The short-term behavioural response of sows, but not gilts, to a social stimulus is related to sow aggressiveness in groups

This study examined relationships between the behavioural response of pregnant gilts (n = 200, gestation 1) and sows (n = 200, gestation 2) to a live, similarly-aged female pig (unfamiliar pig test, UPT) and to a fibre-glass model pig (model pig test, MPT), and aggressive behaviour on the day after mixing (day 2). Sows with a short latency to make contact with an unfamiliar sow in the UPT were more likely to deliver high levels of aggression at day 2 of gestation 2 (P = 0.005), but this relationship was stronger when a model pig was used (P < 0.001). Similarly, sows with a long duration of tactile contact with the model pig in the MPT were more likely to deliver high levels of aggression at day 2 of gestation 2 (P = 0.015), but this relationship was weaker than that between aggression and the latency to contact the model pig. When the terms the latency to contact the unfamiliar pig in the UPT and the model pig in the MPT, as well as the duration of contact with the model pig in the MPT, were included in an overall model of aggression at day 2 of gestation 2, behaviour towards the unfamiliar pig became not statistically significant (P > 0.05). A strong relationship was not apparent with gilts (P > 0.05). Thus, the socially inexperienced pig may not be an ideal model for sow behaviour. This study indicates that sows with a short latency to contact a model pig are more likely to be aggressive when mixed into groups.