Seasonal Changes in Haematocrit in Captive Koalas (Phascolarctos cinereus)

G. M. Cleva, G. M. Stone and R. K. Dickens

Department of Animal Science, University of Sydney, Sydney, NSW 2006, Australia.


Six males and nine females, members of a group of 15 captive koalas, were examined for changes in haematocrit for 22 and 11 months respectively. Plasma protein concentration and plasma osmolarity were also measured in samples that varied widely in haematocrit. Body weight was routinely recorded. In both sexes there were pronounced changes in microhaematocrit with time of year, with elevated values in winter and low values in summer. Microhaematocrit was thus significantly and negatively correlated with maximum and minimum daily temperature. These changes in haematocrit were not associated with changes in body weight, plasma protein concentration or plasma osmolarity. It is suggested that the higher winter haematocrit is a physiological adaptation to the higher energy demands when ambient temperature is reduced.