Physiological changes in brushtail possums, Trichosurus vulpecula, transferred from the wild to captivity

To determine the effect of relocation on the health of possums, the body weights and hormone and immune responses of 11 male and 9 female brushtail possums were monitored following transfer from the environs of Brisbane into an established breeding colony in Brisbane. The possums were monitored weekly for the first 20 weeks of captivity, and their immune responses assessed again 12 months after capture. Over the first 5 weeks of captivity, male possums lost a mean of 8.8% of their original body weight, and females lost 15.3% over the first 6 weeks. Variation between individual possums was evident, and the 11 male possums could be divided into two groups, those that gained weight (number of animals, N = 4) and those that lost weight (N = 7) in captivity. Four males gained weight following capture, and their body weight after 20 weeks of captivity was higher than at capture. The remaining seven males lost weight over the 20 weeks following introduction into captivity, resulting in a lower weight at week 20 than at capture. All of the nine female possums lost weight and were slower to regain weight compared to the males. Plasma cortisol concentrations did not vary greatly over the 20 weeks in male possums, and the mean plasma concentration of cortisol for the 11 male possums was 7.8 ng/ml (number of samples, n = 220). The female possums showed a different pattern. The concentration of cortisol for the nine female possums at week 1 was 34.0 ng/ml, which was significantly higher than 13.3 ng/ml at week 20 (P < 0.016). No significant variation in the mean concentration of plasma thyroxine of 5.7 ng/ml occurred in the 11 male possums over the 20-week period (n = 220). The plasma concentration of thyroxine for the nine female possums was 2.5 ng/ml (n = 54) for the first 6 weeks. At week 6, an increase in the concentration of thyroxine occurred, and a peak concentration of 6.9 ng/ml was reached at week 13. This increase correlated with the females regaining body weight. A low concentration of thyroxine is often associated with stress, thus an increase in the concentration of this hormone, combined with an increase in body weight, may indicate that these females had begun to adjust to their new environment. The seven male possums that lost weight following introduction into captivity displayed a significantly higher concentration of cortisol (9.1 compared with 5.3 ng/ml P < 0.01), and a lower concentration of thyroxine compared to the four males that gained weight following capture (4.7 compared with 7.3 ng/ml, P < 0.005). Over the 20-week period, the total number of white blood cells increased, and the number of neutrophils increased in both males and females. The proliferative response of lymphocytes from male possums to the T-cell mitogen, phytohaemagglutin (PHA) decreased significantly over the 20-week period (P < 0.002). In females an initial decrease in the reactivity of lymphocytes observed over the first 10 weeks was followed by an increase in this response over the remaining 10-week period. Twelve months following capture, the white blood cell parameters of both males and females had returned to similar levels to those of the first 1-5 weeks. The reactivity of lymphocytes from male possums that had been in captivity for 12 months was significantly higher than that of the first 20 weeks of captivity (P < 0.005). Females that had been in captivity for 12 months displayed lymphocyte responses similar to those observed at weeks 16-20. The body weight and hormonal results would suggest that possums undergo a more severe stress response than males immediately following their capture. In contrast, the immune response of males is lower than females and is depressed for a longer period following capture.