Does frustration induce aggression in pigs?

Abstract

Behavioural and pituitary—adrenal responses of pairs of pigs to various competitive situations were investigated. Paired social encounters induced aggressive behaviour and increases in plasma corticosteroid levels in unacquainted pigs. Adreno-cortical activation was more marked in experienced than in inexperienced subordinate animals. Repeated food competition tests in an operant conditioning cage resulted in the development of stable dominance—subordinance relationships in pairs of pigs. Plasma corticosteroid levels were not influenced by the food competition test nor by social ranks. When no more food was delivered following operant responding (extinction), aggression developed between unacquainted pigs submitted by pairs to this aversive situation, but not in acquainted pigs. Adreno-cortical activation was observed only in pairs of unacquainted animals but, contrary to the paired social encounters, its occurrence did not depend on the presence of fighting. These experiments suggests that frustration induces aggression only when this behaviour has a minimal likelihood of occurrence under the experimental conditions studied. Moreover, the presence of a congener with which social bonds have been established plays a protective role against behavioural and physiological consequences of frustration.