The relocation of animals in the wild and into captivity is becoming an increasingly common practice, both for research and for conservation purposes. However, the transfer of animals from their original environment is thought to cause stress and animals that are relocated to a location distant from their site of capture presumably experience additional stress. The aim of this study was to examine the physiological changes that occur in brushtail possums on capture and during the first 20 weeks of captivity to determine whether stress plays a role in the failure of relocated marsupials to survive and reproduce. In addition, the role of the immune system in the greater mortality of adult possums during winter and in the mortality of young possums as they first leave the pouch and at weaning was also examined.
Possums captured in the environs of Brisbane, Adelaide and Armidale were monitored following their relocation into captivity in Brisbane. The survival rate of adult possums and pouch young trapped in the environs of Brisbane was considerably higher than that of the relocated Adelaide and Armidale possums. Over the first 20 weeks of captivity, the survival rate of the Brisbane, Adelaide and Armidale possums was 71% («=48), 47% (w=17) and 62% («=24) respectively. Female possums from both Brisbane and Armidale also had a lower survival rate compared to males. The main cause of death over the first 20 weeks of captivity was bacterial infections whereas many possums that survived longer than 20 weeks died of renal disease. During the first 20 weeks of captivity, possums may have become susceptible to infection as a result of the stress associated with their capture and relocation into captivity. These possums displayed a rapid decrease in body weight, a high plasma concentration of Cortisol and a low concentration of thyroxine suggesting that they were stressed following their transfer into captivity. A decrease in the proportion of lymphocytes to neutrophils and a decrease in the proliferation of lymphocytes was also observed following capture supporting the suggestion that possums became susceptible to infection as a result of a decrease in their immune response.
Brisbane and Armidale possums that survived at least 20 weeks in captivity were also monitored each week over the first 20 weeks of captivity. These possums displayed changes in body weight, hormone concentrations and immune responses suggesting that they experienced stress following capture but gradually recovered from this stress during the final 10 weeks of the 20 week study period. However, possums relocated from the environs of Armidale appeared to experience greater stress than Brisbane possums and required longer to adjust to their new environment. Male possums also appeared to be able to cope with the stress associated with their capture and relocation more readily than females.
The present study also provides evidence that the immaturity of the immune system may contribute to the greater number of deaths of young marsupials as they leave the pouch and at weaning. The numbers and distribution of T and B lymphocytes in the thoracic thymus, spleen and gut associated lymphoid tissue and the proliferation of blood lymphocytes were examined in the developing possum. T lymphocytes were identified in the thymus at day 2 post partum indicating that the thymus is capable of T cell differentiation early in pouch life. However, the numbers of T and B lymphocytes in the spleen and the proliferation of lymphocytes at day 150 post partum was significantly lower than that of the adult indicating that the immune system is immature at the time of weaning and continues to develop after pouch life.
The immune responses of captive adult possums were also compared between summer and winter in order to determine whether the greater number of deaths observed during winter is due to a decrease in the immune response. An increase in the numbers of neutrophils and a decrease in the ratio of lymphocytes to neutrophils was observed during winter in both males and females. This result suggests that possums may experience more difficult conditions during winter than in summer and this may contribute to the greater number of deaths that are observed in winter. However, possums displayed no significant variation in their cell mediated immune response, plasma Cortisol concentration or body weight, suggesting that healthy possums that have adjusted to captivity are able to cope with the normal stressors associated with winter.
Although a breeding colony of Brisbane possums has been maintained in Brisbane for the past 12 years, a breeding colony of relocated Adelaide possums was unable to be established in Brisbane and possums relocated from Armidale were unable to wean their young until their second breeding season in Brisbane. These results suggest that relocated possums may require up to two years in order to adjust sufficiently to their new environment to reproduce successfully.