneously sampled these sub-populations for Batrachochytrium dendrobatidis (Bd), a pathogenic fungus responsible for mass mortalities of amphibians worldwide. The ACTH challenge successfully validated the urinary corticosterone EIA in M. fasciolatus, with a peak urinary corticosterone response to ACTH injection on day 2 and a return to baseline levels by day 6. Polymerase chain reaction (PCR) analysis of 50 individuals returned only 1 positive result for Bd. Simple linear regression showed a strong positive relationship between baseline urinary corticosterone concentrations and altitude and no relationship with body condition. We hypothesize that higher baseline corticosterone concentrations within highland sub-populations of male M. fasciolatus could be associated with increased environmental challenge at high altitudes and geographic range limits. Whether this pattern is an indication of chronic stress in highland populations or life-time fitness and survival, warrants future investigation.

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.

Host stress response is important for the pathogenesis of the deadly amphibian disease, Chytridiomycosis, in Litoria caerulea

Chytridiomycosis, a disease caused by Batrachochytrium dendrobatidis, has contributed to worldwide amphibian population declines; however, the pathogenesis of this disease is still somewhat unclear. Previous studies suggest that infection disrupts cutaneous sodium transport, which leads to hyponatremia and cardiac failure. However, infection is also correlated with unexplained effects on appetite, skin shedding, and white blood cell profiles. Glucocorticoid hormones may be the biochemical connection between these disparate effects, because they regulate ion homeostasis and can also influence appetite, skin shedding, and white blood cells. During a laboratory outbreak of B. dendrobatidis in Australian Green Tree Frogs, Litoria caerulea, we compared frogs showing clinical signs of chytridiomycosis to infected frogs showing no signs of disease and determined that diseased frogs had elevated baseline corticosterone, decreased plasma sodium and potassium, and altered WBC profiles. Diseased frogs also showed evidence of poorer body condition and elevated metabolic rates compared with frogs showing no signs of disease. Prior to displaying signs of disease, we also observed changes in appetite, body mass, and the presence of shed skin associated with infected but not yet diseased frogs. Collectively, these results suggest that elevated baseline corticosterone is associated with chytridiomycosis and correlates with some of the deleterious effects observed during disease development.

A non-invasive stress assay shows that tadpole populations infected with Batrachochytrium dendrobatidis have elevated corticosterone levels

Batrachochytrium dendrobatidis (Bd) is a fungus that causes the disease chytridiomycosis and is associated with widespread amphibian declines. Populations vary in their susceptibility to Bd infections, and the virulence of the infecting lineage can also vary. Both of these factors may manifest as a differential physiological stress response. In addition, variation in disease susceptibility across amphibian populations may be influenced by immunosuppression caused by chronic stress imposed by environmental factors. Here, we use a non-invasive water-borne hormone technique to assess stress levels (corticosterone) of free-living tadpole populations that are infected by Bd. We found that corticosterone release rates were higher in infected populations of two species of tadpoles (Alytes obstetricans and A. muletensis) than in an uninfected population for both species. The relationship between corticosterone and the intensity of infection differed between species, with only the infected A. obstetricans population showing a significant positive correlation. The higher corticosterone release rates found in A. obstetricans may be an outcome of infection by a highly virulent lineage of Bd (BdGPL), whereas A. muletensis is infected with a less virulent lineage (BdCAPE). These results suggest that different lineages of Bd impose different levels of stress on the infected animals, and that this may influence survival. The next step is to determine whether higher corticosterone levels make individuals more susceptible to Bd or if Bd infections drive the higher corticosterone levels.

Effects of natural weathering conditions on faecal cortisol metabolite measurements in the greater bilby (Macrotis lagotis)

Natural weathering conditions can influence faecal cortisol metabolite (FCM) measurements in wildlife if fresh faeces cannot be collected immediately following defaecation. In this study, we evaluated this issue in a threatened Australian marsupial, the greater bilby (Macrotis lagotis). Fresh (<12 h since defaecation) faecal samples (n = 19 pellets per bilby) were collected one morning from seven adult bilbies kept in captivity. One control faecal sample (Day 1) from each bilby was immediately frozen. The remaining faecal pellets were randomly positioned outdoors. Subsequently, we froze one faecal pellet every 24 h for 19 days. FCM levels in bilby faeces were quantified using an enzyme-immunoassay. Mean FCM levels showed variation (daily mean coefficients of variation [CV %]) of 56.83–171.65% over 19 days. Overall, FCM levels were affected by exposure time; however, multiple comparisons showed that no significant change in FCM occurred after environmental exposure (no significant difference in mean FCM between control (Day 1) with any of the exposure days (Days 2–19). Individuals and sex also affected FCM levels. We found no correlation between mean daily CVs with daily minimum–maximum temperatures or rainfall. Our results indicate that FCM in bilby faeces is fairly stable to long-term environmental exposure (19 days). In future, freshly excreted bilby faeces (where the sample maintains a distinct odour for 9–13 days) should be used to study FCM levels in wild bilbies.

