Birth litter sex ratio affects gilt behaviour, endocrine status and reproductive performance

Abstract

This thesis tested the hypothesis that females that developed in a male biased litter would be masculinised and therefore have impaired reproductive performance and be more aggressive than those born into female biased litters. One of the aims of this thesis was to investigate new tools for selection of gilts into the breeding herd. Current selection criteria are minimal, with genetics, weight, and body conformation as the main determinants. We investigated effects of the sex ratio of a gilt’s birth litter, as in other litter bearing species it affects reproduction, behaviour, and physiology. One of the mechanisms that may be involved is the transfer of androgens from male to female fetuses during development which can occur either to neighbouring fetuses or to the whole litter. Therefore, females from male biased litters may have impaired reproduction and be more aggressive due to masculinisation during fetal development. A literature review found evidence that this phenomenon may affect pigs. We defined a male biased litter having ≥60% males (including stillborns) and female biased as ≥60% female. We examined sex ratio effects on induction of ovulation prior to puberty, and the endocrine milieu before and during natural oestrus. We also investigated behaviour by applying anxiety and aggression tests. Phenotypic differences were also assessed by measuring the anogenital distance (AGD; distance from anus to the vulva), which is commonly elongated in females from male biased litters. Interestingly, we found, in contrast to findings from other species, that the AGD of gilts from female biased litters was longer than those from male biased litters at 16 weeks of age. The anogenital distance at 21 weeks of age was then employed in a commercial study to examine associations with gilt fertility through two parities. Gilts with longer anogenital distances reached puberty younger, were more likely to be mated, and had a higher total born alive litter size. Based on our earlier data, we suggest these gilts were likely from female biased litters. In our behavioural studies we found that gilts from male biased litters were bolder as they had a faster emergence time in an arena test, but they also showed trends towards increased aggression with increased scratch scores around weaning and greater likelihood to fight in a resident intruder test. We also found that when stimulated with exogenous gonadotrophins at 18 weeks of age, gilts from male biased litters has a higher ovulation rate. During second oestrus, gilts from male biased litters had impaired ovulatory luteinising hormone surges but no change in tonic luteinising hormone. These results highlight that gilts from male and female biased litters have different reproductive responses. Overall, our research shows that gilts from male biased litters are different to gilts from female biased litters in reproduction, behaviour, and physiology. Currently, it appears that gilts from female biased litters, or those with above average anogenital distances, are better suited to the breeding herd but further commercial work is required to assess the effect that this new selection tool has on overall performance.