Field Metabolic Rate, Water Flux, Food Consumption and Time Budget of Koalas, Phascolarctos Cinereus (Marsupialia: Phascolarctidae) in Victoria.

Abstract

Doubly labelled water measurements in free-ranging adult koalas (9.2 kg) indicated that field metabolic rates averaged 0.434 ml CO2 g-¹h-¹ (equivalent to 2090 kJ per animal per day, or 2.59 X basal metabolic rate). Females (7.8 kg) had significantly higher mass-specific metabolic rates than males (10.8 kg). Percentage apparent assimilation of dietary substances was 56% for dry matter, 52% for energy, 32% for nitrogen, and 66% for water. Feeding rates were about 222 g dry food per animal per day (equivalent to 510 g fresh food per animal per day) in both sexes. However, males had a higher water influx rate (475 ml per animal per day) than females (358 ml per animal per day), suggesting either that males selected more succulent food than females, or that males drank rainwater but females did not. Koalas consumed about twice as much dietary nitrogen as they required for maintenance. They maintained constant body masses, and (presumably) had balanced energy, water and nitrogen budgets during our 20-day study, while eating Eucalyptus ovata foliage. Koalas spent about 4.7 h eating, 4 min travelling, 4.8 h resting while awake and 14.5 h sleeping in a 24-h period. Their activity periods were not obviously restricted to periods of daylight or darkness, but were scattered through the 24 hours. In comparison with free-living, three-toed sloths Bradypus variegatus (4.08 kg) in central America, koalas had significantly higher mass-corrected field metabolic rates (391 kJ kg-0.75 day-¹ for koalas v.209 for sloths), water influx rates (69.9 ml kg-0.80 day-¹ for koalas v. 49.8 for sloths), and feeding rates (42.7 g dry food kg-0.75 day-¹ for koalas v. 21.2 for sloths). Unlike sloths, koalas did not bask in the morning sunshine, and one telemetered koala had a relatively constant body temperature over 24 h (c. 36°C), compared with daily variations between 30 and 38°C in sloths. Population food consumption (g dry food consumed ha-¹ day-¹) was greater for koalas (681 v. 378 for sloths), and koalas consumed most of the leaf production of their preferred food species, E. ovata, which resulted in extensive defoliation of these trees. Although there is similarity in the ecological roles of koalas and sloths, their physiology and behaviour differ substantially.