When the fitness costs and benefits of sons and daughters differ, offspring sex ratio manipulation could be an important reproductive tactic. We explored the effects of environment and maternal caring ability on offspring sex to test four adaptive sex ratio modification hypotheses: the Extrinsic Modification (EMH), Carrying Capacity (CCH), Trivers-Willard (TWH) and Cost-of-Reproduction (CRH) hypotheses. The EMH and CCH propose that environmental conditions shape offspring sex ratios, directly or in interaction with maternal condition. The TWH and CRH predict a positive relationship between maternal condition and production of the costlier sex. The TWH predicts that mothers with superior caring ability should produce more of the sex that can provide the greatest fitness returns from additional maternal allocation, and the CRH proposes that females with limited caring ability should reduce fitness costs by producing the cheaper sex. Repeated measures on 83 known-age eastern grey kangaroos, polygynous marsupials with strong sexual dimorphism, revealed that offspring sex ratio was independent of per capita forage, supporting neither the EMH nor CCH, but was dependent on maternal mass, consistent with the TWH and CCH. Our results, however, cannot clearly identify the ultimate cause of the relationship between maternal mass and greater production of sons. One of the three assumptions of the TWH could not be verified, and mothers of sons suffered only marginal additional fitness costs. Sex ratios in higher vertebrates are likely not solely explained by factors dependent on maternal control.
Description of the methods used to collect the data and get the data ready can be found in the associated paper. Individual IDs have been replaced by randomised numbers.
Description of the variables can be found in the enclosed R script.