The sex biased litter in utero and its effect on post-natal health, development, and reproductive capacity of the commercial pig

Environmental stressors in-utero, including extremes of steroid hormone exposure, can negatively affect reproductive function, the development of offspring, as well as life-course health trajectories in a myriad of species. However, few studies to date have investigated the mechanisms responsible. This thesis tested the hypothesis that a sex ratio biased uterine environment would influence the reproductive capacity of the gilt, and the health and production parameters of pigs within commercial production systems. An initial scoping review showed results from different in-utero sex ratios were highly variable both between and within species, but overarching themes of negative influences of androgenised uterine environments on reproductive outputs, physiological development, and maternal/paternal behaviour were established. Across species, the testosterone biased intra-uterine environment increased aggression whilst decreasing fearful behaviour. I subsequently investigated whether females gestated in biased uterine environments (65% or more of one sex) exhibited altered reproductive potential postnatally. I examined the ovarian reserve and endometrial morphology of these individuals, revealing significant effects to follicle profiles dependent on birth weights, but no effect on endometrial development. A collaborative project utilising commercial pigs investigated whether a hormonally biased environment influenced growth, health, and carcass quality parameters. This found that male and female pigs from a biased uterine environment were more likely to die or be culled before reaching slaughter weight, due to illness and injury, or due to behavioural vices. No effects on growth nor carcass parameters were identified based on uterine bias, but male pigs produced a higher quality carcass than females. In summary, I have provided evidence further supporting an altered reproductive capacity in female pigs and altered mortality causes in pigs from androgenised and oestrogenised litters compared to non-biased litters. This thesis bridges the gap of how the hormonally biased uterine environment may mechanistically contribute to poor reproductive success in breeding sows. Further it is the first study to identify a higher life-course mortality rate in pigs originating from biased litters. It is recommended that the sex ratio of an originating litter, as well as an individual’s birth weight, should be considered when selecting breeding sows.