Constraints on maternal ability to adjust sex ratios in mammals

Sex allocation theory predicts that mothers would benefit from sex-biased differential investment into offspring in relation to their current local condition when it maximizes their lifetime reproductive return. In mammals, however, the extent of the sex bias at birth is often unpredictable, suggesting that mothers may be constrained in their ability to adjust sex ratios. None of the current hypothesized mechanisms of peri-conceptual mammalian maternal sex allocation fully explain the amount of variation observed, and as such I suggested three possible physiological constraints on maternal sex allocation. Firstly, mothers may be constrained by variation in physiological traits, particularly mediated during their own early development through in utero effects, such as testosterone levels and responsiveness to stress. I tested the effects of physiological changes caused by down-regulated stress during in utero development, and showed significant physiological changes in females, as a result of mismatching pre- and post-natal environments, that skewed sex ratios in the next generation. However, artificially lowering the stress of these females at conception will cause the sex ratio to return to parity, as the pre- and postnatal environments match again. Secondly, their physiology may be influenced more proximally, by not only their current condition or ability to invest, but by clinically asymptomatic disease and parasitic infection, particularly manipulative parasites. Lastly, paternal influences such as sperm sex ratios and seminal plasma constituents have been largely overlooked but may influence and constrain maternal ability to adjust sex ratios. I showed evidence of variations in sperm sex ratios, both in the literature and through observational studies where we would expect parity as a result of meiosis during sperm production. I also presented the first evidence of paternal sex allocation, through changes in sperm sex ratios and seminal plasma constituents in relation to coital rate, as a proxy of male attractiveness. The possibility of complementary or antagonistic interactions between maternal and paternal sex allocation should now be accounted for in future research. Overall, my thesis has provided explanations into previously unexplained variation in sex allocation research, and may assist with improvements to conservation breeding and livestock industries, as well as human health developments.