Non-invasive evaluation of physiological stress hormone responses in a captive population of the greater bilby Macrotis lagotis

Captive breeding programs are increasingly being used as a management option for threatened mammals. The greater bilby Macrotis lagotis, for example, is a vulnerable species which is maintained in captivity at several facilities in Australia. Non-invasive evaluation of stress hormones (cortisol in mammals) via excretory metabolites can be used to monitor physiological stress responses of captive individuals. In this study, we validated an enzyme-immunoassay (EIA) to measure cortisol metabolites in fresh faecal samples of adult male and female bilbies (n = 7) held in captivity at the Dreamworld Theme Park, Queensland, Australia. The faecal cortisol EIA was validated via parallelism and the recovery of exogenous cortisol added to pooled faecal extracts (>99% recovery). Female bilbies had higher average faecal cortisol metabolite concentrations and higher day-to-day variation than male bilbies; however, there was no relationship with bilby age. Cortisol metabolites for most individuals varied widely through time, with numerous peaks and troughs in response to long-term stressors (illnesses, injury and reproductive issues) and short-term stressors, such as use in shows at Dreamworld or public displays in local schools, manual restraint and short-term veterinary procedures (e.g. general anaesthesia). Overall, the higher mean cortisol metabolite concentrations of individuals suffering long-term stress was related to a greater response to short-term stressors. This suggests an interaction between responses to short-term and long-term stressors which is perhaps due to habituation and/or facilitation of long-term stressors. Non-invasive faecal monitoring of stress hormones could provide further information on the implications of captive breeding programs and the release of animals reared in captivity.