Ecology, behaviour and growth of sub-adult koalas (Phascolarctos cinereus) on St Bees Island, Central Queensland

Little is known about the growth and behaviours of sub -adult koalas, particularly in a relatively natural environment and stable population. Most studies of this age group have been in over -abundant populations or within fragmented or urban habitats or involve rehabilitated sub -adult koalas. This study looks at sub -adult koalas within a stable population, in an intact, relatively undisturbed environment.

A mathematical model of growth was developed using data from two koalas of known birthdate. The growth of these two sub -adults was assessed against five published models for aging young koalas, based on captive animals. Only one model was effective in estimating the age of free ranging sub -adults on St Bees Island. When data from other juveniles were also included, the rate of growth and the size of individuals at a given age also differed from published data derived largely from captive animals.

Fecundity was relatively low on St Bees Island, particularly when compared with overabundant populations on islands in Victoria. Along with this, there was a high mortality rate, with approximately 50 % of sub -adults perishing in the time between emerging from the mothers' pouch and maturity. These two factors are perhaps contributing to the population dynamics that keep the St Bees Island koala numbers in check. Some limited data of offspring numbers and gender showed some evidence of declining fecundity with individual age. The gender of offspring produced by a female koala throughout her life time was consistent with theTrivers-Willard hypothesis, favouring males in younger, fitter mothers and females in older or less fit mothers.

Maturing sub -adults were radio tracked as they undertook dispersal or extended movements away from the natal area before establishing a home range and breeding. Before becoming independent, juveniles mirrored their mothers ranging and movement behaviour, while remaining within the mothers' territory. After three to five months, the juveniles ventured beyond the mothers' range, in a complex way, before establishing their own home range. Direction and timing of dispersal was varied between individuals and genders. Female post dispersal home ranges were usually close to or overlapping the mothers range area. However, males established home ranges much further from their natal area.

The utilisation of tree species and habitat varied between developmental phases of the juveniles. At the time of independence, and while roaming in unfamiliar areas, they showed significant differences in tree use to that of the maternal females. During the day, Eucalyptus tereticornis (50 %) was the most frequently used species, however the other species used frequently (n = 31) were quite varied and included: Cryptocarya triplinervis, Mallotus philippensis, Ficus spp., Allocasuarina littoralis, and Pouteria sericea. Nocturnally E. tereticornis was used almost exclusively.

Nocturnal tree species utilisation reflected the koalas' diet while daytime utilisation differed significantly. Hence, night observations were needed to fully understand the diet and habitat requirements of koalas.

It was concluded that juvenile koalas have different requirements to adult animals and management practices must incorporate an understanding of these to adequatelysustain populations. This study has revealed previously unrecognised complexities in sub -adult ecology and behaviour.