"Group-housing of pigs can provide benefits for pig welfare, such as interaction with conspecifics and the ability to perform more natural behaviors. Unfortunately, group-housing also presents major welfare concerns. Pigs are often mixed at different production stages based on sex and weight in order to create uniform groups that allow for more efficient resource use by producers. Unfortunately, when unfamiliar pigs are introduced this causes intense aggression as pigs establish a social hierarchy. This increased aggression can persist for 24--48 h after a mixing event and can lead to injury, infection, and stress. Pigs are highly social animals, and as such, have individual differences in behavior and complex social relationships that need to be considered when addressing social aggression. The long-term goals of this project were to identify individual behavior types and understand the role of individual behavior types in social behavior in group-housed pigs. The specific objectives for this research were to understand the role of personality in the management and welfare of pigs through a comprehensive literature search, to identify individual behavior types in group-housed pigs using individual time budgets and behavior tests, and to explore measures of social cohesion in recently mixed pigs as they form a stable social group. Many studies have investigated the role of personality in the management of pigs but as a new field of study there are a number of issues that prevent the advancement of this field into behavioral management of livestock. Despite that, pig personality traits have been related to factors related to pig physiology, housing environment, social behavior, and cognition and therefore there is potential for producers to incorporate pig personality information into their breeding, care, and welfare. Individual pigs vary in their overall behavior, therefore part of this research aimed to compare pig time budgets with duration of aggression at different time points. It was found that pig behavior varies immediately after mix and becomes more consistent at 6 wk after mixing. The amount of time pigs spend on non-aggressive behaviors was related to aggression, particularly time spent inactive and exploring. We also compared duration of aggression with production traits of growth rate, backfat thickness, and loin muscle area. Pigs that are more aggressive at mix and at 3 wk after had slower growth and smaller loin muscle area, suggesting that efforts to reduce aggression should be implemented not only after mixing, but in the weeks following to prevent negative consequences on production traits. One solution to address the issue of aggression is to breed for less aggressive pigs without inadvertently disrupting other behavior traits important in managing pigs. Behavior tests were used to assess traits of fearfulness and response to humans, and these measures were compared with lesion scores, a proxy measure of aggressiveness. Aggressiveness was related to pigs' responses in social and non-social challenges, suggesting there could be correlated suites of behaviors that should be considered when breeding for less aggressive pigs. Another important aspect to consider when addressing issues of aggression in group-housed pigs is their sociality. Social animals display a wide number of behaviors to maintain social bonds. Affiliative and agonistic behaviors were compared at 4 time points and revealed that certain affiliative behaviors are related to less aggression, suggesting potential for selection on positive social behaviors. Overall, the results of this research suggest that the role of individual behavior types in the social cohesion of group-housed pigs is important to consider. Future directions of this research will explore this idea in more depth and aim to guide pig producers on how to manage group-housed pigs in a way that not only reduces aggression but promotes positive social behaviors and good welfare."--Pages ii-iii.
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