PRODUCTION OF MILK AND NUTRITION OF THE DEPENDENT YOUNG OF FREE-RANGING KOALAS (PHASCOLARCTOS-CINEREUS)

The production of milk by lactating females, and energy expenditure an d foliage intake of their dependent young, were investigated in free-r anging koalas. Koalas had the lowest mass specific daily milk-energy p roduction at peak lactation so far recorded in a mammal, but the durat ion of reproduction was 58% longer than the combined marsupial and eut herian average. As a consequence, the total energy input to reproducti on in koalas was similar to that in other mammals. We propose that the prolonged lactation and low daily rate of energy transfer to the youn g by female koalas is an adaptation to the low energy availability fro m their diet of Eucalyptus foliage. Energy requirements (field metabol ic rates) of young koalas were lower than those expected for typical m arsupials (only 60% at permanent pouch exit), which may be a necessary preadaptation that allows the low rate of maternal energy transfer. H owever, the energy requirements of the adult females were no lower tha n expected for marsupials. This pattern of energy requirements and age resulted in a linear relationship between field metabolic rate and ma ss for the koalas in this population. Differences in milk production b etween the years of the study coincided with fluctuations in the avail ability of preferred young foliage, which suggests that lactational ou tput by koalas may be flexible and affected by diet quality. Despite t he interannual differences in milk production, growth of the young was similar in the two years.