Aggression and group size in domesticated pigs, Sus scrofa: ‘when the winner takes it all and the loser is standing small’

Formation of artificial groups of domestic species occurs frequently in todayʹs husbandry systems. These groups are kept in a closed space where the individuals are not able to withdraw from the group, and resources in the environment are most commonly limited and defensible. This creates a competitive environment where aggression and social stress are often seen. We developed a theoretical model that describes how aggression among unacquainted pigs is a function of group size. As the number of potential competitors increases, more individuals will benefit from not getting involved in costly fights. The reason for this is that the probability of being able to monopolize resources diminishes as group size increases. To verify predictions from this model, we compared aggression among unacquainted, weaned pigs in three group sizes (6, 12 and 24 pigs). There were significantly more fights per individual in groups of six and 12 pigs than in groups of 24 pigs, and the proportion of pigs not participating in fights was significantly higher in the largest group. For pigs that were willing to take the risk of getting involved in fights in the largest group size, the duration of each fight was longer than for fights in the smaller groups. These results support the predictions from our theoretical model, and we conclude that pigs are able to alter their behaviour according to how the actual competitive situation changes with group size.