Environmental water relations of the koala, Phascolarctos cinereus, and the importance of the micro-environment in tropical habitats

"The koala, Phascolarctos cinereus, has been introduced to several islands off the coast of Australia. On southern Australian islands the populations often increase rapidly and outstrip the carrying capacity of the habitat. Intriguingly, however, such overpopulation has not been observed in koala populations introduced to islands off the coast of Queensland, so a study of these may provide insights into the reasons for this difference, together with suggesting management strategies for all island populations of this marsupial. The Queensland race of the koala (P. c. adustus) was introduced to St Bees Island, central Queensland, during the early 20th century and has remained undisturbed for over 80 years. As such, St Bees Island offers the opportunity to study various aspects of the ecology and physiology of an apparently stable population of koalas. Furthermore, St Bees Island lies in the warm humid climate zone, where studies of koala physiology have not been performed and where water is likely to be plentiful, unlike most other koala habitats in Australia. To investigate factors that may be contributing to the varying response of koalas to island habitats, water turnover was determined by isotopically labelled water dilution in four seasons. Field metabolic rate was also studied in one season along with a number of environmental parameters that may affect water turnover rates. Water turnover was significantly higher in both winter 2000 and summer 2002 than in winter 2001 and spring 2002. Previous studies at Springsure and Blair Athol in central Queensland were of koalas from the same subspecies and similar latitude but from regions with contrasting rainfall regimes so were compared with the St Bees data to further investigate the proposed limiting nature of browse moisture to koala distribution. Water turnover at St Bees Island was significantly higher during summer than at Blair Athol and Springsure and, setting aside the winter 2000 determination on St Bees Island, no significant difference was found during other seasons. Differences in water turnover among sites appeared to be due to night-time conditions (minimum temperature and relative humidity) during the period of maximum koala activity and it is suggested that the distribution of the koala may be limited coastally by its ability to thermoregulate in conditions of high relative humidity. The high water turnover observed in winter 2000 on St Bees Island was unusual. Comparison of the water turnover and body water content of the two winter seasons (2000 and 2001) suggested that koalas were depositing fat. This area of koala physiology requires more research to investigate the relationship between body water and fat content and to identify conditions under which body fat is deposited and metabolised. Field metabolic rate was determined during October 2002 by the doubly labelled water method. The average metabolic rate was not significantly different to measurements from Springsure during July or Blair Athol during September. It appears that the average energy requirements of koala populations are reasonably constant across central Queensland. Food intake by koalas was estimated from both water turnover rates and field metabolic rates and compared. This comparison on St Bees Island, where access to free water was limited, the koalas diet of Eucalyptus leaves provided ample energy for metabolic demands and food intake was primarily determined by water requirements. Koalas on St Bees Island roosted in non-eucalypt trees on 54% of occasions over all seasons but during summer the use of non-eucalypts rose to 64%. This phenomenon has been noted elsewhere and it has been suggested that the trees provide shelter from the sun and predators. In an effort to quantify the advantage afforded to koalas by non-eucalypt roost trees, telemetry collars were developed that broadcast information about koala activity and the micro-environment. Data from this study indicated that koalas used two strategies to cope with the high summer temperatures experienced on St Bees Island. They either chose non-eucalypt roost trees, or greatly reduced their daytime activity. The second strategy would decrease energy demands and associated water loss." -- abstract