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Abstract Ongoing amphibian population declines are caused by factors such as climate change, habitat destruction, pollution and infectious diseases not limited to chytridiomycosis. Unfortunately, action is taken against these factors once population collapses are underway. To avoid these post hoc responses, wildlife endocrinology aims to analyse physiological mediators that predict future population declines to inform wildlife management. Mediators typically investigated are stress hormones known as glucocorticoids, which are produced by the Hypothalamus—Pituitary—Interrenal axis (HPI axis). The HPI axis is the part of the endocrine system that helps amphibians cope with stress. Chronic increases in glucocorticoids due to stress can lead to immune dysfunction, which makes amphibians more susceptible to infectious diseases. Despite this predictive potential of glucocorticoids, interpretation of glucocorticoid data is confounded by sampling design and type. Glucocorticoid monitoring classically involves blood sampling, which is not widely applicable in amphibians as some are too small or delicate to sample, and repeated samples are often valued. To address this, we tried to validate skin swabbing via corticosterone (CORT) and adrenocorticotropin hormone (ACTH) injections in adults of two amphibian species: Eastern red-spotted newts, Notophthalmus viridescens viridescens, with natural skin infections with Batrachochytrium dendrobatidis (Bd) upon collection in the field, and Northern leopard frogs, Rana (Lithobates) pipiens, raised in captivity and naïve to Bd exposure. Further, we determined the predictive potential of skin glucocorticoids on Bd load in the field via correlations in Eastern red-spotted newts. We found that hormones present in the skin are not related to the HPI axis and poorly predict infection load; however, skin hormone levels strongly predicted survival in captivity. Although skin swabbing is not a valid method to monitor HPI axis function in these species, the hormones present in the skin still play important roles in organismal physiology under stressful conditions relevant to wildlife managers. 

Some of the amphibian populations in Panama are demonstrating slow recovery decades after severe declines caused by the invasion of the fungal pathogenBatrachochytrium dendrobatidis(Bd). However, new species remain to be described and assessed for the mechanisms of disease resilience. We identified seven skin defense peptides from a presumably novel leopard frog species in the Tabasará range, at Buäbti (Llano Tugrí), Ngäbe-Buglé Comarca, and Santa Fe, Veraguas, Panama, herein called the Ngäbe-Buglé leopard frog. Two of the peptides were previously known: brevinin-1BLb fromRana (Lithobates) blairiand a previously hypothesized “ancestral” peptide, ranatuerin-2BPa. We hypothesized that the peptides are active againstBdand shape the microbiome such that the skin bacterial communities are more similar to those of other leopard frogs than of co-occurring host species. Natural mixtures of the collected skin peptides showed a minimum inhibitory concentration againstBdof 100 μg/ml, which was similar to that of other leopard frogs that have been tested. All sampled individuals hosted high intensity of infection withBd. We sampled nine other amphibian species in nearby habitats and found lower prevalence and intensities ofBdinfection. In addition to the pathogen load, the skin microbiomes were examined using 16S rRNA gene targeted amplicon sequencing. When compared to nine co-occurring amphibians, the Ngäbe-Buglé leopard frog had similar skin bacterial richness and anti-Bdfunction, but the skin microbiome structure differed significantly among species. The community composition of the bacterial skin communities was strongly associated with theBdinfection load. In contrast, the skin microbiome composition of the Ngäbe-Buglé leopard frog was similar to that of five North American leopard frog populations and the sympatric and congenericRana (Lithobates) warszewitschii, with 29 of the 46 core bacteria all demonstrating anti-Bdactivity in culture. Because of the highBdinfection load and prevalence in the Ngäbe-Buglé leopard frog, we suggest that treatment to reduce theBdload in this species might reduce the chytridiomycosis risk in the co-occurring amphibian community, but could potentially disrupt the evolution of skin defenses that provide a mechanism for disease resilience in this species.