Testing the application of plasma glucocorticoids and their ratios as biomarkers of acute and chronic stress in rescued wild koala patients: a pilot study

Abstract Koalas (Phascolarctos cinereus) are one of the most iconic marsupial species endemic to Australia. However, their population is declining due to threats including habitat loss, disease, dog attacks, and vehicle collisions. These threats also serve as acute or chronic stressors that impact koala welfare and conservation. Cortisol is widely used as a biomarker to study stress in koalas. However, plasma cortisol concentration is less studied due to its limited ability to assess chronic stress and welfare concerns. Dehydroepiandrosterone sulphate (DHEAS) and dihydrotestosterone (DHT) are biomarkers that could potentially detect chronic stress due to their antagonising and inhibitory effects on cortisol. In this study, we used plasma cortisol and the ratio of DHEAS and DHT to cortisol to assess stress in rescued koalas (n = 10) admitted to RSPCA Queensland. Although no significant differences were found between koalas across all biomarkers and the ratios failed to detect chronic stressors, similar trends were found consistently, suggesting the potential use of the biomarkers to assess stress. Across all biomarkers, the highest medians were found in koalas with Chlamydia-related reproductive disease and oxalate nephrosis and the lowest medians were found in koalas with Chlamydia-related conjunctivitis. Higher medians were also found consistently in females (n = 3) and adult koalas. In addition, insignificant negative correlations were found across all biomarkers between age, weight, and body conditioning scores, except for the positive correlation between weight and cortisol and cortisol:DHT. Overall, the consistency of trends and the insignificant differences found across biomarkers in our study suggested that using a single biomarker to assess chronic stress is insufficient, especially for hospital-based studies limited by sample population. Thus, this pilot study provides first step towards developing a koala-specific allostatic load index based on multiple stress biomarkers to understand chronic stress in rescued koalas. Lay summary Stress in koalas can be challenging for their welfare and conservation. In this study, we tested plasma glucocorticoids and their ratios as biomarkers of acute and chronic stress. Our finding showed ratios of DHEAS and DHT to cortisol are comparable across stress parameters and animal demographic characteristics. This study serves as a foundational framework for developing a stress index based on multiple biomarkers that could be useful tool for koala welfare.