A major problem reported by many commercial producers keeping sows in group housing systems, is that of inter-sow aggression and bullying, due to the establishment and maintenance of a social hierarchy. Even in stalls and tethers, inter-neighbour aggression can be high but the physical consequences for the sow receiving the aggression are negligible, due to the lack of opportunity for physical contact. In a group situation, an inability to prevent attack by another sow can itself lead to poor welfare, in addition to any effects of injury or pain caused by the attack. A sow's ability to control its social environment can not only affect its access to resources (Edwards et al., 1993), but also its physiological state and productivity (Mendl et al., 1992).

The aim of this experiment was to determine the effects that different intensities and degrees of success of agonistic social interaction have on the heart rate of sows, which can be a useful measure of the emotional response of an individual to short-term problems such as social challenge.