The measurement of waterborne corticosterone (CORT) concentrations in amphibians is becoming increasingly common. It is promoted as a minimally invasive method of accurately assessing physiological stress with conservation and management implications. It is important that waterborne CORT be explored thoroughly to validate its use as a measure of health, yet many studies complete only a partial validation. We conducted a multi-pronged validation of waterborne CORT in larval Northern Leopard Frogs, which is a common amphibian model in both the laboratory and wild. Our approach included assay validation, a time-series study to determine when equilibrium is reached, injecting animals with adrenocorticotropic hormone (ACTH), using a handling challenge, and analyzing different developmental stages.
Results/Conclusions
Neither mass nor snout-vent-length were correlated with waterborne CORT in tadpoles. We found that when tadpoles were placed into water, CORT levels reached equilibrium after 15 minutes. Injection with ACTH elevated plasma CORT and waterborne CORT compared to controls. Plasma CORT and waterborne CORT were positively correlated when all ACTH treatment groups were pooled. Handling did not have an effect on CORT. Plasma and waterborne CORT were weakly correlated in controls, but there was no correlation among tadpoles that were handled. There was an effect of developmental stage on CORT. However, only prometamorphic tadpoles showed a correlation between plasma and waterborne CORT. Validating new waterborne hormone assays can be time consuming and expensive. However, the time and cost are worthwhile because using a waterborne assay versus the traditional plasma assays decreases the number of animals sacrificed to research and decreases the amount of stress on animals being studied. Once validated, the use of waterborne hormones to measure CORT levels will give scientists a non-invasive alternative method to using either plasma or whole-body CORT for analysis. This study is a valuable contribution to the field as it helps researchers confidently interpret the meaning of waterborne CORT in larval amphibians.