Inverse urinary corticosterone and testosterone metabolite responses to different durations of restraint in the cane toad (Rhinella marina)
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Non-invasive measurement of urinary corticosterone and testosterone metabolites in amphibians provides opportunities for endocrine studies of responses to physiological and psychological stressors. Typically, corticosterone metabolite concentrations increase in frog urine within 1-2 h of a mild capture and handling stress protocol. However, no study has investigated the effect of duration of manual restraint on the changes in corticosterone and reproductive hormones in amphibians. We quantified urinary corticosterone and testosterone metabolite responses for 8 h following various durations of manual restraint (control, 5, 15 or 30 min) in adult male cane toads (Rhinella marina) under controlled laboratory conditions. All toads had a corticosterone stress response over 8 h to our standard capture and handling stressor. The mean corticosterone stress response was significantly higher after 15 or 30 min restraint in comparison to the control (no restraint) or to 5 min restraint. Manual restraint for 5, 15 or 30 min caused a significant reduction in urinary testosterone concentrations over 8 h. We also provide a novel method of quantifying plasticity in corticosterone stress responses in amphibians with respect to restraint duration using the concept of a "reaction norm". The reaction norm, which was calculated as slope of the regression line of integrated corticosterone response against restraint duration, was 9.69 (pg corticosterone/姠creatinine h)/min for male toads. In summary, corticosterone and testosterone responses to restraint are affected by restraint duration in male toads. Glucocorticoid reaction norms can be applied to study the change in physiological stress hormonal response with respect to restraint duration in other amphibian species.
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