Mothers and forgotten fathers : prenatal effects and paternal influences on mammalian sex ratios

Sex allocation theory predicts that parental fitness is enhanced when parents adjust offspring sex ratios according to the relative fitness returns from sons and daughters. Compared to other taxa, research in mammals exhibits inconsistent empirical results relative to theory. Such inconsistencies could, in part, relate to the degree of prenatal masculinisation or feminisation of female phenotypes altering capacity to adjust sex ratios in adulthood. Further, fathers might also influence offspring sex ratios through sperm sex ratios and seminal plasma composition. My thesis addressed the effects of prenatal masculinisation and feminisation on maternal sex ratios and the scope of paternal influences on sex ratios. I first tested how the degree of prenatal masculinisation or feminisation of female mice (Mus musculus) influenced offspring sex ratios. I found that more feminised females produced more daughters. Thus, I suggest that some inconsistencies between theory and observation in mammalian sex allocation relate to phenotypic variation among females relating to prenatal experience. Secondly, I tested the influence of fathers on sex ratios by quantifying sperm sex ratios in frequently and infrequently mating domestic stallions (Equus ferus caballus). I increased the mating frequency of all stallions to induce sexual exhaustion over three days, mimicking feral stallion mating frequencies. Sperm sex ratios were not at parity. The proportions of X-chromosome-bearing-spermatozoa (CBS) increased in successive ejaculates in frequently mating stallions, while the infrequently mating stallions maintained an X-CBS bias throughout the experimental period. I suggested that Y-CBS are more rapidly depleted than X-CBS with sexual exhaustion in frequently mating stallions. Third, I measured key components of seminal plasma: sex hormones, glucose, major cations and trace elements, and their relationship to X- and Y-CBS in successive ejaculates. Seminal plasma composition varied between ejaculates of the same male; notably, zinc was positively correlated with the proportion of Y-CBS. Zinc is a major antioxidant. Thus, correlation with Y CBS is potentially a protective mechanism as Y-CBS are more susceptible to oxidative damage. Fourth, I investigated how variation in sperm sex ratios relative to mating frequency corresponded to variation in foal sex ratios relative to mating frequency. To test the hypothesis that an increase in mating frequency increases the likelihood of daughters, based on my previous results, I first examined foal sex ratios in a population of feral horses consisting of 198 foals sired by 26 feral stallions. More female foals were born when there were more mares available for stallions to mate at the time of conception, linking a high stallion mating frequency to an increase in daughters. I next obtained 46,486 foaling records from 125 commercially breeding stallions in the New Zealand Thoroughbred Racehorse Studbook. Stallion mating frequency did not predict foal sex, suggesting that mating frequency in managed thoroughbreds is not a driver of foal sex ratio skews. The mixed results indicated paternal sex ratio adjustment in response to mating frequency is complex and contextual, potentially relating to variation in sexual rest days prior to mating or the sexual familiarity versus novelty across multiple mating events. Collectively, these results indicate that the proximal modification of sperm and offspring sex ratios is feasible, thereby demonstrating potential paternal control of sex ratios. In this thesis, I have demonstrated that offspring sex ratios are influenced by the degree of prenatal feminisation of maternal physiology. Moreover, I have shown variation in paternal contributions to sex ratios both between and within males, indicating the need for fathers to also be considered in tests of maternal sex allocation. These results suggest that both factors could explain some of the inconsistencies between observation and theory in mammalian sex allocation research and should be accounted for in future sex allocation research.