BERCOVITCH, Fred B; TOBEY, Jennifer R; Center for Reproduction of Endangered Species, Zoological Society of San Diego; Center for Reproduction of Endangered Species, Zoological Society of San Diego: Maternal Effort and Sex Differences in Koala Joey Development

Maternal effort includes time, energy, and resources devoted to producing and rearing offspring. Sex biased maternal effort is expected when three conditions are met: (1) maternal condition influences offspring condition, (2) offspring condition when weaned predicts adult body condition, and (3) adult body condition has a direct impact on variation in fitness. Collecting the longitudinal data necessary for testing these three foundations for sex-biased investment is difficult in long-lived mammalian species. We examined 18 years of data recorded from koalas living in a captive colony at the San Diego Zoo in order to examine how joey development and maternal condition might be linked. Koalas have a lengthy period of infant dependency, which is accompanied by sex differences in growth rates. Joeys emerge from the pouch at an average age of 28 weeks and are size dimorphic by 52 weeks of age. Adult male koalas are significantly larger than adult female koalas, but our data do not indicate sex-biased maternal effort. Neither maternal condition nor age affected sex ratio at joey emergence and maternal mass did not influence joey mass or survivorship. Given the poor quality diet of koalas, combined with maternal transport of infants who are 25% of maternal mass, we suggest that infant rearing poses an energetic cost on koala females. We propose that when there is a lengthy period of infant dependency, and a high level of maternal expenditure, females will maximize allocation of resources to offspring, regardless of sex.