Are serum cortisol levels a reliable indicator of wellbeing in the tammar wallaby, Macropus eugenii?

Serum corticosteroid levels were measured in six female tammar wallabies (Macropus eugenii) at Macquarie University, NSW, Australia, to assess their reliability as indicators of wellbeing in this species. Animals were challenged with a change in conditions over the course of approximately 3 weeks, comprising (i) isolation and movement; (ii) altered feeding routine, in order to impose a disturbance to homeostasis ("stress"). Blood samples were collected five times during the study period, and analysed for corticosteroid concentration and a number of haematological and biochemical measures. The use of enzyme immunoassay for the measurement of corticosteroids in marsupial serum was validated. We found that there was no significant change in serum corticosteroid concentrations in response to either of the changed conditions suggesting that serum corticosteroid concentrations are not reflective of a change in wellbeing. Cortisol was the dominant serum corticosteroid, with concentrations up to 50 times higher than corticosterone. Significant differences were observed in aspartate amino-transferase, alanine amino-transferase, haemoglobin, total erythrocyte count and mean corpuscular haemoglobin concentration over the period of study. However, no significant correlations emerged to justify any of these measures, or serum corticosteroid levels, as reliable indicators of compromised wellbeing in the tammar wallaby.