Animal personality in the management and welfare of pigs

Personality is defined as individual behavioral differences that are consistent over time and across contexts and is constructed from a number of underlying traits. Over the last 27 years, studies on pig personality have investigated links between personality traits and behavioral and physiological responses. The objective of this paper was to review the literature on personality studies in pigs. Eighty-three peer-reviewed research articles were included. The most common objective of these studies was to identify personality types in pigs by comparing their response across multiple situations. The relationship with physiological responses was the next most common objective. Results were difficult to compare as there was little consistency in terminology or experimental design across studies. Only 24.1% of the studies reported reliability and even fewer explicitly assessed validity. The backtest was the most common test (used in 67.5% of the studies), though it is unclear what specific trait is being measured. Classifying pigs as proactive or reactive personality types using the backtest was common, but the relationship between backtest results and other variables are inconsistent. The human approach, novel object, and food competition tests were also popular methods. Exploration, aggressiveness, reactivity to humans, and fearfulness were the most common personality traits studied in pig populations. There was moderate support for relationships with physiological responses. Personality was related to other behaviors, such as vocalizations and social aggression. Studies on genetic control are promising, with the heritability of personality traits falling within the range seen for other traits already selected for in pigs, suggesting these traits can be considered in breeding programs to improve welfare. Pigs with reactive personality types were more influenced by their housing environment than proactive pigs. Housing influenced reactive pigs' immune response, manipulative oral behavior, response in cognitive tasks, play behavior, and gastric lesions, which has serious implications for the management of pigs. Few studies explored the predictive power of personality traits on future physiological or behavioral outcomes of pigs, however, there is support for the potential use of personality research in improving pig welfare and productivity. In order to move forward with this field, researchers need to agree on consistent terminology and methodologies, and investigate the reliability, validity, and practicality of common personality measures in pigs.