Seasonal variation in water flux, field metabolic rate and food consumption of free-ranging koalas (Phascolarctos cinereus)
Ellis, WAH, Melzer, A, Green, B, Newgrain, K, Hindell, MA & Carrick, FN 1995, Australian Journal of Zoology, vol. 43, pp. 59-68.
Male free-ranging koalas in central Queensland, Australia were found to have field metabolic rates (corrected for mass) that exhibited a variation of 0.382 MJ kg0.75 per day in the winter and 0.329 MJ kg0.75 per day during the summer. Measurements of influx of water in the same koalas were 59.9 mL kg0.8 per day during the summer and 50.8 kg0.8 per day in the winter. This water influx had a positive correlation with the moisture values in the consumed food. Water influx in the winter was lower in Springsure koalas compared to those from Victoria. Feeding rates were discovered to be higher in the winter compared to summer.
Radio-collars were attached to six free-ranging male koalas found near Springsure for telemetry. A blood sample of 2 mL was drawn from the cephalic vein in each of the koalas, with recapture of the koalas occurring after 7-11 days to be weighed and further blood samples collected. The primary species of food consumed was determined through observations of feeding over a period of t13:06:54hree years. Five trees were sampled to identify the leaf moisture by drying to achieve a constant mass. Other findings included that the intake of wet food was lower than previous reports of female koalas of similar weight. A preference for Eucalyptus tereticornis was demonstrated in summer and Eucalyptus crebra favoured in winter.
Factors influencing dietary preferences of koalas for different tree species are poorly understood; however, previous studies have reported a water threshold of 55% and the presence of essential oils to be common among favoured species. This study attempted to investigate the modifications to metabolism in the koala as a response to dietary consumption in varying environments. The observed kolas in the study have been postulated to have an increased turnover of water in the summer as a product of increased water loss by evaporation, which is the primary method used by koalas to mediate heat loss. This is key to survive in the hot climate and is reliant upon high consumption of moist leaves. The water turnover rate in the koalas was linearly associated with food moisture content, which is expected as koalas rarely drink water. The difference in water flux rate of koalas from Queensland and Victoria in winter can be attributed to the strong relationship to moisture of the leaf, which was significantly higher in Victoria. This indicates that koala tree selection is mainly based on nutritional factors. This study suggests that in winter, nutritional requirements regulate intake of water met by digestion of large amounts of dry leaves, therefore resulting in decreased water turnover and increased metabolic rate.
In conclusion, the present study demonstrates that seasonal changes in central Queensland result in adjustment of male koala diet which is characterised by higher energy demands in winter and larger water needs during summer.
Summarised by Alexander Murdoch
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