Responses of conventional pigs and Göttingen miniature pigs in an active choice judgement bias task

Abstract

Pigs are commonly kept in intensive farming systems. Their use as model animals in biomedical research has increased. Both conditions may impact upon their welfare. Recent definitions of welfare emphasize the importance of emotion. Mood congruent biases in judgement have been proposed as proxy measures of emotion in animals; optimistic responses to ambiguous cues are said to reflect positive emotional states while negative biases reflect negative emotional states. We developed a novel active-choice task to measure judgement bias in conventional farm pigs (n=7) and Göttingen minipigs (n=8). Pigs were trained to distinguish a positive tone-cue, from a negative tone-cue, signalling the location (goal-box) of large or small rewards respectively. After learning this discrimination pigs were presented with a series of ambiguous tone-cues and the percentage of choices for the positive goal-box recorded as optimistic responses (phase I). After a 4-week break, pigs were retrained and challenged with a restraint treatment to induce a negative emotional state. Judgement bias was again measured (phase II). After another 2-week break, a third test was performed to assess the effect of repeated testing (phase III). Minipigs learned the initial discrimination faster than conventional pigs but there were no differences in subsequent relearning (Breed: F1,13=21.47, P=0.0005; Phase: F2,26=121.56, P<0.0001; Breed*Phase: F2,26=10.53, P=0.0004). Both groups responded similarly in our judgement bias task (F1,13=2.70, P=0.1241) and both showed a reduction in optimistic responses after the first test (F1.28,16.59=15.08, P=0.0007). This reduction was likely due to learning about the outcomes of ambiguous cues, which potentially masked any effect of the restraint treatment. The comparable performance between the two breeds suggests that the task is suited for both breeds, and that research from one of these breeds can be generalized to the other, across farm and laboratory housing and management conditions. Advantages of an active choice task for assessing judgement bias are discussed, and questions are raised about the suitability of this task for repeated and longitudinal assessment of judgement biases in pigs.