Maternal control of offspring sex is a rich yet controversial topic in evolutionary ecology. The success of sex allocation theory has been largely limited to invertebrates. Sex allocation in birds and mammals has proven more difficult to understand, and remain the next, great challenge. Despite decades of research on these taxa, we are still struggling to understand the occurrence and adaptive explanation of biases in offspring sex ratio, as well as the mechanisms behind biased sex ratios. Many marsupial mammals produce biased sex ratios, and these departures from equality may be more frequent than those recorded in eutherians. The occurrence of sex ratio biases in marsupials supports a role for multiple ecological factors, and marsupial reproductive physiology facilitates proposed mechanisms of maternal sex ratio manipulation and allows novel experimental approaches. In particular, researchers can access metatherian offspring early in development and can manipulate sex ratio in order to test its adaptive value for mothers. Here I cross-fostered offspring by sex in a captive population of Tammar wallabies (Macropus eugenii) prior to any significant maternal investment [5 treatments based on the sex born and the sex fostered by the mother (born/fostered): 1) daughter/son; 2) daughter/daughter; 3) son/son; 4) son/daughter; and 5) mothers that give birth and foster their own offspring (sham manipulation)] to test whether offspring sex has fitness consequences for mothers.

Results/Conclusions In line with the Trivers-Willard Hypothesis I predict a mother that births a son (a mother in “good” condition) will wean an offspring of high sex-specific fitness, regardless of the sex of the fostered young. Similarly, a mother that births a daughter (a mother in “poor” condition) will produce an offspring of low sex-specific fitness, regardless of the foster-offspring sex. My results found a significant effect of birth sex on the likelihood of raising a young to weaning (chi-sq = 5.487, P = 0.0192, df = 1). A mother that birthed a son had an 80% chance of successfully raising her young to weaning regardless of the sex fostered, where a mother that birthed a daughter had a 40% chance of raising her young to weaning.  Additionally, mothers that birthed sons raised offspring that gained weight at a greater rate than mothers that birthed daughters. The study is now being replicated in a wild population that will allow the measurement of both maternal and offspring fitness components over multiple years.