Piggy in the middle: Pig aggression reduced when a bystander pig steps in

Credit: CC0 Public Domain

A small study suggests that when two pigs are fighting, an intervening pig can either reduce the number of attacks by the aggressor or help reduce the victim's anxiety. The study of 104 domestic pigs, published in the journal Animal Cognition, demonstrates how pigs form complex social groups and how they may resolve conflict.

Reconciliation or the introduction of a third-party bystander to reduce further aggression or anxiety are examples of conflict resolution in social animals (known as triadic contacts). These conflict-resolution strategies are essential for maintaining equilibrium in social animal groups and reducing victim anxiety, but their application to domestic pigs is unclear.

Giada Cordoni, Ivan Norscia, and their colleagues from the University of Torino in Turin, Italy observed how 104 pigs at the ethical farm Parva Domus in Turin resolve conflict following a fight. The authors were able to identify the majority of generations of pigs based on their breed, size, and markings, but they also genetically tested 31 pigs of various generations to determine the relatedness of the entire group.

From June to November of 2018, they observed and recorded interactions between the pigs, noting aggressive behaviors such as head-knocking, pushing, biting, and lifting of the victim pig. After each aggressive conflict, the authors observed behavior for three minutes and recorded gender, kinship, and age.

The authors observed that both the aggressor and the victim exhibited reconciliation behaviors, including nose-to-nose contact, sitting in close proximity, and resting their heads on one another. The researchers discovered that both the aggressor and the victim initiated reconciliation behaviors after a conflict. However, they discovered that the proportion of reconciliations was significantly higher in distantly related pigs than in those that were closely related.

Pigs may value relationships based on what they can provide, such as social support, according to the authors' hypothesis. Fighting between closely-related kin (half or full siblings) may cause less harm to social groups because these relationships are viewed as more stable. However, distantly related pigs may be more likely to engage in reconciliation behavior following a fight in order to maintain social support and food access within the group.

Upon observing the resolution of conflicts involving a third-party pig, the authors observed behavioral differences depending on whom the bystander pig approached and interacted with after the conflict. If the bystander approached and interacted with the victim, the number of aggressive behaviors did not change, but the average hourly frequency of anxiety-related behaviors decreased significantly. Symptoms of anxiety included trembling, scratching, chewing with an empty mouth, and yawning. When a bystander pig approached the aggressor, however, the number of aggressive behavior attacks directed at the victim decreased significantly.

When either the aggressor or the victim was a close relative, a greater proportion of bystander pigs intervened in the conflict. This, according to the authors, suggests that pigs value certain relationships and may support closely related relatives.

After the conflict, the victim pig's attempts to approach and interact with a bystander had no effect on reducing post-conflict anxiety behavior or the likelihood of being attacked again. This could be because 95.2% (42 cases) of the bystander pigs did not reciprocate the union when approached by a victim.

The authors caution that this study only includes one group of adult domestic pigs, and thus may not be representative of all pig populations. Future research could examine whether these conflict-resolution strategies are utilized in other contexts.

The fact that pigs engage in reconciliation and triadic contact after conflict suggests, according to the authors, that pigs may possess some socio-emotional regulation abilities to alter their own or others' experience of group conflict.

Journal information: Animal Cognition


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