METABOLISM AND HEAT-BALANCE IN AN ARBOREAL MARSUPIAL, THE KOALA (PHASCOLARCTOS-CINEREUS)

1. The insulation of the pelt of the koala (0.529 °C W-1m2 in still air) is the highest reported for a marsupial. The fur also has a high level of insulative resistance to wind. The dorsal surface is more densely furred and less reflective of solar radiation than the ventral surface and different postural adjustments, particularly in relation to wind direction, allow for insulative flexibility. 2. The basal body temperature of the koala under laboratory conditions was 35.7°C at an ambient temperature of 20°C, rising to 37.0°C at a Tair of 40°C. Although activity raised body temperatures of unrestrained koalas slightly, the mean nycthemeral temperature was 36.0°C; there was no obvious nycthemeral body temperature cycle. 3. The basal metabolic rate (1.21 W kg-1; 1.75 W kg-0.75) occurred at a Tair of 25°C (74% of the predicted marsupial value). The lower critical temperature is about 15°C in summer and about 10°C in winter, the change in critical temperature being linked to seasonal changes in tissue and fur insulation. The major mechanism for evaporative cooling at high ambient temperatures was panting. 4. The combination of a high level of fur insulation, slightly depressed body temperature and reduced level of basal metabolic rate were interpreted as an adaptation to an existence of arboreal folivory extending over a wide latitudinal range. © 1979 Springer-Verlag.