Aggression in group housed sows and fattening pigs

Scott and Fredericson (1951) define agonistic behaviour as ‘the group of behaviouraladjustments associated with fighting, which includes attack, escape, threat,defence, and appeasement’. Agonistic behaviour is generally studied by measuringaggression (fight) or flight in the receiver (Jensen, 1982). Aggressive interactionsbetween pigs are commonly considered to include the following: knock by the head(head-to-head/head-to-body knock); bite (to any part of another pig); chase; paralleland inverse parallel pressing (pressing of the shoulders against each other); andlevering (lifting of the other with the head). Other agonistic behaviours include:threat (usually non-physical actions like gaze, open mouth lunges at another pig);retreat (pig moves rapidly away from another pig that is delivering aggression); andavoidance (pig moves rapidly away from another pig that is not directing anybehaviour at it) (Jensen, 1980, 1982).Under natural conditions, agonistic behaviour arises as an interplay of adaptive social strategies of individuals, thus forming the rules that govern living in social groups (Lorenz, 1966; Lindberg, 2001; East and Hofer, 2010). Periods of high aggression occur when the established social order is disrupted and this can happen frequently under the conditions imposed by commercial farming systems (Velarde, 2007). Due to the consequences of aggression on injuries, stress and productivity (Verdon et al., 2016a), and their probable links to pain and fear (Hemsworth et al., 2015), the welfare implications of aggression are obvious. The factors that contribute to the high levels of aggression observed on commercial pig farms need to be understood so that the consequences of aggression, and subsequently injuries and stress, on negative affective states, such as pain and fear, can be minimised.The design and management of commercial group housing for pigs is complex and with wide disparity. Many features and practices can affect the frequency and duration of pig aggression, and have been reviewed elsewhere (Arey and Edwards, 1998; Barnett et al., 2001; Spoolder et al., 2009; Bench et al., 2013a,b; Verdon et al., 2015a). Rather than re-examining each of these features, this chapter discusses those aspects for which the science has been updated, is best enunciated and/or displays the most potential to reduce aggression. Furthermore, caution is recommended when applying research on immature pigs to the sow because of differences in age, experience, or physiological state (Verdon et al., 2015a). As such, science relating to the gilt or sow will be considered separately to that related to the weaner and grower-finisher pig