The extent to which sex-biased maternal investment characterizes mammals is controversial, with less information available for evaluating patterns of maternal effort in marsupials than in placentals. Koalas Phascolarctos cinereus are sizedimorphic animals with a lengthy period of dependency and they reside in mating systems that might favour sex-biased maternal investment. We examined 18 years of data recorded from koalas living at the San Diego Zoo in order to examine how joey development and maternal condition might be connected. Koalas are pregnant for only 1 month, but joey emergence from the pouch does not occur until 32 weeks of age. Neither maternal condition nor age affected sex ratio at joey emergence, and both sexes had the same survivorship prospects. Koala dams transport and nurse joeys for close to 1 year, at which time the two sexes are size dimorphic. Given the poor-quality diet of koalas, combined with maternal transport of infants who are at least 25% of maternal mass, we suggest that infant rearing poses high energetic costs on koala females. We suggest that ecological and energetic constraints have moulded koala maternal strategies such that females maximize allocation of resources to offspring, regardless of sex, in order to increase prospects for joey survivorship.

We analyzed 18 years of data collected from a breeding colony of captive koalas Phascolarctos cinereus in order to identify factors regulating mating success and to generate expectations about the behavioral ecology of wild koalas. Short-term mate fidelity was associated with reproductive success, but familiarity appeared to diminish reproductive output with time. Male body mass had no effect on reproductive success. Reproductive output was highest when males were slightly older than females. Survivorship of joeys was not dependent on either male or female age. We suggest that sexual selection regulates koala mating tactics primarily in the context of non-agonistic male competition. We hypothesize that field studies will reveal that females in the wild probably capitalize on the link between male vocal and olfactory advertisements and male age as a mechanism to foster positive assortative mating by age.

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