Aggression when pigs are mixed into new social groups has negative impacts on welfare and production. Aggressive behaviour is moderately heritable and could be reduced by genetic selection. The possible wider impacts of selection for reduced aggressiveness on handling traits and activity in the home pen were investigated using 1663 male and female pedigree pigs (898 purebred Yorkshire and 765 Yorkshire × Landrace). Aggressive behaviour was observed over 24 h after pigs were mixed at 10 weeks of age into groups balanced for unfamiliarity and weight. Aggression was highly heritable (duration of involvement in reciprocal fighting h2 = 0.47 ± 0.03, and duration of delivering one-sided aggression h2 = 0.34 ± 0.03). Three weeks after mixing, home pen inactivity (indicated by the frequency of lying) was observed over 24 h. Inactivity was weakly heritable (h2 = 0.05 ± 0.01) but showed no significant genetic association with aggression. Pigs’ behaviour during handling by humans was assessed on entry to, whilst inside and on exit from a weigh crate at both mixing and end of test at 22 weeks. Pigs were generally easy to handle, moving easily into and out of the crate. Scores indicating ‘very difficult to move’ were rare. Handling scores at weighing were weakly heritable (h2 = 0.03 to 0.17), and moderately correlated across the two weighings (rg = 0.28 to 0.76). Aggressive behaviour at mixing was genetically associated with handling at the end of test weighing: pigs that fought and delivered one-sided aggression had handling scores indicating more active behaviour at weighing (e.g. moving quickly into the crate v. fighting rg = 0.41 ± 0.05 and v. bullying rg = 0.60 ± 0.04). Also, there was a genetic association between receiving one-side aggression at mixing and producing high-pitched vocalisations in the weigh crate (rg = 0.78 ± 0.08). Correlated behavioural responses occurring across different challenging situations (e.g. social mixing and human handling) have been described by the concept of animal temperament (also known as coping styles, personality or behavioural syndromes), but this has rarely been demonstrated at the genetic level in farm animals. These findings may have practical implications for the development of breeding programmes aimed at altering animal temperament. Breeding to reduce aggression could result in some reduction in activity at weighing. This would have consequences for animal production, because pigs which are inactive at weighing take longer to move into and out of the weigh crate, and perhaps also for animal welfare.
