A non-invasive water-borne hormone assay for amphibians

Anthropogenic disturbances have been implicated in the rapid decline of amphibians. Disturbances, such as disease and poor water quality, might cause changes in the physiology of amphibians resulting in chronic stress, which can result in decreased growth and development as well as immunosuppression. In amphibians, corticosterone (CORT) is the main hormone released in response to stressors. We took the first steps towards validating a new, non-invasive, technique to assay CORT in amphibians using a water-borne collection method previously used only with fish. In validation of this technique, we found a significant positive correlation between release rates of water-borne CORT and levels of CORT in circulating plasma in adults of the San Marcos salamander, Eurycea nana, and the common midwife toad, Alytes obstetricans. These results indicate that water-borne CORT can be used as a proxy for plasma CORT. Additionally, we examined basic background information on the physiological states of these two species. We found that captive-reared salamanders had significantly lower release rates of CORT than field-collected salamanders. Field-collected salamanders had significantly higher CORT release rates 24 h after capture and transfer to the laboratory. For tadpoles, we found that field-collected tadpoles did not have significantly different CORT release rates than those maintained in the laboratory for four months. Our research indicates that this method of water-borne hormone collection should be viable for many species of amphibians; however, further validation via adrenocorticotropic hormone (ACTH) challenges is required. This method can be a useful tool for assessing the physiological state of laboratory and field populations of amphibians and the effects of urbanization, pesticides and diseases. An important benefit of this method is that it allows for repeated measures of the same individuals and can be less stressful than drawing blood.