Costs of Rearing the Wrong Sex: Cross-Fostering to Manipulate Offspring Sex in Tammar Wallabies

Abstract

Sex allocation theory assumes that offspring sex (son vs. daughter) has consequences for maternal fitness. The most compelling experiment to test this theory would involve manipulating offspring sex and measuring the fitness consequences of having the "wrong" sex. Unfortunately, the logistical challenges of such an experiment limit its application. In tammar wallabies (Macropus eugenii), previous evidence suggests that mothers in good body condition are more likely to produce sons compared to mothers in poor condition, in support of the Trivers-Willard Hypothesis (TW) of condition-dependent sex allocation. More recently, we have found in our population of tammar wallabies that females with seemingly poor access to resources (based on condition loss over the dry summer) are more likely to produce sons, consistent with predictions from the Local Resource Competition (LRC) hypothesis, which proposes that production of sons or daughters is driven by the level of potential competition between mothers and philopatric daughters. We conducted a cross-fostering experiment in free-ranging tammar wallabies to disassociate the effects of rearing and birthing offspring of each sex. This allowed us to test the prediction of the LRC hypothesis that rearing daughters reduces the future direct fitness of mothers post-weaning and the prediction of the TW hypothesis that rearing sons requires more energy during lactation. Overall, we found limited costs to the mother of rearing the "wrong" sex, with switching of offspring sex only reducing the likelihood of a mother having a pouch young the following year. Thus, we found some support for both hypotheses in that rearing an unexpected son or an unexpected daughter both lead to reduced future maternal fitness. The study suggests that there may be context-specific costs associated with rearing the "wrong" sex.

Fig 1. Timeline of study with respect to tammar wallaby reproduction.

This study ran December 2009 to March 2011. We recorded maternal condition in the summer previous to our cross-fostering manipulation (prior to blastocyst reactivation). Then, pouch young were cross-fostered early in the pouch stage (March), and maternal outcomes were measured across lactation (December), across the post-weaning summer and into the following reproductive season (following March). Offspring manipulated in this study could have resulted from eggs fertilized between approximately one year and one month prior to birth.

Fig 2. Effects of cross-fostering offspring sex on maternal body condition and subsequent reproduction in tammar wallabies.

Mothers gain (residual) body condition during lactation (a), which coincides with the season of greatest resources, and lose body condition post-lactation (b) during the dry summers. Values in (a) and (b) are presented as Least Square Means ± SE from the statistical model. The percent of mothers with pouch young early in the following reproductive season (c) was influenced by the cross-fostering treatment, whereas the sex of the pouch young during the following season varied strongly depending on the sex birthed during the experimental year (d). Treatments indicate ‘Birthed Sex / Reared Sex’ of offspring, with F = female and M = male.