Non-invasive reproductive and stress endocrinology in amphibian conservation physiology

Non-invasive endocrinology utilizes non-invasive biological samples (such as faeces, urine, hair, aquatic media, and saliva) for the quantification of hormones in wildlife. Urinary-based enzyme immunoassay (EIA) and radio-immunoassay have enabled the rapid quantification of reproductive and stress hormones in amphibians (Anura: Amphibia). With minimal disturbance, these methods can be used to assess the ovarian and testicular endocrine functions as well as physiological stress in captive and free-living populations. Non-invasive endocrine monitoring has therefore greatly advanced our knowledge of the functioning of the stress endocrine system (the hypothalamo-pituitary-interrenal axis) and the reproductive endocrine system (the hypothalamo-pituitary-gonadal axis) in the amphibian physiological stress response, reproductive ecology, health and welfare, and survival. Biological (physiological) validation is necessary for obtaining the excretory lag time of hormone metabolites. Urinary-based EIA for the major reproductive hormones, estradiol and progesterone in females and testosterone in males, can be used to track the reproductive hormone profiles in relationship to reproductive behaviour and environmental data in free-living anurans. Urinary-based corticosterone metabolite EIA can be used to assess the sublethal impacts of biological stressors (such as invasive species and pathogenic diseases) as well as anthropogenic induced environmental stressors (e.g. extreme temperatures) on free-living populations. Non-invasive endocrine methods can also assist in the diagnosis of success or failure of captive breeding programmes by measuring the longitudinal patterns of changes in reproductive hormones and corticosterone within captive anurans and comparing the endocrine profiles with health records and reproductive behaviour. This review paper focuses on the reproductive and the stress endocrinology of anurans and demonstrates the uses of non-invasive endocrinology for advancing amphibian conservation physiology. It also provides key technical considerations for future research that will increase the accuracy and reliability of the data and the value of non-invasive endocrinology within the conceptual framework of conservation physiology.

Are baseline and short-term corticosterone stress responses in free-living amphibians repeatable?

Amphibians respond to environmental stressors by secreting corticosterone, a stress hormone which promotes physiological and behavioral responses. Capture handling can be used to stimulate physiological stress response in amphibians. The use of single blood sampling and presentation of mean data often limits the quantification of within and between individual variation in baseline and short-term corticosterone stress responses in amphibians. It is important for studies of amphibian physiological ecology to determine whether baseline and short-term corticosterone stress responses are consistent or not. We quantified repeatability (r), a statistical measure of consistency, in baseline and short-term corticosterone stress responses to a standard capture and handling stress protocol in free-living adult male cane toads (Rhinella marina). Corticosterone metabolite concentrations were measured entirely non-invasively in male toad urine samples via an enzyme-immunoassay. During the first sampling occasion, urine samples were collected manually from individual male toads (n=20) immediately upon field capture. Toads were handled for 5min then transferred to plastic bags (constituting a mild stressor), and urine samples were collected hourly over 8h in the field. The toads were resampled for baseline (0h) urine corticosterone with hourly urine sampling over 8h (for quantification of the stress induced corticosterone) at 14 day intervals on three consecutive occasions. Within and between sample variations in urinary corticosterone metabolite concentrations were also quantified. All toads expressed a corticosterone stress response over 8h to our standard capture and handling stress protocol. Variations both within and between toads was higher for corrected integrated corticosterone concentrations than corticosterone concentrations at baseline, 3 or 6h. Baseline urinary corticosterone metabolite concentration of the male toads was highly repeatable (r=0.877) together with high statistical repeatabilities for 3h (r=0.695), 6h (r=0.428) and 8h (r=0.775) corticosterone metabolite concentrations, and for the total and corrected integrated corticosterone responses (r=0.807; r=0.743 respectively). This study highlights that baseline and short-term corticosterone stress responses are repeatable in free-living amphibians. Future studies should utilize this non-invasive tool to explore repeatability among seasons and across years, and determine its functional significance in relation to behavioral ecology and reproduction in amphibians generally.