Towards a task to assess boredom-like states in pigs-Stimulus validation as a basis

Animal boredom is a potentially prevalent, but underresearched animal welfare concern. To study the characteristics of boredom and its welfare consequences, we need to be sure that animals are actually bored and do not suffer from other negatively valenced states like apathy and depression. Animals' responses towards stimuli of different valence (positive, ambiguous, negative) have been suggested to help differentiating between these states. Apathetic animals are hypothesised to show a decreased interest in stimuli of all valences, whereas depressed animals are thought to be less interested in positive stimuli only, due to anhedonia, a key symptom of depression. In contrast, bored animals are hypothesised to show an increased interest in all types of stimuli, including negative ones. To ensure that the applied stimuli are indeed judged as positive, ambiguous or negative by the animals, we aimed to validate the valence of a range of stimuli in domesticated pigs, a species commonly kept under barren and monotonous conditions likely to induce boredom, as a basis for developing a task to distinguish between different negative states. Applying a within-subject design, 39 pigs (20 weaned piglets, 19 gilts) were individually exposed to twelve stimuli pre-classified as positive, ambiguous or negative in an approach-avoidance paradigm. The effects of stimulus, age (piglet, gilt) and their interaction on various approach and avoidance measures were analysed. Stimulus had a statistically significant effect on all measures and the observed pattern was according to pre-classification for many stimuli, but not all, resulting in a re-classification of the valence of five stimuli. The significant interaction between stimulus and age for some outcome measures indicates that age differences should be considered. Our study paves the ground for the selection of stimuli as well as outcome measures of future tasks aiming to differentiate between boredom, depression and apathy in pigs.