Social choices in farm animals: to fight or not to fight?

Abstract

AbstractFrom an animal production and welfare perspective, an important social choice made by farm animals is whether and how vigorously to fight others. The choice to fight (i.e. simultaneous escalated aggression by both contestants) may result in severe injury when unfamiliar animals first meet or compete for highly valued resources. Game theory models of aggressive interactions predict that animals have evolved to stop or avoid fighting when they assess their chances of winning a contest to be poor. Assessment may occur during or before a fight. If it occurs before a fight, reliable cues of fighting ability must exist. If farm animals can establish relative social status by assessment prior to fighting, it may be possible to construct social groups which contain individuals of differing abilities such that most disputes are resolved through assessment and levels of damaging aggression are kept low. Social status appears to be established by assessment rather than fighting in free-ranging deer and sheep when asymmetries exist in reliable cues of their fighting abilities such as roaring rate and horn size. However, there have been few detailed studies of other farmed species and findings are equivocal. Work on young pigs suggests that, in pair-wise encounters, individuals are unable to assess weight-related asymmetries in their abilities without fighting. However, recent studies of groups of pigs suggest that some form of assessment prior to fighting may occur. Individuals were classified as high (H) or low (L) aggressive on the basis of their behaviour in an attack latency test. When litters of H pigs were mixed with litters of L pigs, significantly fewer pairs of unfamiliar pigs fought than when newly mixed groups were made up of litters of H pigs only, or litters of L pigs only. Thus, fighting was least frequent when there was a marked asymmetry in the aggressiveness of unfamiliar individuals. Another study raised the possibility that H and L pigs may be following alternative strategies which, under certain circumstances, are similarly beneficial in welfare and production terms. Further work is required to substantiate these findings and to determine whether aggressiveness is a reliable cue of fighting ability and, if so, how it is manifest and assessed.