The sex ratio of a litter affects the behaviour of its female pigs until at least 16 weeks of age

There are sex differences in behaviour with males tending to be more aggressive and less fearful than females. When females are treated with androgens they display more aggressive, male like, behaviours and this suggests that androgen exposure has a role in the development of aggression. Female pigs born into litters that are more than 60% male (male biased) are likely exposed to greater androgen concentrations in utero than female pigs born into litters that are greater than 60% female (female biased). We hypothesise that the proportion of males in the litter in utero affects female pig behaviour. We predict that gilts from male biased litters will be more aggressive and less anxious than gilts from female biased litters. The behaviour of females from male biased (n = 24) and female biased (n = 30) litters was assessed at day 11 and day 21 of life with an arena test, at weaning (28 days of life) by assessing fight number and duration as well as injury scores and at 16 weeks of age with a resident intruder test. In the day 21 arena test, the pigs from female biased litters took longer to emerge into the arena than pigs from male biased litters (1.13 ± 0.05 (42.2 s) vs. 1.32 ± 0.09 (15.5 s) respectively, P = 0.016). At weaning there was a higher scratch score for piglets from male biased litter (0.84 ± 0.2 vs 0.69 ± 0.15, P = 0.015) and this continued as a trend on the day following weaning (1.76 ± 0.10 vs 1.57 ± 0.13, P = 0.080). At 16 weeks of age in the resident intruder test 48% of gilts from male biased litters initiated a fight compared to 24% of gilts from female biased litters (P = 0.08). Our data suggest that the sex ratio of the birth litter can affect behaviour of gilts, potentially having long term implications for sow aggression.