Field Metabolic Rate, Water Flux, Food Consumption and Time Budget of Koalas, Phascolarctos cinereus (Marsupialia: Phascolarctidae) in Victoria
Kenneth A. Nagy* and Roger W. MartinB
*Laboratory of Biomedical and Environmental Sciences, and Department of Biology, University of California, 900 Veteran .Avenue, Los Angeles, California, 90024, U.S.A.
BDepartment of Zoology, Monash University, Clayton, Vic. 3168.
Doubly labelled water measurements in free-ranging adult koalas (9.2 kg) indicated that field metabolic
rates averaged 0,434 ml C 0 2 g-lh-1 (equivalent to 2090kJ per animal per day, or 2.59X basal
metabolic rate). Females (7.8kg) 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 5 10 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 eatingEucalyptus oL'atafoliage. 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 slothsBradypus variegatus (4.08 kg) in central America,
koalas had significantly higher mass-corrected field metabolic rates (39 1W kg-0.75 day-l for koalas v.
209 for sloths), water influx rates (69.9 ml kg-o,80day-l for koalas v. 49.8 for sloths), and feeding rates
(42.7g dry food kg-0.75 day-1 for koalasv. 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°Cin sloths. Population food consumption
(g dry food consumed ha-1 day-1) was greater for koalas (681v. 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.